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The effect of light intensity and temperature
on photocatalytic water splitting
Stuart James Bell, BEng(Mech)(Hons)
A thesis submitted for the fulfilment of the
requirements for the degree of Doctor of
Philosophy
Faculty of Built Environment and
Engineering, Queensland University of
Technology
June 2011
Keywords
Photocatalysis, photosynthesis, water splitting, solar, solar hydrogen, energy
conversion, hydrogen, hydrogen generation, titanium dioxide, titania, iron
oxide, hematite, light intensity, temperature, reactor design
Abstract
Photocatalytic water splitting is a process which could potentially lead to
commercially viable solar hydrogen production. This thesis uses an
engineering perspective to investigate the technology. The effect of light
intensity and temperature on photocatalytic water splitting was examined to
evaluate the prospect of using solar concentration to increase the feasibility
of the process.
P25 TiO2 films deposited on conducting glass were used as photocatalyst
electrodes and coupled with platinum electrodes which were also deposited
on conducting glass. These films were used to form a photocatalysis cell and
illuminated with a Xenon arc lamp to simulate solar light at intensities up to
50 suns. They were also tested at temperatures between 20°C and 100°C.
The reaction demonstrated a sub-linear relationship with intensity.
Photocurrent was proportional to intensity with an exponential value of 0.627.
Increasing temperature resulted in an exponential relationship. This proved to
follow an Arrhenius relationship with an activation energy of 10.3 kJ mol-1 and
a pre-exponential factor of approximately 8.7×103.
These results then formed the basis of a mathematical model which
extrapolated beyond the range of the experimental tests. This model shows
that the loss of efficiency from performing the reaction under high light
intensity is offset by the increased reaction rate and efficiency from the
associated temperature increase.
This is an important finding for photocatalytic water splitting. It will direct
future research in system design and materials research and may provide an
avenue for the commercialisation of this technology.
Contents
Keywords
Abstract
List of Figures
Glossary of Terms
1.0 Introduction ....................................................................................................... 1
Rationale .................................................................................................... 1
2.0 Literature Review .............................................................................................. 6
2.1 Introduction ....................................................................................... 6
2.2 Solar Energy Absorption ................................................................... 7
2.3 Water splitting thermodynamics and kinetics .................................. 10
2.3.1 Thermodynamics: Gibb’s free energy and electrochemical
potential ................................................................................................ 10
2.3.2 Kinetics: Arrhenius and Butler-Volmer Equation ...................... 13
2.4 Materials for Photocatalysis ............................................................ 17
2.4.1 General .................................................................................... 17
2.4.2 Resistance ............................................................................... 20
2.4.3 Nanostructure and Morphology ................................................ 21
2.4.4 Materials Modification ............................................................... 24
2.5 Solar Collection Systems ................................................................ 27
2.6 Considerations for building a Photocatalytic water splitting system 30
2.6.1 Light Intensity ............................................................................ 32
2.6.2 Temperature and Pressure ....................................................... 35
2.7 Summary ......................................................................................... 38
3.0 Experimental Apparatus and Material ........................................................... 40
3.1 Introduction ...................................................................................... 40
3.2 Solar Simulator and Data Collection setup ...................................... 41
3.3 Reactor Development ...................................................................... 43
3.3.1 Sealed Vessel Test System ...................................................... 44
3.3.2 O-Ring/Clamp Test System ...................................................... 45
3.4 Light Intensity and Pyrometer Calibration ........................................ 49
3.5 IV Curves ......................................................................................... 52
3.6 Standard Photocatalyst ................................................................... 56
3.6.1 Fe2O3 ....................................................................................... 57
3.6.2 TiO2 ........................................................................................... 60
3.6.3 Platinum Counter Electrodes .................................................... 61
3.7 Conclusion ....................................................................................... 62
4.0 Results ............................................................................................................. 63
4.1 Repeatability of experiments ........................................................... 63
4.1.1 Scan rate .................................................................................. 63
4.1.2 Fe2O3 Films ............................................................................... 64
4.1.3 TiO2 Films ................................................................................. 66
4.1.4 Electrolyte ................................................................................ 70
4.2 Light intensity dependence of photocatalysis .................................. 72
4.2.1 Experimental methods .............................................................. 75
4.2.2 Experimental results ................................................................. 75
4.3 Temperature Dependence of Photocatalysis .................................. 81
4.3.1 Experimental methods and setup ............................................. 82
4.3.2 Experimental results ................................................................. 83
4.4 Errors and anomalies ...................................................................... 88
4.4.1 Light intensity ........................................................................... 88
4.4.2 Temperature ............................................................................. 89
5.0 Interpretation of Results, Implications for Scale Up and Practical
System Design ....................................................................................................... 91
5.1 Introduction/literature ...................................................................... 91
5.2 Light Intensity Relationship ............................................................. 92
5.3 Temperature Relationship ............................................................... 96
5.4 Model .............................................................................................. 99
5.4.1 Chart of Model ........................................................................ 100
5.4.2 Heat Balance of Reactor ........................................................ 102
5.4.3 Reactor Pressure ................................................................... 105
5.4.4 Extrapolation of Temperature relationship .............................. 106
5.4.5 Intensity effect on reaction rate .............................................. 109
5.4.6 Intensity and Temperature ...................................................... 110
5.4.7 Current density rate limitation ................................................. 111
5.4.8 Efficiency ................................................................................ 113
5.4.9 Using high performing photocatalyst ....................................... 114
5.4.10 Factors unaccounted for by the model................................. 116
5.5 Comparison to other conversion devices ....................................... 118
5.6 What does this mean for System Design ....................................... 119
5.6.1 Reactor Window ...................................................................... 119
5.6.2 Reactor Design ....................................................................... 120
5.6.3 Hydrogen Embrittlement ......................................................... 123
5.6.4 H2 Solubility at high temperature and pressure ....................... 124
5.6.5 Cost ........................................................................................ 125
5.7 Materials research directions ......................................................... 128
5.8 Summary ....................................................................................... 129
6.0 Conclusion ..................................................................................................... 130
Appendix A
Bibliography
List of Figures
Figure 2-1 Schematic of the hydrogen economy (Department of Industry
Tourism and Resources, 2005) ...................................................................... 7
Figure 2-2: Formation of an electron/hole pair via irradiation by light ............. 8
Figure 2-3: AM1.5 spectrum energy absorption of Si and Fe2O3 .................. 9
Figure 2-4: Enthalpy-temperature diagram of the reaction H2O = H2 + ½O2
(Chao, 1974) ................................................................................................ 11
Figure 2-5: Required overpotential for water splitting (Bockris, 1999) .......... 13
Figure 2-6: Activation energy plot of an exothermic reaction ....................... 14
Figure 2-7: Band levels of common photocatalytic materials (Mills & Le
Hunte, 1997) ................................................................................................ 18
Figure 2-8: Absorption Regions of Fe2O3 and TiO2 in the AM1.5 Spectrum
(Renewable Resource Data Center) ............................................................ 19
Figure 2-9: Various photocatalyst nanostructures; a) dendritic (Ilkay Cesar, et
al., 2009), b) nanotubes (G. K. Mor, et al., 2005), c) nanorods (Wahi et al.,
2005), d) calcined nanoparticles (Sivula et al., 2010). ................................. 23
Figure 2-10: Flat plate solar collector ........................................................... 28
Figure 2-11: a) Fresnel Lens, b) CSIRO Parabolic Reflector, c) Ausra Single
Axis Fresnel Reflector .................................................................................. 29
Figure 2-12: 400MW Ivanpah Solar Electric generating System
(WorleyParsons Ltd, 2008) .......................................................................... 30
Figure 2-13: Concentrating Photovoltaic coupled with High Temperature
Electrolyser (McConnell & Thompson, 2004) ............................................... 32
Figure 2-14: Relationship of Photocurrent to UV Light Intensity for a n-TiO2
electrode in 0.51M HClO4 electrolyte; (right axis, ∆) applied potential of 0V
(SCE), (left axis, ●) +2V (SCE) (Carey & Oliver, 1976). ............................... 34
Figure 2-15: Solar driven water splitting for hydrogen production as a function
of temperature (Licht, 2002) ......................................................................... 36
Figure 3-1: Diagram of Experimental setup .................................................. 41
Figure 3-2 : Absorption spectrum of the 59060filter ...................................... 42
Figure 3-3: Xe Lamp Spectrum, with (red) and without (blue) the filter,
compared to the AM1.5 spectrum (green). ................................................... 42
Figure 3-4: Prototypre Perspex Vessel ......................................................... 43
Figure 3-5: Configuration of Reactor vessel ................................................. 45
Figure 3-6: Schematic of O-Ring test system ............................................... 46
Figure 3-7: Configuration of the O-Ring type Cell Assembly ........................ 47
Figure 3-8: Assembled cell in Sand bath with heater .................................... 48
Figure 3-9: Experimental apparatus under operation ................................... 49
Figure 3-10: Comparison of <430nm light power using the power meter and
actinometry ................................................................................................... 52
Figure 3-11: Example IV Curve for TiO2 Film ............................................... 54
Figure 3-12: SEM images of Inverse Opal Fe2O3 at 160 000x and 24 000x
magnification (FEI Quanta 3D, operator - Dr Wayde Martens) ..................... 58
Figure 3-13: Schematic of Doctor Blading process ...................................... 59
Figure 3-14: FE SEM images of 2 TiO2 films produced using the same
process; a) 250 000x magnification of film 1, b) 250 000x magnification of film
2, c) 80 000x magnification of film 1, d) 80 000x magnification of film 2 (JEOL
7100, operator - Eunice Grinan)................................................................... 61
Figure 4-1: TiO2 IV Curve Scan Rate Validation .......................................... 64
Figure 4-2: a) Stable electrode, b) Cathodic corrosion c) Anodic corrosion d)
both anodic and cathodic corrosion (Gerischer, 1977) ................................. 65
Figure 4-3: UV-Vis for a new and used film ................................................. 67
Figure 4-4: Reduction in Performance from repeated testing (0.5V Bias) .... 67
Figure 4-5: Comparison of Film used with results from Literature (Glasscock,
et al., 2007; Kay, et al., 2006; Ruan, et al., 2006; Sivula, et al., 2010; Wu, et
al., 2009) ...................................................................................................... 69
Figure 4-6: Comparison of Electrolytes (Pure H2O - Red and 0.1M Na2SO4 -
Green) .......................................................................................................... 70
Figure 4-7: Repeated tests using the same electrolyte ................................ 72
Figure 4-8: Effects of Light intensity reported in literature: a) Nogueira &
Jardim (1996); b) Huang et al. (1999); c) Jiang et al. (2001); d) Lim et al.
(2000). ......................................................................................................... 74
Figure 4-9: IV curves for Cell 4.4 ................................................................. 76
Figure 4-10: IV curves for Cell 4.5 ............................................................... 76
Figure 4-11: IV curves for Cell 4.6 ............................................................... 77
Figure 4-12: IV Curves with mean and standard deviation .......................... 78
Figure 4-13: Photocurrents at various intensities for the 3 films at 0.5V
applied bias .................................................................................................. 79
Figure 4-14: Photocurrent vs intensity at 1.0V bias ...................................... 80
Figure 4-15: Quantum efficiency (<430nm) of intensity experiments at 0.5V
applied bias .................................................................................................. 81
Figure 4-16: Maxwell-Boltzmann distrubution at increasing temperature (S.
Zumdahl, 1993) ............................................................................................ 84
Figure 4-17: I-V Curves for Cell 4.7 at various temperatures........................ 84
Figure 4-18: I-V Curves for Cell 4.8 at various temperatures........................ 85
Figure 4-19: Photocurrent of Cell 4.7 ............................................................ 86
Figure 4-20: Photocurrent of Cell 4.8 ............................................................ 86
Figure 4-21: 0.5V applied bias photocurrent vs temperature ........................ 87
Figure 4-22: Quantum efficiencies at 0.5V Applied ....................................... 88
Figure 5-1: Log-Log plot of photocurrents at various intensities for 3 films at
0.5V applied bias .......................................................................................... 93
Figure 5-2: Experimental results compared to those found by Carey and
Oliver, (1976) ................................................................................................ 94
Figure 5-3: Log of 0.5V Applied photocurrents vs 1/ Temperature ............... 96
Figure 5-4: 0.5V Applied photocurrents vs Temperature .............................. 97
Figure 5-5: Temperature predicted by model at various solar concentrations
................................................................................................................... 104
Figure 5-6: Arrhenius predicted photocurrent ............................................. 108
Figure 5-7: Calculated rate coefficient, k(T) over temperatures predicted . 108
Figure 5-8: Photocurrents predicted from model compared to experimental
data ............................................................................................................ 109
Figure 5-9: Predicted photocurrents j(I) (intensity), j(T) (temperature) and J
(combined temperature and light intensity) vs light intensity ...................... 110
Figure 5-10: Limiting Current compared to predicted current ..................... 112
Figure 5-11: Conversion efficiency of light energy to hydrogen with respect
light intensity .............................................................................................. 114
Figure 5-12: Currents predicted using 1% efficient photocatalyst .............. 115
Figure 5-13: Standard I-V Curve at room temperature ............................... 117
Figure 5-14: Gibb’s free energy and potential change over temperatures
calculated from our model .......................................................................... 118
Figure 5-15: Assembled Cylinder Reactor ................................................. 122
Figure 5-16: Cylinder Reactor Exploded View ........................................... 122
Figure 5-17: Projected cost of Heliostat concentration (Sargent & Lundy LLC
Consulting Group, 2003) ............................................................................ 127
Glossary of Terms
List of Abbreviations
AM1.5 Air mass 1.5
CB Conduction band
Fe2O3 Iron oxide
FTO Fluorine doped tin oxide (SnO:F)
IV Current - Voltage
Na2SO4 Sodium sulphate
P25 Grade of TiO2
PMMA Poly(methyl methacrylate)
Pt Platinum
PV Photovoltaic
QE Quantum efficiency
SEM Scanning electron microscope
S.T.P. Standard temperature and pressure
TiO2 Titanium dioxide
UV Ultraviolet
UV-vis Ultraviolet to visible spectroscopy
VB Valence band
Xe Xenon
List of Symbols
A Pre-exponential factor
Surface area (cm-2)
C Concentration (mol. L-1)
Coulomb (A s)
c Speed of light in vacuum (m s-1)
D Diffusivity
d Path length (m)
E Potential (V)
EA Activation energy (J)
EG Band gap (eV)
eV Electron volt (eV)
F Faraday’s Constant (C mol.-1)
ΔG Gibbs free energy (kJ mol.-1)
h Planck’s constant (J s)
Heat transfer coefficient
ΔH Change in Enthalpy (kJ mol.-1)
hv Photon energy (J)
I Light intensity (mW cm-2-)
i Current (A)
j Photocurrent density (A cm-2)
k Reaction rate constant
Thermal conductivity (W m-1 K-1)
kB Boltzmann’s constant (J K-1)
L Diffusion length (m)
Characteristic length (m)
M Molar (mol. L-1)
N Number of photons
n Number of atoms per molecule
Number of tests
NA Avogadro’s number (mol.-1)
Nu Nusselt number
Pr Prandtl number
q Charge (C)
Q Heat transfer rate (J s-1)
R Universal gas constant (J K-1 mol.-1)
r Reaction rate (s-1)
Degradation rate (n-1)
Re Reynolds number
ΔS Change in Entropy (kJ mol.-1)
T Temperature (°C or K)
t Time (s)
V Voltage (V)
Volume (cm3)
Velocity (m s-1)
ν Kinematic viscosity (m2 s-1)
List of Greek Symbols
α Thermal diffusivity (m2 s-1)
β Transfer coefficient (symmetry factor)
δ Diffusion layer thickness (m)
ε Extinction coefficient
Emissivity
η Efficiency (%)
Φ Yield
λ Wavelength (nm)
μ Dynamic viscosity (N s m-2)
ρ Fluid density (kg m-3)
σ Stefan-Boltzmann constant (W m-2 K-4)
Statement of original authorship
The work contained within this thesis has not been previously submitted to
meet the requirements for an award at this or any other higher education
institution. To the best of my knowledge and belief, this thesis contains no
material previously published or written by any other person except where
due reference is made.
Signature: .
Date: 15/03/2012 .
Acknowledgments
Firstly I would like to thank my supervisors, Associate Professor Geoff Will
and Professor John Bell, for their input into this work.
I would also like to thank my friends and colleagues from O401 for their
support and help throughout this PhD.
Finally I wish to thank my friends and family, because without them I wouldn’t
have gotten this far.
1
1.0 Introduction
This PhD investigation was undertaken to investigate how light intensity and
temperature affect’s photocatalytic reactions, specifically the photocatalytic
water splitting reaction. The implications of these investigations where
applied to system design and implementation, to maximise the feasibility of
photocatalysis for water splitting.
Rationale
This work was initiated by asking the question; “how would a photocatalytic
water splitting system be built?”
There are two general approaches which could be applied to a photocatalytic
system for splitting water (disregarding materials aspects). The first approach
is to use a one sun, flat plate style system with a cheap photocatalyst and
2
large illuminated area. The second approach is to incorporate solar
concentrators and focus light on a small reactor yielding product at a high
rate.
Both of these approaches have been used in the more mature solar energy
field of photovoltaics, where both one sun and multiple sun systems are at
the commercial level. Concentrated light systems for photovoltaics however,
lose efficiency as the temperature increases and must be actively cooled.
Photocatalytic water splitting however is a chemical reaction which, unlike the
photovoltaic effect, will accelerate with increasing temperature. The Gibbs
free energy required to split water is reduced at high temperature as thermal
energy contributes to the energy required for the reaction. Furthermore, at
high temperatures the resistance of the cell is decreased.
These trends suggest that the previously unused infrared energy in the solar
spectrum could heat a reactor and contribute to the reaction. This will allow
the use of a portion of the solar spectrum previously unexploited by quantum
solar energy conversion devices.
A literature survey identified some significant gaps in this area, in particular
experimental studies. Therefore the following research questions were
developed:
Physical parameters
1. What effect does increasing the incident light intensity have on the
rate of reaction?
3
2. What effect does high temperature and the required pressure increase
have on the rate of reaction?
3. How does the change in the energy required to split water with
increasing temperature affect photocatalytic water splitting?
System
4. How can these results be applied to the engineering of practical
systems?
5. What problems will be encountered by increasing light intensity, and
subsequently temperature, for practical systems and how can they be
addressed?
Thus this thesis consists of two sections; investigating the impact of physical
parameters on the reaction (chapters 3 – 5), and the implications of these
results for practical system design (chapter 6).
This thesis, and the experimental study upon which it is based, attempts to
answer these research questions and interpret their implications towards
developing photocatalytic water splitting as a commercially viable technology.
The chapters of this thesis are listed below with a brief explanation of their
contents and purpose.
Summary of chapters
Chapter 2 – Literature Review
Chapter 2 presents a literature survey into the pertinent areas of this
investigation. This includes an introduction into the basics of photocatalysis,
4
the thermodynamics of water splitting, a materials overview, a discussion of
solar energy collection and a review of existing literature regarding
photocatalytic water splitting at high temperature and light intensity.
Chapter 3 - Experimental Apparatus and Materials
Chapter 3 discusses the development and layout of the experimental
apparatus used for the experimental portion of this study. It describes the
solar simulator and data collection setup, reactor development, parameter
manipulation and photocatalyst development and characterisation.
Chapter 4 - Results
This chapter presents the experimental results of this work. The acquisition
of stable photocatalyst materials, the effect of the electrolyte and
experimental methods are discussed here. These are important for
establishing repeatable and reliable results. This is followed by the
presentation of light intensity and temperature test results. These
experiments form one of the major contributions of this work, and to the best
of the authors knowledge have not been studied previously.
Chapter 5 –Interpretation of results, implications for Scale Up and System
Design
This chapter is devoted to the analysis of the experimental results. It includes
the interpretation of the results and the development of a model to describe
those results and extrapolate outside the range of parameters tested. This is
followed by a discussion into reactor design and materials research
directions.
5
Chapter 6 - Conclusion
Chapter 6 summarises the work and findings of this thesis. It compares the
outcomes of the work to the research question, acknowledges limitations of
the work and comments on future directions for work in this area.
6
2.0 Literature Review
2.1 Introduction
Energy has been a major political, economic and environmental issue
throughout the 20th century and will become the number one issue of the 21st
century. The amount of energy that falls on the earth in sunlight is 3 × 1024
joules per year, or 10,000 times that of the world’s usage (Grätzel, 2001)
meaning that large scale conversion of solar energy to a usable form is a
potential solution to future energy needs.
Conventional methods for capturing solar energy (solar thermal and
photovoltaics) suffer from limited capability to efficiently store that energy
over periods greater than a few hours. Solar to hydrogen energy conversion
7
could provide energy storage in a form easily used for stationary power
generation, aviation, land transportation, heating and in remote areas
(Department of Industry Tourism and Resources, 2005). Photocatalytic water
splitting is a technology that could produce renewable hydrogen using solar
energy and water to contribute to a hydrogen economy (Figure 2-1)
Figure 2-1 Schematic of the hydrogen economy (Department of Industry
Tourism and Resources, 2005)
2.2 Solar Energy Absorption
Photocatalysis is defined as the acceleration of a reaction in the presence of
a light activated catalyst (Mills & Le Hunte, 1997). A photocatalyst is the
material acting as the catalyst in the reaction, usually a metal oxide
semiconductor. Photocatalysts utilise a phenomena known as the Becquerel
8
effect to produce a charge separation, which drives the catalytic reaction.
Simply put, this effect is the transfer of the energy from a photon to an
electron in the valence band (VB), promoting the electron to the conduction
band (CB) of the material (Figure 2-2). This same effect is utilised by
photovoltaics and other quantum solar energy conversion devices to produce
electric current.
Figure 2-2: Formation of an electron/hole pair via irradiation by light
The charge separation is dependent on the band gap of the material and the
energy of the incident photon (Mills & Le Hunte, 1997). If the gap between
the lowest free energy state in the conduction band and the original valence
position of the electron is larger than the energy of the incident photon, then
the promoted electron will fall back to the valence band releasing its energy
as heat. If the transferred energy is greater than that required to promote the
electron above the lowest free energy state in the conduction band, then the
electron will settle in the conduction band, lose its excess energy and form a
separated electron-hole pair.
9
This selective absorption of photons producing a charge separation has
some important ramifications for solar energy conversion. As the solar
spectrum spans a large range of photon energies (4.4 – 0.3 eV), photons
with an energy less than the band gap of the photodevice will not be
absorbed (Mills & Le Hunte, 1997). Furthermore, extra energy provided by
the absorbed photon above that of the semiconductor bandgap is also lost.
The maximum light absorption for 3 semiconductors commonly used in
photoconversion devices are outlined in Figure 2-3. The energy absorbed by
Si (1.1eV bandgap, red), Fe2O3 (2.2eV bandgap, green) and TiO2 (3.2eV
bandgap, blue) is depicted by the area under their respective curves. It can
be seen that Si absorbs further towards the infrared portion of the spectrum
but less of the energy in the higher energy light. TiO2 however, collects much
more of the energy in the light it absorbs, but only absorbs a small portion of
the spectrum.
Figure 2-3: AM1.5 spectrum energy absorption of Si and Fe2O3
10
The area under the curves corresponds to 50.4% for Si, 24.1% for Fe2O3 and
5.0% for TiO2, of the energy in the AM1.5 solar spectrum. This is an
important factor when considering materials for quantum solar energy
conversion.
2.3 Water splitting thermodynamics and kinetics
2.3.1 Thermodynamics: Gibb’s free energy and electrochemical potential
The splitting of water using a quantum device is further limited by the
thermodynamics of the reaction. This reaction occurs via 2 half reactions at
electrically opposite electrodes (Nowotny, Sorrell, Bak, & Sheppard, 2005):
eHOOHhv
442 22 Anode
OHHeOH 222 22 Cathode
The Gibbs free energy required for this reaction is 237.1kJ/mol at standard
temperature and pressure (Chase, 1998). This equates to a water splitting
potential of 1.23eV, but when electrode/electrolyte interface losses, and
overpotential requirements are considered it requires a bandgap around 1.7 -
1.9 eV to produce a significant reaction rate (Nowotny, Sorrell, Sheppard, &
Bak, 2005). This is greater than the optimum bandgap for solar energy
harvesting.
The Gibbs free energy is reliant on the enthalpy change (ΔH), the entropy
change (ΔS) of the reaction and the temperature of the system (T), given by
(S. S. Zumdahl, 1993):
11
STHG Equation 2.1
This equation demonstrates that as temperature increases, thermal energy
reduces the Gibbs free energy required for the reaction. This shifts the
equilibrium of the reaction towards the products and is described graphically
in Figure 2-4.
Figure 2-4: Enthalpy-temperature diagram of the reaction H2O = H2 + ½O2
(Chao, 1974)
The reduction in work required (ΔG) for the reaction, due to the contribution
of the T·ΔS term, has resulted in several technologies. High temperature
electrolysis is based upon the premise that heat is approximately 1/3 of the
cost of electricity, so performing the electrolysis at high temperature yields
more economically competitive electrolysis of water (Bockris, 1999).
12
Thermolysis is a technique where water is heated to such a temperature and
pressure that the Gibbs free energy is zero, and it dissociates of its own
accord (Chase, 1998). This suggests that heat from a renewable resource -
such as the sun - could be used to reduce the overpotential required to split
water. This could allow the use of photodevices with band gaps closer to the
optimum for the solar spectrum, resulting in more efficient solar collection
(Licht, 2002; Licht, 2003; Licht, Halperin, Kalina, Zidman, & Halperin, 2003).
It would also make use of the otherwise wasted infrared section of the
spectrum.
The chemical potential for the water splitting reaction is important, as it
corresponds to the minimum voltage that must be applied to drive the
endothermic reaction (S. S. Zumdahl, 1993). The Gibbs free energy is
directly related to the electrochemical potential by the following modified
Nernst equation:
OHH EFG22
.20
Equation 2.2
Where: F is the Faraday constant (9.649 ×104 C mol-1) and EH2O is the
potential required to split water. As stated above, the required potential to
split water is 1.23eV at standard temperature and pressure. Figure 2-5
shows the effect of temperature on this potential.
The water splitting potential is directly related to the band gap required by a
photocatalyst to split water. The band gap must be greater than 1.23eV and
its band levels must span the water reduction and oxidation (redox) potentials
for the reaction to occur at STP (Ni, Leung, Leung, & Sumathy, 2007).
13
Figure 2-5: Required overpotential for water splitting (Bockris, 1999)
2.3.2 Kinetics: Arrhenius and Butler-Volmer Equation
The speed at which a chemical reaction occurs is described by the kinetics of
the reaction. Many parameters affect reaction kinetics including temperature,
species of the reactant and their concentration. As the reaction rate is
important for our study, then the effect of temperature on the reaction kinetic
must be ascertained.
The change in the reaction rate with temperature is described by the
Arrhenius equation which holds for all reactions (S. S. Zumdahl, 1993). The
Arrhenius equation is an empirical equation which uses the activation energy
(EA) and a collision frequency term (A, also called the pre-exponential factor)
to determine the reaction rate at different temperatures. The other factors in
the Arrhenius equation are temperature (T), the universal gas constant (R)
and the rate constant of the equation (k).
OV
ER
PO
TE
NT
IAL
14
RT
EA
eAk
Equation 2.3
The activation energy is the amount of energy required to cause a reaction to
proceed (Figure 2-6) whilst the frequency factor describes the probability of
an effective collision resulting in a reaction (S. S. Zumdahl, 1993). The
collision frequency accounts for the need for two molecules to come into
contact and “collide” before a reaction can occur. As temperature increases,
the frequency of collisions also increases due to molecules possessing more
kinetic energy and moving faster. Additionally, the number of molecules
possessing enough energy to overcome the activation energy is also
increased. Therefore, an increase in temperature will result in higher reaction
rate. In relation to the Arrhenius equation above, the temperature factor
increases, reducing the negative exponential term and subsequently
increasing the reaction rate
Figure 2-6: Activation energy plot of an exothermic reaction
15
Photocatalysis occurs at the interface between and solid semiconductor and
an electrolyte (Hoffmann, Martin, Choi, & Bahnemann, 1995; Mills & Le
Hunte, 1997). This commonly takes place in one of two situations, a
photocatalyst particle suspended in a solution (i.e. a slurry reactor) or with
the photocatalyst attached to an electrode. The latter situation is relevant to
this investigation.
Reactions that occur at electrodes in solution are electrochemical reactions
and are generally investigated via their voltage and current characteristics.
The Butler-Volmer equation describes the relationship between electric
current (i), applied potential (E) and concentration of the reduced and
oxidised species (CR and CO) for a reversible reaction (Bard & Faulkner,
2001).
Equation 2.4
Where: F is the Faraday constant, A is the electrode area, k0 is the standard
rate constant at Eeq which is the equilibrium potential, α is the transfer
coefficient/symmetry factor between 0 and 1, R is the universal gas constant
and T is the temperature.
The Butler-Volmer equation shows that the equilibrium exchange rate of a
reversible reaction becomes further from equal the greater the applied
electrode potential. For example, when E - Eeq = 0 (i.e. equilibrium), both the
terms relating to the reduced and oxidised species have the same value. If a
voltage is applied, the reaction will favour one of these species and a current
will be produced. This means that greater the overpotential (i.e. the greater
16
the difference between E and Eeq), the faster the reaction rate. The
overpotential effectively “drives” the reaction.
An increase in temperature has the effect of favouring the oxidation reaction
as the oxidation term is increased when its negative exponential factor is
decreased. The negative reduction term however, is decreased as its positive
exponential factor is increased. Higher temperature will also lower the
required potential for splitting water, due to the lower Gibbs free energy,
which increases the overpotential for any applied voltage (Licht, 2002; Licht,
2003, 2005a; Licht, et al., 2003).
Another kinetic factor affected by temperature is the limiting current of the
electrode/electrolyte interface. The limiting current is the maximum current
flux that can pass between the electrode and electrolyte, and is described by
the following equation (Gerasimov & Rozenfeld, 1956):
nFDCiLim.
Equation 2.5
Where: .Limi is the limiting current density, n is the number of electrons
transferred in the reaction, F is Faraday’s constant, D is the diffusion
coefficient, C the bulk solution concentration and the diffusion layer
thickness.
Of these factors, both the diffusion coefficient and the diffusion layer
thickness are influenced by temperature. The diffusion coefficient increases
with temperature whilst the diffusion layer thickness is reduced. This means
that the maximum current that can pass between the electrode and the
electrolyte increases with temperature (Gerasimov & Rozenfeld, 1956).
17
Also of note, is that the conductivity of a semiconductor is enhanced by
increasing temperature, as more thermal energy allows easier promotion to
the conduction band (Callister, 2000). In an electrolyte, the conductivity is
reliant on the mobility of ions through the solution. This is also faster at higher
temperature, resulting in higher electrolyte conductivity (Callister, 2000).
2.4 Materials for Photocatalysis
2.4.1 General
There are a number of factors influencing how effective a material is for
photocatalysis. Firstly, a semiconductor with a bandgap larger than the
required potential to split water (1.23eV at STP) is required. Also, the
conduction band potential must be more negative than the water reduction
potential and the valence band potential must be more positive than the
water oxidation potential (Bockris, 1999; Mills & Le Hunte, 1997; Ni, et al.,
2007). Figure 2-7 gives an overview of some of the commonly studied
photocatalysts.
Some of the smaller band gap metal oxides such as tungsten oxide and iron
oxide do not span the redox potentials. In order for them to dissociate water,
extra potential must be applied (Glasscock, Barnes, Plumb, & Savvides,
2007). This means that energy must be put into the system for the reaction to
proceed. However, lower bandgap semiconductors can absorb more of the
solar spectrum, which increases the possible energy conversion efficiency.
18
Figure 2-7: Band levels of common photocatalytic materials (Mills & Le Hunte,
1997)
The greater possible efficiency of lower band gap semiconductors is due to
the shape of the solar spectrum. The absorbance regions of anatase TiO2
and Hematite Fe2O3 are shown in Figure 2-8. Anatase - with a band gap of
3.2eV - can absorb light at wavelengths shorter than 388nm, or 5.4% of the
AM1.5 spectrum. Hematite however - at 2.2eV – absorbs wavelengths below
564nm, which corresponds to 29.6% of the AM1.5 spectrum. The maximum
theoretical efficiency of these photocatalysts however, is 24.1% for Fe2O3
and 5.0% for TiO2 for the AM1.5 spectrum. This is because the
semiconductor only absorbs energy equal to its band gap, the rest being lost
as heat (Figure 2-3).
19
Figure 2-8: Absorption Regions of Fe2O3 and TiO2 in the AM1.5 Spectrum
(Renewable Resource Data Center)
Another important factor when choosing a photocatalyst material is
photocorrosion. Photocorrosion is a process - which occurs concurrent to
photocatalysis – resulting in the loss of photocatalytic performance (Mills &
Le Hunte, 1997; Ni, et al., 2007). Cadmium Sulfide (CdS) has a band gap of
2.5 eV situated favourably for the dissociation of water (Figure 2-7).
However, it suffers from deactivation of its catalytic capability due to a
corrosion reaction that competes with the water splitting reaction
(Ashokkumar, 1998):
SCdCdShvb 22
A photocorrosion model developed by Gerischer (1977) and discussed in
more detail in section 3.6.1; states that any semiconductor material that does
not span the water splitting potentials is susceptible to corrosion under light.
This includes the Fe2O3 and WO3 materials depicted in Figure 2-7. For
20
materials that do not span the potentials, the favourability of the corrosion
reaction in comparison to the water splitting reaction determines its
photocorrodibility.
2.4.2 Resistance
How easily electrons move through a semiconductor can significantly affect
its performance as a photocatalyst. This is usually described by the diffusion
length of the material (Ld), which is the average distance charges (i.e.
electron or hole) will move before recombination. It is related to the diffusivity
(D) and charge lifetime (t) via the equation:
DtLd 2
Equation 2.6
Kennedy and Frese (1978) found that the diffusion length for Hematite was 2
- 4 nm, whereas TiO2 has a diffusion length of around 100nm. This low
charge mobility has been the major disadvantage of using hematite as a
photocatalyst. It has lead to doping hematite with various elements to
increase the material’s conductivity. Early investigations found that Ti, Sn,
and Zr act as electron donors and Nb and Ta as double electron donors. Ca,
Cu, Mg and Ni generated holes whilst Mn and Cr become electron trapping
sites (Shinar & Kennedy, 1982). Recent studies doping with Ti and Si has
acquired some recent successful results (I. Cesar, Kay, Gonzalez Martinez,
& Gratzel, 2006; Ilkay Cesar, Sivula, Kay, Zboril, & Grätzel, 2009;
Glasscock, et al., 2007; Kay, Cesar, & Grätzel, 2006). Pt has also been used
as an electron donor (Hu et al., 2008).
21
2.4.3 Nanostructure and Morphology
The nanostructure of the photocatalyst has a large affect on its performance
and generally, greater surface area means better performance (Hoffmann, et
al., 1995; Mills & Le Hunte, 1997; Ni, et al., 2007). This is due to a number of
reasons. Firstly, most photocatalytic reactions require a site for the reaction
to occur and high surface area means more active sites are available. Porous
structures also increase light scattering in the material and subsequently the
amount of light absorbed. Also, semiconductor nanostructure dimensions
below the sum of the diffusion layer and depletion layer widths increases
electron/hole separation, reducing recombination (Marín, Hamstra, &
Vanmaekelbergh, 1996).
High surface area materials can be achieved using many different methods
to produce various structures. This is generally approached in two ways:
Solution based deposition - where the material is prepared then
coated to a substrate whilst in a solution (eg. dip coating, doctor
blading, spin coating, spray coating or spray pyrolysis) (Arabatzis et
al., 2002; Nazeeruddin et al., 1993).
Growing the material on the substrate (eg. electrodeposition, vapour
deposition, sputtering) (Ilkay Cesar, et al., 2009; Glasscock, et al.,
2007).
These methods can also be combined (eg: growing nanotubes then
depositing them in a solution). Synthesis of materials can result in a number
of different structures, these include; nanoparticles - such as nanosperes,
22
nanotubes and nanorods, dendritic growth structures and inverse opal
lattices.
Crystal properties of a photocatalyst are another morphological factor that
influences performance. These properties include the size, orientation and
electronic structure of the crystal, active sites on the surface and their
exposure to reactants, and electronic interactions between the crystals and
grain boundaries (Mills & Le Hunte, 1997).
Some examples of methods of photocatalyst synthesis are shown in Figure
2-9 and briefly described below.
Figure 2-9 a) shows a Si doped, hematite Fe2O3 dendritic type nanostructure
grown on a substrate using atmospheric pressure chemical vapour
deposition (APCVD) (Ilkay Cesar, et al., 2009; Kay, et al., 2006). Its feature
size was controlled by the deposition temperature, allowing the distance that
charges have to travel to the electrolyte interface to be minimised. However,
the film is thick enough to absorb a majority of the light.
Figure 2-9 b) depicts a titanium dioxide nanotube array grown electrolytically
on a titanium sheet (G. K. Mor, Shankar, Paulose, Varghese, & Grimes,
2005; Gopal K. Mor, Varghese, Paulose, Shankar, & Grimes, 2006; Ruan,
Paulose, Varghese, & Grimes, 2006). The wall thickness and length of the
tubes were controlled using the bath temperature, allowing porosity and film
thickness to be optimised.
23
Figure 2-9: Various photocatalyst nanostructures; a) dendritic (Ilkay Cesar, et
al., 2009), b) nanotubes (G. K. Mor, et al., 2005), c) nanorods (Wahi et al., 2005),
d) calcined nanoparticles (Sivula et al., 2010).
The nanorods presented in Figure 2-9 c) were synthesised by Wahi et al.
(2005). They used various solution based methods to produce nanoparticles
with various physical properties, including; size, shape, surface area, crystal
structure and phase composition. They reported that particle size, surface
area, crystal phase and exposed crystal orientation all affected the
performance of the photocatalysts for pollutant degredation reactions.
Sivula et al. (2010) produced the mesoporous electrodes displayed in Figure
2-9 d). The films were manufactured by doctor blading a colloid solution on a
substrate, allowing to dry, then heating twice (400°C, and 700 or 800°C
24
respectively). The second sintering temperature had a large affect on the
average particle size and consequently the photocurrent.
2.4.4 Materials Modification
There have been many techniques developed to modify photocatalyst
materials and improve their performance. Some of the most common
methods are discussed below.
Noble Metal Loading
Noble metal loading is the practice of depositing small amounts of noble
metals (platinum, gold, silver, etc) onto the surface of the photocatalyst. As
noble metals have lower Fermi energy levels than the photocatalyst
semiconductor, they form what is known as a Schottky barrier (Linsebigler,
Lu, & Yates, 1995). This Schottky barrier allows electrons to pass from the
catalyst to the noble metal but not the other way, effectively separating them
from holes and reducing recombination. This is called “charge trapping”. After
an electron is “trapped” the metal particle forms an effective reduction site
due to its own photocatalytic ability, whilst the holes remaining in the
photocatalyst perform oxidation reactions (Jakob, Levanon, & Kamat, 2003).
Noble metal loading for photocatalytic reactions was first demonstrated by
Sato & White (1980) who used platinised, powdered TiO2 to decompose
water in the presence of CO and form gaseous hydrogen.
Ion Doping
Cations and anions can also be added to the semiconductor bulk to improve
its photocatalytic ability. This is achieved by narrowing the semiconductor
bandgap, or by imposing mid-gap bands in the forbidden zone of the
25
semiconductor (Nowotny, Sorrell, Sheppard, et al., 2005). This means that
the electronic properties of the photocatalyst can be tuned whilst still
retaining favourable properties of the material - such as stability and
structure. Examples include; band gap modification for spectral absorption,
lowering electrical resistance and introducing active sites on the material’s
surface.
Doping with cations of higher valences than that of the base-metal in the
oxide (i.e. Ti4+ in TiO2) creates n-type doping, whereas doping with ions of
lower valences results in p-type doping. However, dopants that are located
too far from the surface of the semiconductor, or if the concentration is too
high, can increase recombination (Carp, Huisman, & Reller, 2004; Ni, et al.,
2007). Doping with anions is less likely to form recombination centres as they
replace the O2- ions in the TiO2 lattice producing a band shift which reduces
the bandgap (Asahi, Morikawa, Ohwaki, Aoki, & Taga, 2001).
Metal Ion Implantation
High energy transition metal ions implanted into the semiconductor lattice
substitute themselves into the base-metal ion lattice positions after
calcination. This results in a narrowing of the semiconductor bandgap similar
to ion doping, but preventing undesired impurities, and with greater control
over film thickness and crystallinity (Ni, et al., 2007).
Studies into the implantation of ions (such as V, Cr, Mn and Fe) into TiO2
found a red shift in the absorption spectrum of the material. This allowed the
utilisation of visible light for photocatalytic reactions (Anpo et al., 2001; H.
Yamashita et al., 2003; Hiromi Yamashita et al., 2002).
26
Dye Sensitisation
Dye sensitisation involves coating the semiconductor surface with a dye
which, when illuminated injects electrons into the semiconductor’s conduction
band. The semiconductor then becomes a charge separator and uses the
electron to perform the reactions, or in the case of a dye sensitised solar cell
drive an external circuit (O'Regan & Grätzel, 1991).
Dhanalakshmi, Latha, Anandan, & Maruthamuthu (2001) investigated the
effects of the catalyst amount, dye concentration and Pt loading on TiO2 for
hydrogen production in a slurry system. They found that increasing the
catalyst and Pt loading quantities above certain amounts, or adsorbing dye
molecules onto the TiO2 surface did not further increase H2 evolution rate.
Composites
Composite photocatalysts use semiconductors with different bandgaps to
absorb more of the solar spectrum. Generally, a small bandgap
semiconductor injects electrons into a large bandgap semiconductor. This
also results in greater charge separation and less recombination. Electrons
can either be produced solely by the small bandgap semiconductor or by
both semiconductors (Carp, et al., 2004)
The photocatalytic ability of a CdS-TiO2 nanocomposite film was investigated
by So, Kim, & Moon (2004). The inclusion of CdS extended the optical
absorption of the film to over 500nm. A ratio of about 0.8 CdS to 0.2 TiO2
was found to produce the highest photocurrent under solar simulated light.
27
Multiple Junction Cells
Multiple junction cells are commonly investigated in the photovoltaic area and
can obtain efficiencies over 40% (Martin A. Green, Emery, Hishikawa, &
Warta, 2011). They obtain such efficiency by multiple semiconductor
junctions to collect the light. This enables collection of light over a range of
wavelengths and voltages. As this type of photovoltaic is complex to produce,
it is mainly used in solar concentrating systems where small cell area and
high efficiency is required. Theoretical maximum efficiencies range from 50%
at 1000 suns for a two junction system (bandgaps of 1.64 and 0.96 eV) to
72% for a 36 junction cell (Henry, 1980). An AlGaAs/SiRuO2/Ptblack cell is
claimed to have achieved 18.3% efficiency for water splitting under 1 sun
AM0 illumination (Licht, 2001).
Therefore, when choosing a photocatalyst a number of factors must be
considered to obtain the most appropriate material. These factors include
spectral absorption range of the solar spectrum, band gap, locations of bands
relative to the water splitting potentials, electrical resistance, control over
nanostructure and crystallinity, stability, cost and ease of production. The
most commonly studied photocatalysts are Titanium Dioxide (TiO2), Iron
Oxide (Fe2O3) and Tungsten Oxide (WO3) as they have the most favourable
compromises between these factors.
2.5 Solar Collection Systems
There are two general types of solar collection systems: non-concentrating
and concentrating. Non-concentrating systems can be as simple as a flat
28
plate angled towards the equator at the local latitude (Figure 2-10), or employ
complex 1 and 2 axis solar tracking to capture more light.
Figure 2-10: Flat plate solar collector
Other forms of low or non-concentrating systems include V type, or
compound parabolic collectors. These are generally trough style
concentrators and use curved or flat plates to distribute light onto a collection
tube (Bandala, Arancibia-Bulnes, Orozco, & Estrada, 2004; Sixto Malato et
al., 2003; S. Malato et al., 2002; McLoughlin, Ibanez, Gernjak, Rodriguez, &
Gill, 2004). They are usually used in water purification or solar thermal
applications.
Parabolic concentrators use a parabolic arc shape to concentrate light to a
focal point (Figure 2-11: b). They come in two forms – single-facet; where
one reflector membrane is formed into the parabolic contour – and multi-
faceted; where a number of shaped reflectors are mounted together (Alpert
et al., 1991). They require very precise shaping and can only collect direct
sunlight, so must use 2-axis tracking. This makes them expensive in
comparison to some more recent systems.
29
Fresnel lens and reflector concentrators are thinner, easier to manufacture,
require less material and precision tooling and are subsequently cheaper
than parabolic systems. They work by dividing the reflector into concentric
rings which reflect to the same point. This gives them a reasonable
approximation of a parabolic lens whilst allowing large apertures and smaller
focal lengths. Ausra use a similar setup - single axis Fresnel reflectors
(Figure 2-11: c) - in their solar thermal systems (Ausra Inc., 2011).
Figure 2-11: a) Fresnel Lens, b) CSIRO Parabolic Reflector, c) Ausra Single
Axis Fresnel Reflector
Heliostat fields use many independent reflectors which are controlled to track
the sun and reflect to a focal point. These systems are typically used in large
scale solar concentration where high concentration is required. They are
generally cheaper than parabolic reflectors.
Solar Systems Pty Ltd are using a heliostat field for a concentrated
photovoltaic plant to be built in Victoria, Australia (Solar Systems Pty Ltd).
Another project using heliostat fields is the 400MW Ivanpah Solar Electric
Generating System in California (Figure 2-12).
30
Figure 2-12: 400MW Ivanpah Solar Electric generating System (WorleyParsons
Ltd, 2008)
2.6 Considerations for building a Photocatalytic water splitting system
The most important consideration when developing a photocatalytic water
splitting system is the efficiency of the photocatalyst. This is because it
determines the amount of photocatalyst required to produce the hydrogen at
the desired rate. For example, a photocatalyst with 10% energy efficiency at
1 sun will require approximately 56 m2 of illuminated photocatalyst to produce
hydrogen at a rate of 5kW (with no other losses).
This example assumes optimum production; the actual area required for a
5kW system would be much greater. Variations in light levels – due to
weather and time of day – will significantly affect reaction rate. System
related factors will also affect the production rate. These include; energy
losses through the window, losses in collecting the hydrogen, useful catalyst
lifetime and pumping and compression losses. If a concentrated light was
considered, then the losses due to these factors diminish.
For instance, a concentrated light system will require a much smaller reactor.
If the same 10% efficiency 2 was maintained using 100 suns, then the
illuminated photocatalyst area would be 0.56 m2 for a 5 kW system. This
31
means less catalyst is needed; allowing the use of a more sophisticated (and
potentially expensive) catalyst and cheaper replacement if required. It would
also make capture of evolved gases easier and sealing against leakage more
effective (especially pertinent when dealing with hydrogen).
Using concentrated light to improve the economic feasibility of solar energy
conversion devices is not a new concept. It has been thoroughly investigated
for photovoltaic applications. Noteworthy investigations include a 3-junction
InGaP/InGaAs/Ge cell with an efficiency of 37.5% (Yamaguchi, Takamoto, &
Araki, 2006) and a recently announced record efficiency of 41.1% efficiency
by the Fraunhofer institute in January 2009 (Zubi, Bernal-Agustín, &
Fracastoro, 2009). An Australian group headed by Professor Andrew Blakers
has also investigated silicon based concentrated light photovoltaic solar cells
for a number of years (Blakers, 2000; M. A. Green, Blakers, Wenham, et al.,
1987; M. A. Green, Blakers, Zhao, et al., 1987; M. A. Green et al., 1989; M.
A. Green et al., 1986; M. A. Green, Zhao, Wang, & Blakers, 1990; Zhao,
Wang, Blakers, & Green, 1988). This technology has reached the level where
it is becoming commercialised, for instance the $420 million project to be built
in Victoria (Solar Systems Pty Ltd).
Concentrated light photovoltaics coupled with electrolysis for solar hydrogen
production has also been investigated. An investigation into using a spectral
splitter to separate infrared and visible wavelengths, then provide heat and
electricity for high temperature electrolysis has been undertaken (Figure
2-13) (McConnell & Thompson, 2004). Theoretical studies predict possible
efficiencies around the 50% mark for such a system; a finding that is
applicable to both photocatalytic water splitting and photovoltaics coupled
32
with high temperature electrolysis (Licht, 2002; Licht, 2003, 2005a, 2005b;
Licht, et al., 2003). Licht’s work is discussed in more detail in section 2.6.2.
Figure 2-13: Concentrating Photovoltaic coupled with High Temperature
Electrolyser (McConnell & Thompson, 2004)
A search of the literature for the effects of light intensity and temperature on
photocatalytic water splitting reactions was undertaken and a gap in the
knowledge in this area was found. The existing literature is discussed below.
2.6.1 Light Intensity
Early theoretical investigations into the maximum limit of photochemical solar
energy conversion include Ross and Hsiao (1977) and Bolton (1978). Their
work was expanded upon by Bilchak et al. (1980) who published an
investigation into how light intensity and temperature effects the maximum
theoretical efficiency for single and multiple bandgap photoconverters (using
the AM1.2 spectrum). Their results for single junction photoconverters are
shown in Table 1. It is noted that theoretical efficiencies increase linearly with
33
the logarithm of the light intensity at constant temperature, and decrease
linearly with temperature at a constant intensity.
Table 1: Maximum theoretical efficiencies for a single band gap system over
various light intensities and temperatures using the AM1.2 spectrum (Bilchak,
et al., 1980).
These studies are purely theoretical and minimal experimental investigations
for the water splitting reaction have been reported in the literature. An early
experimental study by Carey & Oliver (1976) used an Argon ion laser to
illuminate a TiO2 electrode with UV light (351 & 364nm) at intensities up to
400mW/cm2 (approximately 80 suns). They found that the photocurrent
response was non-linear at intensities above 12mW/cm2 (approximately 6
suns) (Figure 2-14). They attributed this non-linearity to a reduction in
quantum efficiency at higher intensities, due to increased recombination
rates.
The only other experimental study found used K4Nb6O17 (3.5 eV bandgap) as
the photocatalyst in a suspension (Tabata, Ohnishi, Yagasaki, Ippommatsu,
& Domen, 1994). The evolution rate at light intensities up to 16 suns UV
equivalent was measured by volume and chromatographically for
composition. Hydrogen evolution rates proportional to I0.92 at low intensities
(<0.1mW/cm2 UV), and proportional to I0.52 at higher intensities (1-
34
100mW/cm2 UV) were reported. The near linear relationship at low intensities
was ascribed to low recombination; whereas at high intensities recombination
became dominant, leading to the half order relationship.
Figure 2-14: Relationship of Photocurrent to UV Light Intensity for a n-TiO2
electrode in 0.51M HClO4 electrolyte; (right axis, ∆) applied potential of 0V
(SCE), (left axis, ●) +2V (SCE) (Carey & Oliver, 1976).
Linear response to light intensity for degradation reactions at low intensity
were reported by Nogueira & Jardim, (1996) Huang et al., (1999) and Jiang,
Zhao, Jia, Cao, & John, (2001). Lim, Jeong, Kim, & Gyenis, (2000) found a
similar result to Tabata et al. (1994) for NO decomposition on TiO2, a first
order relationship at low intensity and a half order relationship at high
intensity.
35
2.6.2 Temperature and Pressure
Licht performed a series of theoretical studies into coupling electrochemical
water splitting processes with thermal energy from the sun (Licht, 2002; Licht,
2003, 2005a, 2005b; Licht, et al., 2003). His results - which are applicable to
both photovoltaic + electrolysis and photoelectrochemical systems - are
summarised in Figure 2-15. The upper portion of the figure represents the
maximum overall solar conversion efficiencies (ηsolar-max) at 1 bar and 500
bar, over various temperatures and for different quantum efficiencies (ηphot).
The lower portion of the figure illustrates the dependence of the minimum
bandgap on reactor temperature under AM1.5 insolation, at 1 bar for varying
thermal capture efficiencies (ηheat) and quantum efficiencies (ηphot).
These results show that thermodynamically, water splitting efficiencies
should be enhanced significantly by adding heat from the sun to the system.
This is due to the reduction in water splitting potential as temperature
increases. This effect was not considered by the earlier studies of Ross and
Hsiao, (1977), Bolton, (1978) and Bilchak et al., (1980).
36
Figure 2-15: Solar driven water splitting for hydrogen production as a function
of temperature (Licht, 2002)
37
Experimental studies into conducting photocatalytic reactions at elevated
temperature include Hong, Park, & Han, (2009), who reported an increase in
water splitting rate with TiO2 nanotube photocatalysts at temperatures
approaching 100°C. However, their catalysts degraded above 75°C. The
effect of both light intensity and temperature was investigated Katakis,
Mitsopoulou, & Vrachnou (1994). They used a Tungsten based catalyst and
an electron acceptor in the electrolyte, and varied the temperature. Whilst
they reported that light intensity had no affect on performance, they did find
that the reaction yield increased threefold from 20°C to 70°C. Also, an
Arrhenius relationship was identified by Harvey, Rudham, & Ward (1983), for
the photocatayltic degradation of alcohols by rutile TiO2.
Whilst these articles give us a basic concept of how photocatalytic water
splitting is affected by temperature and light intensity, a number of gaps exist.
This includes the exact nature of light intensity and temperature’s relationship
with photocatalysis, both independently and together. Also, many factors -
such as recombination, reaction potential and charge transfer kinetics - and
their contribution to this relationship are only vaguely understood at this time.
Additionally, no recent experimental studies have been reported which focus
on the effect of light intensity and temperature on photocatalytic water
splitting reactions, over a significant range.
There are a couple of explanations for the scarcity of literature on increasing
light intensity and temperature for photocatalytic energy conversion. Firstly,
high light intensity generally leads to lower quantum efficiency, which is
undesirable. Secondly, high light intensity from sunlight causes the cell to
heat up. As water vaporises at 100°C at atmospheric pressure,
38
photocatalysis of liquid water above this temperature requires a pressure
vessel with a window for illumination. This presents considerable
experimental challenges for investigators. Some of these include:
Engineering a window on a pressurised reaction vessel.
Determining the values of important parameters (light intensity and
temperature) at the surface of the photocatalyst.
Producing stable photocatalytic materials to obtain repeatable results.
Measuring photocurrent without interference from other reactions
(such as corrosion of electrical interconnects).
The outstanding question is whether the decrease in efficiency at high light
intensity and the greater complexity of the system required, will be offset by
more favourable thermodynamics and higher reaction rates with increased
temperature.
2.7 Summary
Most investigations into photocatalytic water splitting have concentrated on
improving the materials aspects of the process. Far less attention has been
focussed on the practical and engineering aspects of the operation of a
system. Two parameters which will have a great affect on both the design of
a practical system and the materials required for that system - light intensity
and temperature - have not been thoroughly investigated. Some of the
advantages for performing photoelectrolysis reactions at increased light
intensity and temperature are summarised below:
39
The photocatalytic process is fundamentally reliant on incident light
intensity, thus increasing the intensity will increase the reaction rate.
As temperature increases, the reaction rate of the reaction will
increase due to faster kinetics and lower resistance.
Increased reaction rates will lead to more effective product capture
and process.
Better confinement and control of the reaction means less catalyst and
associated reactor infrastructure is required
The required electrical potential for the reaction decreases with
increasing temperature. This increases the capacity of the reaction to
proceed.
The objective of this work is to conduct an experimental investigation to
establish the effect of light intensity and temperature on photocatalytic water
splitting. This will make a valuable contribution and inform future work and
research directions.
40
3.0 Experimental Apparatus and Material
3.1 Introduction
A major component of this project was the development of an experimental
apparatus and testing protocol for the acquisition of repeatable results under
varying conditions. The key components are:
a light source and measurement apparatus
a reactor which can be used to test at high temperatures whilst
allowing light to illuminate the photocatalyst; and
a reliable, reasonably active and easily replicated photocatalyst with
which to carry out the experiments.
41
This chapter describes the development of the experimental apparatus and
materials required to obtain reliable results. Reliable and repeatable results
were required before testing the effect of light intensity and temperature on
the reaction.
Figure 3-1: Diagram of Experimental setup
3.2 Solar Simulator and Data Collection setup
Measurements were carried out using the experimental setup described in
Figure 3-1. The solar simulator consisted of a 150W Ozone free Xenon Lamp
supplied by Oriel Instruments (model No. 6255) followed by an Oriel 59060
Band pass filter (transmission range 300-800nm, Figure 3-2). The spectrum
of the light obtained from this device before and after the filter is depicted in
Figure 3-3. This light was focussed above a locating tray, into which could be
loaded either a power meter (Newport 1918-C with Newport 818P-015-19
thermopile detector) or the test cell. This locating tray ensured that the power
42
meter and the cell were situated in identical positions for each measurement.
A thin sheet of metal was placed in front of the cell to shield it during dark
measurements. Also, a Keithley 236 source-measure unit was used to
control the voltage and measure the current produced by the cell. This
information was relayed to a computer, recorded and used to produce
current-voltage (IV) plots.
Figure 3-2 : Absorption spectrum of the 59060filter
Figure 3-3: Xe Lamp Spectrum, with (red) and without (blue) the filter,
compared to the AM1.5 spectrum (green).
43
3.3 Reactor Development
The development of a reactor with which to carry out the experiments was a
major focus of this project. There were a number of constraints and
considerations which were applied at the beginning of the design process.
These included:
Temperature range 25-180°C
Pressure range 1-10 Bar
Ability to illuminate the photoanode
Ability to control the voltage and measure the current of the cell
Figure 3-4: Prototype Perspex Vessel
The first prototype reactor developed consisted of two Perspex cylinders,
bolted together and filled with electrolyte (Figure 3-4). Each electrode was
attached to the inside of its own cylinder and a Nafion sheet separated the
cylinders. The major problem encountered with this prototype was the
44
absorption of the light through the Perspex and water before reaching the
photoanode. Also, it was inconvenient to assemble. This reactor was
intended as a learning exercise to identify the requirements of the reactor
vessel, thus no sealing was incorporated and heating was not considered.
3.3.1 Sealed Vessel Test System
A second reactor was then designed and built (Figure 3-5). This system had
an open window to allow light to back illuminate the photoanode directly. It
had two sections for easier assembly, was machined from Lexcen to
withstand higher temperatures and incorporated O-Ring seals to prevent
leaks under pressure. Lexcen is chemically inert in the electrolyte.
This approach however, encountered major difficulties in establishing a
reliable electrical connection to the electrodes through the vessel wall. This
connection is required for measurement and manipulation of the current and
voltage of the cell. Currents occurring when metal in the electrical
connections corroded in the aggressive electrolyte, effectively “drowned out”
photocurrents. Also, the operation of this vessel at high temperature and
pressure added more complication to the electrical connection issue. For our
experiments a wire was soldered to the conducting glass using an ultrasonic
soldering iron (MBR Electronics USS-9200). The wire was passed through a
small hole in the vessel wall which was sealed with silicon. This was only a
temporary solution however and this vessel was never tested at temperature
or pressure - due to the difficulties encountered with corrosive currents.
Electrical feed throughs would be a possible solution.
45
Figure 3-5: Configuration of Reactor vessel
3.3.2 O-Ring/Clamp Test System
With these sealed vessel problems noted, it was decided that a simplification
of the reactor was required. A simple cell consisting of the photoanode and
counter electrode separated by a rubber O-Ring and clamped together
(depicted in Figure 3-6 and Figure 3-7) was used for the rest of the
investigation.
The cell was formed by placing a rubber O-ring between a fresh photoanode
and a fresh counter electrode then clamping the system together with binder
clips. Electrolyte was injected through the o-ring into the reaction space with
a needle and syringe. A second needle was used to allow enclosed air to
escape. These components were matched, ascribed a cell number and only
tested together to ensure that all tests were comparable. The cell was back
46
illuminated from the photoanode side through the glass, and masked with an
appropriately sized aperture (usually 6.25mm in diameter - resulting in an
illuminated area of 0.307cm2). Before assembly electrical contacts were
soldered onto the conducting surface with an ultrasonic soldering iron and
low temperature solder.
Figure 3-6: Schematic of O-Ring test system
The use of the ultrasonic soldering iron was a valuable addition to the
reliability of the measurements. Attaching traditional solder to a conducting
glass surface is unreliable, and without a soldered surface the alligator clips
used to connect the cell to the Keithley produced erratic and inconsistent
contacts.
With the addition of ultrasonic soldered contacts this cell arrangement allows
for simple assembly and disassembly, reliable electrical connection and fast
heating. It is however, vulnerable to leaking and fracturing of the glass
electrodes under the pressures required for testing over 100°C. Also, small
47
reaction volumes necessitated frequent replacement of the electrolyte. This is
discussed more in section 4.1.4.
Figure 3-7: Configuration of the O-Ring type Cell Assembly
One of the goals of this project was to test these cells at varying
temperatures. This was initially done using an oil bath and heater plate.
However, this meant that the cell was immersed in oil. Upon removal and
disassembly the oil invariably found its way onto the reactive surfaces,
coating them and introducing impurities onto the electrode surfaces. This
meant the electrodes could only be used for one test. Furthermore, the oil
acted as a light absorbing layer in front of the photocatalyst, introducing an
undesirable variable to the light illuminating the cell . Thus, the bath shown in
Figure 3-8 was developed and sand used as a heat transfer medium. Heating
was provided by a simple resistance heater which was controlled manually.
48
The temperature of the cell was measured using a thermocouple placed next
to the O-ring of the cell and between the glass electrodes. The temperature
was allowed to come to equilibrium for every test, ensuring that the reaction
space was at the same temperature as the thermocouple. This thermal
equilibrium was confirmed by inserting an additional thermocouple inside the
reaction chamber, heating the rig to equilibrium and verifying that the
temperature read by both thermocouples matched.
A situating apparatus was built to allow precise positioning of the cell (Figure
3-9). This device allowed movement in 3 dimensions and ensured that the
pyrometer and cell are located in the same position for accurate and
consistent measurement of cell illumination.
Figure 3-8: Assembled cell in Sand bath with heater
49
Figure 3-9: Experimental apparatus under operation
3.4 Light Intensity and Pyrometer Calibration
The light source used was a150W Ozone free Xenon Lamp supplied by Oriel
Instruments (model No. 6255) in a solar simulator housing with collimated
output (Oriel model: 96000). The light intensity was controlled in two ways;
using the lens inbuilt in the housing to focus or defocus the light as required,
or by moving the cell along the focal axis. The highly focussed light retained
an image of the lamp, which limited the maximum intensity of the light.
The UV power of the light was measured by two methods. The first involved
using a pyrometer to measure to total power and multiplying that value by the
percentage of UV (<430nm due to TiO2’s absorbance) in the incident
spectrum (Figure 3-3).
50
The second method for determining the UV power of the light was potassium
ferrioxalate actinometry described by Murov, Carmichael, & Hug (1993). This
technique was used to calibrate the pyrometer. It involved illuminating a
solution of two chemicals - which react in UV light - and measuring how much
reacted over a specific time period.
The reactants used were Iron (III) Sulfate and Potassium Oxalate. A known
volume and concentration of this solution was reacted for intervals up to 1
minute, to produce Fe2+ ions according to the reaction:
Equation 3.1
This reaction is very sensitive to light in the UV region, having an absorbtivity
of 1 at wavelengths up to 400nm. Also, its quantum yield is accurately
known.
An aliquot of the irradiated solution was removed, complexed with
phenanthroline, diluted with a buffer of acetic acid and its absorbance at
510nm measured. This absorbance was compared to a blank which had not
undergone irradiation to determine the change in absorbance due to the
illumination. This process was repeated at 10 points over the range of
achievable light intensity which was also measured using the power meter.
The incident photon rate (I) was determined using the absorbance values (A),
the quantum yield (Φλ), extinction coefficient (ε), path length (d), illumination
time (t), volume of the aliquot (V1), volume of irradiated solution (V2), volume
of the dilution (V3), and the following modification of the Beer-Lambert Law
(Murov, et al., 1993):
51
Equation 3.2
As the distribution of the incident spectrum is known (Figure 3-3), the
intensity of the light in the UV can be found by calculating the rate of incident
photons at each wavelength, relative to the total rate of photons incident
across the spectrum. Each photon at a specific wavelength corresponds to
an energy which, when multiplied by its specific incident photon rate and
summed across the spectrum, gives the total energy flux of the light - light
intensity in W/m2.
The actinometry data was then plotted against the intensities measured by
the power meter (Figure 3-10). A non-linear relationship between the
intensity measured by the power meter and that measured by actinometry
exists. As the actinometry data is far more accurate than the power meter
data, it is taken to be a true measurement of the light intensity and used as a
calibration for the pyrometer. For practical reasons the power meter was
used for intensity measurements during the experiments.
For ease of interpretation light intensity is expressed in suns. As the
photocatalyst used only absorbs UV light, the power in the UV portion of the
AM1.5 spectrum (6.15mW/cm2) was used as a reference, from which the
equivalent suns delivered by the system could be calculated. This allowed us
to develop a method for calculating the UV equivalent suns of our incident
light, using the power meter measurement:
Equation 3.3
52
Equation 3.4
Figure 3-10: Comparison of <430nm light power using the power meter and
actinometry
Where: IUV is the UV power of the solar simulator, IP is the intensity
measured by the power meter, IAM1.5 is the UV power of the AM1.5 spectrum
(6.15mW/cm2), Isun equiv. is the power incident on the sample in suns
equivalent to the AM1.5 spectrum.
3.5 IV Curves
There are two methods by which the performance of a photocatalytic water
splitting device can be evaluated. The first is by measuring the current-
voltage characteristics of the device. Current-voltage, or IV curves, measure
the response of the cell over a range of voltages. They are a simple, quick,
y = 1.80E-04x2 + 2.68E-01x R² = 9.98E-01
0
50
100
150
200
250
300
350
400
450
500
0 100 200 300 400 500 600 700 800 900 1000
Total intensity measured with power meter (mW/cm2)
UV
Lig
ht
Inte
nsi
ty f
rom
act
ino
met
ry (
mW
/cm
2)
Calibration of power meter using actinometry
53
reliable and accurate method for performance characterisation of films that
have been deposited on conducting surfaces. As such IV curves are the most
common method used for the testing of cell style photo-devices (Duret &
Gratzel, 2005; Kay, et al., 2006; Khan, Al-Shahry, & William B. Ingler, 2002;
G. K. Mor, et al., 2005; Gopal K. Mor, et al., 2006; O'Regan & Grätzel, 1991;
Ruan, et al., 2006; Sivula, et al., 2010). The disadvantage of this testing
method is that the hydrogen production is not being measured directly, and
must be calculated and inferred. Also, the resistance of the film, the
substrate, the electrolyte and the connections between them will affect the
measurement.
The second method for performance measurement is to evaluate the amount
of gas produced by the system. This method allows the production rate of the
system to be measured directly, but as the volumes produced are generally
small, long evolution times are often required. Therefore time dependent
characteristics of the system are missed. This technique is most commonly
used in slurry type systems, as IV curves are impossible in such an
arrangement (Bamwenda & Arakawa, 2001; Bamwenda, Tsubota,
Nakamura, & Haruta, 1995; Gurunathan, 2000, 2004; Gurunathan,
Maruthamuthu, & Sastri, 1997; Kiwi & Gratzel, 1986; Nada, Barakat, Hamed,
Mohamed, & Veziroglu, 2005).
54
Figure 3-11: Example IV Curve for TiO2 Film
Some example IV curves are presented in Figure 3-11. The dark
measurement (black line) produces very little current until a bias of
approximately 1.7V, when the current increases dramatically. This is the
voltage where electrolysis begins to occur, overcoming the water splitting and
boundary/interface potentials. The light curve (red line) has two switch-on
voltages, one at approximately 0.1V and one at a similar voltage to the dark
curve. The first is associated with the conduction band of the titania’s position
relative to the H+/H2 reaction potential (see Figure 2-7). As the conduction
band of the titania it is only slightly above the H+/H2 potential, it only requires
a small voltage ‘push’ for the reduction reaction to occur at higher rates. The
second switch-on voltage is again due to electrolysis. The photocurrent at a
specific voltage is the difference between the dark and light curve values at
that voltage.
55
The photocurrent is an important value as it can be used to calculate the
hydrogen production rate; and in conjunction with the light intensity, the
efficiency of the cell. The rate of hydrogen production is calculated using the
following equation:
Equation 3.5
where: rH2 is the rate of hydrogen production in moles s-1 cm-2, jphoto is the
photocurrent in A cm-2, C is the number of electrons in one Coulomb of
charge (6.24×1018), n is the number of atoms in molecule (H2 = 2) and NA is
Avogadro’s number (6.02×1023).
The calculation of the efficiency of the system at a specific light intensity
employs this equation (Murphy et al., 2006):
Equation 3.6
where: η is the efficiency, EWS is the water splitting potential per electron
(1.23V at S.T.P.), VB is the applied bias in Volts and I is the power of the light
in W.cm-2.
Quantum efficiency is another method used to assess the system’s ability to
utilize light. It is calculated by comparing the number of electrons in the
photocurrent to the photon flux of the incident light; via the following equation:
Equation 3.7
56
where: Jphoto = the photocurrent in A cm-2, C is the number of electrons in 1
coulomb (6.24×1018), is the photon flux s-1 cm-2 of the incident light
with a wavelength below 430nm. This was found by dividing the UV power of
the incident light (IUV) by the average energy of an incident photon with a
wavelength between 290nm and 430nm ( vh ):
where ( vh ) is obtained via the summation:
Equation 3.8
where: h = planck’s constant (6.63×10-34 J.s-1), c is the speed of light in a
vacuum (3.00×108 m.s-1), hv(λ) is the number of photons at a specific
wavelength of the incident light, 290
450hv is the total number of photons of the
incident light between 290nm and 450nm.
As the quantum efficiency is calculated for UV light only, it does not take into
consideration visible and infra red radiation.
3.6 Standard Photocatalyst
The electrodes used in this study were produced by depositing material onto
FTO glass substrates. Fe2O3 and TiO2 Photoanodes were produced in a
number of ways which are discussed below. The aim of this section was the
development of electrodes and cells which would perform consistently and
reliably.
Equation 3.9
57
3.6.1 Fe2O3
The first photocatalyst produced was hematite (α-Fe2O3), a form of Iron
Oxide. Hematite has a band gap of 2.2eV (Mills & Le Hunte, 1997) and can
absorb up to 30% of the solar spectrum. This, coupled with its reported
resistance to photocorrosion (Kay, et al., 2006; Satsangia, Kumaria, Singha,
Shrivastavb, & Dassb, 2007; Sivula, et al., 2010) made it a primary candidate
for a robust and easily prepared photocatalyst with good performance. Also,
it does not require quartz optical components.
However - as mentioned in section 2.4.2 - Fe2O3 has a very short diffusion
length for charge carriers, around 2 – 4nm (Kennedy & Frese Jr., 1978). This
typically results in high recombination rates when used for photocatalysis.
We proposed to address this problem by producing a highly porous
photocatalyst using an inverse opal technique developed by fellow
researchers at QUT (Martens et al., 2007). The thin lattice walls and high
surface area (Figure 3-12) typical of these materials means less distance that
photo-excited charges have to travel to reach a surface/electrolyte interface
and react.
Inverse opal and non-inverse opal Hematite materials were produced in a
number of different ways. Firstly a solution, usually consisting of iron nitrate
(Fe(NO3)3) dissolved in methanol or water, was prepared. To this was added
varying amounts of Poly(methyl methacrylate) (PMMA) spheres. The PMMA
spheres were produced by polymerising methyl methacrylate in solution at
temperature.
58
Figure 3-12: SEM images of Inverse Opal Fe2O3 at 160 000x and 24 000x
magnification (FEI Quanta 3D, operator - Dr Wayde Martens)
These PMMA/Fe(NO3)3 solutions were then spread on the surface of a FTO
coated glass substrate using a number of methods; doctor blading, spray
coating and spray pyrolysis:
Doctor blading is a coating technique depicted in Figure 3-13. The
substrate is secured to a flat surface using tape, ensuring that some of
the tape is above the surface of the substrate (Figure 3-13a). A drop of
solution is then placed at the top of the taped substrate area (Figure
3-13b) then spread using a glass rod drawn across the surface whilst
pressed against the tape (Figure 3-13c and Figure 3-13d). The
substrate surface, glass rod and tape thickness form a volume over
the spreading area which the solution fills (Nazeeruddin, et al., 1993).
These samples are then allowed to dry and the calcined in a furnace
for 4hrs at 450°C.
59
Figure 3-13: Schematic of Doctor Blading process
Spray coating uses an airbrush gun to atomise the solution and direct
it towards the substrate. Multiple coatings were required with this
technique with each coat being allowed to dry before the application of
the next. After the coating had been built up to the desired thickness
the sample was calcined in a furnace for 4hrs at 450°C (Li & Li, 2003).
Spray pyrolysis is a similar technique to spray coating. The same
airbrush gun and layering technique are used but in this case the
substrate is heated to 450°C during the spraying process. This allows
each coat to calcine before the application of the next (Acosta,
Martinez, Lopez, & Magana, 2005; Joseph, Gopchandran, Thomas,
Koshy, & Vaidyan, 1999).
60
Finally, an ultrasonic soldering iron was used to apply an electrical contact to
the conducting surface of the FTO coated glass. This enabled an alligator
clamp to form a reliable contact with the electrode.
3.6.2 TiO2
The TiO2 which was used in this investigation was P25 grade obtained from
Degussa. It was deposited on FTO conducting glass (obtained from Dyesol)
by the following method. Raw P25 was ground in a mortar and pestle and
added to methanol to form a suspension of 0.231g per ml. This suspension
was then sonicated for 30 minutes. Deposition was by doctor blading onto
cleaned (detergent, acetone, de-mineralised water then air dried) FTO glass.
After the methanol had evaporated the films were placed in a furnace and
calcined for 4 hours at 450°C. Electrical contacts were attached to the
electrode using ultrasonic soldering.
The films obtained from this process had even coverage and good
adherence to the glass surface. Even after multiple testing and rinsing there
was no visible damage to the film. Scratching of the surface with a fingernail
or blade was required for material to be visibly removed.
Field emission scanning electron microscopy (FE SEM) images of two films
manufactured in different batches show minor differences between the films;
mainly in the agglomeration of the P25 particles. This is shown by the
difference between the films in Figure 3-14 a) and Figure 3-14 b), where the
particles are more densely packed. Figure 3-14 c) and Figure 3-14 d) also
show this greater agglomeration in the second film. This agglomeration
disparity is probably due to slightly different oven temperatures, resulting in
61
more melting of the particles together. This leads to lower surface area, but
better electrical conductivity in the film. Whilst the film production process
was kept as consistent as possible, slight differences between films are
unavoidable and subsequent differences in film performance will occur.
Figure 3-14: FE SEM images of 2 TiO2 films produced using the same process;
a) 250 000x magnification of film 1, b) 250 000x magnification of film 2, c) 80
000x magnification of film 1, d) 80 000x magnification of film 2 (JEOL 7100,
operator - Eunice Grinan).
3.6.3 Platinum Counter Electrodes
Counter electrodes were produced by submerging FTO glass in chloroplatinic
acid (approximately 2.1 × 10-3 M concentration) with silver chloride reference
and platinum counter electrodes. The voltage was scanned from 0V to -0.8V
then held at -0.8V for 30 seconds for electrodeposition. This process was
62
repeated 5 times per electrode. Finally ultrasonic soldering was used to
attach electrical contacts to the electrode.
3.7 Conclusion
This chapter has outlined the development of the experimental apparatus
and procedures used to conduct the light intensity and temperature
photocatalysis studies presented in Chapters 4 and 5. A number of problems
were identified during this work and solutions found for them.
63
4.0 Results
This chapter consists of three major sections. The first is a description of the
reliability of the tests, the stability of the materials and the experimental
methods used to obtain repeatability of the results. The second section
presents the results pertaining to the light intensity experiments and the third
section presents the results from varying temperature.
4.1 Repeatability of experiments
4.1.1 Scan rate
In this study current–voltage curves were obtained by scanning from the
lowest to highest voltage, at a sample rate of 20 mV s-1, using the Keithley
236 source-measure unit. Figure 4-1 shows some IV curves compared to
steady state data. The thick red line is the IV curve under illumination and the
black line is the IV curve without illumination. The diamond markers show the
64
system at steady state under illumination (light blue) and without illumination
(dark blue). The steady state was found by conducting experiments where
the cell was illuminated, allowed to reach equilibrium, then shuttered and
again allowed to reach equilibrium. The raw data from these experiments are
included in Appendix A-1. The steady state values are similar to the IV curve.
This means that scans conducted at a rate of 20 mV s-1 are sufficiently slow
to ensure that the system approximates steady state when sampled.
Figure 4-1: TiO2 IV Curve Scan Rate Validation
4.1.2 Fe2O3 Films
The performance of the hematite materials was below that required to make
this research meaningful. More importantly however, these materials did not
display the stability, reliability, mechanical strength and scratch resistance
required for this investigation. A hematite test has been included in Appendix
A-2.
0.00E+00
5.00E-05
1.00E-04
1.50E-04
2.00E-04
2.50E-04
-0.5 0 0.5 1 1.5 2 2.5
Light Curve
Dark Curve
Steady State Light
Steady state Dark
Applied Voltage (V)
Cu
rren
t (A
)
Comparison of IV Curves to Steady state
65
The literature pertaining to Hematite as a photocatalyst appears to overlook
the stability of the films over repeated tests. This is very important for
photocatalysis as a material that degrades is of limited benefit for a practical
system. Many studies, Kay & Grätzel (2006) and Sivula et al. (2010) for
instance, do not report on the performance of the photocatalyst over long
periods or over repeated tests. Stability is mentioned as a materials choice
factor, but not discussed further. The photocatalyst’s stability however, is an
important factor in this study, as without a stable photocatalyst repeated
testing of a sample will produce varying results. This will make comparison
between experiments run under different conditions difficult or impossible and
compromise the light intensity and temperature investigations of this study.
Figure 4-2: a) Stable electrode, b) Cathodic corrosion c) Anodic corrosion d)
both anodic and cathodic corrosion (Gerischer, 1977)
A simple model of the electrode’s stability was developed by Gerischer
(1977). This model uses the positions of the conduction and valence band on
a potential diagram, relative to the reduction and oxidation potentials of the
water splitting half reactions to evaluate stability (Figure 4-2). A photocatalyst
is stable if the band gap spans the redox potentials. If either or both of the
semiconductor’s bands lies between the half reaction potentials then it is
unstable and subject to corrosion. In this instance its corrodibility is
66
determined by the favourability of the corrosion reaction in comparison to the
water splitting reaction.
Iron Oxide has a conduction band which lies below the reduction potential of
water and is therefore susceptible to corrosion. It was also clear during the
experiments with Fe2O3 that the performance of the films being tested was
changing with repeated experiments. This unreliability, coupled with the low
performance, complexity of manufacture and fragility lead us to investigate a
different material to use as a standard photocatalyst.
4.1.3 TiO2 Films
The TiO2 films were used when it became clear that the Fe2O3 films would
not suit our purposes. The aim for these films was stability, robustness,
simplicity of manufacture and adequate performance for the production of
meaningful results.
The UV-vis absorption spectrum for a P25 TiO2 film shows that absorbance
starts around 400nm as expected. The absorbance at wavelengths above
450nm is probably due to light scattering from the film. There is minimal
difference between the new and used films, suggesting that by conducting
experiments with the film dopants are not being introduced or the absorbance
region changed.
The stability of these TiO2 films was found experimentally. They were
repeatedly tested under high light intensity conditions (approx. 28 suns UV
Equivalent) at room temperature. The cells were disassembled, rinsed and
reassembled between every test to ensure that polarisation of the electrolyte
did not affect the measured performance. These tests were repeated 12-15
67
times under illumination, with dark current tests preceding and succeeding
the illuminated tests. The scans were undertaken from 0 to 1.0V at a scan
rate of 20 mV s-1. The currents obtained at 0.5V are presented in Figure 4-4.
The raw data from these tests are included in Appendix A-3, Appendix A-4
and Appendix A-5.
Figure 4-3: UV-Vis for a new and used film
Figure 4-4: Reduction in Performance from repeated testing (0.5V Bias)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
350 450 550 650 750
Used Film Unused Film
P25 TiO2 UVvis absorbance plot (normalised)
y = -3.89E-06x + 3.06E-04 R² = 7.28E-01
y = -3.03E-06x + 2.25E-04 R² = 8.15E-01
y = -3.28E-06x + 2.50E-04 R² = 7.97E-01
0.00E+00
5.00E-05
1.00E-04
1.50E-04
2.00E-04
2.50E-04
3.00E-04
3.50E-04
0 5 10 15 20
Cell 4.1
Cell 4.2
Cell 4.3
Test number
Cu
rren
t (A
)
Currents obtained at 0.5V from repeated tests
68
Figure 4-4 shows that each cell’s performance degraded at a more or less
steady rate. The data was fitted with a linear function and the rate of
degradation per test calculated from this. Cells 4.1, 4.2 and 4.3 lost 1.19%,
1.25% and 1.30% performance per test respectively, giving a mean
degradation rate of approximately 1.25% per test. The following equation was
developed to account for this degradation in cell performance:
11
n
dfi rjj
Equation 4.1
Where ji and jf are the initial and final photocurrent densities, rd is the rate of
degradation per test and n is the number of times the film has been tested
(under light).
Obviously there is some difference between these cells - almost 30%.
However, most of the other tests performed under these conditions produced
results within this range. These variations are probably due to slight
differences between films - from solution preparation, doctor blading and
calcinations processes - leading to minor disparities in film thickness,
morphology and electronic characteristics. For instance, the difference in
particle agglomeration between the 2 films shown in Figure 3-14 will lead to a
lower surface area, but greater electronic conductivity in the more highly
agglomerated film. Both of these factors are known to affect photocatalyst
performance.
The performance of the TiO2 films is compared to some other photocatalysts
described in the literature in Figure 4-5. The photocurrent produced by the
P25 TiO2 film under high intensity light (52 suns) is depicted by the solid
69
green line. The difference between the switch on voltage for our P25 film and
the other TiO2 films is due to our cell being a 2 electrode system (i.e. no
reference electrode), whilst the films of Ruan et al. (2006) and Wu et al.
(2009) were tested with a Ag/AgCl reference electrode. This produced a
potential difference of approximately 0.5V between the measured switch on
voltages of the films.
Whilst the catalyst used in the work does not perform as well as some other
materials, it still displays sufficient photoactivity that its response to varying
light intensity and temperature can be measured. It should also be noted that
it was developed for simplicity, stability and consistency of manufacture,
rather than efficiency.
Figure 4-5: Comparison of Film used with results from Literature (Glasscock,
et al., 2007; Kay, et al., 2006; Ruan, et al., 2006; Sivula, et al., 2010; Wu, et al.,
2009)
-0.0002
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
-1 -0.5 0 0.5 1 1.5 2
Undoped x20, Glasscock (2007)
Si Doped, Glasscock (2007)
TI Doped, Glasscock (2007)
P25 TiO2 Film (Light)
P25 TiO2 Film (Dark)
Colloidal Fe2O3, Gratzel (2010)
TiO2, Wu et. Al. 2009
Applied Voltage (V)
Cu
rren
t (A
)
Comparison Of Our Film to Films Published in Literature
70
4.1.4 Electrolyte
The electrolyte used in the measurement cells can affect the photocatalytic
response significantly. Initially deionised water was used as an electrolyte in
order to better simulate a practical system. This yielded IV Curves shown in
Figure 4-6 (red). Na2SO4 was then added and the green curves were
obtained.
Figure 4-6: Comparison of Electrolytes (Pure H2O - Red and 0.1M Na2SO4 -
Green)
The different shape of the electrolyte curves is attributed to the difference in
conductivity of the two solutions. Deionised water has a lower conductivity
than the Na2SO4 solution (10.04μS/cm for deionised water compared to
15.61mS/cm for 0.1M Na2SO4). The shape of the pure water curve suggests
that ohmic resistance from ionic transfer through the solution limited the
-5.00E-05
-5.00E-19
5.00E-05
1.00E-04
1.50E-04
2.00E-04
-1 -0.5 0 0.5 1 1.5 2 2.5
Dark, water
Dark, Na2SO4
1.53 Suns, water
1.53 suns, Na2SO4
4.17 suns, water
3.96 suns, Na2SO4
8.01 suns, water
8.17 suns, Na2SO4
23.8 suns, water
23.1 suns, Na2SO4
35.5 suns, water
34.4 suns, Na2SO4
Pure H2O (red) vs 1M Na2SO4 (green) Electrolyte
Cu
rren
t (A
)
Voltage (V)
71
current. The Na2SO4 solution however, reaches a plateau early in the voltage
scan and is therefore being limited by a different process. Tests with other
electrolytes (KOH and H2SO4) did not change the results from those obtained
with the Na2SO4 solution. Therefore, the process limiting the reaction is not
related to the electrolyte.
It was found however, that repeated testing using the same electrolyte did
not produce consistent results. The current degraded as the total charge
transferred increased. The effect of repeated testing without replacing the
electrolyte, using a 3ml electrolyte volume is illustrated in Figure 4-7.
Figure 4-7 shows that the performance steadily declines - much like in Figure
4-4 - up to around test 10 before a step develops in the 0.5 - 1.0V region.
Also of note is the growing reverse current being experienced at -0.5V at the
beginning of each test. This has been attributed to a polarisation effect in the
electrolyte, which increases the electromotive potential for the reverse
reaction; thus reducing the photocatalytic reaction. If a reference electrode
could have been used in our test rig then this problem would have been
greatly reduced.
In order to reduce the influence of this polarisation of the electrolyte, it was
decided to increase the volume of the electrolyte. O-rings 3mm thick by
38mm in diameter - resulting in approximately 3ml of electrolyte - were used
for all subsequent tests. This reduced the polarisation effect, however the
cells were also dismantled between each test and the electrolyte replaced to
ensured all testing began under identical electrolytic conditions.
72
Figure 4-7: Repeated tests using the same electrolyte
4.2 Light intensity dependence of photocatalysis
This section presents the results of the light intensity experiments. Tests
were undertaken at light intensities ranging from 0 to 52 suns, using a cell
consisting of a P25 TiO2 film, Pt counter electrode and Na2SO4 electrolyte as
discussed in chapter 3.
Section 2.6.1 established that there have been relatively few studies
undertaken into the effect of light intensity on photocatalytic water splitting.
Carey & Oliver (1976) used an argon laser to illuminate their cell with UV light
up to 400mW/cm2 (approximately 65 suns). They found a non-linear
response to light intensity over this range, but did not reach saturation or
propose any type of relationship.
-2.50E-04
-2.00E-04
-1.50E-04
-1.00E-04
-5.00E-05
0.00E+00
5.00E-05
1.00E-04
1.50E-04
2.00E-04
2.50E-04
3.00E-04
-1 -0.5 0 0.5 1 1.5 2 2.5
Initial Dark test
Light test 1#
Light test #2
Light test #3
Light test #4
Light test #5
Light test #10
Light test #15
Light test #20
Light test #25
Light test #30
Light test #37
Final Dark test Applied Voltage (V)
Cu
rren
t (A
) Repeated Tests without replacing Electrolyte
73
Tabata, Ohnishi, Yagasaki, Ippommatsu, & Domen (1994) conducted a study
using a suspension of K4Nb6O17 as the photocatalyst, a Xe lamp for low
intensity tests (<0.1mW/cm2 UV) and Hg lamp for high intensity tests (1-
100mW/cm2 UV, or ≈16 suns UV equivalent). They found that the hydrogen
evolution rate was proportional to I0.92 at low intensity, and proportional to I0.52
at high intensity. They proposed a model to describe this relationship; a linear
relationship exists at low intensities before recombination becomes dominant
at high intensities half order relationship.
A more recent study (Ruan, et al., 2006) tested TiO2 nanotube array
photocatalysts under two different light conditions; a UV source with a power
of 98mW.cm-2 (approximately 16.5 suns) and an AM1.5 solar simulator (1
sun). The photocurrent reported under the high intensity UV source is over
26 times greater than attained using the AM1.5 source. Whilst this
investigation establishes no relationship to light intensity, it does suggest that
photocatalytic water splitting can be undertaken under high light intensity
conditions with little or no reduction in efficiency. In fact, this result is much
higher than would be suggested by a linear relationship.
Nogueira & Jardim, (1996) Huang et al., (1999) and Jiang, Zhao, Jia, Cao, &
John, (2001) all reported linear responses to light intensity at irradiations
levels up to 1 sun, for water decontamination of various pollutants in different
configurations (Figure 4-8; a), b) and c) respectively). A study by Lim, Jeong,
Kim, & Gyenis, (2000) into the decomposition of NO by TiO2 in flowing gas,
described the relationship between reaction rate and light intensity in two
regimes; first order at low intensities and half order at higher intensities
(Figure 4-8; d).
74
Figure 4-8: Effects of Light intensity reported in literature: a) Nogueira &
Jardim (1996); b) Huang et al. (1999); c) Jiang et al. (2001); d) Lim et al. (2000).
The linear regime at low light intensities described in Figure 4-8; d) was
attributed to photogenerated electron-hole pairs being consumed by chemical
reactions faster than they can recombine. As the intensity is increased
however, so too does the density of the charges in the material and the
recombination of electron-hole pairs becomes dominant, causing the half-
order regime.
Another explanation for this regime shift could be that the charge transfer
rate from the electrode to the electrolyte becomes limiting. This could be due
to insufficient mass transfer through the electrolyte resulting in insufficient
reactants at the electrolyte/electrode interface (Meng, Huang, Wu, Wang, &
75
Qian, 2002). This again would result in higher recombination and non-
linearity with increasing light intensity.
4.2.1 Experimental methods
The light intensity experiments were undertaken using the O-Ring/clamp
experimental rig described in section 3.3.2. The cell was assembled and
placed in the sand bath vessel (without sand). The power meter and lens
inside the lamp housing were then used to adjust the light intensity to the
desired level. The intensity was recorded and the power meter replaced with
the sand bath/cell assembly. The Keithley 236 source-measure unit was
attached and an IV scan was performed between 0 and 1V at a rate of 20 mV
s-1. After the scan, the cell was disassembled, rinsed with deionised water
and re-assembled with new electrolyte for the next test. The intensity of the
light was initially zero (a dark curve) and increased with each subsequent test
until the maximum light intensity was attained. A second dark curve was
conducted to conclude the testing. The same TiO2 photoanode and platinum
counter electrode were used for each light intensity test and a 6.25mm
aperture was used for all experiments. This set of tests was repeated three
times, each time with a new photoanode and counter electrode prepared
according to the method described in section 3.6.
4.2.2 Experimental results
4.2.2.1 IV Curves
The experimental results consist of IV curves conducted over a range of light
power per aperture area. The tests were repeated three times and the curves
for each cell are displayed in Figure 4-9, Figure 4-10 and Figure 4-11.
76
Figure 4-9: IV curves for Cell 4.4, first light intensity experimental repeat
Figure 4-10: IV curves for Cell 4.5, second light intensity experimental repeat
0.0E+00
5.0E-05
1.0E-04
1.5E-04
2.0E-04
2.5E-04
3.0E-04
0 0.2 0.4 0.6 0.8 1
Dark Current 1
0.618 Suns
1.26 Suns
2.61 Suns
5.30 Suns
11.5 Suns
18.5 Suns
26.4 Suns
37.6 Suns
51.9 Suns
Dark Current 2 Applied Voltage (V)
Cu
rren
t (A
) IV Curves for Cell 4.4
-5.00E-05
-9.00E-19
5.00E-05
1.00E-04
1.50E-04
2.00E-04
2.50E-04
3.00E-04
3.50E-04
4.00E-04
0 0.2 0.4 0.6 0.8 1
Dark Current 1
0.618 Suns
1.26 Suns
2.56 Suns
5.30 Suns
11.5 Suns
18.5 Suns
26.4 Suns
36.8 Suns
51.9 Suns
Dark Current 2 Applied Voltage (V)
Cu
rre
nt
(A)
IV Curves for Cell 4.5
77
Figure 4-11: IV curves for Cell 4.6, third light intensity experimental repeat
These graphs show that as light intensity increases, the current also
increases. They also show that above about 0.5V of applied voltage, the
current saturates. However, the voltage at which this saturation occurs is
higher as light intensity increases.
The results of these experiments have been combined, the mean and
standard deviations calculated and shown in Figure 4-12. There is a
significant difference between the performances of each cell. These
disparities are attributed to differences in the film preparation, deposition and
calcination processes.
-5.00E-05
0.00E+00
5.00E-05
1.00E-04
1.50E-04
2.00E-04
2.50E-04
3.00E-04
3.50E-04
0 0.2 0.4 0.6 0.8 1
Dark Current 1
0.647 Suns
1.26 Suns
2.55 Suns
5.28 Suns
11.5 Suns
18.5 Suns
26.4 Suns
37.9 Suns
51.9 Suns
Dark Current 2 Applied Voltage (V)
Cu
rren
t (A
)
IV Curves for Cell 4.6
78
Figure 4-12: IV Curves with mean and standard deviation
79
4.2.2.2 Current/intensity curves
Figure 4-13 shows the data from the above IV curves at 0.5V applied bias.
The data presented in this graph has been converted into photocatalytic
current density (A/cm2) by subtracting the dark current from the light current
and dividing by the illuminated area (0.307cm2). They have also been
corrected for degradation in the film from the testing using the equation and
rd value presented in section 4.1.3.
Figure 4-13: Photocurrents at various intensities for the 3 films at 0.5V applied
bias
This graph shows what appear to be two different regimes. Below
approximately 5 suns the slope of the data is higher than above 5 suns.
There are essentially two sections on this graph; one between 0 and 5 suns,
and another between 15 and 50 suns.
The data at 1.0V applied has been plotted and compared to the mean value
at 0.5V bias shown above (Figure 4-14). This is included to show that
0.00E+00
2.00E-04
4.00E-04
6.00E-04
8.00E-04
1.00E-03
1.20E-03
0 10 20 30 40 50 60
Cell 4.4
Cell 4.5
Cell 4.6
Mean
UV Equivalent Intensity (Suns)
Ph
oto
curr
ent
(A/c
m2)
Light Intensity Tests at 0.5V
80
increasing the bias to 1.0V the shape and magnitude of the data changes
very little. This is due to the virtually flat response to applied voltage above
0.5V. The 0.5V biased system is thus considered to represent the system
sufficiently for this investigation.
Figure 4-14: Photocurrent vs intensity at 1.0V bias
4.2.2.3 Quantum Efficiency
The quantum efficiency results are presented in Figure 4-15. This figure
shows that as light intensity increases quantum efficiency drops, before
stabilising at approximately 1%. This is between 20% and 25% of the
quantum efficiency value acquired under low light intensity.
The quantum efficiency data shows a steady decrease in the ratio of photons
which produce a reaction, as light intensity increases. However, above
approximately 10 – 20 suns, the relationship approaches a steady value with
intensity. This quantum efficiency data illustrates the non-linearity of the
0.00E+00
2.00E-04
4.00E-04
6.00E-04
8.00E-04
1.00E-03
1.20E-03
1.40E-03
0 10 20 30 40 50 60
0.5V Mean
Cell 4.4, 1.0V
Cell 4.5, 1.0V
Cell 4.5, 1.0V
UV Equivalent Intensity (Suns)
Ph
oto
curr
ent
(A/c
m2)
Light Intensity Tests at 1.0V, compared to 0.5V Mean
81
intensity relationship, but suggests as intensity increases the relationship
approaches linearity.
Figure 4-15: Quantum efficiency (<430nm) of intensity experiments at 0.5V
applied bias
4.3 Temperature Dependence of Photocatalysis
As described in sections 2.3.1 and 2.3.2, an increase in temperature reduces
the Gibbs free energy of the water splitting reaction and speeds up the
reaction kinetics. Both of these effects mean that at higher temperatures the
photocatalytic water splitting reaction will be faster and require less energy.
Studies into the effect of temperature on photocatalytic water splitting are
infrequent. Licht et al. undertook a number of theoretical studies into coupling
high temperatures with solar quantum energy conversion devices (Licht,
2002; Licht, 2003, 2005a, 2005b; Licht, et al., 2003). He concluded that water
splitting efficiencies would be thermodynamically improved at elevated
temperatures. A recent study by Hong, Park, & Han, (2009) reported
0.00%
1.00%
2.00%
3.00%
4.00%
5.00%
6.00%
0 20 40 60
Q.E. Cell 4.4
Q.E. Cell 4.5
Q.E. Cell 4.6
Mean Q.E.
UV Equivalent Light intensity (Suns)
<450nm Quantum Efficiency of Cells 4.4, 4.5 and 4.6
Qu
antu
m E
ffic
ien
cy
82
significant increases in water splitting with TiO2 nanotube photocatalysts at
temperatures up to 100°C. However, the nanotube structure collapsed with
continuous operation above 75°C. Also, Katakis, Mitsopoulou, & Vrachnou,
(1994) acquired a 3 times greater reaction yield by increasing the
temperature from 20°C to 70°C using a tungsten based photocatalyst.
Electrolytic water splitting at high temperature, using heat supplied externally
(generally waste heat from a power station or a geothermal source) is
commonly investigated. By heating the electrolyte to temperatures
approaching 1100°C, the amount of electrical energy required to split water is
reduced as the heat contributes energy towards the reaction ( Figure 2-5). As
heat is generally a cheaper form of energy then electricity, this process
reduces the cost of the electrolysis. The state of the art of high temperature
electrolysis was reviewed by Hauch, Ebbesen, Jensen, & Mogensen (2008).
Photocatalytic degradation of pollutants over small ranges of temperature
has also been studied. Harvey, Rudham, & Ward, (1983) found excellent
Arrhenius plots were acquired over a 275 - 313 K (2 – 40 ºC) range for the
oxidation of alcohols by rutile. Herrmann (1999) however, states that above
80°C, when water approaches its boiling point, the exothermic adsorption of
a reactant becomes unfavourable and rate limits the reaction. However this
effect only applies to degradation reactions.
4.3.1 Experimental methods and setup
Experiments investigating the temperature dependence of photocatalysis
were undertaken using the O-ring/Clamp experimental setup described in
section 3.3.2. First, the cell was assembled and electrolyte injected into the
83
reaction space. Then the cell was placed in the sand bath, a resistance
heater inserted behind and sand added to fill the bath. Keithley alligator clips
were attached to the soldered contacts and a thermocouple inserted into the
sand between the electrode plates (beside the O-ring).
Voltage was applied to the resistance heater to heat the cell until the desired
temperature was reached. The sample was illuminated for the light tests and
remained unilluminated during the dark tests and heating period. The cell
was cooled, disassembled and reassembled with new electrolyte between
every test.
4.3.2 Experimental results
4.3.2.1 I-V Curves
The I-V curves from the temperature experiments are presented in Figure
4-17 and Figure 4-18. They show that as temperature is increased, so too
does both the dark and light currents. As these tests were undertaken under
different light intensities (cell 5.1 – 36 suns and cell 5.2 – 44 suns) a mean
and standard deviation cannot be calculated for them.
Of interest in these plots are the dark curves at higher temperature are
significantly increased above a bias of 0.5V. The velocity of molecules in
solution, which is directly related to kinetic energy, is described by a Maxwell-
Boltzmann energy distribution (Figure 4-16). This means that the number of
molecules with energies above the activation energy (i.e. area under the
distribution above a specific energy) is significantly larger with increasing
temperature. Therefore more molecules have energies which allow them to
react at biases below that predicted by the potential model of the system.
84
Figure 4-16: Maxwell-Boltzmann distrubution at increasing temperature (S.
Zumdahl, 1993)
Figure 4-17: I-V Curves for Cell 4.7 at various temperatures
0.00E+00
1.00E-04
2.00E-04
3.00E-04
4.00E-04
5.00E-04
6.00E-04
0 0.2 0.4 0.6 0.8 1
20C, Dark
20C, 36 suns
42C, Dark
42C, 36 suns
68C, Dark
68C, 36 suns
92C, Dark
92C, 36 suns Applied Voltage (V)
Cu
rren
t (A
)
Temperature Tests Cell 4.7
85
Figure 4-18: I-V Curves for Cell 4.8 at various temperatures
4.3.2.2 Photocurrent against voltage
When the dark currents are subtracted from the light currents, a plot showing
the photocatalytic contribution to the water splitting current is acquired (
Figure 4-19 and
Figure 4-20). This photocurrent peaks around 0.4 - 0.6V applied bias, before
increasing dark currents reduce light’s contribution to the photocurrent.
The 0.5V applied bias data has been plotted against temperature in Figure
4-21. This data shows an above linear response to temperature, suggesting
an exponential relationship.
0.00E+00
1.00E-04
2.00E-04
3.00E-04
4.00E-04
5.00E-04
6.00E-04
7.00E-04
0 0.2 0.4 0.6 0.8 1
22C, Dark
22C, 44 suns 46C, Dark
43C, 44 suns 70C, Dark
73C, 44 suns 96C, Dark
102C, 44 suns
Applied Voltage (V)
Cu
rren
t (A
)
Temperature Tests for Cell 4.8
Dark Curves
Light Curves
86
Figure 4-19: Photocurrent of Cell 4.7
Figure 4-20: Photocurrent of Cell 4.8
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0.0014
0.0016
0.0018
0.002
0 0.2 0.4 0.6 0.8 1
22 C
42 C
68 C
92C
Applied Voltage (V)
Ph
oto
curr
ent
(A/c
m2
) Photocurrent (Cell 4.7)
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0.0014
0.0016
0.0018
0.002
0 0.2 0.4 0.6 0.8 1
22C
43C
73C
102C
Applied Voltage (V)
Ph
oto
curr
ent
(A/c
m2
)
Photocurrent (Cell 4.8)
87
Figure 4-21: 0.5V applied bias photocurrent vs temperature
4.3.2.3 Quantum Efficiency
The quantum efficiencies of the photocatalysts at each temperature were
calculated using the method described in section 3.5. They are presented in
Figure 4-22. The photocurrent is from the current vs temperature data above
(Figure 4-21) and also suggests an exponential relationship. The quantum
efficiency starts at approximately 1% at 22°C; as the reaction rate increases
with temperature the efficiency also increases - reaching 2 - 2.5%. The
difference between the two catalysts is greater in this case however, because
the lower light intensity used on Cell 4.7 acquired similar photocurrents to
Cell 4.8. This data is normalised with respect to light intensity so a mean and
standard deviation can be calculated. These are included in Figure 4-22.
0.00E+00
2.00E-04
4.00E-04
6.00E-04
8.00E-04
1.00E-03
1.20E-03
1.40E-03
1.60E-03
1.80E-03
0 20 40 60 80 100 120
Cell 4.7, 0.5V Bias
Cell 4.8, 0.5V Bias
Temperature (°C)
Ph
oto
curr
ent
(A/c
m2)
0.5 V Applied Temperature Plot
88
Figure 4-22: Quantum efficiencies at 0.5V Applied
4.4 Errors and anomalies
There was a reasonable amount of variation between the photocatalytic films
tested. The stability data presented in section 4.1.3 shows differences of
almost 30% between two different films. This data also illustrates an activity
decrease of approximately 1.2 - 1.3% per test. Also, tests undertaken
consecutively on the same film, can display significant discrepancies. These
are probably due to minor changes in the positioning of the films in the
light,and small variations in electrolyte volume.
4.4.1 Light intensity
The intensity testing was affected by a number of factors. Firstly the power of
the lamp was a major limiting factor with a maximum output of 530mW. This
light contained significant intensity gradients in its profile at high
magnification (due to the image of the filament), which limited the maximum
concentration to around 50 suns.
0.00%
0.50%
1.00%
1.50%
2.00%
2.50%
3.00%
0 20 40 60 80 100 120
Cell 5.1
Cell 5.2
Mean
Temperature (°C)
Qu
antu
m E
ffic
ien
cy
0.5 V Applied Quantum efficiency
89
Also, the non-linearity of the pyrometer (section 3.4) required a function to
describe its response. This function is an approximation over a certain
defined range within which all of these tests were conducted. However, it
introduces another source of uncertainty into the experiments. Due to the
accuracy of the actinometry method, it is expected this uncertainty is minimal.
4.4.2 Temperature
Conducting photocatalytic water splitting experiments at high temperatures
also introduced a number of errors and problems. Firstly, the temperature
measurement is subject to errors for a few different reasons. The
thermocouple could not be placed inside the cell in direct contact with the
electrolyte, as sealing the cell would not have been possible. Also, the metal
of the thermocouple may have reacted with the electrolyte and affected the
measurement. Thus the thermocouple was placed beside the O-Ring
between the cell electrodes in order measure the temperature as close to the
reaction as possible. There were temperature gradients within the cell and
sand bath heating arrangement, leading to differences in the temperatures
around the cell. These temperature gradients were minimised by allowing the
cell to come to equilibrium before conducting the experiment.
A major issue was encountered when attempting to carry out experiments
above 100°C with the glass cell and O-Ring setup. At temperatures around
110-125°C either the O-Ring would leak, or the glass would crack allowing
the electrolyte to vaporise and escape. A number of methods were tried to
reduce the occurrence of these ruptures including; doubling the thickness by
laminating the glass electrodes, and using ‘plate’ style clamps to support the
glass evenly. Unfortunately these methods did not work and experimental
90
data significantly above 100°C could not be obtained. Using a fully sealed
pressure vessel with a quartz window appears to be the only way to conduct
photocatalytic reactions at temperatures above 100°C.
91
5.0 Interpretation of Results, Implications for
Scale Up and Practical System Design
5.1 Introduction/literature
In order to understand the results presented in chapter 4, they need to be
interpreted in a form that can be compared to other types of solar energy
conversion. This chapter identifies and models the trends in the experiments,
then extrapolates from the data to form predictions over a greater range than
could be tested. These results and how they affect practical systems is
discussed including advantages, disadvantages and recommendations.
The use of concentrated light for solar energy conversion is not a new
concept. Extensive research and development for high intensity photovoltaics
and solar thermal electricity generation has been conducted. This means
92
there is a wealth of experience available in the solar energy industry for the
design and construction of concentrated solar infrastructure. Commonly used
solar concentrators in the industry include; heliostats, parabolic reflectors and
Fresnel lenses and reflectors (Alpert, et al., 1991; Yamaguchi, et al., 2006;
Zubi, et al., 2009).
High temperature electrolysis is also an area relevant to this study. The
electrode and electrolyte materials, reactor, kinetics and operations of this
technology are very similar to those that would be required for concentrated
light and high temperature photocatalysis. The most common high
temperature electrolysis methods are alkaline and solid oxide electrolysis
(Fujiwara et al., 2008; Hauch, et al., 2008).
5.2 Light Intensity Relationship
The light intensity data was presented in 4.2.2 of chapter 4. Figure 4-13 was
modified into a log-log graph and presented in Figure 5.1. It was found that
the data approximatedlinearity with an R2 value of 0.9687. A linear plot on a
log-log graph produces an equation in the following form:
Equation 5.1
Where m is the slope, and b is the intercept.
Least squares fit of our data gave a slope of 0.627 and an intercept of -4.14;
resulting in the following equation:
Equation 5.2
93
Figure 5-1: Log-Log plot of photocurrents at various intensities for 3 films at
0.5V applied bias
The high accuracy of the fit that this equation has with the experimental data
indicates that photocurrent is proportional to I0.627 for these films (i.e:
). A plot of the fitted equation against linear intensity has been
included in Figure 5-2.
This relationship is similar to that reported in the literature. Tabata et al.
(1994) found a relationship proportional to I0.52 at high light intensities for
water splitting. Also, Lim et al. (2000) reported an exponential value of 0.47
for NO decomposition. The exponential value of 0.627 obtained by our study
is slightly higher than those reported values. The experimental data from this
study is compared to that of Carey and Oliver, (1976) in Figure 5-2.
1.00E-05
1.00E-04
1.00E-03
1.00E-02
0.1 1 10 100
Cell 4.4
Cell 4.5
Cell 4.6
F(x)=
UV Equivalent Intensity (Log(Suns))
Ph
oto
curr
ent
(Lo
g(A
))
Log-Log Plot of Photocurrent vs Intensity with fitted equation
(I0.627)(10-4.14) R2 = 0.969
94
Figure 5-2: Experimental results compared to those found by Carey and
Oliver, (1976)
The data presented by Carey & Oliver (1976) using titania displays a similar
relationship with intensity to our data. Their 0V applied bias data is compared
to our 0.5V applied bias data in Figure 5-2. The line fitted to the Carey &
Oliver (1976) data on a log-log graph has a slope of 0.395, an intercept of
-3.442 and an R2 value of 0.9985, or:
Equation 5.3
The exponential value of 0.395 is slightly below those found in more recent
studies.
Sub-linear relationships are generally attributed to either recombination of
photo generated charges, or reactant mass transfer limitations. Either of
these explanations could apply to our system. It must be noted however, that
-5.00E-04
5.00E-18
5.00E-04
1.00E-03
1.50E-03
2.00E-03
0 10 20 30 40 50 60
Cell 4.4
Cell 4.5
Cell 4.6
F(x)=
Carey & Oliver, (1976)
F(x) =
UV Equivalent Intensity (Suns)
Ph
oto
curr
ent
(A/c
m2
) Experimental results compared to Carey & Oliver (1976)
(I0.627)(10-4.137)
(I0.395)(10-3442)
95
as our reactant is water (with a concentration of 55.56M) which will make the
mass transfer rate much higher than in degradation reactions where the
reactant concentration is low. Thus, this sub-linearity is probably due to
higher recombination at high intensities.
The spread of the three exponential terms (0.395, 0.52 and 0.627 found by
Carey and Oliver (1976), Tabata et al. (1994) and this work respectively)
raises some questions. Is this exponential term related to the materials used
in some way? Could it be affected by particle size? As thin film technology
has progressed in its ability to produce films with particle sizes approaching
the diffusion length of the material, the exponent appears to increase. This
exponent may be directly related to charge recombination in the
photocatalyst and independent of performance. If so then it could be an
important factor in determining the photocatalyst’s ability to use generated
charges effectively. If the particle size is reduced further, it may result in
reduced recombination, as the distance which charges have to migrate to
perform reactions approaches the diffusion length of the material. This could
result in an increase in the exponent of the light intensity relationship,
pushing the relationship towards linearity.
As the effect of light intensity on photocatalytic water splitting is not well
understood, this study forms a valuable contribution to the area. Light
intensity is an important parameter for system design and this study shows
that light intensities up to 50 suns does not saturate the photocatalyst. This
makes high intensity solar water splitting a plausible proposition. The
development of photocatalysts for use in high intensity light will be essential
for the commercialisation of this technology.
96
5.3 Temperature Relationship
The intensity data presented in chapter 4 (Figure 4-21) at 0.5V has been
plotted on a graph showing ln(k) vs 1/T, where k=Jphoto/[A]. (Figure 5-3).
Figure 5-3: Log of 0.5V Applied photocurrents vs 1/ Temperature
The R2 values of lines fitted to this data are above 0.97. From these fitted
lines the actvation energy (EA) and pre-exponential factor (A) were found
using:
Equation 5.4
Equation 5.5
An activation energy of approximately 10.3kJ.mol.-1 and a pre-exponential
factor of approximately 8.7×103 was obtained. Hisatomi et al. (2010) found
activation energies of 8 and 15kJ.mol.-1 for photocatalytic water splitting on
Zn:Ga2O3 catalysts loaded with Rh2-yCryO3 or Ni respectively. An earlier
y = -1.24E+03x - 7.04E+00 R² = 9.85E-01
y = -1.26E+03x - 7.06E+00 R² = 9.77E-01
-11.4
-11.2
-11
-10.8
-10.6
-10.4
-10.2
0.0025 0.0027 0.0029 0.0031 0.0033 0.0035
Cell 4.7
Cell 4.8
ln(k
)
1/T
Temperature plot, 0.5 V Bias
97
study using metal electrodes obtained activation energies of 56kJ/mol. and
204kJ.mol.-1 for Ni and Hg respectively. Our value of approximately
10kJ.mol.-1 is similar to those found by Hisatomi et al. (2010) for the water
splitting reaction using a photocatalytic electrode.
Using the Arrhenius equation ( ) and the exponential factor and
activation energy it was possible to fit a curve to the photocurrent data. This
is presented in Figure 5-4.
Figure 5-4: 0.5V Applied photocurrents vs Temperature
These values for the pre-exponential factor and activation energy fit the
experimental data well, as expected from the high R2 values. This suggests
that the relationship to temperature observed here is mostly due to the
Arrhenius kinetics. Any contribution from the lowering of the Gibbs free
energy with temperature is likely to be too small to be observed. This means
that the increase in reaction rate with temperature is due to an increase in
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0.0014
0.0016
0.0018
0.002
0 20 40 60 80 100 120
Cell 4.7, 0.5V Bias
Cell 4.8, 0.5V Bias
Arrhenius Plot (cell 4.7)
Arrhenius Plot (cell 4.8)
Temperature (°C)
Ph
oto
curr
ent
(A/c
m2)
0.5 V Applied Temperature Plot
98
available heat energy, this gives the reactants more energy and enhances
their ability to overcome the activation energy. This results in an increased
reaction rate and more hydrogen production at higher temperatures.
Our results do not extend far beyond the boiling point for water at 1
atmosphere; however there is no indication that this relationship will not
continue above this temperature providing a liquid electrolyte is maintained.
The next phase change for water is at the critical point (22.09MPa,
374.14°C), the point above which there is no distinction between a liquid and
a vapour. At temperatures greater than the critical temperature the electrolyte
will resemble a superheated vapour (Cengel & Boles, 2002) and this will
have an unknown affect on the photocatalysis reaction.
The theory states (section 2.3.1) that there is a reduction in the reaction’s
change in Gibbs free energy with temperature, according to Equation 2.1.
This asserts that increased thermal energy reduces the required energy for
the reaction. Our range of temperature was from approximately 20-100°C,
equating to a change of approximately 10kJ.mol.-1; or about 4% in the Gibbs
free energy of the reaction. Considering the uncertainties in the
measurements and photocatalysts used and the small amount of data
generated, this factor and its effect on the reaction has not been evaluated.
Experimental investigations into the effect of temperature on photocatalytic
water splitting have not previously been reported in the literature. Due to the
advantages of using higher temperatures to increase hydrogen production
rates, and the possibility of making use of the infrared portion of the solar
99
spectrum as a heat source, this work is a significant initial contribution to the
field.
The photocatalysts used in this investigation showed little effect from use at
elevated temperature. Further investigations into this subject should
endeavour to perform photocatlytic water splitting at temperatures above
100°C and pressures above 1 atmosphere. Also, the affect of high
temperature photocatalysis on materials and the Gibbs free energy are of
interest for future studies.
5.4 Model
This section discusses a model for extrapolating the results presented in
chapter 4. This model predicts the possible performance of photocatalytic
water splitting under concentrated light and high temperature conditions.
These results are important as they will help us compare with other solar
conversion systems, and evaluate the practicality of photocatalytic water
splitting.
Mathematical modelling of photocatalytic water splitting systems is not often
presented in the literature. One article by Jianhu, Yitung, Robert, & Shanthi
(2008) presented a model based on the Butler-Volmer equation. Our model
takes the slightly different approach of basing the model on the experimental
data and empirical relationships discussed in sections 5.2 and 5.3.
Some initial assumptions have been made for this model. These are:
light intensity and temperature relationships hold over the range of this
model
100
the optical properties of the reactor window are not changed by
increasing temperature and pressure
every two electrons passing through the circuit produce one hydrogen
molecule
all infrared radiation is absorbed by the reactor
5.4.1 Chart of Model
A flow chart describing the mathematical model presented in this chapter is
presented below. This flow chart depicts the order in which the calculations
should be carried out, and how they relate to each other.
The model consists of three major parts; the inputs (red), the extrapolations
(green) and the output (blue). The inputs are very conditional to outside
parameters such as weather, reactor and concentrator design, electrolyte
and solar irradiation. The extrapolations are the major contribution of this
work as they use the inputs to predict system performance over a range of
operating parameters. The outputs display’s model results in a form that can
be easily understood and used to compare between different solar energy
conversion technologies.
101
Temperature
rate constant
k(T)
Combine temperature
and intensity
relationships
Predicted photocurrent
Light concentration
and spectrum
Power density of
light over a range
of Intensity
Rate of heat
loss from
reactor
Reactor
equilibrium
temperature
Intensity
relationship
Exponential
relationship to
temperature
(Arrhenius)
Electrolyte current
density rate limit
Results
Findings
of work
Heat
Balance
Input light
power
Kinetics
Predicted hydrogen
production rate
102
5.4.2 Heat Balance of Reactor
The extrapolation section of this model relies upon the reactor temperature
and power of the light illuminating the photocatalyst as inputs. These two
parameters are linked and can be calculated analytically. Whilst these
parameters are important, the method of their calculation is not of major
significance to this study. They are reliant on factors such as weather, reactor
and concentrator design (which would vary greatly between system layouts),
geography, materials, etc. Therefore an arbitrary design has been used,
consisting of a cylindrical reaction vessel incorporating a circular quartz
window of diameter, D. The power incident on the quartz window can be
calculated from:
4
2DII IRinc Equation 5.6
Where: IRI = Infrared power in the AM1.5 Spectrum from 700-4000nm, and
IncI = power incident on the aperture. Only the Infrared power has been used
for this calculation because the visible and UV light is expected to be used by
the photocatalyst (with the possible inclusion of a photovoltaic).
When the reactor’s temperature is at steady state the incident solar power
will equal the rate of heat lost from the reactor (i.e. .. Totinc QI ). Therefore, by
treating the reactor as a flat circular plate and estimating the rate of heat loss,
the steady state temperature of the reactor can be calculated.
The heat loss rate due to convection and radiation is estimated by applying
the following equations (Cengel, 2006):
103
... RadConvTot QQQ
Equation 5.7
where:
fSConv TThAQ .
Equation 5.8
and
44
. fSRad TTAQ
Equation 5.9
When: .TotQ is the total Heat loss rate, .ConvQ is the heat loss rate due to
convection, .RadQ is the heat loss rate due to radiation, h is the convection
heat transfer coefficient, A is the surface area, ST is the surface
temperature, fT is the bulk fluid temperature (ambient), is the surface
emissivity, and is the Stefan-Boltzmann constant.
The calculation of the heat transfer constant ( h ) requires the application of
empirical equations and design conditions developed for specific geometries
and heat loss mechanisms. We used a window diameter of 30cm and forced
convection from a 5m/s wind over a turbulent circular plate to acquire
estimated temperatures which could be attained by such a system.
As the design conditions and empirical equations used are arbitrary and
specific to reactor design and situation, they have not been included. These
are basic engineering heat transfer calculations and would need to be
modified to describe any reactor used for such a system regardless.
Furthermore, any system that is built will need to find the actual heat balance
experimentally in the commissioning phase and optimise the control systems
104
from there. The temperatures found by this model (Figure 5-5) were used as
inputs for the extrapolations section to provide a range of conditions over
which predictions could be made.
Also, the Ts + Ts4 term in the total heat loss rate equation makes it non-trivial
to solve for Ts algebraically. Therefore, a heat loss rate was calculated for the
reactor over a range of temperatures. A polynomial was then fitted to the
heat loss data to estimate Ts for a given rate of heat loss. As the rate of heat
loss is equal to the incident light power at steady state, this polynomial was
applied to the solar insolation data at concentrations between 1 and 100 suns
to determine equilibrium temperatures.
Figure 5-5: Temperature predicted by model at various solar concentrations
The calculated temperatures are compared to temperatures found
experimentally by Suter, Tomeš, Weidenkaff, & Steinfeld (2010) in Figure
0.0
100.0
200.0
300.0
400.0
500.0
600.0
700.0
800.0
900.0
0 20 40 60 80 100 120
Model
C. Suter, 2010
Concentration (AM1.5 Suns)
Rec
ieve
r Te
mp
erat
ure
(K
)
Modelled temperature of 30cm dia. reactor
105
5-5. These estimated temperatures are similar to those found experimentally
in their study providing confidence that they are realistic for such a system.
The thermal efficiency was calculated to be approximately 60% for the
temperatures attained by our model. This is realistic for solar thermal
technologies.
This section of the model is open to much variation, but allows us to gain
some insight into the temperatures and pressures at which a system could be
operated. Also, the temperature of the reactor may require control if practical
considerations require it (such as optimal temperature and pressure for the
reaction, or maximum pressure and temperature of the reactor).
Finally, the critical temperature for water - after which a change of state
occurs and the water becomes a supercritical fluid - occurs at 647K (374°C)
and 22.09 MPa. Above this temperature water displays properties of both a
liquid and a gas. Our calculations show this temperature being achieved at
50.4 suns. The temperature experiments were not conducted above 100°C,
so the affect of this state change upon the reaction is unclear. Therefore we
limited our model to 100 suns in order to provide a more realistic
extrapolation of the data. This limit to 100 suns also increases confidence in
our intensity extrapolation.
5.4.3 Reactor Pressure
In order for water to remain as a liquid at temperatures above 100°C the
reactor must be pressurised. The pressures required to retain water as a
liquid at the temperatures calculated by our model are displayed in the table
below (Cengel & Boles, 2002).
106
Table 2: Pressure of reactor for liquid water at various temperatures (* critical
temperature for water)
Intensity (suns) Temperature (K) Pressure (MPa)
1 313.2 0.1
5 351.0 0.1
10 395.0 0.212
20 473.1 1.56
30 539.7 5.22
50 646.3 21.9
50.4* 647.1* 22.09*
75 744.0 22.09
100 817.2 22.09
The pressure that the reactor will be subjected to is not only an important
design parameter, it is also important for the efficiency of the system. This is
discussed in more detail in section 5.6.
5.4.4 Extrapolation of Temperature relationship
The relationship to temperature was found to be exponential in section 5.3.
The relationship matches that of the Arrhenius equation, which describes the
107
reaction rate as the number of collisions with enough energy to overcome the
activation energy.
RT
EA
eAk
Equation 2.3
Where:
k is the reaction rate coefficient, T is the temperature, A is the pre-
exponential factor, and AE is the activation energy.
Section 5.3 found AE to be approximately 10kJ.mol.-1 and A to be
approximately 8.6×103. The photocurrent is calculated by multiplying the rate
constant (k) by the concentration of water ([H2O] = 55.56M).
Equation 5.10
This relationship is extrapolated over the range of temperature and plotted on
Figure 5-6.
The photocurrents at each temperature are divided by the photocurrent at
20°C to normalise the effect of temperature on the reaction rate over all
intensities. This normalised coefficient is denoted k(T), where:
Equation 5.11
k(T) is plotted in Figure 5-7 and has the same shape as the curve in Figure
5-6. Its application to the model is explained in section 5.4.6.
108
Figure 5-6: Arrhenius predicted photocurrent
Figure 5-7: Calculated rate coefficient, k(T) over temperatures predicted
0
0.002
0.004
0.006
0.008
0.01
0.012
0 100 200 300 400 500 600
Model j(T)
Cell 5.1
Cell 5.2
Temperature (°C)
Ph
oto
curr
ent
(A.c
m-2
) Arrhenius Model Photocurrent Prediction
0
2
4
6
8
10
12
14
16
0 100 200 300 400 500 600
k(T)
Temperature (°C)
k(T)
Arrhenius Model k(T) Prediction
109
5.4.5 Intensity effect on reaction rate
This investigation found no saturation with increasing light intensity up to 50
suns. The data was best fitted with the relationship derived in section 5.2:
Equation 5.12
Where j(I) is photocurrent in A.cm-2 and Isun equiv. is light intensity in AM1.5 UV
equivalent suns.
This model is extrapolated and compared to experimental data in Figure 5-8.
It is extrapolated to 100 suns - twice that tested - to keep predictions within a
reasonable range.
Figure 5-8: Photocurrents predicted from model compared to experimental
data
0.00E+00
2.00E-04
4.00E-04
6.00E-04
8.00E-04
1.00E-03
1.20E-03
1.40E-03
0 20 40 60 80 100 120
Cell 4.1
Cell 4.2
Cell 4.3
Model j(I) Ph
oto
curr
ent
(A.c
m-2
)
Intensity (suns)
Light Intensity Model Photocurrent Prediction
110
5.4.6 Intensity and Temperature
To combine the temperature and light intensity relationships the current
predicted by the light intensity (j(I)) is multiplied by the reaction rate
coefficient (k(T)) at the steady temperature of the reactor at the associated
light concentration (found in section 5.4.2):
Equation 5.13
This gives the predicted current at temperature, T, under light intensity, I. If
the relationship between light intensity and temperature is defined then this
equation could be described as a function of light intensity alone. As our
model does not focus on the light intensity/temperature relationship then it
has been described using both light intensity and temperature independent of
each other.
Figure 5-9: Predicted photocurrents j(I) (intensity), j(T) (temperature) and J
(combined temperature and light intensity) vs light intensity
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02
0 20 40 60 80 100 120
j(I)
J
j(T)
Light Intensity (suns)
Ph
oto
curr
ent
(A.c
m-2
)
Currents calculated from model
111
These equations are applied to the intensities and temperatures of our model
and plotted on Figure 5-9.
Figure 5-9 shows that, according to our model, increasing reactor
temperature has a greater affect on reaction rate than intensity alone. This
finding however, is based on an extrapolation from data acquired at
temperatures between 20 and 100°C. Therefore, further experimental
studies into the effect of temperature on photocatalytic water splitting need to
be undertaken to corroborate this finding.
5.4.7 Current density rate limitation
The current passing through an electrolyte is limited by the rate at which ions
migrate through the solution. As the photocurrent cannot exceed the limiting
current of the electrolyte then this factor must be appraised in our model. The
limiting current of an electrolyte is calculated by the following equation:
nFDCiLim.
Equation 2.5
Where: .Limi is the limiting current density, n is the number of electrons
transferred in the reaction, D is the diffusion coefficient, C is the bulk
solution concentration and is the diffusion layer thickness.
The diffusion coefficient and the diffusion layer thickness both change with
temperature. The diffusion coefficient increases with temperature following
the relationship (Gerasimov & Rozenfeld, 1956):
r
TkD B
6
Equation 5.14
112
Where: r is the radius of the diffusing molecule and Bk is the Boltzmann
constant.
The thickness of the diffusion layer in an unmixed electrolyte reduces with
temperature according to Gerasimov & Rozenfeld (1956). They measured
the limiting current in a salt electrolyte system, at temperatures between 20
and 95°C with a known diffusion coefficient. The diffusion layer thickness was
then calculated. The values reported were extrapolated for our model to
estimate the diffusion layer thickness up to the critical temperature of water. It
is acknowledged that this method for estimating diffusion layer thickness is
imprecise; however information on the behaviour of this factor in high
temperature aqueous systems is sparse.
Figure 5-10: Limiting Current compared to predicted current
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0 20 40 60 80 100 120
Model Photocurrent (J)
Limiting current
Light Intensity (suns)
Ph
oto
curr
ent
(A.c
m-2
)
Model predicted current compared to Limiting current
113
The estimated rate limiting current for the temperatures in our model is
compared to the calculated photocurrent in Figure 5-10. As the limiting
current is greater than the predicted photocurrent density of our catalyst we
do not believe this will limit the reaction.
5.4.8 Efficiency
The energy conversion efficiency of the system allows direct comparison
between solar conversion devices. The efficiency was calculated from the
modelled data by converting the predicted current density (j), into an amount
of energy per unit time (the energy stored or output) and comparing to the
power density of the incident light (the energy input).
Equation 3.6
Where: η is the efficiency, EWS is the water splitting potential per electron
(1.23V at S.T.P.), VB is the applied Bias in Volts and I is the power of the light
in W.cm-2.
The predicted efficiency of the photocatalyst over varying light intensity and
temperature, at 0.5V applied bias is presented in Figure 5-11.
Figure 5-11 shows that the efficiency of the system initially drops then
increases with intensity and temperature. The sub-linear relationship to light
intensity is initially dominant, before the effect of temperature increases the
efficiency at intensities above 10 suns. Therefore, performing photocatalytic
water splitting under solar light concentrated over 10 suns increases the
efficiency of the process.
114
Figure 5-11: Conversion efficiency of light energy to hydrogen with respect
light intensity
5.4.9 Using high performing photocatalyst
To better understand the implications of this model it was applied to a
hypothetical photocatalyst with an efficiency of 1% (total AM1.5 insolation at
1 sun intensity, 0V applied bias and 20°C). This is a higher photoactivity then
the photocatalyst used in this investigation, but lower than many reported in
the literature (Kay, et al., 2006; Ruan, et al., 2006). It is also one tenth of the
10% efficiency commercial target for photocatalytic water splitting
(Department of Industry Tourism and Resources, 2005).
The shape of the resultant curve (Figure 5-12) is the same as that reported
above (due to the same temperatures and constants being used) but the
magnitude is higher.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0 20 40 60 80 100 120
Intensity (suns)
Ene
rgy
Co
nve
rsio
n E
ffic
ien
cy (
%)
Predicted Efficiency
115
According to this model, a 30cm in diameter reactor of a 1% efficient
material, being operated at 50 suns and 646K (373°C, just below critical
temperature for water), would be capable of producing approximately 300kJ
of H2 per hour. This is 2g or 28ml of liquid hydrogen.
The predicted efficiency of this hypothetical catalyst increases from 1.3% at 1
sun (40°C) to almost 2.5% under 100 sun illumination. This 1.9 fold increase
is effectively the heat (infrared) in the solar spectrum contributing to the
reaction and driving up the conversion efficiency. This introduces the
possibility of using a more robust but lower performing photocatalyst to
achieve the same efficiency by operating at elevated temperature.
Figure 5-12: Currents predicted using 1% efficient photocatalyst; light
intensity predicted photocurrent (red), temperature predicted photocurrent
(blue), model predicted photocurrent (black) and limiting current (purple).
0
0.05
0.1
0.15
0.2
0.25
0.3
0 20 40 60 80 100 120
j(I)
j(T)
J
Limiting Current
1% efficient photocatalyst
Light Intensity (suns)
Ph
oto
curr
ent
(A.c
m-2
)
116
Of note is that if the limiting current calculated above is used to calculate a
maximum possible efficiency for the system (in 0.1M Na2SO4 electrolyte at
40°C) we find a maximum efficiency of 7.3%. This is below the NREL
efficiency target, but dependent on the electrolyte used.
Also, a TiO2 photocatalyst only absorbs wavelengths below about 430nm,
and only the infrared portion of the spectrum is considered for the heating by
this model. This leaves the visible wavelengths (approximately 40% of the
solar spectrum) unused. This portion of the spectrum may be exploited by
including a beam splitting device to separate the spectrum into wavelength
bands, then directing the visible wavelengths to illuminate photovoltaic cells.
This would make greater use of the already concentrated sunlight and add
value to the system. Also, the photovoltaics could be used to apply potential
to the water splitting reactor and increase conversion rate.
5.4.10 Factors unaccounted for by the model
There are some factors which are not accounted for by this model. Firstly, the
application of potential across the electrodes has a major effect on the
photocurrent. The data presented is for 0.5V applied voltage. This voltage
was chosen as it is the start of the “flat” portion of the I-V curves (Figure
5-13). This region extends from approximately 0.5V to 1.5V. It is preceded by
a linear region, which is attributed to the Ohmic resistance of the cell under
light, and succeeded by an electrolysis tail. Our model applies to the “flat,”
rate limited region of the IV curve.
117
Figure 5-13: Standard I-V Curve at room temperature
The second factor not accounted for in this model is the decrease in the
change in Gibbs free energy as temperature increases (Figure 5-14). As this
factor is reduced the required potential for water to dissociate also reduces
(from 1.23eV at 25°C, to 0.87eV at 544°C). The reduction in the potential of
the reaction means that at any specific applied bias, the dissociation of water
will occur more readily. This will lead to a greater overpotential at any applied
bias and subsequently higher reaction rates and greater photon to current
conversion efficiency. This factor was excluded, as its effect on photocatalytic
water splitting could not be ascertained from our experiments.
0
0.00005
0.0001
0.00015
0.0002
0.00025
0.0003
0.00035
0.0004
0 0.5 1 1.5 2 2.5
Applied Voltage (V)
Cu
rren
t (A
)
Standard IV Curve
Linear Ohmic region
“Flat” rate limited region
Electrolysis
118
Figure 5-14: Gibb’s free energy and potential change over temperatures
calculated from our model
5.5 Comparison to other conversion devices
Our photocatalyst efficiency went from 0.08% at S.T.P. (measured) to 0.16%
at 544°C and 100 suns (calculated with the model). A hypothetical 1%
efficient photocatalysts at S.T.P. would increase its efficiency to around 2.5%
at 100 suns under the conditions in the model. The higher temperature not
only offsets the decrease in efficiency from the high intensity light, it doubles
the efficiency of the reaction
Conibeer & Richards, (2007) compared the hydrogen production method of a
photovoltaic coupled with electrolysis, to direct photoelectrolysis. They
reported solar to hydrogen efficiencies of 5 - 21% for the PV/Electrolysis
systems, depending on the type of PV and electrolysis employed. They
decided that a figure of about 9% best described this technology.
0
0.2
0.4
0.6
0.8
1
1.2
0.0
50.0
100.0
150.0
200.0
250.0
300 400 500 600 700 800 900
Temperature (K)
ΔG
(kJ
mo
l.-1)
Gibbs free energy vs temperature
Po
ten
tial
(e
V)
119
Photoelectrolysis efficiencies ranged from 15% for a multijunction design, to
1.5 and 4% for TiO2 devices. A figure of 5% was put forward as a reasonable
efficiency for a photoelectrolysis system.
If we apply this 5% representative efficiency to our model we acquire a
theoretical efficiency of about 12% at 100 suns and 544°C. As reported by
Licht (2002; Licht, 2003, 2005a, 2005b; Licht, et al., 2003) and discussed in
section 2.6; the theoretical maximum efficiency for a photoconversion device
under high light intensity and temperature conditions approaches 50%. Our
findings corroborate the concept that a photoelectrolysis device could be
operated at high efficiencies, making it a possible alternative to
PV/electrolysis systems.
5.6 What does this mean for System Design
One of the goals of this investigation was the consideration of how high light
intensity and temperature influence reactor design. Some of the factors
already mentioned include the corrosion of exposed metal electrical
connections, pressurising the reactor and the integrity of the illumination
window. Some more considerations are discussed in the following sections.
5.6.1 Reactor Window
The inclusion of windows in a pressurised and heated reactor, through which
the reactor can be observed or illuminated is within the bounds of current
technology. The Parr Instrument Company for instance, offer screw in or
integral windows on their reactor vessels. These windows are made from
quartz or sapphire and are rated up to 34 MPa pressure, which is well above
the pressures required for super-critical water (22 MPa). They have a
maximum operating temperature of 275°C due to the seal material. For
120
sealing above this temperature carbon or metal seals must be used (Mayer,
1972).
5.6.2 Reactor Design
Using high light intensity and temperature for photocatalytic water splitting
reactions has some functional advantages. Firstly, the plant infrastructure
required for the system (i.e. concentrators, controls, reactors, etc) are
independent of the photocatalyst. This means that if the photocatalyst is
damaged, degraded, or superseded it can be replaced at significantly less
cost than a one sun system would require. It also allows the use of a more
sophisticated and expensive photocatalyst as less area is illuminated and
subsequently less catalyst required.
Another important advantage for such a system is the collection of product
gases. The large areas needed for a one sun system will distribute the
reaction; this would result in significant sealing and high precision
manufacturing requirements over a large area and volume. Using a
concentrated light system however, would allow the evolution of product
gases in a much smaller volume. This means the sealing and high precision
manufacture are confined to a small reactor, thus reducing the cost of this
component. This is doubly important when one of the gases being captured
is hydrogen, a molecule that is renowned for its sealing requirements.
The production method is an important consideration for system design. The
obvious choice is to run the reactor as a continuous production system, with
electrolyte flowing over the photoanode and counter electrode. However,
most experiments in photocatalytic water splitting use batch production style
121
apparatus. Continuous systems are quite common in photocatalytic water
purification and generally consist of a pump recirculating the water across the
photocatalyst in either a thin “sheet” (Bekbolet, Lindner, Weichgrebe, &
Bahnemann, 1996; Feitz, Boyden, & Waite, 2000; Franke & Franke, 1999;
Nogueira & Jardim, 1996) or through exposed tubes (McLoughlin et al., 2004;
Robert, Piscopo, Heintz, & Weber, 1999).
This highlights another advantage of a high intensity and temperature
system. A large reaction area will result in greater water pressure loss and
high flow rates. Therefore a larger volume and pressure pump will be
required, increasing the parasitic energy loss and cost of the system.
Another consideration is the transfer of ions between the electrodes to
complete the circuit. As the anode will produce O2 and the cathode H2, it
would be beneficial to physically separate the evolution spaces. This will
allow each gas to be collected separately at a high purity and reduce the
possibility of an explosive mixture. This could be facilitated through the use of
ion transfer membranes developed for high temperature electrolysis and fuel
cell applications - such as porous ceramics.
Figure 5-15 and Figure 5-16 below give an example of what a system might
look like. It includes some possible solutions to problems and some design
considerations are pointed out.
122
Figure 5-15: Assembled Cylinder Reactor
Figure 5-16: Cylinder Reactor Exploded View
123
This reactor layout has 0.09 m2 of Illuminated area and is made up of
multiple cylindrical components which bolt together. These components
would be sealed using gaskets between each component. Its manufacture
would require relatively simple machining methods.
This design uses a thick quartz window to allow light to enter the reactor. It
needs to resist the high pressures that will be encountered whilst operating at
high temperature. The quartz window sits inside a front plate and is held in
place by the separator plate. This separator plate defines the thickness of the
water film on the surface of the photoanode and is where inlet/outlet
manifolds and hydrogen gas collection systems would be situated. Behind
this is the photoanode; which is accessible from outside the reactor for
electrical connection and has cut out sections for ion transfer. The electrode
separator plate houses the Nafion membrane and also with ion transfer
cutouts. This plate would need to be electrically isolated from the two
electrodes that it divides. Also, the electrode separator plate defines the
thickness of the water at the counter electrode and is where inlet/outlet
manifolds and oxygen gas collection systems would be placed. Finally, the
counter electrode and backing plate complete the reactor.
5.6.3 Hydrogen Embrittlement
Diffusion of Hydrogen into metal under high temperatures and concentration
gradients, both of which would exist in this system, can lead to hydrogen gas
forming in micro-voids in the material. This results in reduced ductility, tensile
strength and possible cracking of the metal. Appropriate material selection is
required for components in the hydrogen reactor. Susceptible materials
include high strength steel, Nickel base alloys, Ductile and low strength steel,
124
pure nickel and titanium alloys. Stable materials are generally Aluminium
alloys, Austenitic stainless steel and Copper. Aside from materials selection,
prevention methods such as the removal of notches, smooth surface finish,
surface coatings and periodic inspection reduce the likelihood of
embrittlement (Jewett, Walter, Chandler, & Frohmberg, 1973)
5.6.4 H2 Solubility at high temperature and pressure
The solubility of produced gases under operational conditions will greatly
affect the design of a system. For instance, under 1 Bar and sub 100°C the
solubility of H2 gas in water is around 0 - 0.02 cm3/g. As pressure increasesat
temperatures above 100°C, the solubility of H2 varies greatly. At 370°C (just
below the critical point for water) and 300 Bar, 18.2 cm3/g is soluble and at
500 Bar this increases to 62.0 cm3/g (Baranenko & Kirov, 1989).
This is obviously a massive difference between possible operational
conditions especially for the collection of the product gas. Obviously at low
temperature and pressure, the low solubility will allow evolved gases to be
collected directly from the reaction chamber. Under high temperature and
pressure conditions however, this may not be the best solution. A gas
evolution chamber separate to the reactor may be required, where
temperature and pressure is reduced to allow the product gases to
effervesce from solution. Of course this would have the disadvantage of
releasing the heat and pressure energy of the fluid.
These factors will have a large influence on whether the system is most
effective running as a continuous, or batch type system. Also, often stated as
a shortcoming of hydrogen as a fuel, is the amount of energy required to
125
compress or liquefy it for storage. A sealed, totally water filled system will
effectively pressurise itself (as there is no volume for vapour to occupy as
liquids are incompressible) when heated above boiling point. As the heat
comes from the sun - a renewable resource - the pressure acquired is a
beneficial offshoot of the high temperature reaction. If the pressure was held
constant during the gas evolution step and the temperature reduced then a
large proportion of the gas would effervesce without a pressure loss. This
however, would also be associated with a significant energy loss.
Also, products dissolved in electrolyte affect the ability for the reaction to
progress, because they increase the back reaction. However, the solubilities
of hydrogen and oxygen at the critical temperature and pressure of water are
low enough that the back reaction is negligible.
5.6.5 Cost
As costing a system at this stage of research would be far too complex, a
simple cost comparison between a flat plate collector system and a
concentrated solar light system was undertaken. An example $100 000
investment, at an interest rate of 12% compounded monthly with a 5 year pay
back period, produces a monthly repayment of $2224.44. Therefore, a
system has to produce this revenue per month in order to pay for itself in 5
years. McConnell & Thompson (2004) reported the following hydrogen
production costs for a number of different processes (Table 3):
126
Table 3: Hydrogen production cost comparison (McConnell & Thompson,
2004)
Process Hydrogen production
cost ($/kg)
Gas reformation 1.15
Wind Electrolysis 3.10
Nuclear Electrolysis 1.48
PV Flat-Plate Electrolysis 7.40
Concentrated PV Electrolysis 3.63
The amount of hydrogen needed to be produced per month, in order to meet
the repayment, at hydrogen prices from 1 - 4.87 $/kg, is presenting in Table
4. These were chosen as they are similar to those in Table 3. 4.87 $/kg is the
energy equivalent to paying 1.20 $/L for petrol/gasoline. This is followed by
the area of photocatalyst required to produce this amount in two systems; a
one sun system, operating at 10% efficiency (the commercial target for this
technology), the other is a 100 sun system running at 544°C, with an
efficiency projected by our model to be 20%.
Table 4: Areas required for a 1 sun and a 100 sun system at various hydrogen
prices
H2 prices (/kg) = $1 $2 $3 $4 $4.87
kg/month 2224.44 1112.22 741.48 556.11 457.22
1 Sun system, 10% Efficiency
m2 required 2690.79 1345.39 896.93 672.70 553.07
100 Sun system, 20% Efficiency
m2 required
13.45 6.73 4.48 3.36 2.77
Concentrator area 1345.39 672.70 448.46 336.35 276.54
The predicted doubling in photocatalyst efficiency reduces the area required
to produce the same amount of hydrogen by the same factor. This will
significantly reduce system costs due to the reduction in materials required to
127
cover the area. Additionally, a solar reflector will be significantly cheaper to
produce per unit area than a flat plate collector as the flat plate collector will
need to consist of a complex glass-electrolyte-photocatalyst-substrate-back
plate style laminated system. Conversely, a concentrated light photocatalytic
reactor would be an expensive unit to produce and install. This reactor
however, would be more easily serviced than a one sun system, and allows
the replacement and upgrade of the photocatalyst if degraded or superseded.
If this occurred to a flat plate reactor it would likely require replacement of the
entire reactor.
Figure 5-17: Projected cost of Heliostat concentration (Sargent & Lundy LLC
Consulting Group, 2003)
Moreover, the cost of solar concentration and solar energy is dropping. NREL
(National Renewable Energy Lab) projected cost for Heliostats are falling
from $145/m2 in 2004 to $107/m2 in 2010 to $76/m2 in 2020. These cost
reductions are summarised in Figure 5-17 and are a result of technology
128
improvement, scaling to larger heliostats and higher production volumes
(Sargent & Lundy LLC Consulting Group, 2003). Concentrating technologies
of all types are following similar paths, as are associated plant costs.
More research into photocatalytic water splitting at high temperature and light
intensity is required to ascertain the cost per kg of hydrogen for such a
system. This will allow the analysis of the feasibility of this technology and
better direct research efforts in the field.
5.7 Materials research directions
Undertaking photocatalytic water splitting at high light intensity, temperature
and reaction rates will alter the requirements of photocatalytic materials. The
first material requirement is its temperature and pressure resistance.
Obviously the photocatalyst will be required to operate under these
conditions and therefore it must be stable at them. Most photocatalysts are
metal oxides and tend to be stable at moderately high temperatures. Above
these temperatures their crystallinity can change (amorphous TiO2 forms
anatase at approx 400° C for example). This could be an important factor
limiting photocatalysis above certain temperatures. Also, the photocatalyst
will be in an aqueous environment, so its solubility at operational temperature
must be considered also. Titania’s solubility in water was calculated to be
relatively low and constant (approximately 6×10-4 M) in neutral pH at
temperatures up to 327°C (Atashfaraz et al., 2007). This suggests that TiO2
should be stable in low pH water solutions at temperatures up to the critical
point of water.
129
Another material requirement is the ability to absorb light at high intensities,
and use the energy in that light. This presents considerable challenges
similar to those faced by high intensity photovoltaics. Additionally, the
capacity for high electrode/electrolyte interface current densities is needed.
Another factor which may affect high surface area photoanodes is the
significant mechanical forces from large volume changes – such as bubble
formation – which may destroy the structure of the material.
Also of importance is the long term stability of the material when operating.
This often goes unreported in many high performing photocatalyst papers but
is obviously very important for a photocatalytic system. Materials research
should focus on producing high efficiency photocatalysts that are stable over
significant working lifetimes.
5.8 Summary
The affect of light intensity and temperature on photocatalytic water splitting
reactions was analysed and a model developed in this chapter. The results
from this model show that high light intensity and temperature increases the
performance and efficiency of the reaction. This could be a promising avenue
for the commercialisation of photocatalytic water splitting.
A simple design of a reactor also was presented, and factors relevant to the
design of the reactor discussed. Finally, the implications of concentrated light
and high temperature on materials research directions were discussed.
130
6.0 Conclusion
This study was undertaken to investigate operation of photocatalytic water
splitting systems under concentrated sunlight at elevated temperatures.
Those results were then applied to system design. In order to accomplish this
we developed apparatus and methods for testing photocatalysis under
various light intensities and at different temperatures.
This study endeavoured to answer the research questions laid out in Chapter
1. These consisted of two areas of investigation; the effect of physical
parameters and its relevance to a practical system.
Physical parameters
1. What effect does increasing the incident light intensity have on the
rate of reaction?
131
2. What effect does high temperature and the required pressure increase
have on the rate of reaction?
3. How does the change in the energy required to split water with
increasing temperature affect photocatalytic water splitting?
System
4. How can these results be applied to the engineering of practical
systems?
5. What problems will be encountered by increasing light intensity, and
subsequently temperature, for practical systems and how can they be
addressed?
Questions 1, 4 and 5 were investigated successfully whilst question 3 could
not be addressed. Question 2 was partially answered, the relationship
between reaction rate and temperature was established but pressure could
not be accounted for.
The experimental investigation into the effect of light intensity was expected
to find a saturation of the response at high intensities. However, our results
showed no such saturation up to 50 suns and a sub-linear relationship
proportional to intensity with an exponential term of 0.627. This means that
the efficiency of the reaction is reduced with increasing light intensity.
The temperature results show that increasing the temperature of the reaction
increases the rate and efficiency of the reaction. This increase resembles an
Arrhenius relationship. This means that thermal energy contributes to the
132
activation energy of the water splitting reaction and increases the collision
frequency of reactant molecules. Unfortunately these tests could not be
performed at temperatures above 100°C due to the glass test cells being
unable to withstand the increase in pressure above the boiling point.
These experimental results were used to develop a model describing the
relationship between light intensity, temperature and reaction rate. This
model estimates the performance of photocatalysis at solar light intensities
up to 100 suns and approximately 550°C. This model found that the increase
in efficiency due to temperature outweighs the associated loss in efficiency
from concentrating the solar light.
The results from the model, and the experiences gained in the apparatus and
methods development, were applied to reactor design. Problems that may be
encountered are highlighted and some solutions to them proposed. These
include electrical connections corroding in the electrolyte, inserting an
illuminating window into the reactor, hydrogen embrittlement of the reactor
material and catalyst solubility and stability at operational conditions.
Future work on this area should focus on extending the range over which
both temperature and light intensity have been tested. Testing above 100°C
and at light intensities greater than 50 suns are needed to expand upon the
findings of this work. In order to conduct tests above 100°C a sealed
pressure vessel with a quartz or sapphire window is recommended. Also, a
higher performing photocatalyst will produce more meaningful the results.
The influence of photocatalyst particle size on the response to light intensity
is another area requiring further investigation. If particle size is positively
133
linked to the exponent of the light intensity relationship then this will be
important when optimising materials for concentrated light.
If the concept of operating a photocatalytic water splitting system is proven to
be feasible, then the development of photocatalyst materials for the purpose
is important. Heat and pressure tolerance, high current density, absorption
and efficiency under concentrated light, and material stability are some of the
major properties required by a material for use in such a system.
134
Appendix A
Appendix A-1: Raw Data for Scan Rate Validation (Figure 4-1)
Appendix A-1 is a combination of two different types of measurements
conducted on a cell produced using the method and apparatus described in
Chapter 3.0. On/Off measurements: where voltage is applied and the cell is
illuminated then unilluminated once steady state has been reached in each
case, and a light and dark IV curve. The data from these tests has been
arranged on the graph so that the point at which the cell is unilluminated in
the On/Off tests is approximately at the same voltage on the x-axis as the
voltage at which test has been carried out. This allows a direct comparison
between the steady state current on the illuminated On/Off test with the IV
curve at the equivalent current/voltage. In all 4 cases these currents and
voltages are similar in both types of tests.
0.00E+00
5.00E-05
1.00E-04
1.50E-04
2.00E-04
2.50E-04
3.00E-04
-1 -0.5 0 0.5 1 1.5 2 2.5
34.5 Suns IV Curve
Dark IV Curve
On/Off 0.5V bias
On/Off 1.0V bias
On/Off 1.5V bias
On/Off 2.0V bias
Applied Voltage (V)
Cu
rren
t (A
)
Comparison of IV Curves to On/Off Tests at various applied voltages
135
Appendix A-2: Hematite stability test
Appendix A-2 shows the degradation of a 2.5% Ti doped Fe2O3 sample over
repeated testing. This test was carried out using the apparatus described in
Chapter 3.0 and a doctor bladed Fe2O3 inverse opal film produced using the
method described in section 3.6.1 with the inclusion of a 2.5% wt fraction of
Titanium using a Titanium oxychloride precursor solution. These tests were
undertaken using purified water as the electrolyte and approximately 1 sun
illumination.
-1.00E-05
-5.00E-06
0.00E+00
5.00E-06
1.00E-05
1.50E-05
2.00E-05
2.50E-05
3.00E-05
-0.4 -0.2 0 0.2 0.4 0.6 0.8
Dark Test
Light test #1
Light test #2
Light test #3
Light test #4
Light test #5
Light test #6
Light test #7
Light test #8
Light test #9
Light test #10
2.5% Ti doped Fe2O3 Repeat Tests
Cu
rren
t (A
)
Applied Bias (V)
136
Appendix A-3, Appendix A-4 and Appendix A-5 show the raw data used to
produce Figure 4-4 displayed in IV curve format. This data was produced
using the apparatus and materials described in Chapter 3.0 and method
described in section 4.1.3.
Appendix A-3: Cell 4.1 Stability Raw Data (Figure 4-4)
0.00E+00
5.00E-05
1.00E-04
1.50E-04
2.00E-04
2.50E-04
3.00E-04
3.50E-04
4.00E-04
0 0.2 0.4 0.6 0.8 1
0 Suns
28 Suns, Test 1
28 Suns, Test 2
28 Suns, Test 3
28 Suns, Test 4
28 Suns, Test 5
28 Suns, Test 6
28 Suns, Test 7
28 Suns, Test 8
28 Suns, Test 9
28 Suns, Test 10
28 Suns, Test 11
28 Suns, Test 12
28 Suns, Test 13
28 Suns, Test 14
28 Suns, Test 15
0 Suns
Stability Test - Cell 4.1
Cu
rren
t (A
)
Applied Voltage (V)
137
Appendix A-4: Cell 4.2 Stability Raw Data (Figure 4 4)
Appendix A-5: Cell 4.3 Stability Raw Data (Figure 4 4)
0.00E+00
5.00E-05
1.00E-04
1.50E-04
2.00E-04
2.50E-04
0 0.2 0.4 0.6 0.8 1
0 Suns
28 Suns, Test 1
28 Suns, Test 2
28 Suns, Test 3
28 Suns, Test 4
28 Suns, Test 5
28 Suns, Test 6
28 Suns, Test 7
28 Suns, Test 8
28 Suns, Test 9
28 Suns, Test 10 28 Suns, Test 11
Stability Test - Cell 4.2
Cu
rren
t (A
)
Applied Voltage (V)
0.00E+00
5.00E-05
1.00E-04
1.50E-04
2.00E-04
2.50E-04
3.00E-04
0 0.2 0.4 0.6 0.8 1
0 Suns
28 Suns, Test 1
28 Suns, Test 2
28 Suns, Test 3
28 Suns, Test 4
28 Suns, Test 5
28 Suns, Test 6
28 Suns, Test 7
28 Suns, Test 8
28 Suns, Test 9
28 Suns, Test 10
28 Suns, Test 11
28 Suns, Test 12
28 Suns, Test 13
0 Suns
Stability Test - Cell 4.3
Cu
rren
t (A
)
Applied Voltage (V)
138
Bibliography
Alpert, D. J., Mancini, T. R., Houser, R. M., Grossman, J. W., Schissel, P., Carasso, M., et al. (1991). Solar concentrator development in the United States. Solar Energy Materials, 24(1-4), 307-319.
Ashokkumar, M. (1998). An overview on semiconductor particulate systems for photoproduction of hydrogen. International Journal of Hydrogen Energy, 23(6), 427-438.
Baranenko, V. I., & Kirov, V. S. (1989). Solubility of hydrogen in water in a broad temperature and pressure range. Atomic Energy, 66(1), 30-34.
Bekbolet, M., Lindner, M., Weichgrebe, D., & Bahnemann, D. W. (1996). Photocatalytic detoxification with the thin-film fixed-bed reactor (TFFBR): Clean-up of highly polluted landfill effluents using a novel TiO2-photocatalyst. Solar Energy, 56(5), 455-469.
Bilchak, R. V., Connolly, J. S., & Bolton, J. R. (1980). Effects of Temperature and intensity on thermodynamic limits for efficiencies of photochemical conversion of solar energy. Paper presented at the AS/ISES 1980 Annual Meeting, Phoenix, Ariz., June 2-6.
Blakers, A. W. (2000). Efficient 20-50 sun concentrator cells. Paper presented at the European Photovoltaic Solar Energy Conference, Glasgow, United Kingdom.
Bockris, J. O. M. (1999). Hydrogen economy in the future. International Journal of Hydrogen Energy, 24(1), 1-15.
Bolton, J. R. (1978). Solar Fuels. Science, 202(4369), 705-711.
Carey, J. H., & Oliver, B. G. (1976). Intensity effects in the electrochemical photolysis of water at the titanium dioxide electrode. Nature, 259(5544), 554-556.
Carp, O., Huisman, C. L., & Reller, A. (2004). Photoinduced reactivity of titanium dioxide. Progress in Solid State Chemistry, 32(1-2), 33-177.
Cengel, Y. A., & Boles, M. A. (2002). Thermodynamics: An Engineering Approach (4th ed.). New York: McGraw-Hill.
Feitz, A. J., Boyden, B. H., & Waite, T. D. (2000). Evaluation of two solar pilot scale fixed-bed photocatalytic reactors. Water Research, 34(16), 3927-3932.
Franke, R., & Franke, C. (1999). Model reactor for photocatalytic degradation of persistent chemicals in ponds and waste water. Chemosphere, 39(15), 2651-2659.
Fujiwara, S., Kasai, S., Yamauchi, H., Yamada, K., Makino, S., Matsunaga, K., et al. (2008). Hydrogen production by high temperature electrolysis with nuclear reactor. Progress in Nuclear Energy, 50(2-6), 422-426.
139
Glasscock, J. A., Barnes, P. R. F., Plumb, I. C., & Savvides, N. (2007). Enhancement of Photoelectrochemical Hydrogen Production from Hematite Thin Films by the Introduction of Ti and Si. Journal of Physical Chemistry C, 111(44), 16477 - 16488.
Grätzel, M. (2001). Photoelectrochemical cells. Nature, 414, 338 - 344.
Green, M. A., Blakers, A. W., Wenham, S. R., Jianhua, Z., Chong, C. M., Taouk, M., et al. (1987). Improvements in flat-plate and concentrator silicon cell efficiency. Paper presented at the IEEE Photovoltaic Specialists Conference.
Green, M. A., Blakers, A. W., Zhao, J., Taouk, M., Narayanan, S., & Campbell, P. (1987). High-efficiency silicon concentrator solar cells. Kensington, Australia.: Jt. Microelectron. Res. Cento. Document Number)
Green, M. A., Blakers, A. W., Zhao, J., Wang, A., Milne, A. M., Dai, X., et al. (1989). High-efficiency silicon concentrator solar cell research. Kensington, Australia.: Sol. Photovoltaic Lab., New South Wales Univ.o. Document Number)
Green, M. A., Taouk, M., Blakers, A. W., Narayanan, S., Zhao, J., & Campbell, P. (1986). A 23.6% efficient low-resistivity silicon concentrator solar cell. Applied Physics Letters, 49(4), 194-195.
Green, M. A., Zhao, J., Wang, A., & Blakers, A. W. (1990). Silicon concentrator solar cell development. Kensington, Australia: New South Wales Univ.o. Document Number)
Harvey, P. R., Rudham, R., & Ward, S. (1983). Photocatalytic oxidation of liquid alcohols and binary alcohol mixtures by rutile. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 79(12), 2975-2981.
Hauch, A., Ebbesen, S. D., Jensen, S. H., & Mogensen, M. (2008). Highly efficient high temperature electrolysis. Journal of Materials Chemistry, 18(20), 2331-2340.
Henry, C. H. (1980). Limiting efficiencies of ideal single and multiole ebergy gap terrestrial solar cells. Journal of Applied Physics, 51(8), 4494 - 4500.
Herrmann, J.-M. (1999). Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants. Catalysis Today, 53(1), 115-129.
Hong, W. S., Park, J. H., & Han, G. Y. (2009). Optimization of Conditions for Hydrogen Production from Anodized TiO2 Nanotube-Based Photoelectrochemical CellsArticle Journal of Nanoscience and Nanotechnology, 9(12), 7293-7297.
Huang, A., Cao, L., Chen, J., Spiess, F.-J., Suib, S. L., Obee, T. N., et al. (1999). Photocatalytic Degradation of Triethylamine on Titanium Oxide Thin Films. Journal of Catalysis, 188(1), 40-47.
140
Jakob, M., Levanon, H., & Kamat, P. V. (2003). Charge Distribution between UV-Irradiated TiO2 and Gold Nanoparticles: Determination of Shift in the Fermi Level. Nano Letters, 3(3), 353-358.
Jewett, R. P., Walter, R. J., Chandler, W. T., & Frohmberg, R. P. (1973). Hydrogen environment embrittlement of metals (No. NASA-CR-2163 ): NASAo. Document Number)
Jiang, D., Zhao, H., Jia, Z., Cao, J., & John, R. (2001). Photoelectrochemical behaviour of methanol oxidation at nanoporous TiO2 film electrodes. Journal of Photochemistry and Photobiology A: Chemistry, 144(2-3), 197-204.
Jianhu, N., Yitung, C., Robert, F. B., & Shanthi, K. (2008). A Photoelectrochemical Model of Proton Exchange Water Electrolysis for Hydrogen Production. Journal of Heat Transfer, 130(4), 042409.
Katakis, D., Mitsopoulou, C., & Vrachnou, E. (1994). Photocatalytic splitting of water: increase in conversion and energy storage efficiency. Journal of Photochemistry and Photobiology A: Chemistry, 81(2), 103-106.
Kay, A., & Grätzel, I. C. M. (2006). New Benchmark for Water Photooxidation by Nanostructured -Fe2O3 Films. Journal of the American Chemical Society, 128(49), 15714 -15721.
Kennedy, J. H., & Frese Jr., K. W. (1978). Photooxidation of Water at -Fe2O3 Electrodes. Journal of the Electrochemical Society, 125(5), 709 - 714.
Licht, S. (2001). Multiple Band Gap Semiconductor/Electrolyte Solar Energy Conversion. J. Phys. Chem. B, 105(27), 6281-6294.
Licht, S. (2002). Efficient solar generation of hydrogen fuel - a fundamental analysis. Electrochemistry Communications, 4(10), 790-795.
Licht, S. (2003). Solar Water Splitting To Generate Hydrogen Fuel: Photothermal Electrochemical Analysis. Journal of Physical Chemistry B, 107, 4253 - 4260.
Licht, S. (2005a). Solar water splitting to generate hydrogen fuel--a photothermal electrochemical analysis. International Journal of Hydrogen Energy, 30(5), 459-470.
Licht, S. (2005b). Thermochemical solar hydrogen generation. Chemical Communications(37), 4635 - 4646.
Licht, S., Halperin, L., Kalina, M., Zidman, M., & Halperin, N. (2003). Electrochemical potential tuned solar water splitting. Chemical Communications, 3006 - 3007.
Lim, T. H., Jeong, S. M., Kim, S. D., & Gyenis, J. (2000). Photocatalytic decomposition of NO by TiO2 particles. Journal of Photochemistry and Photobiology A: Chemistry, 134(3), 209-217.
141
Linsebigler, A. L., Lu, G., & Yates, J. T., Jr. (1995). Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results. Chemical Reviews, 95(3), 735-758.
Marín, F. I., Hamstra, M. A., & Vanmaekelbergh, D. (1996). Greatly Enhanced Sub-Bandgap Photocurrent in Porous GaP Photoanodes. Journal of the Electrochemical Society, 143(3), 1137-1142.
McConnell, R., & Thompson, J. (2004). Generating Hydrogen through Water Electrolysis Using Concentrator Photovoltaics. Paper presented at the 2004 DOE Solar Energy Technologies Program Review Meeting.
McLoughlin, O. A., Kehoe, S. C., McGuigan, K. G., Duffy, E. F., Touati, F. A., Gernjak, W., et al. (2004). Solar disinfection of contaminated water: a comparison of three small-scale reactors. Solar Energy, 77(5), 657-664.
Mills, A., & Le Hunte, S. (1997). An overview of semiconductor photocatalysis. Journal of Photochemistry and Photobiology A: Chemistry, 108(1), 1-35.
Ni, M., Leung, M. K. H., Leung, D. Y. C., & Sumathy, K. (2007). A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production. Renewable and Sustainable Energy Reviews, 11(3), 401-425.
Nogueira, R. F. P., & Jardim, W. F. (1996). TiO2-fixed-bed reactor for water decontamination using solar light. Solar Energy, 56(5), 471-477.
Nowotny, J., Sorrell, C. C., Sheppard, L. R., & Bak, T. (2005). Solar-hydrogen: Environmentally safe fuel for the future. International Journal of Hydrogen Energy, 30(5), 521-544.
O'Regan, B., & Grätzel, M. (1991). A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal Ti0(2) Films. Nature, 353(6346), 737.
Renewable Resource Data Center. Reference Solar Spectral Irradiance: Air Mass 1.5. from http://rredc.nrel.gov/solar/spectra/am1.5/
Robert, D., Piscopo, A., Heintz, O., & Weber, J. V. (1999). Photocatalytic detoxification with TiO2 supported on glass-fibre by using artificial and natural light. Catalysis Today, 54(2-3), 291-296.
Ross, R. T., & Hsiao, T.-L. (1977). Limits on the yield of photochemical solar energy conversion. Journal of Applied Physics, 48(11), 4783 - 4785.
Ruan, C., Paulose, M., Varghese, O. K., & Grimes, C. A. (2006). Enhanced photoelectrochemical-response in highly ordered TiO2 nanotube-arrays anodized in boric acid containing electrolyte. Solar Energy Materials and Solar Cells, 90(9), 1283-1295.
Sargent & Lundy LLC Consulting Group. (2003). Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts (No. 34440). Chicago, Illinois. (NREL o. Document Number)
142
Shockley, W., & Queisser, H. J. (1961). Detailed Balance Limit of Efficiency of p-n Junction Solar Cells. Journal of Applied Physics, 32(3), 510-519.
Sivula, K., Zboril, R., Formal, F. L., Robert, R., Weidenkaff, A., Tucek, J., et al. (2010). Photoelectrochemical Water Splitting with Mesoporous Hematite Prepared by a Solution-Based Colloidal Approach. Journal of the American Chemical Society, 132(21), 7436–7444.
Wu, K.-R., Hung, C.-H., & Tsai, M.-H. (2009). Effects of ITO thin films on microstructural and photocatalytic properties of layered TiO2/ITO films prepared via an extended deposition period. Applied Catalysis B: Environmental, 92(3-4), 357-366.
Yamaguchi, M., Takamoto, T., & Araki, K. (2006). Super high-efficiency multi-junction and concentrator solar cells. Solar Energy Materials and Solar Cells, 90(18-19), 3068-3077.
Zhao, J., Wang, A., Blakers, A. W., & Green, M. A. (1988). High efficiency prismatic cover silicon concentrator solar cells. Paper presented at the IEEE Photovoltaic Specialists Conference.
Zubi, G., Bernal-Agustín, J. L., & Fracastoro, G. V. (2009). High concentration photovoltaic systems applying III-V cells. Renewable and Sustainable Energy Reviews, 13(9), 2645-2652. Bamwenda, G. R., & Arakawa, H. (2001). The photoinduced evolution of O2 and H2 from a WO3 aqueous suspension in the presence of Ce4+/Ce3+. Solar Energy Materials and Solar Cells, 70(1), 1-14.
Bamwenda, G. R., Tsubota, S., Nakamura, T., & Haruta, M. (1995). Photoassisted hydrogen production from a water-ethanol solution: a comparison of activities of Au---TiO2 and Pt---TiO2. Journal of Photochemistry and Photobiology A: Chemistry, 89(2), 177-189.
Duret, A., & Gratzel, M. (2005). Visible Light-Induced Water Oxidation on Mesoscopic Films Made by Ultrasonic Spray Pyrolysis. Journal of Physical Chemistry B, 109(36), 17184 -17191.
Gerischer, H. (1977). The photoelectrochemical cell: principles, energetics and electrode stability. Paper presented at the Electrochemical Society (Semicond. Liq.-Junction Sol. Cells).
Glasscock, J. A., Barnes, P. R. F., Plumb, I. C., & Savvides, N. (2007). Enhancement of Photoelectrochemical Hydrogen Production from Hematite Thin Films by the Introduction of Ti and Si. Journal of Physical Chemistry C, 111(44), 16477 - 16488.
Gurunathan, K. (2000). Photobiocatalytic production of hydrogen using sensitized TiO2-MV2+ system coupled Rhodopseudomonas capsulata. Journal of Molecular Catalysis A: Chemical, 156(1-2), 59-67.
Gurunathan, K. (2004). Photocatalytic hydrogen production using transition metal ions-doped [gamma]-Bi2O3 semiconductor particles. International Journal of Hydrogen Energy, 29(9), 933-940.
143
Gurunathan, K., Maruthamuthu, P., & Sastri, M. V. C. (1997). Photocatalytic hydrogen production by dye-sensitized Pt/SnO2 and Pt/SnO2/RuO2 in aqueous methyl viologen solution. International Journal of Hydrogen Energy, 22(1), 57-62.
Kay, A., & Grätzel, I. C. M. (2006). New Benchmark for Water Photooxidation by Nanostructured -Fe2O3 Films. Journal of the American Chemical Society, 128(49), 15714 -15721.
Kennedy, J. H., & Frese Jr., K. W. (1978). Photooxidation of Water at -Fe2O3 Electrodes. Journal of the Electrochemical Society, 125(5), 709 - 714.
Khan, S. U. M., Al-Shahry, M., & William B. Ingler, J. (2002). Efficient Photochemical Water Splitting by a Chemically Modified n-TiO2. Science, 297, 2243-2245.
Kiwi, J., & Gratzel, M. (1986). Heterogeneous photocatalysis: enhanced dihydrogen production in titanium dioxide dispersions under irradiation. The effect of magnesium promoter at the semiconductor interface. Journal of Physical Chemistry, 90(4), 637-640.
Martens, W., Will, G., Frost, R., Bell, J., Kloprogge, T., Vezzoli, M., et al. (2007). Australia Patent No. AU 2006317512. PCT Int. Appl.
Mills, A., & Le Hunte, S. (1997). An overview of semiconductor photocatalysis. Journal of Photochemistry and Photobiology A: Chemistry, 108(1), 1-35.
Mor, G. K., Shankar, K., Paulose, M., Varghese, O. K., & Grimes, C. A. (2005). Enhanced Photocleavage of Water Using Titania Nanotube Arrays. Nano Letters, 5(1), 191-195.
Mor, G. K., Varghese, O. K., Paulose, M., Shankar, K., & Grimes, C. A. (2006). A review on highly ordered, vertically oriented TiO2 nanotube arrays: Fabrication, material properties, and solar energy applications. Solar Energy Materials and Solar Cells, 90(14), 2011-2075.
Murov, S. L., Carmichael, I., & Hug, G. L. (1993). Handbook of photochemistry (2nd ed. ed.). New York: M. Dekker.
Nada, A. A., Barakat, M. H., Hamed, H. A., Mohamed, N. R., & Veziroglu, T. N. (2005). Studies on the photocatalytic hydrogen production using suspended modified TiO2 photocatalysts. International Journal of Hydrogen Energy, 30(7), 687-691.
O'Regan, B., & Grätzel, M. (1991). A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal Ti0(2) Films. Nature, 353(6346), 737.
Ruan, C., Paulose, M., Varghese, O. K., & Grimes, C. A. (2006). Enhanced photoelectrochemical-response in highly ordered TiO2 nanotube-arrays anodized in boric acid containing electrolyte. Solar Energy Materials and Solar Cells, 90(9), 1283-1295.
144
Satsangia, V. R., Kumaria, S., Singha, A. P., Shrivastavb, R., & Dassb, S. (2007). Nanostructured hematite for photoelectrochemical generation of hydrogen. International Journal of Hydrogen Energy.
Sivula, K., Zboril, R., Formal, F. L., Robert, R., Weidenkaff, A., Tucek, J., et al. (2010). Photoelectrochemical Water Splitting with Mesoporous Hematite Prepared by a Solution-Based Colloidal Approach. Journal of the American Chemical Society, 132(21), 7436–7444.
Wu, K.-R., Hung, C.-H., & Tsai, M.-H. (2009). Effects of ITO thin films on microstructural and photocatalytic properties of layered TiO2/ITO films prepared via an extended deposition period. Applied Catalysis B: Environmental, 92(3-4), 357-366.
Cengel, Y. A., & Boles, M. A. (2002). Thermodynamics: An Engineering Approach (4th ed.). New York: McGraw-Hill.
Harvey, P. R., Rudham, R., & Ward, S. (1983). Photocatalytic oxidation of liquid alcohols and binary alcohol mixtures by rutile. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 79(12), 2975-2981.
Hauch, A., Ebbesen, S. D., Jensen, S. H., & Mogensen, M. (2008). Highly efficient high temperature electrolysis. Journal of Materials Chemistry, 18(20), 2331-2340.
Herrmann, J.-M. (1999). Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants. Catalysis Today, 53(1), 115-129.
Hisatomi, T., Miyazaki, K., Takanabe, K., Maeda, K., Kubota, J., Sakata, Y., et al. (2010). Isotopic and kinetic assessment of photocatalytic water splitting on Zn-added Ga2O3 photocatalyst loaded with Rh2-yCryO3 cocatalyst. Chemical Physics Letters, 486(4-6), 144-146.
Hong, W. S., Park, J. H., & Han, G. Y. (2009). Optimization of Conditions for Hydrogen Production from Anodized TiO2 Nanotube-Based Photoelectrochemical CellsArticle Journal of Nanoscience and Nanotechnology, 9(12), 7293-7297.
Katakis, D., Mitsopoulou, C., & Vrachnou, E. (1994). Photocatalytic splitting of water: increase in conversion and energy storage efficiency. Journal of Photochemistry and Photobiology A: Chemistry, 81(2), 103-106.
Licht, S. (2002). Efficient solar generation of hydrogen fuel - a fundamental analysis. Electrochemistry Communications, 4(10), 790-795.
Licht, S. (2003). Solar Water Splitting To Generate Hydrogen Fuel: Photothermal Electrochemical Analysis. Journal of Physical Chemistry B, 107, 4253 - 4260.
Licht, S. (2005a). Solar water splitting to generate hydrogen fuel--a photothermal electrochemical analysis. International Journal of Hydrogen Energy, 30(5), 459-470.
145
Licht, S. (2005b). Thermochemical solar hydrogen generation. Chemical Communications(37), 4635 - 4646.
Licht, S., Halperin, L., Kalina, M., Zidman, M., & Halperin, N. (2003). Electrochemical potential tuned solar water splitting. Chemical Communications, 3006 - 3007. Acosta, D. R., Martinez, A. I., Lopez, A. A., & Magana, C. R. (2005). Titanium dioxide thin films: the effect of the preparation method in their photocatalytic properties. Journal of Molecular Catalysis A: Chemical, 228(1-2), 183-188.
Alpert, D. J., Mancini, T. R., Houser, R. M., Grossman, J. W., Schissel, P., Carasso, M., et al. (1991). Solar concentrator development in the United States. Solar Energy Materials, 24(1-4), 307-319.
Anpo, M., Kishiguchi, S., Ichihashi, Y., Takeuchi, M., Yamashita, H., Ikeue, K., et al. (2001). The design and development of second-generation titanium oxide photocatalysts able to operate under visible light irradiation by applying a metal ion-implantation method. Research on Chemical Intermediates, 27(4-5), 459 - 467.
Arabatzis, I. M., Antonaraki, S., Stergiopoulos, T., Hiskia, A., Papaconstantinou, E., Bernard, M. C., et al. (2002). Preparation, characterization and photocatalytic activity of nanocrystalline thin film TiO2 catalysts towards 3,5-dichlorophenol degradation. Journal of Photochemistry and Photobiology A: Chemistry, 149(1-3), 237-245.
Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K., & Taga, Y. (2001). Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides. Science, 293, 269-271.
Ashokkumar, M. (1998). An overview on semiconductor particulate systems for photoproduction of hydrogen. International Journal of Hydrogen Energy, 23(6), 427-438.
Atashfaraz, M., Shariaty-Niassar, M., Ohara, S., Minami, K., Umetsu, M., Naka, T., et al. (2007). Effect of titanium dioxide solubility on the formation of BaTiO3 nanoparticles in supercritical water. Fluid Phase Equilibria, 257(2), 233-237.
Ausra Inc. (2011). Ausra Australia. from http://www.ausra.com.au/
Bamwenda, G. R., & Arakawa, H. (2001). The photoinduced evolution of O2 and H2 from a WO3 aqueous suspension in the presence of Ce4+/Ce3+. Solar Energy Materials and Solar Cells, 70(1), 1-14.
Bamwenda, G. R., Tsubota, S., Nakamura, T., & Haruta, M. (1995). Photoassisted hydrogen production from a water-ethanol solution: a comparison of activities of Au---TiO2 and Pt---TiO2. Journal of Photochemistry and Photobiology A: Chemistry, 89(2), 177-189.
Bandala, E. R., Arancibia-Bulnes, C. A., Orozco, S. L., & Estrada, C. A. (2004). Solar photoreactors comparison based on oxalic acid photocatalytic degradation. Solar Energy, 77(5), 503-512.
Baranenko, V. I., & Kirov, V. S. (1989). Solubility of hydrogen in water in a broad temperature and pressure range. Atomic Energy, 66(1), 30-34.
146
Bard, A. J., & Faulkner, L. R. (2001). Electrochemical methods : fundamentals and applications (2nd ed.). John Wiley: New York.
Bekbolet, M., Lindner, M., Weichgrebe, D., & Bahnemann, D. W. (1996). Photocatalytic detoxification with the thin-film fixed-bed reactor (TFFBR): Clean-up of highly polluted landfill effluents using a novel TiO2-photocatalyst. Solar Energy, 56(5), 455-469.
Bilchak, R. V., Connolly, J. S., & Bolton, J. R. (1980). Effects of Temperature and intensity on thermodynamic limits for efficiencies of photochemical conversion of solar energy. Paper presented at the AS/ISES 1980 Annual Meeting, Phoenix, Ariz., June 2-6.
Blakers, A. W. (2000). Efficient 20-50 sun concentrator cells. Paper presented at the European Photovoltaic Solar Energy Conference, Glasgow, United Kingdom.
Bockris, J. O. M. (1999). Hydrogen economy in the future. International Journal of Hydrogen Energy, 24(1), 1-15.
Bolton, J. R. (1978). Solar Fuels. Science, 202(4369), 705-711.
Callister, W. D. (2000). Materials science and engineering : an introduction (5th ed.). New York: Wiley.
Carey, J. H., & Oliver, B. G. (1976). Intensity effects in the electrochemical photolysis of water at the titanium dioxide electrode. Nature, 259(5544), 554-556.
Carp, O., Huisman, C. L., & Reller, A. (2004). Photoinduced reactivity of titanium dioxide. Progress in Solid State Chemistry, 32(1-2), 33-177.
Cengel, Y. A. (2006). Heat and Mass Transfer: A Practical Approach. New York: McGraw-Hill.
Cengel, Y. A., & Boles, M. A. (2002). Thermodynamics: An Engineering Approach (4th ed.). New York: McGraw-Hill.
Cesar, I., Kay, A., Gonzalez Martinez, J. A., & Gratzel, M. (2006). Translucent Thin Film Fe2O3 Photoanodes for Efficient Water Splitting by Sunlight: Nanostructure-Directing Effect of Si-Doping. Journal of the American Chemical Society, 128(14), 4582-4583.
Cesar, I., Sivula, K., Kay, A., Zboril, R., & Grätzel, M. (2009). Influence of Feature Size, Film Thickness, and Silicon Doping on the Performance of Nanostructured Hematite Photoanodes for Solar Water Splitting. The Journal of Physical Chemistry C, 113(2), 772-782.
Chase, M. ( 1998). NIST-JANAF Thermochemical Tables, (4th Edition ed.): American Institute of Physics.
Conibeer, G. J., & Richards, B. S. (2007). A comparison of PV/electrolyser and photoelectrolytic technologies for use in solar to hydrogen energy storage systems. International Journal of Hydrogen Energy, 32(14), 2703-2711.
Department of Industry Tourism and Resources. (2005). Australian Hydrogen Activity: Australian Government.
147
Dhanalakshmi, K. B., Latha, S., Anandan, S., & Maruthamuthu, P. (2001). Dye sensitized hydrogen evolution from water. International Journal of Hydrogen Energy, 26(7), 669-674.
Duret, A., & Gratzel, M. (2005). Visible Light-Induced Water Oxidation on Mesoscopic Films Made by Ultrasonic Spray Pyrolysis. Journal of Physical Chemistry B, 109(36), 17184 -17191.
Feitz, A. J., Boyden, B. H., & Waite, T. D. (2000). Evaluation of two solar pilot scale fixed-bed photocatalytic reactors. Water Research, 34(16), 3927-3932.
Franke, R., & Franke, C. (1999). Model reactor for photocatalytic degradation of persistent chemicals in ponds and waste water. Chemosphere, 39(15), 2651-2659.
Fujiwara, S., Kasai, S., Yamauchi, H., Yamada, K., Makino, S., Matsunaga, K., et al. (2008). Hydrogen production by high temperature electrolysis with nuclear reactor. Progress in Nuclear Energy, 50(2-6), 422-426.
Gerasimov, V. V., & Rozenfeld, I. L. (1956). Effect of temperature on the diffusion current and the thickness of the diffusion layer. Russian Chemical Bulletin, 5(7), 797-801.
Gerischer, H. (1977). The photoelectrochemical cell: principles, energetics and electrode stability. Paper presented at the Electrochemical Society (Semicond. Liq.-Junction Sol. Cells).
Glasscock, J. A., Barnes, P. R. F., Plumb, I. C., & Savvides, N. (2007). Enhancement of Photoelectrochemical Hydrogen Production from Hematite Thin Films by the Introduction of Ti and Si. Journal of Physical Chemistry C, 111(44), 16477 - 16488.
Grätzel, M. (2001). Photoelectrochemical cells. Nature, 414, 338 - 344.
Green, M. A., Blakers, A. W., Wenham, S. R., Jianhua, Z., Chong, C. M., Taouk, M., et al. (1987). Improvements in flat-plate and concentrator silicon cell efficiency. Paper presented at the IEEE Photovoltaic Specialists Conference.
Green, M. A., Blakers, A. W., Zhao, J., Taouk, M., Narayanan, S., & Campbell, P. (1987). High-efficiency silicon concentrator solar cells. Kensington, Australia.: Jt. Microelectron. Res. Cent.
Green, M. A., Blakers, A. W., Zhao, J., Wang, A., Milne, A. M., Dai, X., et al. (1989). High-efficiency silicon concentrator solar cell research. Kensington, Australia.: Sol. Photovoltaic Lab., New South Wales Univ.
Green, M. A., Emery, K., Hishikawa, Y., & Warta, W. (2011). Solar cell efficiency tables (version 37). Progress in Photovoltaics: Research and Applications, 19(1), 84-92.
Green, M. A., Taouk, M., Blakers, A. W., Narayanan, S., Zhao, J., & Campbell, P. (1986). A 23.6% efficient low-resistivity silicon concentrator solar cell. Applied Physics Letters, 49(4), 194-195.
148
Green, M. A., Zhao, J., Wang, A., & Blakers, A. W. (1990). Silicon concentrator solar cell development. Kensington, Australia: New South Wales Univ.
Gurunathan, K. (2000). Photobiocatalytic production of hydrogen using sensitized TiO2-MV2+ system coupled Rhodopseudomonas capsulata. Journal of Molecular Catalysis A: Chemical, 156(1-2), 59-67.
Gurunathan, K. (2004). Photocatalytic hydrogen production using transition metal ions-doped [gamma]-Bi2O3 semiconductor particles. International Journal of Hydrogen Energy, 29(9), 933-940.
Gurunathan, K., Maruthamuthu, P., & Sastri, M. V. C. (1997). Photocatalytic hydrogen production by dye-sensitized Pt/SnO2 and Pt/SnO2/RuO2 in aqueous methyl viologen solution. International Journal of Hydrogen Energy, 22(1), 57-62.
Harvey, P. R., Rudham, R., & Ward, S. (1983). Photocatalytic oxidation of liquid alcohols and binary alcohol mixtures by rutile. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 79(12), 2975-2981.
Hauch, A., Ebbesen, S. D., Jensen, S. H., & Mogensen, M. (2008). Highly efficient high temperature electrolysis. Journal of Materials Chemistry, 18(20), 2331-2340.
Henry, C. H. (1980). Limiting efficiencies of ideal single and multiole ebergy gap terrestrial solar cells. Journal of Applied Physics, 51(8), 4494 - 4500.
Herrmann, J.-M. (1999). Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants. Catalysis Today, 53(1), 115-129.
Hisatomi, T., Miyazaki, K., Takanabe, K., Maeda, K., Kubota, J., Sakata, Y., et al. (2010). Isotopic and kinetic assessment of photocatalytic water splitting on Zn-added Ga2O3 photocatalyst loaded with Rh2-yCryO3 cocatalyst. Chemical Physics Letters, 486(4-6), 144-146.
Hoffmann, M. R., Martin, S. T., Choi, W., & Bahnemann, D. W. (1995). Environmental Applications of Semiconductor Photocatalysis. Chemical Reviews, 95(1), 69-96.
Hong, W. S., Park, J. H., & Han, G. Y. (2009). Optimization of Conditions for Hydrogen Production from Anodized TiO2 Nanotube-Based Photoelectrochemical CellsArticle Journal of Nanoscience and Nanotechnology, 9(12), 7293-7297.
Hu, Y.-S., Kleiman-Shwarsctein, A., Forman, A. J., Hazen, D., Park, J.-N., & McFarland, E. W. (2008). Pt-Doped α-Fe2O3 Thin Films Active for Photoelectrochemical Water Splitting. Chemistry of Materials, 20(12), 3803-3805.
Huang, A., Cao, L., Chen, J., Spiess, F.-J., Suib, S. L., Obee, T. N., et al. (1999). Photocatalytic Degradation of Triethylamine on Titanium Oxide Thin Films. Journal of Catalysis, 188(1), 40-47.
149
Jakob, M., Levanon, H., & Kamat, P. V. (2003). Charge Distribution between UV-Irradiated TiO2 and Gold Nanoparticles: Determination of Shift in the Fermi Level. Nano Letters, 3(3), 353-358.
Jewett, R. P., Walter, R. J., Chandler, W. T., & Frohmberg, R. P. (1973). Hydrogen environment embrittlement of metals (No. NASA-CR-2163 ): NASA.
Jiang, D., Zhao, H., Jia, Z., Cao, J., & John, R. (2001). Photoelectrochemical behaviour of methanol oxidation at nanoporous TiO2 film electrodes. Journal of Photochemistry and Photobiology A: Chemistry, 144(2-3), 197-204.
Jianhu, N., Yitung, C., Robert, F. B., & Shanthi, K. (2008). A Photoelectrochemical Model of Proton Exchange Water Electrolysis for Hydrogen Production. Journal of Heat Transfer, 130(4), 042409.
Joseph, B., Gopchandran, K. G., Thomas, P. V., Koshy, P., & Vaidyan, V. K. (1999). A study on the chemical spray deposition of zinc oxide thin films and their structural and electrical properties. Materials Chemistry and Physics, 58(1), 71-77.
Katakis, D., Mitsopoulou, C., & Vrachnou, E. (1994). Photocatalytic splitting of water: increase in conversion and energy storage efficiency. Journal of Photochemistry and Photobiology A: Chemistry, 81(2), 103-106.
Kay, A., Cesar, I., & Grätzel, M. (2006). New Benchmark for Water Photooxidation by Nanostructured -Fe2O3 Films. Journal of the American Chemical Society, 128(49), 15714 -15721.
Kennedy, J. H., & Frese Jr., K. W. (1978). Photooxidation of Water at -Fe2O3 Electrodes. Journal of the Electrochemical Society, 125(5), 709 - 714.
Khan, S. U. M., Al-Shahry, M., & William B. Ingler, J. (2002). Efficient Photochemical Water Splitting by a Chemically Modified n-TiO2. Science, 297, 2243-2245.
Kiwi, J., & Gratzel, M. (1986). Heterogeneous photocatalysis: enhanced dihydrogen production in titanium dioxide dispersions under irradiation. The effect of magnesium promoter at the semiconductor interface. Journal of Physical Chemistry, 90(4), 637-640.
Li, C.-J., & Li, W.-Y. (2003). Deposition characteristics of titanium coating in cold spraying. Surface and Coatings Technology, 167(2-3), 278-283.
Licht, S. (2001). Multiple Band Gap Semiconductor/Electrolyte Solar Energy Conversion. J. Phys. Chem. B, 105(27), 6281-6294.
Licht, S. (2002). Efficient solar generation of hydrogen fuel - a fundamental analysis. Electrochemistry Communications, 4(10), 790-795.
Licht, S. (2003). Solar Water Splitting To Generate Hydrogen Fuel: Photothermal Electrochemical Analysis. Journal of Physical Chemistry B, 107, 4253 - 4260.
Licht, S. (2005a). Solar water splitting to generate hydrogen fuel--a photothermal electrochemical analysis. International Journal of Hydrogen Energy, 30(5), 459-470.
150
Licht, S. (2005b). Thermochemical solar hydrogen generation. Chemical Communications(37), 4635 - 4646.
Licht, S., Halperin, L., Kalina, M., Zidman, M., & Halperin, N. (2003). Electrochemical potential tuned solar water splitting. Chemical Communications, 3006 - 3007.
Lim, T. H., Jeong, S. M., Kim, S. D., & Gyenis, J. (2000). Photocatalytic decomposition of NO by TiO2 particles. Journal of Photochemistry and Photobiology A: Chemistry, 134(3), 209-217.
Linsebigler, A. L., Lu, G., & Yates, J. T., Jr. (1995). Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results. Chemical Reviews, 95(3), 735-758.
Malato, S., Blanco, J., Vidal, A., Alarcon, D., Maldonado, M. I., Caceres, J., et al. (2003). Applied studies in solar photocatalytic detoxification: an overview. Solar Energy, 75(4), 329-336.
Malato, S., Blanco, J., Vidal, A., Fernandez, P., Caceres, J., Trincado, P., et al. (2002). New large solar photocatalytic plant: set-up and preliminary results. Chemosphere, 47(3), 235-240.
Marín, F. I., Hamstra, M. A., & Vanmaekelbergh, D. (1996). Greatly Enhanced Sub-Bandgap Photocurrent in Porous GaP Photoanodes. Journal of the Electrochemical Society, 143(3), 1137-1142.
Martens, W., Will, G., Frost, R., Bell, J., Kloprogge, T., Vezzoli, M., et al. (2007). Australia Patent No. AU 2006317512. PCT Int. Appl.
Mayer, E. (1972). Mechanical seals (2nd edition ed.). London: Butterworths & Co (Publishers) Ltd.
McConnell, R., & Thompson, J. (2004). Generating Hydrogen through Water Electrolysis Using Concentrator Photovoltaics. Paper presented at the 2004 DOE Solar Energy Technologies Program Review Meeting.
McLoughlin, O. A., Ibanez, P. F., Gernjak, W., Rodriguez, S. M., & Gill, L. W. (2004). Photocatalytic disinfection of water using low cost compound parabolic collectors. Solar Energy, 77(5), 625-633.
McLoughlin, O. A., Kehoe, S. C., McGuigan, K. G., Duffy, E. F., Touati, F. A., Gernjak, W., et al. (2004). Solar disinfection of contaminated water: a comparison of three small-scale reactors. Solar Energy, 77(5), 657-664.
Meng, Y., Huang, X., Wu, Y., Wang, X., & Qian, Y. (2002). Kinetic study and modeling on photocatalytic degradation of para-chlorobenzoate at different light intensities. Environmental Pollution, 117(2), 307-313.
Mills, A., & Le Hunte, S. (1997). An overview of semiconductor photocatalysis. Journal of Photochemistry and Photobiology A: Chemistry, 108(1), 1-35.
Mor, G. K., Shankar, K., Paulose, M., Varghese, O. K., & Grimes, C. A. (2005). Enhanced Photocleavage of Water Using Titania Nanotube Arrays. Nano Letters, 5(1), 191-195.
151
Mor, G. K., Varghese, O. K., Paulose, M., Shankar, K., & Grimes, C. A. (2006). A review on highly ordered, vertically oriented TiO2 nanotube arrays: Fabrication, material properties, and solar energy applications. Solar Energy Materials and Solar Cells, 90(14), 2011-2075.
Murov, S. L., Carmichael, I., & Hug, G. L. (1993). Handbook of photochemistry (2nd ed. ed.). New York: M. Dekker.
Murphy, A. B., Barnes, P. R. F., Randeniya, L. K., Plumb, I. C., Grey, I. E., Horne, M. D., et al. (2006). Efficiency of solar water splitting using semiconductor electrodes. International Journal of Hydrogen Energy, In Press, Corrected Proof.
Nada, A. A., Barakat, M. H., Hamed, H. A., Mohamed, N. R., & Veziroglu, T. N. (2005). Studies on the photocatalytic hydrogen production using suspended modified TiO2 photocatalysts. International Journal of Hydrogen Energy, 30(7), 687-691.
Nazeeruddin, M. K., Kay, A., Rodicio, I., Humphry-Baker, R., Mueller, E., Liska, P., et al. (1993). Conversion of light to electricity by cis-X2bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) charge-transfer sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes. Journal of the American Chemical Society, 115(14), 6382-6390.
Ni, M., Leung, M. K. H., Leung, D. Y. C., & Sumathy, K. (2007). A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production. Renewable and Sustainable Energy Reviews, 11(3), 401-425.
Nogueira, R. F. P., & Jardim, W. F. (1996). TiO2-fixed-bed reactor for water decontamination using solar light. Solar Energy, 56(5), 471-477.
Nowotny, J., Sorrell, C. C., Bak, T., & Sheppard, L. R. (2005). Solar-hydrogen: Unresolved problems in solid-state science. Solar Energy, 78(5), 593-602.
Nowotny, J., Sorrell, C. C., Sheppard, L. R., & Bak, T. (2005). Solar-hydrogen: Environmentally safe fuel for the future. International Journal of Hydrogen Energy, 30(5), 521-544.
O'Regan, B., & Grätzel, M. (1991). A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal Ti0(2) Films. Nature, 353(6346), 737.
Renewable Resource Data Center. Reference Solar Spectral Irradiance: Air Mass 1.5. from http://rredc.nrel.gov/solar/spectra/am1.5/
Robert, D., Piscopo, A., Heintz, O., & Weber, J. V. (1999). Photocatalytic detoxification with TiO2 supported on glass-fibre by using artificial and natural light. Catalysis Today, 54(2-3), 291-296.
Ross, R. T., & Hsiao, T.-L. (1977). Limits on the yield of photochemical solar energy conversion. Journal of Applied Physics, 48(11), 4783 - 4785.
Ruan, C., Paulose, M., Varghese, O. K., & Grimes, C. A. (2006). Enhanced photoelectrochemical-response in highly ordered TiO2 nanotube-
152
arrays anodized in boric acid containing electrolyte. Solar Energy Materials and Solar Cells, 90(9), 1283-1295.
Sargent & Lundy LLC Consulting Group. (2003). Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts (No. 34440). Chicago, Illinois.
Sato, S., & White, J. M. (1980). Photoassisted water-gas shift reaction over platinized titanium dioxide catalysts. Journal of the American Chemical Society, 102(24), 7206-7210.
Satsangia, V. R., Kumaria, S., Singha, A. P., Shrivastavb, R., & Dassb, S. (2007). Nanostructured hematite for photoelectrochemical generation of hydrogen. International Journal of Hydrogen Energy.
Shinar, R., & Kennedy, J. H. (1982). Photoactivity of doped α-Fe2O3 electrodes. Solar Energy Materials, 6(3), 323-335.
Sivula, K., Zboril, R., Formal, F. L., Robert, R., Weidenkaff, A., Tucek, J., et al. (2010). Photoelectrochemical Water Splitting with Mesoporous Hematite Prepared by a Solution-Based Colloidal Approach. Journal of the American Chemical Society, 132(21), 7436–7444.
So, W.-W., Kim, K.-J., & Moon, S.-J. (2004). Photo-production of hydrogen over the CdS-TiO2 nano-composite particulate films treated with TiCl4. International Journal of Hydrogen Energy, 29(3), 229-234.
Solar Systems Pty Ltd. 154MW Victorian Project from http://www.solarsystems.com.au/154MWVictorianProject.html
Suter, C., Tomeš, P., Weidenkaff, A., & Steinfeld, A. (2010). Heat Transfer and Geometrical Analysis of Thermoelectric Converters Driven by Concentrated Solar Radiation. Materials, 3(4), 2735-2752.
Tabata, S., Ohnishi, H., Yagasaki, E., Ippommatsu, M., & Domen, K. (1994). Light-intensity dependence in photocatalytic decomposition of water over K4Nb6O17 catalyst. Catalysis Letters, 28(2), 417-422.
Wahi, R. K., Yu, W. W., Liu, Y., Mejia, M. L., Falkner, J. C., Nolte, W., et al. (2005). Photodegradation of Congo Red catalyzed by nanosized TiO2. Journal of Molecular Catalysis A: Chemical, 242(1-2), 48-56.
WorleyParsons Ltd. (2008). Ivanpah Solar Electric Generating System. from http://www.worleyparsons.com/Projects/Pages/IvanpahSolarElectricGeneratingSystem.aspx
Wu, K.-R., Hung, C.-H., & Tsai, M.-H. (2009). Effects of ITO thin films on microstructural and photocatalytic properties of layered TiO2/ITO films prepared via an extended deposition period. Applied Catalysis B: Environmental, 92(3-4), 357-366.
Yamaguchi, M., Takamoto, T., & Araki, K. (2006). Super high-efficiency multi-junction and concentrator solar cells. Solar Energy Materials and Solar Cells, 90(18-19), 3068-3077.
Yamashita, H., Harada, M., Misaka, J., Nakao, H., Takeuchi, M., & Anpo, M. (2003). Application of ion beams for preparation of TiO2 thin film photocatalysts operatable under visible light irradiation: Ion-assisted
153
deposition and metal ion-implantation. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 206, 889-892.
Yamashita, H., Harada, M., Misaka, J., Takeuchi, M., Ikeue, K., & Anpo, M. (2002). Degradation of propanol diluted in water under visible light irradiation using metal ion-implanted titanium dioxide photocatalysts. Journal of Photochemistry and Photobiology A: Chemistry, 148(1-3), 257-261.
Zhao, J., Wang, A., Blakers, A. W., & Green, M. A. (1988). High efficiency prismatic cover silicon concentrator solar cells. Paper presented at the IEEE Photovoltaic Specialists Conference.
Zubi, G., Bernal-Agustín, J. L., & Fracastoro, G. V. (2009). High concentration photovoltaic systems applying III-V cells. Renewable and Sustainable Energy Reviews, 13(9), 2645-2652.
Zumdahl, S. (1993). Chemistry (3rd Edition ed.). Lexington, Massachusetts: D. C. Heath and Company.
Zumdahl, S. S. (1993). Chemistry (3rd ed.). Lexington, Massachusetts: D. C. Heath and Company.
