Ce0.6Mn0.3Fe0.1O2-d as an Alternative Cathode Material for High

Ce 0.6Mn 0.3Fe 0.1O 2-d as an Alternative Cathode Material for HighTemperature Steam Electrolysis Using LaGaO 3-based Oxide ElectrolyteKohei Hosoi a ,Takaaki Sakai a ,b ,Shintaro Ida a ,c ,Tatsumi Ishihara a ,c ,*aDepartment of Applied Chemistry,Faculty of Engineering,Kyushu University,Motooka 744,Nishi-ku,Fukuoka 819-0395,Japan bCenter for Molecular Systems (CMS),Kyushu University,744Motooka,Nishi-ku,Fukuoka 819-0395,Japan cInternational Institute for Carbon-Neutral Energy Research (WPI-I 2CNER),Kyushu University,Motooka 744,Nishi-ku,Fukuoka 819-0395,JapanA R T I C L E I N F OArticle history:Received 8December 2015Received in revised form 17February 2016Accepted 18February 2016Available online 23February 2016Keywords:steam electrolysis oxide cathode SOEC LaGaO 3CeO 2A B S T R A C TCe 0.6Mn 0.3Fe 0.1O 2-d (CMF)has been used as a new oxide cathode of solid oxide electrolysis cell (SOEC)using LaGaO 3-based electrolyte for H 2production under a feeding gas composition of 20%steam/1%H 2/79%Ar.CMF cathode exhibits good electrolysis performance and well-performed hydrogen production rate with the Faraday ’s law at 1173-973K.Through impedance measurement under a DC polarized condition,the studies show that polarization resistance and ohmic resistance decreased with increasing DC bias potential.Furthermore,CMF showed good stability,and no signi ficant aggregation and delamination were noticed during the electrolysis measurement.These results suggest that CMF can be a promising oxide cathode candidate for intermediate temperature SOEC.ã2016Elsevier Ltd.All rights reserved.1.IntroductionSolid oxide fuel cell (SOFC)has been attracting much attention as powerful energy conversion device because of its high energy conversion ef ficiency and fuel flexibility [1].Recently,solid oxide electrolysis cell (SOEC),a reversible type of SOFC,provide an ef ficient way to produce clean hydrogen from high-temperature steam [2–5].The components of SOEC almost similar with those of SOFC.Therefore,if SOFC and SOEC can be operated in one cell,it would be a potential strategy for energy conversion application between electricity and fuel production (hydrogen in this case),providing an ef ficient energy storage system [6,7].Ni-based anode materials,which have been widely used in the conventional SOFC,are also adopted as cathode materials in SOEC due to their good catalytic property and high electronic conductivity,However,Ni-based cathodes have low stability in high pO 2atmosphere under a electrolysis operating condition of repeated redox cycles,resulting in occurrence of delamination and aggregation [8].In addition,the Ni-based electrodes are required to be operated with a small amount of H 2to avoid oxidation,while this leads to a long pre-operation process.To deal with the issues,oxide cathodes have been of investigated recently instead of Ni-based materials [9–14]due to good stability for redox cycles and noneed of the H 2flow for reduction.However,there are several limitations of oxide materials,such as low catalytic property and low electric conductivity,which can not only lead to low electrolysis performance and limit the development of oxide electrode in SOEC.Therefore,a oxide cathode material with appropriate electric conductivity and catalytic property is highly required.In previous work,a mixed conducting oxide,Mn-and Fe-doped CeO 2(Ce 0.6Mn 0.3Fe 0.1O 2-d ,CMF),has been reported as a new oxide anode for SOFC [15].The catalytic activity of CMF for electrode reaction was reasonably high for SOFC anode application,although its electrical conductivity is lower than that of typical perovskites oxide electrode materials (10À1S/cm at 1173K)[16].In addition,this material has n-type conduction under reducing atmosphere [17].Thus,its slightly low electric conductivity is expected to be increased when it is used for SOEC;its electronic conductivity can be increased by the electrochemical reduction under the steam electrolysis condition.In this work,we further investigated the cathode performance of CMF SOEC in comparison with the property of the conventional Ni-based cathodes.2.ExperimentalCe 0.6Mn 0.3Fe 0.1O 2-d (CMF)cathode powder was prepared by the following method;Ce(NO 3)3Á6H 2O,Mn(NO 3)2Á6H 2O,Fe (NO 3)3Á9H 2O were dissolved into deionized water.After drying the nitrate solution under a constant stirring,the resultant was*Corresponding author.Tel.:+81928022870;fax:+81928022871.E-mail address:ishihara@cstf.kyushu-u.ac.jp (T.Ishihara)./10.1016/j.electacta.2016.02.1210013-4686/ã2016Elsevier Ltd.All rights reserved.Electrochimica Acta 194(2016)473–479Contents lists available at ScienceDirectElectrochimica Actaj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c taprecalcined in a ventilated decomposition furnace at673K for2h to remove the nitrogen oxides(NO x)completely,and was calcined in a furnace at1273K for6h.Ba0.6La0.4CoO3-d(BLC)anode powder was prepared by a conventional solid-state reaction method.The stoichiometric amounts of BaCO3,La2O3,and Co3O4were manually mixed using an Al2O3mortar and pestle,followed by a precalcination at1173K for3h.The precalcined powder was held for a manually intermediate grinding,then following a calcination process at1473K for6h.La0.9Sr0.1Ga0.8Mg0.2O3-d(LSGM)electrolyte was also prepared by a solid-state reaction[18].The stoichiometric amounts of La2O3, SrCO3,Ga2O3and MgO were mixed by a manual grinding with the Al2O3mortar and pestle.The obtained powder mixture was precalcined at1173K for6h and then isostatically pressed into disk (2.0cm in diameter)at275MPa for30min.The disk was sintered at1773K for6h in static air and polished down to a thickness of 300m m.CMF and BLC were screen-printed on each face of the obtained LSGM disk and calcined at1373K for1h(effective electrode area: approximately0.2cm2).Platinum electrode was used as reference electrode and set close to the anode.For comparison,Ni-Ce0.8Sm0.2O2-d(SDC)(Ni-SDC:60-40wt%)cathode cell was also prepared.Thus,two sets of cell configurations of CMF/LSGM/BLC (denoted as cell-A)and Ni-SDC/LSGM/BLC(cell-B)were prepared for the investigation in this work.Steam electrolysis performance of the cells was measured by a four-probe method in a single-cell testing setup,which has been reported previously[19].The surface of both cathode and anode was covered with Pt mesh as a current collector.In the case of cell-A using CMF as a cathode,a better contact with the Pt mesh is required due to low conductivity of CMF oxide,as mentioned previously.Thus,to address this problem,Pt mesh current collector was put into the cathode layer by the following approach.After the screen printing of CMF ink,the Pt mesh was embedded into the top of the electrode before drying the ink,followed by calcination at 1373K for1h.By this approach,the prepared CMF electrode can have a well-contact with the Pt current collector.Steam with1%H2 and Ar(steam:H2:Ar=20:1:79vol%)was fed to the cathode side and a sweep gas of air to the anode side;the total gasflow rate was constantly kept to100ml/min in both sides.On the other hand,a Ni/SDC composite was used as a cathode in cell-B,and reduced by a constant H2flow at1173K before electrolysis operation.During the electrolysis measurements,a feeding composition of steam with1%H2and Ar(steam:H2: Ar=20:1:79vol%)was fed at cathode side and air at anode side as a sweep gas.The total gasflow rate at both sides of the cell was kept at100ml/min.The cell testing was operated by a galvanostat(HAL-3001,Hokuto Denko Corp.)and the terminal voltage was measured using a digital multimeter(R6451A,Advantest Corp.).Steam was generated in an evaporation chamber where water was supplied by a micropump(LC-20AT,Shimadzu Corp.).The formation rate of H2was analyzed using gas chromatograph(GC) with a thermal conductive detector(GC-8A,Shimadzu Corp.).The H2production was measured three times at each operated current in order to minimize the experimental error.Impedance spectrum analysis was characterized using the impedance/gain-phase analyzer(Solartron type1260,Solartron Co.,Ltd.)combined with the electrochemical interface(Solartron type1287,Sorlatron Co., Ltd.).Furthermore,a long-term stability of CMF cathode was measured over90h at275mA/cm2under a feeding gas composi-tion of20%steam/80%Ar gas at1073K.After the electrochemical measurements,the phase stability of the tested cell was conducted by X-ray diffraction(XRD)analysis and scanning electron microscope(SEM)observation.The crystal structure of CMF was identified by XRD(RINT2500,Rigaku).The microstructures of cathode and cross-section images of cathode/electrolyte were characterized byfield emission scanning electron microscope(FE-SEM,Versa3D,FEI).Long-term stability measurements?3.Results and discussionsFig.1shows the I–V curves of CMF/LSGM/BLC(cell-A)and Ni-SDC/LSGM/BLC(cell-B)measured from1173K to973K under a gas composition of20%steam/1%H2/79%Ar.As shown in Fig.1,the current densities of the cell-A was reached to1.50,0.61,0.21A/ cm2at1.5V at1173,1073,973K,pared to the cell-B,cell-A shows the slightly lower performance but this perfor-mance is also considerable.In general,using oxides as fuel electrodes in either SOFC or SOEC shows considerably lower performance than Ni based electrodes.However,the CMF oxide cathode presents good electrochemical performance,which suggests that CMF is highly active for steam electrolysis and can be suitable in the SOEC operation.Hydrogen formation rates of the cell-A is demonstrated in Fig.2 and plotted as a function of current density from1173to973K.The Fig.1.I-V curves at respective temperatures for the cell-A and cell-B under an atmosphere of20%steam/1%H2/79%Ar.474K.Hosoi et al./Electrochimica Acta194(2016)473–479Fig.2.H 2formation rates plotted with a function of current density for cell-A at respective temperatures.The percentage of the generated H 2rate in comparison with the theoretical values from Faraday ’s law.(SD:Standarddeviation).Fig.3.AC impedance spectra of cell-A under 20%steam/1%H 2/79%Ar atmosphere under a DC bias at 0V,(a-1)the overall cell and (b-2)cathode side.(a-2)and (b-2)are an enlarged view.K.Hosoi et al./Electrochimica Acta 194(2016)473–479475H 2formation was observed from the potential of 0.9V at 1173K,and the observed formation rate well follows the Faraday ’s law at each operation temperature.A current ef ficiency of nearly 100%on this cell was reached under these experimental conditions (in particular,97.0%was achieved under a high applied voltage of 1.42V at 1173K).It is clear that the cell-A using CMF cathodes can achieve reasonably high current density with a very good conversion rate of H 2production.The electrolysis performance of the cell-A using the CMF cathode was characterized by impedance spectroscopy.Fig.3(a)shows the impedance spectra of overall resistance of the cell-A without applied potential at operated temperatures (1173to 973K)under 20%steam/1%H 2/79%Ar.The ohmic resistance was higher than the estimated resistance of 300m m LSGM electrolyte in the examined temperature range.This is because the electronic conductivity of CMF is not suf ficiently high,resulting in high current collecting resistance.Fig.3(b)shows the impedance spectra of the cathode side of the cell at the same measuring paring the results shown in Fig.3(a)and 3(b),the cathodic polarization resistance dominates the electrode resis-tance of the cell.In other words,the contribution of polarization resistance at anode is much smaller than that at cathode.Fig.4(a)and (b)show the impedance spectra of the cell-A measured at 1173K when applying a range of DC potentials from 0to 1.2V.With increasing potentials,both polarization and ohmic resistances were decreased.This behavior of the decreasing polarization resistance by the increasing potentials is similar to the behavior of the impedance results reported by Yue et al.[20].The decrease in the ohmic resistance of the cell-A can be considered to be due to the increase in electronic conductivity of CMF.As discussed previously,CMF is an n-type semiconductor and its electronic conductivity increases with decreasing oxygen partial pressure.Thus,the resistance of CMF is decreased by an electrochemical reduction under the polarized conditions.The impedance spectra of cell-B can be found in Fig.5for comparison.In comparison with the spectra in Fig.3,the ohmic resistance of the cell-B was slightly larger than that of the cell-A.This may be attributed to re-oxidation of Ni-based electrode or the different setup of the Pt current collector at cathode.In this case,Pt current collector was just mechanically pressed to the cathode,which is described previously in the experimental section.Furthermore,considering the pO 2in the atmosphere of 1%H 2-20%H 2O,re-oxidation of Ni may not occur seriously.Thus.Fig.4.AC impedance spectra of cell-A at various DC bias potentials at 1173K under a steam electrolysis;an enlarged view is also demonstrated in(b).Fig.5.AC impedance spectra of cell-B under an atmosphere of 20%steam/1%H 2/79%Ar at a DC bias of 0V;(b)is an enlarged view.476K.Hosoi et al./Electrochimica Acta 194(2016)473–479Fig.6.AC impedance spectra of cell-B under steam electrolysis at different DC bias potentials at 1173K;an enlarged view is also demonstrated in(b).Fig.7.Plots of polarization resistance (R pol )of cell-A and B as a function of different bias at 1173K.Fig.8.XRD patterns of the CMF cathode of the cell-A (a)before and (b)after steam electrolysis.K.Hosoi et al./Electrochimica Acta 194(2016)473–479477the large ohmic resistance could be assigned to the aggregation of Ni and/or the different setup of Pt current collector.Fig.6shows the impedance plots of the cell-B under applied DC potentials (0$1.2V)at 1173K.With increasing potentials,the ohmic resistance which is estimated from x-axis intercept of impedance semicircles at high frequency region and the polariza-tion resistance was decreased.Decrease in ohmic resistance suggests that the re-oxidized Ni was reduced electrochemically under polarized condition.The polarization resistances (R pol )of cell-A and cell-B were estimated from semicircles of impedance spectra and plotted with a function of applied potentials at 1173K in pared to cell-B,the polarization resistance of cell-A is signi ficantly decreased with increasing bias potential and achieved to be a reasonable small value at a high applied potential condition of 1.2V.These results suggest that CMF can present appropriate conducting andactive behavior under the operated steam electrolysis conditions.Under polarized condition,CMF is reduced,and lose oxygen content,then leading to an increase in the content of oxygen vacancy.Therefore,the surface activity of CMF may be improved by applied DC potentials due to providing the increasing number of active site and charge.Fig.8shows the XRD analyses of the CMF cathode before and after the steam electrolysis,respectively.The thickness of the CMF cathode was around 15m m in the case of cell-A.The CMF phase still remains after the steam electrolysis since no alternative phases or deposited oxides,e.g.Mn 2O 3,Fe 2O 3were identi fied,as can be clearly seen in the XRD scan Fig.8.In addition,the cross-section images between cathode and electrolyte were observed for cell-A before and after steam electrolysis using FE-SEM technique (Fig.9).The images show no considerable aggregation and delamination for CMFcathode.Fig.9.SEM images of the CMF cathode (a,c)before and (b,d)after steamelectrolysis.Fig.10.Long-term operation of the cell-A under a constant current at 1073K.478K.Hosoi et al./Electrochimica Acta 194(2016)473–479This result implies that CMF has superior stability in steam electrolysis process.Furthermore,a long-term stability measurement was con-ducted to the cell-A under constant current density(275mA/cm2), showing a steady potential during the operation in96h(Fig.10). These results suggest that CMF is stable and suitable under the operation of steam electrolysis conditions.Since CMF is originally developed for anode of SOFC,it is expected the risible operation of CMF as fuel electrode,and solid oxide reversible cells using CMF fuel electrode is now studied.Results will be reported in future.4.ConclusionsCeO2doped with Mn and Fe has been studied as an oxide cathode for steam electrolysis.It was found that H2formation rate of CMF cathode was in a good agreement with the theoretical value estimated by Faraday’s law at the operation temperature from 1173to973K.The CMF cathode exhibited a similar cathodic polarization resistance with Ni-SDC at1.5V.Also,the polarization resistance of the cell using CMF cathode was significantly decreased with increasing DC potential.Good phase and long-term stability of CMF under steam electrolysis have been confirmed by the series of electrochemical and material inves-tigations.Therefore,we suggest that CMF can be a potential cathode material for steam electrolysis.AcknowledgementThis study wasfinancially supported in part by a Grant-in-Aid for Scientific Research(S),No.24226016from Ministry of Education,Sports,Culture,Science,and Technology(MEXT),Japan and New Energy Development Organization(NEDO). 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