SPECIAL ISSUE ON NANO ENERGY TECHNOLOGIES

Electrochemical study of nanostructured electrode forlow-temperature solid oxide fuel cell (LTSOFC)Ghazanfar Abbas1,4, Rizwan Raza2,4,*,†, M. Ashfaq2, M.Ashraf Chaudhry3, Ajmal Khan3,Imran Ahmad3 and Bin Zhu4

1Department of Physics, COMSATS Institute of Information Technology, Islamabad, 44000, Pakistan2Department of Physics, COMSATS Institute of Information Technology, Lahore, 54000, Pakistan3Department of Physics, Bahauddin Zakaria University (BZU), Multan, 60800, Pakistan4Department of Energy Technology, Royal Institute of Technology, KTH, Stockholm, 100 44, Sweden

SUMMARY

Zn-based nanostructured Ba0.05Cu0.25Fe0.10Zn0.60O (BCFZ) oxide electrode material was synthesized by solid-state reaction forlow-temperature solid oxide fuel cell. The cell was fabricated by sandwiching NK-CDC electrolyte between BCFZ electrodesby dry press technique, and its performance was assessed. The maximum power density of 741.87mW-cm�2 was achieved at550°C. The crystal structure and morphology were characterized by X-ray diffractometer (XRD) and SEM. The particle sizewas calculated to be 25 nm applying Scherer's formula from XRD data. Electronic conductivities were measured with thefour-probeDCmethodunderhydrogenandair atmosphere.ACElectrochemical ImpedanceSpectroscopyof theBCFZoxide elec-trode was also measured in hydrogen atmosphere at 450°C. Copyright © 2013 JohnWiley & Sons, Ltd.

KEY WORDS

Zn-based electrode; new electrodes; solid oxide fuel cell; nanostructured electrode; efficient device; anode

Correspondence

*Rizwan Raza, Department of Energy Technology, KTH, Sweden and COMSATS Institute of Information Technology, Lahore, Pakistan.†E-mail: razahussaioni1980@yahoo.com

Received 2 January 2013; Revised 22 June 2013; Accepted 26 June 2013

1. INTRODUCTION

Ni-based electrodes have been used for solid oxide fuelcell with Yttria Stabilized Zirconia (YSZ) electrolytes formany decades. Ni-zirconia cermet electrode possesses highelectrochemical behavior, excellent catalytic activity andapproachable electrical conductivity [1]. However, thismaterial displays some drawbacks with the passage of timedue to its high working temperature. Moreover, the deposi-tion of carbon layer on Ni-YSZ cermet electrode is anotherbig issue particularly, when using hydro carbon fuelsinstead of pure hydrogen as well as high cost. By the virtueof the deposition of carbon layer, the Ni-cermet electrodesare oxidized which decreases the life of fuel cell. In orderto get rid of these problems, the developing of newelectrodes for solid oxide fuel cells instead of Ni is an im-mense requirement and challenge for fuel cell community.Numerous materials have been evaluated and synthesizedwhich can work in the temperature range of 600–800°C.Those materials fall in the category of intermediate tempera-ture solid oxide fuel cells (ITSOFCs) and exhibit a reliableperformance as well as conductivity [2–4]. Lowering of theworking/operating temperature of solid oxide fuel cells can

improve the stability of the cell and has much R & D interest[5]. Obviously, to maintain the low temperature as comparedto 1000°C is an easy job, and it also carries an advantage oflow cost.

The function of anode is to split the hydrogen (H2) fuel intoions (protons and electrons) [6]. It has been reported that thehighest electrical conductivity, electrochemical activity inorder to oxidize the fuel, suitable porosity at themicrostructurelevel, thermal stability, electrode's morphology and thecompatibility with electrolyte are the primary requirementsof a good anodic material for SOFC [7–12]. In order to fulfilland improve these basic requirements for SOFC electrodes,nanostructuring is feasible solution. A nanostructure designhas changed the orientation of solid oxide fuel cell researchand development [13]. Zhu and co-workers [14] reported thatuse of nanostructured exhibits high ionic conduction at lowertemperature as compared to the temperature of conventionalbulk material LTSOFC.

Electrode materials for traditional SOFCs based on YttriaStabilized Zirconia (YSZ) are composites of Ni-YSZ [15].Electrolyte materials play key role for solid oxide fuel cellperformance; therefore, the use of those electrolytematerials, which can provide higher ionic conductivity at

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. (2013)

Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.3090

Copyright © 2013 John Wiley & Sons, Ltd.

low temperature is very suitable. It has been provedexperimentally that Gadolinium-doped Ceria (GDC),Samarium-doped Ceria (SDC) or Calcium-doped Ceria(CDC) can yield 0.1 S-cm�1 ionic conductivity at 600,700 or 800°C, whereas YSZ yields this value of ionicconductivity at 1000°C [16–20]. Fang [8] reported that amaximum power density of 388mW-cm�2 has been foundin Ni-SDC cermets at 750°C.

The present study is focused on a new zinc (Zn)-based electrode materials. The consisted composition ofBa0.05Cu0.25Fe0.10Zn0.60O (BCFZ) has been suggestedas an electrode material for low-temperature solid oxidefuel cell. It can be assumed that the small amount of ironworks as a catalyst, while the presence of Cu shows anability of its pure metallic behavior. It has been reportedthat the use of barium oxide prevents the material fromcorrosion during the working process of the material,which absorbs the water and facilitate water-radiatedcarbon removal reactions, which may be a precursor toenhance its life time [21]. Nickel (Ni) has been replacedinto Zn element which has given better results atcomparatively low temperature (in the range of 400–550°C).This new electrode was demonstrated to show betterperformance than that of the state-of-art electrode, e.g.Ni-SDC anode and La0.6Sr0.4Co0.2Fe0.8O3�δ LSCF cathode.

2. EXPERIMENTAL

Solid-state reaction (dry method) was applied to synthesizethe nanostructured BCFZ oxide powder. The stoichiometricmolar ratio of the composition BaCO3, CuCO3.Cu(OH)2, Fe(NO3)3.9H2O and Zn(NO3)2.6H2O (Sigma Aldrich, USA)was ground in a mortar with pestle to make the precursorhomogeneous. This homogenous precursor was then sinteredat 800°C for 4 h and allowed to cool with the furnace. Thesintered powder was again ground for 30 min by adding smallamount of carbon to produce porosity.

CDC coated with Sodium–Potassium Carbonates(NK-CDC) electrolyte was prepared by co-precipita-tion method. The whole procedure has been discussedin our previous work [18].In this work, Cerium Nitratehexahydrate Ce(NO3)3.6H2O (Sigma Aldrich, USA)and Calcium Nitrate tetra hydrate Ca(NO3)2.4H2O(Sigma Aldrich, USA) were used as starting materials.Later on, 20 wt. % amount of this NK-CDC electrolytewas mixed with the electrode (BCFZ) material to forma composite electrode, which was in turn employed fortesting the performance of the fuel cell.

In order to measure the electrical conductivity of theBCFZ material, a pellet of pure BCFZ oxide powderhaving 13mm diameter and 3mm thickness wasprepared by dry press technique under a pressure of280 kg-cm�2 by hydraulic press machine. The pelletwas sintered at 650°C for 1 h. Silver paste was coatedby brush on both sides of the pellet to make electricalcontact. DC conductivities were measured in hydrogenand air atmosphere by implementing KD 2531 Digital

Micro-ohmmeter, China. The following formula wasused to calculate conductivity;

σ ¼ L=RA (1)

Where σ is the conductivity, L is the thickness of thepellet, R is the internal resistance and A is the activearea of the pellet. The active area of the pellet wasconsidered to be 0.64 cm2.

A symmetrical three-component fuel cell wasprepared to measure the electrochemical performance.BCFZ-NKCDC/NKCDC/BCFZ-NKCDC were pouredone by one in a die (13mm diameter), each time slightlypressed in the same sequence and finally pressed at apressure of 280 kg-cm�2 with a hydraulic press usingdry press technique. In the newly formed cell, BCFZ-NKCDC works as symmetrical electrode and NKCDCas electrolyte. The thickness of the cell was controlledat 0.9mm followed by 0.4mm anode, 0.3mm electrolyteand 0.2mm cathode in thickness contained. Thus,composed three layers pellet/cell was then sintered at650°C for half an hour. Silver paste was painted on bothexternal sides of the cell to facilitate electrical contact.

The fuel cell performance was measured by providinghydrogen as a fuel at anode side and air as oxidant atcathode side under variable resistance load using fuel celltesting unit (L-43, China). Under each resistance load, thedata of open circuit voltage (OCV) and current wererecorded in the temperature range of 400 to 550°C with aninterval of 50°C, and I–V curves were drawn at each temper-ature. Power density was calculated from these curves, andI–P curves also were drawn. The hydrogen gas flow wascontrolled at a rate of 100ml/min under 1 atm pressure.

The X-ray diffractometer (XRD) pattern of sinteredBCFZ (electrode) was recorded by using D/Max-3ARegaku XRD with Cu Kα radiation (λ= 1.5418Å), 35 kVvoltage and 30 mA current at room temperature. Thestructure of the material was determined from XRD patternthrough JCPDs cards. The crystallite size (Dβ) wascalculated from line-broadening peaks of XRD patternsusing Scherer's equation;

Dβ ¼ 0:89λ=βCosθ (2)

Where λ and β are wavelength and full width halfmaximum (FWHM), respectively.

In order to analyze the microscopic view of the pre-pared BCFZ electrode, SEM (Philips XL-30) was used.The detailed microstructure analysis and morphology in-cluding the size and shape were recorded. ElectrochemicalImpedance Spectroscopy (EIS) measurements of BCFZelectrode were analyzed at hydrogen atmosphere by usingAuto Versa STAT 4 (Princeton Applied Research, USA).The frequency is varied from 0.01Hz to 1.00 MHz under10mV. Experimental and simulated curves were drawnin the light of ZSim Win Demo version 3.20 Software bythe adjustment of LRQ(CR) equivalent circuit, where L,

Electrochemical study of nanostructured electrode for LTSOFC)G. Abbas et al.

Int. J. Energy Res. (2013) © 2013 John Wiley & Sons, Ltd.DOI: 10.1002/er

R, Q and C show the inductance, resistance, charge andcapacitance of the material, respectively.

3. RESULTS AND DISCUSSIONS

The X-ray diffraction patterns of BCFZ electrode areshown in Figure 1. The patterns in Figure. 1 exhibit thatall elements are completely shifted into Zn during sinteringprocess, except three peaks that exhibit the BaZnO2 phasesaccording to JCPDs 01-074-0137, which indicate that thematerial has two-phase structure. No other peaks of copperand ferric were found in XRD pattern according to JCPDscard No. 36-1451. The peaks of Figure 1 are indexed, andthe structural analysis emphasizes that the structure ishexagonal. The particle size of the BCFZ electrode wascalculated from the XRD data applying Scherer's formulaand found to be 25 nm. It has been identified from theXRD study of material that the sintering temperature of800°C is suitable to create nanostructure crystallinestructure. The image of surface morphology has beenshown in Figure 2. It has been found that the particle sizesof BCFZ electrode are in the range of 20–50 nm, this is agood agreement of the XRD result. It can be clearlyobserved from SEM micrograph that there are many poresin the material. The porous structure provides super way totransport electrons and oxygen ions from anode to cathodeor vice versa, and gas [22]. The DC electrical conductivityof BCFZ electrode has been measured at hydrogen atmo-sphere, and the corresponding results are shown in Figure 3(a). BCFZ electrode displays a maximum electricalconductivity of 6.15 S-cm�1 at temperature of 300°C inhydrogen atmosphere. It has been reported in our previouswork [18] that NK-CDC electrolyte possesses an ionicconductivity of 0.1 S-cm�1 at 600°C. Since, BCFZ elec-trode has more than 60 times higher conductivity than thatof NKCDC electrolyte, which concerns the electricalcompatibility of BCFZ electrode and NK-CDC electrolyte

at interface for ion transportation. Figure 3(a) indicates thatBCFZ material exhibits the metallic conduction behaviordue to decrease in conductivity at elevated temperatureFigure 1. X-ray diffraction pattern of BCFZ oxide electrodematerial.

Figure 2. SEM micrograph of BCFZ oxide electrode.

Figure 3. (a) DC conductivity of BCFZ electrode at hydrogenatmosphere. (b) DC conductivity of BCFZ electrode at air

atmosphere.

Electrochemical study of nanostructured electrode for LTSOFC) G. Abbas et al.

Int. J. Energy Res. (2013) © 2013 John Wiley & Sons, Ltd.DOI: 10.1002/er

at hydrogen atmosphere, which emphasizes the presenceof electronic conduction in BCFZ electrode. The electri-cal DC conductivity of BCFZ electrode at air atmospherewas also measured and is shown in Figure 3(b). Thefigure shows that the conductivity increases, whentemperature decreases from 600 to 300°C, which alsoexhibits the same metallic conduction mechanism. Zhoureported [19] that the electrical conductivity at airatmosphere undergoes a semiconductor-like conductionbehavior to metal-like conduction behavior at 425°C.The conductivity subsequently decreases with increasingtemperature beyond 425°C. It can be specified that areaspecific resistance decreases with the threshold conduc-tivity of electrode at air as well as hydrogen atmosphere.The low area specific resistance helps to transport H+/O2-

at either anode–electrolyte or cathode–electrolyte

interfaces, then electrode can be used either anode orcathode [23]. BCFZ material has 2.34 S-cm�1 conductivityat air atmosphere, so in this present work, we havepractically demonstrated its use as anode as well as cathodeby fabricating a symmetric fuel cell. The activation energyof BCFZ electrode for conduction has been obtained byplotting electrical conductivity data in Arrhenius relationin hydrogen atmosphere using formula below:

σ ¼ A=T exp �Ea=kTð Þ (3)

Where σ is electrical conductivity, T is temperature inKelvin, A is the exponential factor, k is Boltzman's constantand Ea is the activation energy. The activation energy playsan important role to evaluate performance for oxygendiffusion and oxygen ion conductivity in electrode/cathodematerial [24]. The low value of activation energy 0.21 eVemphasizes the high catalytic activity of the BCFZ elec-trode material. The achievement of low activation energy

Figure 4. Arrhenius plot of DC conductivity at hydrogenatmosphere.

Figure 5. AC Electrochemical Impedance Spectroscopy (EIS) of80BCFZ-20NKCDC electrode, experimental and simulated curve

in the frequency range of 0.01Hz to 1MHz. Figure 7. Short-term stability of the single cell at 550°C.

Figure 6. Fuel cell performance of BCFZ-NKCDC/NKCDC/BCFZ-NKCDC.

Electrochemical study of nanostructured electrode for LTSOFC)G. Abbas et al.

Int. J. Energy Res. (2013) © 2013 John Wiley & Sons, Ltd.DOI: 10.1002/er

is a result of decrease in average particle size (nanostruc-tured materials as compared to bulk materials). The resulthas been shown in Figure 4, and the linear fit graph hasbeen plotted and shown in inset of Figure 4.

Figure 5 shows the AC EIS analysis of composite elec-trode 80BCFZ-20NKCDC over the temperature 450°C athydrogen atmosphere. The frequency was adjusted in therange of 0.01Hz to 1MHz. The imaginary part ofimpedance is plotted versus the real part of the impedance.The experimental result exhibits two semicircles. A part ofarc in high-frequency region and a depressed arc in low-frequency region has been observed in 80BCFZ-20NKCDC electrode material. The depressed arc in thespectra revealed the maximum anode contribution andfound that the high-frequency arc corresponds to chargetransfer resistance which, includes the oxide ions diffusionin the electrode and transportation of oxygen ions fromtriple phase boundary into the electrolyte lattice. Low-frequency arc corresponds to a number of otherresistances such as device, holder and wires etc. [25].The experimental curve was simulated with an equivalentcircuit LRQ(CR) using ZSim Win Demo version 3.2Software for the interpretation of EIS data, where L, R, Cand Q denote the inductance, ohmic resistance, capacitanceand charge, respectively. It has been observed in fittingbetween the experimental and the simulated data that highfrequency is generally good, and at low frequencies, thesimulated data slightly deviate from the experimentalvalues [26].

Figure 6 shows the performance of a single sym-metric fuel cell consisting of three consecutive layersof BCFZ-NKCDC anode, NKCDC electrolyte andBCFZ-NKCDC cathode. The maximum power densi-ties 741.87 mW-cm�2, 717mW-cm�2, 715mW-cm�2

and 636mW-cm�2 at temperatures of 550°C, 500°C,450°C and 400°C were obtained, respectively. TheOCV values were observed to be 1.07 V, 1.02 V,1.02 V and 1 V at the same temperatures, respectively.The obtained power density of 741.87 mW-cm�2 at550°C based on BCFZ electrode was found greaterthan that of fuel cell having maximum power densityof 470 mW-cm�2 at elevated temperature of 800°Cusing conventional Ni-YSZ anode and conventionalLSCF cathode [15].

The stability for OCV measurements of the fuel cellbased on BCFZ-NKCDC composite electrode wasrecorded for 24 h with a regular interval of half an hourcontinuously. The results of measurement were shown inFigure 7. The cell yielded approximately one volt output,which remains constant during the experiment performedat temperature 550°C.

4. CONCLUSION

In summary, the hexagonal structure Ba0.15Cu0.15Fe0.1Zn0.6Owas successfully achieved by solid-state reaction method andcharacterized as a novel Ni-free symmetrical electrode for

the low-temperature SOFCs. The sintering temperature (800°C for 4 h) produced good crystallinity. BCFZ electrode hasmetal-like conduction phenomenon with a maximum conduc-tivity value of 6.15S-cm�1 and 2.34S-cm�1 at H2 and airatmosphere, respectively. The low value of 0.21 eV activationenergy indicates that nanostructure enhances the electricalconductivity as well as performance of the cell. The presenceof depressed semi-circle in EIS analysis shows that themaximum contribution for cell performance comes fromBCFZ electrode. The maximum values of OCV and powerdensity were achieved 1.07V and 741.87mW-cm�2 attemperature 550°C, respectively. Hence, its average value ofOCV was recorded as 1V at 550°C for a short-term stabilitytest, which shows that the BCFZ electrode is another potentialcandidate electrode material for the solid oxide fuel cell. TheBCFZ electrode based on doped ceria electrolytes employeda new concept of Ni free symmetrical fuel cell and proved tobe a valid alternative to the traditional SOFC configurationswith improved fuel cell performance.

ACKNOWLEDGEMENTS

This work is supported by funding of Higher EducationCommission, Islamabad, Pakistan. HEC Pakistan has facil-itated the above scholar to work at the Department of En-ergy Technology, KTH Sweden, through InternationalResearch Support Initiative Program (IRSIP). The Swedishagency for Innovation Systems (VINNOVA) and SwedishEnergy Agency (STEM) through industrialization projectsare also acknowledged.

NOMENCLATURE

AC = Alternate CurrentCDC = Calcium-Doped CeriaDC = Direct CurrentEIS =Electrochemical ImpedanceSpectroscopyFWHM = Full Width Half MaximumGDC = Gadolinium-Doped CeriaITSOFCs = Intermediate Temperature Solid Oxide

Fuel CellsJCPDS = Joint Committee on Powder Diffraction

StandardsLTSOFCs = Low-Temperature Solid Oxide Fuel

CellsNK-CDC = Sodium-Potassium carbonated Calcium-

Doped CeriaOCV = Open Circuit VoltageR& D = Research and Developmentrpm round per minuteSDC = Samarium-Doped CeriaSEM = Scanning Electron MicroscopySOFCs = Solid Oxide Fuel CellsUSA = United State of AmericaXRD = X-Ray DiffractionYSZ = Yttria Stabilized Zirconia

Electrochemical study of nanostructured electrode for LTSOFC) G. Abbas et al.

Int. J. Energy Res. (2013) © 2013 John Wiley & Sons, Ltd.DOI: 10.1002/er

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Int. J. Energy Res. (2013) © 2013 John Wiley & Sons, Ltd.DOI: 10.1002/er