Structural and electrical characterisation of nanostructure electrodes for SOFCs

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Structural and electrical characterisation ofnanostructure electrodes for SOFCs

Muhammad Ashfaq Ahmad a, Nadeem Akrama, Rizwan Raza a,b,*aDepartment of Physics, COMSATS Institute of Information Technology, 54000 Lahore, PakistanbDepartment of Energy Technology, Royal Institute of Technology (KTH), 10044 Stockholm, Sweden

a r t i c l e i n f o

Article history:

Received 17 May 2013

Received in revised form

11 October 2013

Accepted 22 October 2013

Available online xxx

Keywords:

AC/DC techniques

Intermediate temperature

Nanocomposite

Conductivity

Alternative energy resources

* Corresponding author. Department of P87907403.

E-mail address: [email protected]

Please cite this article in press as: AhmadSOFCs, International Journal of Hydrogen

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.10.1

a b s t r a c t

This paper reports the effects of sintering temperature on structure, particle size and

conductivity of electrodes (Sn0.2Zn0.8Fe0.2O & Sn0.8Zn0.2Fe0.2O). The electrode material was

prepared by the chemical method combining a solid state reaction. Structural analyses

were performed using X-ray diffraction and scanning electron microscopy. The particle

size of the material obtained using Scherrer’s formula was 50e60 nm and the nano-

structure’s surface was studied using electrochemical characterisations tools. Electrical

conductivity was determined using the 4-probe DC method, which was compared with the

4-probe AC method. These results suggest a promising substitute for the conventional

electrodes of solid oxide fuel cells (SOFCs). It is known that a sintering temperature above

1000 �C causes an increase in density and a reduction of porosity. Therefore, we optimised

the sintering temperature at 1000 �C and obtained electrical conductivity of about 5 S cm�1.

Thus, this electrode could play a vital role in the development of high performance SOFCs

at intermediate temperatures.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction The solid oxide fuel cell (SOFS) is one kind of fuel cell that

The demand for alternative energy sources is increasing

rapidly due to depleting fossil fuel resources. The use of fossil

fuel resources creates pollution, which affects the global

climate. To overcome these problems,much research effort in

recent decades has been put into alternative sources of energy

[1e10]. A fuel cell is one of the best alternative sources of

energy. It is defined as “an electrochemical device which can

convert chemical energy into electrical energy without any

combustion process” [1e14] and it is of considerable impor-

tance owing to the world energy crisis. Furthermore, this

technology has received a great deal of interest because it is

pollution free and environmentally friendly [10e15].

hysics, COMSATS Institu

om (R. Raza).

MA, et al., Structural aEnergy (2013), http://dx

2013, Hydrogen Energy P24

converts chemical energy into electricity, heat and powerwith

water as a by-product [1]. The SOFS is a pollution-free device

with no mechanical parts. It comprises three components: an

anode, an electrolyte and a cathode, each of which has its own

characteristics. In particular, the properties of the anode

should be highly electrocatalytic, highly conductive and sta-

ble. For many decades, nickel-cermet electrodes have been

used; however, there are many problems, because of which,

SOFCs are still not commercialised [2e4]. Unfortunately, the

current cost of SOFCs is too high and thus, a reduction of its

cost is an important issue [3,5]. There is a need to develop a

stable material that can operate at low temperatures, which

will make it possible to lower the costs. To develop high

te of Information Technology, 54000 Lahore, Pakistan. Tel.: þ46

nd electrical characterisation of nanostructure electrodes for.doi.org/10.1016/j.ijhydene.2013.10.124

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

mailto:[email protected]

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http://dx.doi.org/10.1016/j.ijhydene.2013.10.124



Fig. 1 e Synthesis procedure of electrode synthesis using

Pechini method.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 3 ) 1e52

performance LT-SOFCs, a highly electronic and compatible

anode is required. There are a number of conventional ma-

terials that have been developed as electrodes for SOFS, e.g.,

Ni-cermet [6,7] but all conventional developed materials

operate at high temperatures (800e1000 �C). Because of this

problem, it has not been possible to commercialise SOFCs.

An optimised microstructure and dopant concentration is

necessary to extend the electrical conductivity of the anode

[8]. To study the development of anode materials, it is

assumed that fuel cell performance is dependent on the

microstructure of the anode material. This can be achieved

through composition optimisation and the skill of anode

fabrication [3]. Therefore, development of a new electrode for

SOFCs, which can operate at low temperatures, is a significant

challenge for the fuel cell community. To address this chal-

lenge, in this particular research work, SZF1 (Sn0.2Zn0.8Fe0.2O)

& SZF2 (Sn0.8Zn0.2Fe0.2O) electrodes were prepared. The high-

est conductivity for the composite electrolyte has been found

by different research groups at low temperatures of about

300 �C [14e22].

The purpose of the present study is to develop such an

electrode that can work efficiently at intermediate tempera-

tures and to characterise this electrode both electrically and

structurally. We compare the two different samples, SZF1 and

SZF2 and analyse their characteristics using X-ray diffraction

(XRD), scanning electron microscopy (SEM) and advanced

techniques of AC impedance spectroscopy. The electrical

conductivity was found using DC conductivity with the 4-

probe DC method.

2. Materials and methods

2.1. Sample preparation

The powder for the electrode nanostructure was prepared

using the Pechini method. The details of the preparation and

method of synthesis are presented in Fig. 1. Stoichiometric

amounts of SnCl (99.99%, Aldrich), Zn (NO3)2$6H2O (99.99%,

Aldrich) and Fe (NO3)3$9H2O (99%, Merck) were used as start-

ing materials. After mixing these elements, dilute nitric acid

was added to them in a cylinder and stirred on a hotplate at

80 �C, until the solutions became a chelate by undergoing

polyesterification. After drying the chelate, it becomes a

powder, which was placed into crucibles. The powder was

sintered in a furnace for six hours at 1000 �C. Using the

chemical method, two samples were prepared with different

compositions: Sn0.2Zn0.8Fe0.2O and Sn0.8Zn0.2Fe0.2O. The de-

tails of the preparation and synthesis presented in Fig. 1.

2.2. Structural and morphological analysis

These samples were studied for their crystal structure using

an X-ray diffractometer (RIGAKU, XRD-D/Max-IIA, Ni filter)

over the range of 20e80� using Cu Ka radiation with a wave-

length of 1.540562 �A. All the data were analysed using the

Rietveld analysis technique and the “Powder X” software. The

analyses of the morphology and microstructure were deter-

mined using an ultra 55 Scanning Electron Microscope oper-

ated at 25 kV in the secondary electron image mode.

Please cite this article in press as: Ahmad MA, et al., Structural aSOFCs, International Journal of Hydrogen Energy (2013), http://dx

2.3. Electrochemical impedance spectra (EIS)

The electrochemical impedance spectrum of the samples in

both hydrogen (H2) and an air atmosphere were analysed

using a frequency response analyser (Solartron 1260) in the

range of 1 HZ to 100 KHZ at 600 �C. The potentiostatic tech-

nique was used for obtaining the spectra. The curve fitting of

the obtained results was drawn from experimental data using

ZView software.

2.3.1. Conductivity measurementsFor the conductivity measurements, a pellet of the prepared

samples was pressed at 250 Mpa to a size of 15 mm. Platinum

paste was used on both sides of the pellet to facilitate current

collection. The flow rates of hydrogen gas and air were within

the range of 90e100 ml min�1 at 1 atm pressure on the anode

and cathode of the cell.

� The AC conductivity was calculated from the EIS data at

different temperatures in both hydrogen and air atmo-

sphere. The results were attributed in Arrhenius plot.

� The DC conductivity of the material was found using the

conventional 4-probe DC method with a Keithley instru-

ment (Taiwan) from 450 to 700 �C.

2.3.2. Fuel cell performanceThe fuel cell performance was executed in the temperature

range of 400e600 �C. The electrochemical performance of the

fuel cell was observed using natural gas and hydrogen gas as

fuel. The gas flow rate, adjusted with the help of a gas flow

meter, was 100 ml min�1 at 1 atm pressure. The open circuit

voltage and the voltages and currents after applying the load

resistance were measured with the help of a fuel cell testing

instrument (SM-102, Tianjin, China). The characteristics

curves were plotted between I (current)eV (voltage) and I




Fig. 2 e X-ray Diffraction pattern and comparison between two samples.

i n t e rn a t i o n a l j o u rn a l o f h y d r o g e n en e r g y x x x ( 2 0 1 3 ) 1e5 3

(current)eP (power) for the nanocomposite electrolyte Ba-SDC

and were called the characteristic curves for the analysis of

performance at temperatures of 400e600 �C.

3. Results and discussions

3.1. Crystal and microstructure analysis

The phase structure of both samples was analysed and is

presented in Fig. 2. Using Scherrer’s formula, the particle size

was calculated as being in the range of 30e80 nm for sample

SZF1 and 10e80 nm for SZF2. Sample SZF2 has a particle size

of 8 nm for tin oxide (SnO2) and 80 nm for ironezinc oxide

(Fe2ZnO4). The XRD analysis of sample SZF1 shows a single

phase with good structure andmore Zn oxide, which has good

compatibility for the solid electrolytes used in SOFCs.

The XRD analysis of sample SZF2 revealed two phases: one

is tetragonal for tin oxide and the second is cubic for ironezinc

oxide. The XRD pattern shown in Fig. 2 reveals that the lower

content of zinc oxide has the two theta (2q) angle shifted to-

wards a lower angle. The comparison of the structural pa-

rameters for the two different samples and space group from

the Rietveld analysis are shown in Table 1. The morphology

and microstructure of sample SZF1 are displayed in Fig. 3. It

can be noted that the particle size is 30e80 nm and that the

SEM shows that the material exhibits homogeneity.

Table 1 e Structural parameters of electrodes.

Parameters Sample SZF1

Phase Fe2ZnO4 SnO2

Crystal structure Cubic Tetragonal

Space group Fd-3m P42/mnm

a (�A) 8.43 4.737

b (�A) 8.43 4.737

c (�A) 8.43 3.185

Calculated density (g/cm3) 5.34 7


3.2. Electrochemical impedance analysis

The EIS analysis of both samples are represented in two ways:

a Nyquist plot and a Bode plot, which are shown in Fig. 4(a)

and (b), where Zim is the negative imaginary part of the

Fig. 3 e Image of electrode microstructures.




4.8 5.0 5.2 5.4

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Zim

Zre

AC Impedance in H2

0 200000 400000 600000 800000 1000000

-6

-4

-2

0

2

4

6

Zre,

Zim

Frequency, Hz

Zim

Zre

Bode Plot in H2 atmosphere

a

b

Fig. 4 e Electrochemical impedance spectrum (EIS).

Fig. 5 e a) Conductivity measurements of as-prepared

electrode in air atmosphere. (b) Conductivity

measurements of as-prepared electrode in H2 atmosphere.

0 250 500 750 1000 1250 1500

0.0

0.2

0.4

0.6

0.8

1.0

0 200 400 600 800 1000 1200 1400 1600 1800

0

100

200

300

400

500

SZF-2

Pow

er d

ensi

ty (m

W/c

m2 )

Cel

l vol

tage

(V)

Current density (mA/cm2

)

SZF-1

Fig. 6 e Fuel cell performance @ 580 �C with H2 gas.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 3 ) 1e54

impedance and Zre is the real part of the impedance. In

Fig. 4(a), the curve (semi-circle) of the Nyquist plot shows the

mixed conduction and lower resistance of the material.

The Bode plot (Fig. 4(b)) is plotted with the y-axis repre-

senting absolute impedance jZj, as well as the phase shift and

frequency is shown on the x-axis. Thus, we can obtain infor-

mation about the conductivity, diffusion and ionic behaviour

of the material. The curves in Fig. 4(b), for sample SZF2, show

the electronic behaviour at both higher and lower frequency.

3.3. Conductivity analysis

The mixed conductivity (electronic and ionic) was calculated

within the range of 400e700 �C. The Arrhenius equation was

used to calculate the conductivity of the material as follows:

s ¼ s0 expð � Ea=RTÞ;where so is a pre-exponential factor, which is constant for

charge density carriers. Ea is the activation energy for the ionic

migration, R is the real gas constant and T is the absolute

temperature. The conductivity of material SZF1 is shown in

Fig. 5(a) and (b) in both atmospheres. To calculate the activa-

tion energy, a linear fit has been performed, which is shown in

the inset.


The activation energy for SZF1 was determined as 0.22 and

0.58 in air and in a hydrogen atmosphere, respectively. For

SZF2, it was found to be 0.2 and 0.5 in air and in a hydrogen

atmosphere, respectively.

It has been observed that the conductivity of sample SZF1

is higher than sample SZF2 in both atmospheres. This




i n t e rn a t i o n a l j o u rn a l o f h y d r o g e n en e r g y x x x ( 2 0 1 3 ) 1e5 5

material, having an excess of zinc oxide in the sample, ismore

electronically conductive and has greater stability for SOFCs.

3.4. Fuel cell performance

Fig. 6 shows the fuel cell performance with hydrogen gas and

IeV (currentevoltage) and IeP (currentepower) density char-

acteristics. The maximum power density of sample SZF1 was

obtained at 560 mW cm�2 at 580 �C. Therefore, it has been

observed that the performance of the fuel cell using the SZF1

electrode was enhanced significantly at the operating

temperatures.

4. Conclusion

A tin- and zinc-based electrode has been synthesised suc-

cessfully using the Pechini method. It has been found that the

SFZ2 sample has very good structure and conductivity and is

very suitable for use with SOFCs at intermediate tempera-

tures. From the analysis, it is found that there is good agree-

ment of the structure between the XRD and results. Similarly,

a good agreement is observed between the conductivities of

the materials by both the AC and DC methods.

It has been determined that the higher zinc content en-

hances the conductivity, as observed in the results. It is

concluded that the sample SZF1 is amaterialmore suitable for

use as an electrode for SOFCs. Based on these findings, the

proposed electrode is recommended as a promising substitute

for the conventional electrodes of SOFCs.

Acknowledgements

The Higher Education Commission (HEC), Pakistan and

COMSATS Institute of Information Technology, Lahore, are

acknowledged for awarding travel grant. We acknowledge to

Dr. Bin Zhu helping us characterise the material and for

valuable comments.

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Structural and electrical characterisation of nanostructure electrodes for SOFCs