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ByDr. Sarika Phadke-KelkarNational Chemical Laboratory24-March-2011Solar Powered Hydrogen Generation: Systems, Materials and Performance

1OutlineEnergy CrisisAlternative FuelsHydrogen as fuelHydrogen production from water using solar energyPhoto-chemical decomposition of waterPhoto-electro-chemical water splittingMaterials: Selection criteria, important candidatesCurrent Status & Future Trend

So that brings us to the outline of my talk.I am mainly going to focus on two photovoltaic applications..both are photoelectrochemical applicationsfirst is DSSC and the second is PEC h2 generationWe will see what metal oxides are currently explored for these applications and their properties are being tuned for performance improvement

Recently there has been a lot of interest in the multicomponent binary oxides OR ternary oxide.such as ZnSnO4, cadmium stannet, etc.these metal oxides have already shown good potential as transparent conducting oxides and

Towards the end we will see what are the future trends in this areaespecially in DSSC.there is a big push to make flexible DSSC devices in order to make them more light, less expensive and be applicable on curved surfaces.but this imposes new challenges on the metal oxide coating present in the DSSC device..so we will how these issues could be handled.

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This slide shows you a satellite image of the world at night..and as you can see here, north east America, Europe, India, China, japan..these are the countries which are most lit and this of course tells than these are the zones which have highest industrialization and highest CO2 emissions..and we have already started experiencing the bad side effects of all thiswith global warming and.changing weather patterns and all..

But the important thing to note is, all this technology and industrialization has become a part of our lifeand in order to progress further we do need all this. and we will continue to need this all more and moreand at the heart of all this is the energy or electricity..which we will need more and more as we progressbut how we obtain this energy is the real questionbecause the fossil fuel stocks are scarce so unless we draw this enrgy from a sustainable/renewble source, we will not be able to survive.

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Historical and Projected Variations in Earths Surface TemperatureYears

IPCC ReportsEnergy Demand in present and near future* Present : 12.8 TW 2050 : 28-35 TW

* Needs at least 16 TW Bio : 2 TW Wind : 2 TW Atomic : 8 TW (8000 power plant) Fossil : 2 TW

* Solar: 160,000 TW

20102020

Therefore Energy generation is one of the most important issue to address for scientists and engineers.and lot of effort is geared toward it

From this slide we can see that the energy demand will go to almost 2 or 3 fold by 2050 and.none of the natural resources such as bio, wind, and fossil fuelswill be able to meet up this demandand as we see here only solar energy has the potential of meeting this future demanddue to its abundant supply on earth

We at NCL, are very actively working on this problem.we mainly focus on different Metal oxide nanomaterialsandtry to tune therei properties by playing with their electronic band structure, morphology, and surface chemistry to improve their performance for photovoltaic applications..

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HydrogenHydrogen, a gas, will play an important role in developing sustainable transportation in the United States, because in the future it may be produced in virtually unlimited quantities using renewable resources. Hydrogen and oxygen from air fed into a proton exchange membrane fuel cell produce enough electricity to power an electric automobile, without producing harmful emissions. The only byproduct of a hydrogen fuel cell is water. Currently there are no original equipment manufacturer vehicles available for sale to the general public. Experts estimate that in approximately 10-20 years hydrogen vehicles, and the infrastructure to support them, will start to make an impact.

Applications of Hydrogen FuelWhat is a Fuel Cell?A Fuel Cell is an electrochemical device that combines hydrogen and oxygen to produce electricity, with water and heat as its by-product.

11http://www.fuelcells.org/

How can Fuel Cell technology be used?TransportationAll major automakers are working to commercialize a fuel cell carAutomakers and experts speculate that a fuel cell vehicle will be commercialized by 201050 fuel cell buses are currently in use in North and South America, Europe, Asia and AustraliaTrains, planes, boats, scooters, forklifts and even bicycles are utilizing fuel cell technology as well

12http://www.fuelcells.org/basics/apps.htmlhttp://en.wikipedia.org/wiki/Hydrogen_vehicleHow can Fuel Cell technology be used? Stationary Power StationsOver 2,500 fuel cell systems have been installed all over the world in hospitals, nursing homes, hotels, office buildings, schools and utility power plantsMost of these systems are either connected to the electric grid to provide supplemental power and backup assurance or as a grid-independent generator for locations that are inaccessible by power lines13http://www.fuelcells.org/basics/apps.htmlHow can Fuel Cell technology be used?TelecommunicationsDue to computers, the Internet and sophisticated communication networks there is a need for an incredibly reliable power sourceFuel Cells have been proven to be 99.999% reliable14http://www.fuelcells.org/basics/apps.htmlHow can Fuel Cell technology be used?Micro PowerConsumer electronics could gain drastically longer battery power with Fuel Cell technologyCell phones can be powered for 30 days without rechargingLaptops can be powered for 20 hours without recharging

15http://www.fuelcells.org/basics/apps.htmlHydrogen ProductionThe biggest challenge regarding hydrogen production is the costReducing the cost of hydrogen production so as to compete in the transportation sector with conventional fuels on a per-mile basis is a significant hurdle to Fuel Cells success in the commercial marketplaceHydrogen ProductionThere are three general categories of Hydrogen productionThermal ProcessesElectrolyte ProcessesPhotocatalyticProcesses17http://www1.eere.energy.gov/hydrogenandfuelcells/production/current_technology.htmlHydrogen ProductionPhotocatalyticProcessesUses light energy to split water into hydrogen and oxygenThese processes are in the very early stages of research but offer the possibility of hydrogen production which is cost effective and has a low environmental impactTwo types: a) Photochemical b) Photo-electro-chemical18http://www1.eere.energy.gov/hydrogenandfuelcells/production/photo_processes.htmlPhoto-catalytic water splitting1. Direct Water Splitting:

2. Water Splitting using photo-electrochemical cell (PEC): pnH+/H2e-e-h+h1h+h2O2H2O/O2hH2TCO with ohmic contact

Experimental setupDirect Water Splitting:

PEC water splitting:

Potentiostat

MetalP-type semiconductorP-type semiconductorN-type semiconductorN-type semiconductorMetalPhto-electrochemistry of water decompositionBasic principle-In the most simple terms, the principle of photo- electrochemical water decomposition is based on the conversion of light energy into electricity within a cell involving two electrodes, immersed in an aqueous electrolyte, of which at least one is made of a semiconductor exposed to light and able to absorb the light. This electricity is then used for water electrolysis. 21Reaction Mechanism2h 2e + 2h+ (1)2h+ + H2O(liquid) 1/2O2(gas) + 2H+ (2)2H+ + 2e H2(gas) (3)Overall Reaction2h + H2O(liquid) 1/2O2(gas) + H2(gas)

= 1.23 eVElectrochemical decomposition of water is possible when EMF of cell 1.23 VLight results in intrinsic ionization of n-type semiconducting materials over the band gap, leading to the formation of electrons in the conduction band and electron holes in the valence band:Reaction (1) may take place when the energy of pho- tons (h) is equal to or larger than the band gap. An elec- triceld at the electrode=electrolyte interface is required in order to avoid recombination of these charge carriers. This may be achieved through modication of the potential at the electrode=electrolyte interface.This process takes place at the photo-anode=electrolyte interface. Gaseous oxygen evolves at the photo-anode and the hydrogen ions migrate to the cathode through the internal circuit (aqueous electrolyte). Simultaneously, the electrons, generated as a result of Reaction (1) at the photo-anode, are transferred over the external circuit to the cathode, resulting in the reduction of hydrogen ions into gaseous hydrogen:where G0is the standard free enthalpy per mole of (H2 O)Reaction (4)=237:141 kJ=mol; NA =Avogadros number= 6:022 1023 mol1.the electrochemical decomposition of water is possible when the electromotive force of the cell (EMF) is equal to or larger than 1:23 V.22

Band model representationABCDMaterials Aspects of PECTwo main functions of photoelectrodesOptical function: maximum absorption of solar energyCatalytic function: water decomposition

Desired properties of photoelectrodesBandgapFlatband potentialSchottky barrierElectrical resistanceHelmholtz potentialCorrosion resistanceMicrostructure

Band structure of photoelectrode material

-The band gap of the photo-electrode has a critical impact on the energy conversion of photons [62,63]. That is, only the photons of energy equal to or larger than that of the band gap may be absorbed and used for conversion. The maximal conversion eciency of photovoltaic devices may be achieved at band gaps in the range 1.01:4 eV; this will be discussed subsequently in Section 8.2.3.-Theoretically, the lowest limit for the band gap of a PECs photo-anode is determined by the energy required to split the water molecule (1:23 eV), which is de- termined by the photon ux as represented by the integral of J1 J2 . Accordingly, this photon ux, within this part of the spectrum, is not available for conversion owing to the theoretical energy limit of 1:23 eV [62].

polarization within the PEC; recombination of the photo-excited electronhole pairs; resistance of the electrodes; resistance of the electrical connections; voltage losses at the contacts.

The estimated value of these combined losses is 0:8 eV ( J2 J3 ); this part of the spectrum is not available for con- version. Therefore, the optimal energy range in terms of the photons available for conversion is 2 eV. This situation is represented in Fig. 9 by the integral of J1 J3.In consequence, the energy corresponding to the photon ux J3 in Fig. 9 is available for conversion. However, the availability of this energy is contingent upon the use of a photo-anode with band gap of 2 eV. Unfortunately, oxide semiconductors that have such a band gap, such as Fe2O3, are susceptible to corrosion, as will be discussed subse- quently in Section 5.6.

25MetalsNo band gapOnly reduction or oxidationDepends on the band position

InsulatorsHigh band gapHigh energy requirement

MetalsVBCBVBCBVBCBH+/H2H2O/O2InsulatorsSCEWHY SEMICONDUCTOR ?2626For conventional redox reactions, one is interested in either reduction or oxidation of a substrate.

For example consider that one were interested in the oxidation of Fe2+ ions to Fe 3+ ions then the oxidizing agent that can carry out this oxidation is chosen from the relative potentials of the oxidizing agent with respect to the redox potential of Fe2+/Fe3+ redox couple.

The oxidizing agent chosen should have more positive potential with respect to Fe3+/Fe2+ couple so as to affect the oxidation, while the oxidizing agent undergoes reduction spontaneously. This situation throws open a number of possible oxidizing agents from which one of them can be easily chosen. Concepts Why semiconductors are chosen as photo-catalysts?27Bandgap

-These data are shown in terms of their energies compared to the vacuum level and the normal hydrogen electrode (NHE) level in an aqueous solution of pH = 2 [64]. Un- fortunately, the most promising materials from the view- point of the band gap width, such as Fe2O3 (Eg = 2:3 eV) [65], GaP (Eg = 2:23 eV) [66], and GaAs (Eg = 1:4 eV) [66], are not stable in aqueous environments and so exhibit signicant corrosion by water. Therefore, these materials are not suitable as photo-electrodes in aqueous environments. The most promising oxide materials, which are corrosion resistant, include TiO2 and SrTiO3 [714,17, 2035, 3751].

-As discussed in Section 4.1, the optimal band gap for high- performance photo-electrodes is 2 eV [10,22,27,51,58,59]. Such a material, which satises this requirement and is corrosion resistant, is not available commercially. There- fore, there is a need to process such a material.

-One possibility by which this can be achieved is through the imposition of a band located 2 eV below the conduction band. Experimentally, this impurity band can be achieved through the heavy doping of TiO2 with aliovalent ions. As seen in Fig. 14 [68,69], the most promising dopant to use is V4+=5+, which forms the solid solution (Ti1xVx)O2 [47,48,70]. However, these reports are not in agreement concerning the eect of doping on the electrochemical properties of TiO2. Philips et al. [70] have observed that, although the addition of 30 mol% V to TiO2 results in a reduction in the band gap to 1:99 eV, the formation of (Ti0:7 V0:3 )O2hadadetrimentaleectonthephoto-activity due to a substantial increase in the at band potential by 1 V). As a result, this necessitated the imposition of an adequate external bias voltage.

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Flatband potential

-The at-band potential, Ufb, is the potential that has to be imposed over the electrode=electrolyte interface in order to make the bands at [22,51,58]. This potential is an im- portant quantity in photo-electrode reactions. Specically, the process of water photo-electrolysis may take place when the at-band potential is higher than the redox potential of the H+=H2 couple [22,51,58]. The at-band potential may be modied to the desired level through surface chemistry [48,49].

-photo-cells equipped with a photo-anode made of materials with negative at-band potentials (relative to the redox potential of the H+=H2 couple, which depends on the pH) can split the water molecule without the imposition of a bias. 30Other important parametersElectrical Conductivity

Helmholtz Potential Barrier

Corrosion Resistance:Electrochemical corrosion resistancePhotocorrosion resistanceDissolution

-When a semiconducting photo-electrode material is immersed in a liquid electrolyte (in which the chemicalpotential of the electrons is determined by the H+=H2 redox potential), the charge transfer at the solid=liquid interface results in charging of the surface layer of the semiconductor. The charge transfer from the semiconductor to the electrolyte leads to the formation of a surface charge and results in upwards band bending, forming a potential barrier, as shown in Fig. 16. This barrier is similar to that of the solid=solid interface, shown in Fig. 5. This surface charge is compensated by a charge of the opposite sign, which is induced in the electrolyte within a localized layer, known as the Helmholtz layer. It is 1 nm thick and is formed of oriented water molecule dipoles and electrolyte ions adsorbed at the electrode surface [51,73,74]. The height of this potential barrier, known as the Helmholtz barrier, is determined by the nature of the aqueous environment of the electrolyte and the properties of the photo-electrode surface.

The performance characteristics of PECs depend, to a large extent, on the height of the Helmholtz barrier [51]. Therefore, it is essential to obtain further infor- mation on (i) the eect of the specic properties of the electrode=electrolyte interface on the height of the barrier and (ii) the determination of the eect of the Helmholtz barrier on the eciency of the photo-electrochemical process.31Criterion for PE corrosion stability

Free enthalpy of oxidation reactionPhoto anodeFree enthalpy of reduction reactionPhoto cathodeWhat modifications?various conceptual principles have been incorporated into typical TiO2 system so as to make this system responsive to longer wavelength radiations. These efforts can be classified as follows:Dye sensitizationSurface modification of the semiconductor to improve the stability Multi layer systems (coupled semiconductors)Doping of wide band gap semiconductors like TiO2 by nitrogen, carbon and Sulphur New semiconductors with metal 3d valence band instead of Oxide 2p contribution Sensitization by doping.All these attempts can be understood in terms of some kind sensitization and hence the route of charge transfer has been extended and hence the efficiency could not be increased considerably. In spite of these options being elucidated, success appears to beeluding the researchers. 33Conditions to be satisfied?The band edges of the electrode must overlap with the acceptor and donor states of water decomposition reaction, thus necessitating that the electrodes should at least have a band gap of 1.23 V, the reversible thermodynamic decomposition potential of water. This situation necessarily means that appropriate semiconductors alone are acceptable as electrode materials for water

The charge transfer from the surface of the semiconductor must be fast enough to prevent photo corrosion and shift of the band edges resulting in loss of photon energy.34 without deterioration of the stability

should increase charge transfer processes at the interface

should improvements in the efficiency

ENGINEERING THE SEMICONDUCTOR ELECTRONIC STRUCTURES35

Positions of bands of semiconductors relative to the standard potentials of several redox couples36 Identifying and designing new semiconductor materials with considerable conversion efficiency and stability

Constructing multilayer systems or using sensitizing dyes - increase absorption of solar radiation

Formulating multi-junction systems or coupled systems - optimize and utilize the possible regions of solar radiation

Developing nanosize systems - efficiently dissociate waterTHE AVAILABLE OPPORTUNITIES37 high surface area

morphology

presence of surface states

wide band gap

position of the VB & CB edge

CdS appropriate choice for the hydrogen productioneVADVANTAGES OF SEMICONDUCTOR NANOPARTICLES38The opportunitiesThe opportunities that are obviously available as such now include the following:

Identifying and designing new semiconductor materials with considerable conversion efficiency and stabilityConstructing multilayer systems or using sensitizing dyes so as to increase absorption of solar radiation.Formulating multi-junction systems or coupled systems so as to optimize and utilize the possible regions of solar radiation.Developing catalytic systems which can efficiently dissociate water.39Opportunities evolvedDeposition techniques have been considerably perfected and hence can be exploited in various other applications like in thin film technology especially for various devices and sensory applications. The knowledge of the defect chemistry has been considerably improved and developed.Optical collectors, mirrors and all optical analysis capability have increased which can be exploited in many other future optical devices.The understanding of the electronic structure of materials has been advanced and this has helped to our background in materials chemistry.Many electrodes have been developed, which can be a useful for all other kinds of electrochemical devices.

40Limited success Why?The main reasons for this limited success in all these directions are due to:The electronic structure of the semiconductor controls the reaction and engineering these electronic structures without deterioration of the stability of the resulting system appears to be a difficult proposition.The most obvious thermodynamic barriers to the reaction and the thermodynamic balances that can be achieved in these processes give little scope for remarkable improvements in the efficiency of the systems as they have been conceived and operated. Totally new formulations which can still satisfy the existing thermodynamic barriers have to be devised.The charge transfer processes at the interface, even though a well studied subject in electrochemistry has to be understood more explicitly, in terms of interfacial energetics as well as kinetics. Till such an explicit knowledge is available, designing systems will have to be based on trial and error rather than based on sound logical scientific reasoning.

41Nanocrystalline (mainly oxides like TiO2, ZnO, SnO and Nb2O5 or chalcogenides like CdSe) mesoscopic semiconductor materials with high internal surface area If a dye were to be adsorbed as a monolayer, enough can be retained on a given area of the electrode so as to absorb the entire incident light.

Since the particle sizes involved are small, there is no significant local electric field and hence the photo-response is mainly contributed by the charge transfer with the redox couple.

Two factors essentially contribute to the photo-voltage observed, namely, the contact between the nano crystalline oxide and the back contact of these materials as well as the Fermi level shift of the semiconductor as a result of electron injection from the semiconductor.

42Another aspect of thee nano crystalline state is the alteration of the band gap to larger values as compared to the bulk material which may facilitate both the oxidation/reduction reactions that cannot normally proceed on bulk semiconductors.

The response of a single crystal anatase can be compared with that of the meso-porous TiO2 film sensitized by ruthenium complex (cis RuL2 (SCN)2, where L is 2-2bipyridyl-4-4dicarboxlate).

The incident photon to current conversion efficiency (IPCE) is only 0.13% at 530 nm ( the absorption maximum for the sensitizer) for the single crystal electrode while in the nano crystalline state the value is 88% showing nearly 600-700 times higher value. 43This increase is due to better light harvesting capacity of the dye sensitized nano crystalline material but also due to mesoscpic film texture favouring photo-generation and collection of charge carriers .

It is clear therefore that the nano crystalline state in combination with suitable sensitization is one another alternative which is worth investigating. 44The second option is to promote water splitting in the visible range using Tandem ells. In this a thin film of a nanocrystalline WO3 or Fe2O3 may serve as top electrode absorbing blue part of the solar spectrum. The positive holes generated oxidize water to oxygen4h+ + 2H2O --- O2 + 4 H+

The electrons in the conduction band are fed to the second photo system consisting of the dye sensitized nano crystalline TiO2 and since this is placed below the top layer it absorbs the green or red part of the solar spectrum that is transmitted through the top electrode. The photo voltage generated in the second photo system favours hydrogen generation by the reaction4H+ + 4e- --- 2H2

The overall reaction is the splitting of water utilizing visible light. The situation is similar to what is obtained in photosynthesis45Dye sensitized solid hetero-junctions and extremely thin absorber solar cells have also been designed with light absorber and charge transport material being selected independently so as to optimize solar energy harvesting and high photovoltaic output. However, the conversion efficiencies of these configurations have not been remarkably high.

Soft junctions, especially organic solar cells, based on interpenetrating polymer networks, polymer/fullerene blends, halogen doped organic crystals and a variety of conducting polymers have been examined. Though the conversion efficiency of incident photons is high, the performance of the cell declined rapidly. Long term stability will be a stumbling block for large scale application of polymer solar cells.46New OpportunitiesNew semi-conducting materials with conversion efficiencies and stability have been identified. These are not only simple oxides, sulphides but also multi-component oxides based on perovskites and spinels.

Multilayer configurations have been proposed for absorption of different wavelength regions. In these systems the control of the thickness of each layer has been mainly focused on.47New OpportunitiesSensitization by dyes and other anchored molecular species has been suggested as an alternative to extend the wavelength region of absorption.

The coupled systems, thus giving rise to multi-junctions is another approach which is being pursued in recent times with some success

Activation of semiconductors by suitable catalysts for water decomposition has always fascinated scientists and this has resulted in various metal or metal oxide (catalysts) loaded semi conductors being used as photo-anodes48New opportunities (Contd)Recently a combinatorial electrochemical synthesis and characterization route has been considered for developing tungsten based mixed metal oxides and this has thrown open yet another opportunity to quickly screen and evaluate the performances of a variety of systems and to evolve suitable composition-function relationships which can be used to predict appropriate compositions for the desired manifestations of the functions. It has been shown that each of these concepts, though has its own merits and innovations, has not yielded the desired levels of efficiency. The main reason for this failure appears to be that it is still not yet possible to modulate the electronic structure of the semiconductor in the required directions as well as control the electron transfer process in the desired direction.49PREPARATION OF CdS NANOPARTICLES1 g of Zeolite (HY, H, HZSM-5)1 M Cd(NO3)2 , stirred for 24 h, washed with water Cd / Zeolite1 M Na2S solution, stirred for 12 h, washed with waterCdS / Zeolite48 % HF, washed with waterCdS Nanoparticles50XRD PATTERN OF CdS

M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy (In press)51Debye Scherrer EquationdSPACING AND CRYSTALLITE SIZE

= diffraction angle T = Crystallite size = wave length = FWHMd-spacing ()Catalyst(0 0 2)(1 0 1)(1 1 2)CrystalliteSize(nm)CdS (bulk)1.521.792.9721.7CdS (bulk)(HF treated)1.521.792.9321.7CdS-Y1.531.792.968.8CdS-1.521.782.938.6

CdS-Z1.521.792.977.252UV VISIBLE SPECTRA OF CdS SAMPLESSamples Band Gap (eV)CdS Z

CdS Y

CdS -

Bulk CdS 2.38

2.27

2.21

2.13

M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy (In press)53PHOTOCATALYTIC PRODUCTION OF HYDROGEN35ml of 0.24 M Na2S and 0.35 M Na2SO3 in Quartz cell0.1 g CdS400 W Hg lampN2 gas purged before the reaction and constant stirring Hydrogen gas was collected overwater in the gas burette54AMOUNT OF HYDROGEN EVOLVED BY CdS PHOTOCATALYST

55TEM IMAGE OF CdS NANOPARTICLESCatalystParticle Size (nm)Surfacearea (m2/g)Rate of hydrogenproduction ( moles /h)CdS - Y8.836102CdS - Z64668CdS - 112667CdS - Bulk23 1445

CdS-Z

CdS- 100 nm

100 nmCdS-Z56SCANNING ELECTRON MICROGRAPHS57

CdS-Z

CdS-Y

CdS-

CdS- bulkPHOTOCATALYSIS ON Pt/TiO2 INTERFACE Electrons are transferred to metal surface Reduction of H+ ions takes place at the metal surface The holes move into the other side of semiconductor The oxidation takes place at the semiconductor surface

TiO2PtVacuum levelAq. SolAq. SolpH=0pH = 7H+/H2C.BV.BEFT.Sakata, et al Chem. Phys.Lett. 88 (1982) 5058MECHANISM OF RECOMBINATION REDUCTION BY METAL DOPINGConduction Bande- e- e- e- e- e- e- e- e- e- e- e-Valence Bandh+ h+ h+ h+ h+ h+ h+ h+ h+ h+Electron/hole pairrecombinationElectron/hole pair generatione-(M)