- Email: [email protected]
Electrochemical stability of La0.6Sr0.4Co0.2Fe0.8O3−δ-infiltrated YSZ oxygen electrode for reversible solid oxide fuel cells
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Electrochemical stability of La0.6Sr0.4Co0.2Fe0.8O3¡dinfiltrated YSZ oxygen electrode for reversible solid oxide fuel cells Hui Fan a,b, Michael Keane b, Na Li b, Dan Tang a, Prabhakar Singh b, Minfang Han a,* a
Union Research Center of Fuel Cell, School of Chemical & Environment Engineering, China University of Mining & Technology, Beijing 100083, China b University of Connecticut, Center for Clean Energy Engineering, 44 Weaver Rd., Storrs, CT 06269-5233, United States
article info
abstract
Article history:
La0.6Sr0.4Co0.2Fe0.8O3d (LSCF)-YSZ (yttria stabilized zirconia) oxygen electrodes were
Received 31 March 2014
developed by an infiltration process for reversible solid oxide fuel cells (RSOFCs). Electro-
Received in revised form
chemical performance of the LSCF-YSZ composite oxygen electrode was investigated in
22 May 2014
both fuel cell and steam electrolysis modes. Galvanostatic polarization operated at
Accepted 23 May 2014
±600 mA cm2 and 750 C showed that the cell has a voltage degradation rate of 3.4% and
Available online 30 July 2014
4.9% for fuel cell mode and steam electrolysis mode, respectively. Post-test SEM (scanning electronic microscopy) analysis of the electrodes indicates that the agglomeration of
Keywords:
infiltrated LSCF particles is possibly responsible for the performance degradation of the
Lanthanum strontium cobalt ferrite
cell.
Infiltration
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Reversible solid oxide fuel cell Solid oxide electrolysis cell
Introduction Reversible solid oxide fuel cells (RSOFCs) can work as solid oxide fuel cells (SOFCs) to release electric energy using hydrogen gas and as solid oxide electrolysis cells (SOECs) to store excess electrical energy into chemical energy (such as hydrogen gas) [1e3]. Materials employed into RSOFCs are similar to those used for SOFCs. Ni-based cermet and yttria stabilized zirconia (YSZ) are the most widely used hydrogen electrode and electrolyte materials, respectively [4e7]. In
terms of oxygen electrodes, composite lanthanum strontium manganite (LSM) has been proposed to operate reversibly under SOFC and SOEC modes [8,9]. However, significant degradation has been reported on the LSM oxygen electrode under SOEC operation condition [10e12]. Mixed ionic and electronic compound (MIEC) La0.6Sr0.4Co0.2Fe0.8O3d (LSCF) has shown high catalytic activity toward the oxygen reduction. Nevertheless, high resistance zirconate phases such as SrZrO3 and La2Zr2O7 are often formed at the LSCF-YSZ interface above 1000 C, which deteriorates the cell performance [13e15]. A new approach of
* Corresponding author. Tel./fax: þ86 010 62331098. E-mail address: [email protected] (M. Han). http://dx.doi.org/10.1016/j.ijhydene.2014.05.149 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
14072
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
solution infiltration has recently attracted more interests for the development of nano-structured electrodes, which can bypass the high processing temperature via the deposition of catalytically and/or electrochemically active nanoparticles into a rigid and prepared electrolyte scaffold [16e18]. Low calcination temperature results in formation of nano-sized particles and minimizes possible reactions and/or interdiffusion between the infiltrated and scaffold phases. Infiltrated oxygen electrodes have been investigated to yield low polarization resistance in solid oxide fuel cells by achieving nanoscale structures. Nano-structured LSCF-YSZ cathode has been reported by infiltrating LSCF into YSZ backbone [19]. LSCF perovskite phase can be formed at temperature as low as 700 C. However, the stability of the infiltrated LSCF-YSZ cathode was not reported. Study by Shah et al. [20] has investigated infiltration of LSCF into porous Gd-doped Ceria (GDC) scaffold to prepare nano-scale solid oxide fuel cell cathodes. It was found that decreasing the LSCF firing temperature and increasing the infiltrated LSCF loading reduced the polarization resistance of the cell. Nie et al. [21] have demonstrated that porous LSCF electrodes were coated with nano-sized Sm0.2Ce0.8O1.95d (SDC) particles by a one-step infiltration process. It indicated that the polarization resistance of LSCF electrode was significantly reduced due to the SDC infiltration. Furthermore, SDC infiltration enhanced cell stability under SOFC operating conditions. In addition, SOFC performance was significantly enhanced by surface modification through the solution-based infiltration process [22]. An SSC solution was infiltrated into an LSCF backbone to considerably improve the performance of LSCF electrode [23]. It has been reported that both electro-catalytic activity and stability of LSCF cathode have been increased by modification using active Mn-based catalyst coatings [24,25]. In an investigation by Yoon et al. [26], an infiltration technique was utilized to uniformly coat nano-sized LSCF particles into La0.8Sr0.2Ga0.8Mg0.2O3d (LSGM) scaffold. Low firing temperature prevented the formation of undesired secondary phases such as LaSrGaO4. Moreover, the nano LSCF-infiltrated LSGM electrode had improved oxygen reduction, which was attributed to an increase in the triple phase boundary (TPB) regions. As well as for SOFCs, LSCF is also the most commonly used materials for SOEC application. LSCF þ SDC/LSCF double layer-type oxygen electrode has been developed for reversible SOFCs, to exhibit high performance [27]. An SOEC using a CGO inter-diffusion barrier between YSZ electrolyte layer and LSCF:CGO oxygen electrode has been tested in galvanostatic H2O/CO2 co-electrolysis condition [28]. Compared to LSM:YSZ based cells, a lower overall degradation was shown for the cell with the LSCF:CGO oxygen electrode. Schiller et al. [29] have investigated an LSCF oxygen electrode for metal supported SOECs. A low degradation rate was shown for high temperature steam electrolysis after long-term test. The hydrogen electrode was mainly responsible to enhanced polarization resistance during the electrolysis operation. The overpotential of an LSCF oxygen electrode was studied under SOEC and SOFC operating modes [30]. It was shown that the oxygen electrode has higher activity as an SOFC cathode than that as an SOEC anode. Relative to LSF and LSM-YSZ oxygen electrodes, the LSCF oxygen electrode exhibited higher electrochemical performance. However, no studies were focused on
electrochemical stability of the LSCF composite oxygen electrodes for both reversible SOFC and SOEC operation under respective inlet gas conditions. In this study, nano-structured LSCF-YSZ oxygen electrode was prepared by an infiltration process for RSOFCs. Electrochemical performance of the cell was investigated under both fuel cell and steam electrolysis modes. Low steam content fuel gas (3% H2O) and high steam content mixture gas (50% H2O) were introduced for fuel cell and steam electrolysis operation, respectively. Galvanostatic charge/discharge cycles using respective inlet gas composition were performed to evaluate stability of the LSCF-infiltrated YSZ oxygen electrode for reversible fuel cell/steam electrolysis operation. Failure mechanism of the RSOFC was proposed based on post-test SEM analysis.
Experimental Fabrication of porous RSOFC substrates Porous RSOFC substrates consisted of Ni-YSZ hydrogen electrode supports, thin YSZ electrolytes, and porous YSZ layer. The substrates were fabricated by tape casting and hot isostatic pressing techniques, as mentioned before [31]. For fabricating the NiO-YSZ electrode, commercial powders of NiO, YSZ (8 mol% yttria stabilized zirconia, with a median particle size of 0.114 mm) and graphite (Furunda Zirconium Material Co. Ltd., China) pore former were mixed and ballmilled in a weight ratio of 50:50:10 with appropriate amounts of ethanol-butanone solvent, caster oil dispersant, dibutyl phthalate (DBP) plasticizer, and polyvinyl butyral (PVB) binder. A two-stage milling process [31] was adopted. Firstly, NiO and YSZ powders were dispersed in mixed solvent and dispersant via ball-milling for 15e20 h. Secondly, a homogenous mixture including remaining solvent, plasticizer, and binder formed by thermal dissolution was added, and the resultant slurry was then ball milled for another 24 h before adding the last constituent of graphite. For preparing the YSZ electrolyte and the YSZ layer, the same way of tape casting as in the case of NiO-YSZ substrates was used excepted for no addition of NiO and graphite powders. Porous YSZ layers were prepared with higher ratio of pore formers. Through study on different weight ratios of graphite to YSZ, 50 wt% was thought as an optimal proportion. A tape casting machine (DR-150, made in Japan) was used for separately casting the NiO-YSZ substrate, the electrolyte, and the porous YSZ layer. These tapes were dried in air at room temperature for 12 h. One sheet of NiO-YSZ support (~300 mm), YSZ electrolyte layer (20e30 mm), and porous YSZ layer (~70 mm) were stacked together under a vacuum condition and laminated at 20 MPa for 10 min using a hot isostatic press (30Tm Shanxi, China) to form a NiO-YSZ supported tri-layer structure. The resulting tri-layer tape was punched to discs, and then co-sintered at 1300 C for 10 h in order to densify the electrolyte layer, during which graphite was burned out, leaving a well-formed porous YSZ network. The microstructural characteristics of the fired tri-layer tapes were performed using a scanning electron microscope, and the estimated porosity of the porous YSZ layer was 40e50%.
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
Single-cell preparation An infiltration solution of LSCF (La0.6Sr0.4Co0.2Fe0.8O3d) precursor was prepared by dissolving stoichiometric amounts of La(NO3)3$6H2O, Sr(NO3)2, Co(NO3)2$6H2O and Fe(NO3)3$9H2O into deionized water and ethanol [4]. Ethanol has been proved to be an effective additive to lower surface tension of the infiltration solution on backbone [23]. Glycol was used as chelating agent to form LSCF perovskite phase with small average grain size at low firing temperature [32]. Infiltration was carried out by inserting the LSCF precursor solution into the pre-treated porous YSZ layer. The infiltrated samples were dried at room temperature, and then fired at 450 C for 1 h to decompose the nitrate. In order to increase the LSCF loading into the porous layer, repeated infiltration was performed, followed by firing at 450 C for 1 h after each infiltration. The LSCF infiltrated YSZ electrode was eventually sintered at 900 C for 2 h to prepare the NiO-YSZ/YSZ/LSCF-YSZ RSOFCs with an LSCF loading amount of ~40%. Study by Liu et al. [33] has investigated the effect of different catalyst (Sm0.5Sr0.5CoO3d, SSC) loading into LSCF backbone on electrode polarization resistances. Generally, high electrode performance was achieved with high catalyst loading weight ratio (~40e50%) in the infiltration process [31].
14073
Results and discussion XRD characterization of LSCF-infiltrated oxygen electrode Fig. 1 shows XRD patterns of LSCF-infiltrated YSZ oxygen electrode sintered at 900 C. For reference, the XRD diffractions of porous YSZ matrix and pure perovskite LSCF powders are also displayed in Fig. 1. In comparison with XRD patterns of YSZ matrix and pure perovskite LSCF, the XRD pattern of LSCF-infiltrated YSZ indicates that LSCF perovskite phase can be formed in porous YSZ layer from the infiltration precursor solution after firing at 900 C. No impurity phases were detected, which indicated that reactions between LSCF and YSZ can be avoided at this temperature. No reactions between LSCF and YSZ at a calcination temperature of 900 C have been demonstrated by Adijanto et al., and lanthanum and strontium zirconates were produced at the interface and over 1100 C [34]. In infiltration process, the processing temperature required for formation of the desired crystal structure plays a key role in the size and morphology of the nanoparticles [35]. The phase formation of LSCF nano-particles and their chemical compatibility with YSZ backbone are examined at 900 C via XRD analysis. The LSCF-YSZ oxygen electrode is therefore of high phase-purity.
Characterization of RSOFCs Microscopy of RSOFCs Silver paste (Ag ink, Beijing, China) was applied as a current collector onto electrode surfaces. The RSOFCs were mounted on an alumina tube, sealed using ceramic paste (Aramco-552, USA) as sealant, and tested in a temperature-controlled tube furnace. The area of electrolyte and hydrogen electrode was about 1.5 cm2 and the external area of oxygen electrode was 0.2 cm2. Electrochemical performance of the cell was measured from 700 C to 800 C under both fuel cell and steam electrolysis modes. For fuel cell testing, 80 sccm (standard cubic centimeters per minute) flow rate of humidified hydrogen gas (3% H2O þ 97% H2, volume fraction) was fed to the hydrogen electrode, while the oxygen electrode was exposed to ambient air. For steam electrolysis operation, 50% H2O, 25% H2 and 25% Ar were introduced into the Ni-based hydrogen electrode. Similarly, the oxygen electrode was left open to atmospheric environment. Polarization curves and electrochemical impedance spectra (EIS) under open circuit were conducted using an IM6 Electrochemical Workstation (ZAHNER, Germany). For EIS test, the frequency range was from 100 mHz to 100 kHz, and the AC amplitude was 20 mV. In addition, the crystallographic phase of the infiltrated LSCF was verified by XRD analysis using a high power X-ray diffractometer (XRD, PANalytical X0 Pert PRO, Netherlands) with CuKa radiation. To understand the operational stability of the RSOFC under SOFC and SOEC modes, repeated fuel cell and steam electrolysis cycles were galvanostatically measured via an Arbin test system (Arbin MSTAT4) at 750 C, meanwhile the variation of charge/discharge voltage with time at a constant current density of ±600 mA cm2 was recorded. The morphology of the RSOFCs and LSCF infiltrated oxygen electrode as prepared and after galvanostatic polarization was characterized using a scanning electron microscope (SEM, JEOL, JSM 6700F).
Fig. 2 shows the SEM micrographs of the fractured crosssection of a tri-layer NiO-YSZ/YSZ/LSCF-YSZ cell. The cell was composed of a porous NiO-YSZ hydrogen electrode support (~300 mm), a dense YSZ electrolyte (~20 mm), and a porous LSCF-infiltrated YSZ oxygen electrode (~70 mm). It indicated that the co-sintering at 1300 C for 10 h forms dense YSZ electrolyte and porous YSZ electrode backbone. Estimated porosity of the porous YSZ layer was 40e50% in our previous study [31]. Both the highly porous hydrogen and oxygen electrodes are in good contact with the dense electrolyte. SEM
Fig. 1 e X-ray diffraction patterns of LSCF-YSZ oxygen electrode prepared by LSCF infiltration method after sintering at 900 C, porous YSZ matrix, and pure perovskite LSCF powders.
14074
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
the nano-sized LSCF particles were uniformly distributed on the surface of the porous YSZ structure via in-situ calcination at 900 C. Fine microstructure of formed particles is beneficial for increasing the active area of TPB (triple phase boundary). To control the morphology and microstructure of infiltrationformed phase, surface tension of the precursor solution was modified by wetting properties [23]. Water was an excellent solvent for most metal nitrate precursors due to its high polarity [36], however the aqueous solution exhibits inferior wetting properties on perovskite electrodes [37], due to the high surface tension. Ethanol has been proven to be an effective additive to nitrate precursor solution to lower its surface tension, resulted into uniform and continuous coating formation [23]. The addition of ethanol into the aqueous solution remarkably improves the wetting characteristic.
Fuel cell evaluation Fig. 3 shows initial electrochemical characterization of LSCFinfiltrated Ni-YSZ/YSZ/LSCF-YSZ button cell under fuel cell mode. The typical voltage and power density as a function of current density at 800 Ce700 C are shown in Fig. 3 (a). For the fuel cell operation, 97% H2 and 3% H2O at a flow rate of 80 sccm
Fig. 2 e SEM micrographs of fractured cross-sections of (a) LSCF-infiltrated NiO-YSZ/YSZ/LSCF-YSZ RSOFC, (b) porous YSZ backbone, and (c) LSCF-infiltrated YSZ oxygen electrode.
micrographs of the porous YSZ structure before and after LSCF-infiltration treatment are displayed in Fig. 2 (a) and (b). Prior to the infiltration, the porous YSZ matrix was well sintered, forming a rigid three-dimensional network. The unmodified YSZ backbone has clean surface and cleanly visible grain boundaries. After infiltration of the precursor solution,
Fig. 3 e (a) Voltage and power density vs current density curves, and (b) Nyquist electrochemical impedance spectra (EIS) plots at open circuit for Ni-YSZ/YSZ/LSCF-YSZ cell under fuel cell mode.
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
were used as fuels for a single cell. The open circuit voltage (OCV) is 1.072 V, 1.099 V, and 1.108 V at the temperature from 800 C to 700 C, respectively, indicating a good sealing was achieved for the test of the cells. These measured values are slightly lower than theoretical OCVs of 1.103 V (800 C), 1.111 V (750 C), and 1.112 V (700 C), which were predicted from Nernst equation [38]. The maximum power density decreases from 900 mW cm2 at 800 C to 640 mW cm2 at 700 C. Chang et al. [39] have investigated that NiO-YSZ anode supported SOFCs with screen-printed LSCF composite cathode exhibited a peak power density of 409 mW cm2 at 800 C. It indicated that SOFCs with LSCF infiltrated cathode possessed a better electrocatalytic activity and electrochemical performance [19]. Nyquist electrochemical impedance spectra (EIS) plots for the fuel cell at OCV are displayed in Fig. 3 (b). The EIS plots were composed of a high-frequency arc and a low-frequency arc. The former is usually attributed to charge transfer in composite electrode and the latter to mass transport [40]. With increasing temperature, a pronounced increase in the high-frequency arc was observed, whereas the low frequency arc has low temperature dependence. The ohmic resistance RU obtained from the intersection of the impedance curve and X-axis at high frequency refers to the sum of the electrolyte resistance of ionic transfer, lead resistance of electronic transfer, and electrode/electrolyte contact resistance [39], wherein the lead resistance can be negligible using silver wire as current collectors. The polarization resistance Rp, determined from the differences between high- and low-frequency intercepts on EIS is attributed to mass transport properties of gas and charge transfer resistance, which includes intrinsically electro-catalytic characteristic of the composite electrode materials, ionic transport, TPB length, and grain sizes of electrolyte and electrocatalyst [41]. It has been shown in Fig. 3 (b) that the RU values are 0.24 U cm2, 0.26 U cm2, and 0.29 U cm2 at 800 Ce700 C, respectively. The Rp values are 0.21 U cm2, 0.26 U cm2, and 0.34 U cm2 at the temperatures from 800 C to 700 C, respectively. The infiltrated LSCF cathodes generally exhibited lower Rp values than LSCF prepared by co-firing of powder and screen printing technique [19,42], due mainly to an increase in surface area from the nanoparticles [37].
14075
electrolysis current density increases, which indicates hydrogen production is improved. During steam electrolysis, oxygen gas is firstly adsorbed on the electrode surface and reduced at the TPB sites to produce oxygen ions and consume electrons. The oxygen ions are then transported through oxygen ion conductor such as YSZ to hydrogen electrodes [44]. It has been reported that the principal electrode polarization losses were usually ascribed to the generation and transport of oxygen ions within the porous oxygen electrode structure [45]. Typical Nyquist electrochemical impedance spectra at open circuits for the cells under the steam electrolysis mode are shown in Fig. 4 (b). The total cell resistance was 0.48 U cm2 and 0.33 U cm2 at 700 C and 800 C, respectively. The RU values are 0.24 U cm2, 0.26 U cm2 and 0.28 U cm2 at 800 Ce700 C, respectively, which are similar to those obtained under fuel cell mode. The Rp values are 0.2 U cm2 at 700 C and 0.08 U cm2 at 800 C. The electrode polarization resistance is mainly attributed to the charge transfer reaction at the electrode/ electrolyte interface, the adsorption/desorption, and diffusion inside the porous electrode [46].
Fuel cell/steam electrolysis cycles To evaluate the performance durability of the LSCF-infiltrated YSZ oxygen electrode under both fuel cell and steam
Steam electrolysis performance After initial evaluation in the fuel cell mode, the current direction was changed for operation in steam electrolysis mode. Fig. 4 shows steam electrolysis performance of the Ni-YSZ/ YSZ/LSCF-YSZ cells at 800 Ce700 C. For steam electrolysis operation, 50% H2O, 25% H2 and 25% Ar at a total flow rate of 150 sccm were introduced into the hydrogen electrode, and the oxygen electrode was left in air atmosphere. Current vs voltage curves taken as function of temperature are displayed in Fig. 4 (a). Measured OCV values of the cell in the steam electrolysis operation are 0.919 V, 0.937 V, and 0.956 V at the temperature of 800 C, 750 C, and 700 C, respectively. The measured OCVs are influenced by steam/hydrogen ratio and temperature, as expected from the Nernst equation [43]. The current densities of the cells at an electrolysis voltage of 1.3 V were 1.14 A cm2, 0.98 A cm2, and 0.72 A cm2 within the temperature range of 800 Ce700 C, respectively. With increasing the temperature at the same voltage, the
Fig. 4 e (a) Voltage vs current density curves, and (b) Nyquist EIS plots at open circuit for Ni-YSZ/YSZ/LSCF-YSZ cells under steam electrolysis mode.
14076
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
electrolysis modes, the Ni-YSZ/YSZ/LSCF-YSZ cells were subjected to galvanostatic polarization at ±600 mA cm2 and 750 C. Repeated charge/discharge cycles (1 h of steam electrolysis and 1 h of fuel cell each) under respective operational condition are shown in Fig. 5. During the 8 cycles, the discharge voltage was gradually decreased from 0.82 V to 0.78 V and the charge voltage increased from 1.19 V to 1.23 V. The corresponding voltage degradation of the cell is 3.4% and 4.9% for fuel cell mode and electrolysis mode, respectively. Marina et al. [47] studied electrode performance in reversible solid oxide fuel cells. They observed that the electrodes exhibited generally higher losses in the SOEC mode as compared to the SOFC mode. Investigations by Hauch et al. [7] showed that cells in SOEC mode had much higher degradation than those in SOFC mode. No reports on RSOFCs in literatures include repeated charge/discharge cycles under respective fuel gas/steam electrolysis atmosphere for evaluating the performance durability. By altering inlet gas mixture to the hydrogen electrode, the fuel cell and steam electrolysis operations were alternatively achieved for reversibility evaluation. Furthermore, it is necessary to find evidence for improving the reversible durability of the cell.
Microstructural characterization of pre/post-test cells The electrochemical performance of the cells is closely related to their microstructure. For investigating degradation mechanism of the RSOFCs after repeated discharge/charge cycles, post-test microstructure of the infiltrated LSCF-YSZ oxygen electrode is shown in Fig. 6 (b). An SEM image of as-prepared LSCF-infiltrated YSZ oxygen electrode is displayed as a reference in Fig. 6 (a). Prior to the polarization, the infiltrated LSCF particles were uniformly distributed on the porous YSZ backbone. After being polarized for 8 discharge/charge cycles, the LSCF particles were dramatically agglomerated. A longterm performance degradation is usually associated with the coarsening of the microstructure [48]. The agglomeration of LSCF particles further leaded to a reduction in both the number of TPBs and electrical conductivity [49].
Fig. 6 e SEM images of LSCF-infiltrated YSZ oxygen electrodes (a) as-prepared, (b) after fuel cell/steam electrolysis cycles.
Agglomeration also decreases the porosity of infiltrated LSCFYSZ electrode, and affects the mass transfer process of the oxygen reduction reaction associated with the adsorption and diffusion of oxygen at the gas/electrode interface and the surface diffusion of the oxygen species [50,51]. These results demonstrate that the agglomeration of LSCF particles was possibly responsible for the performance degradation of the Ni-YSZ/YSZ/LSCF-YSZ RSOFCs.
Conclusions
Fig. 5 e Repeated charge/discharge cycles of Ni-YSZ/YSZ/ LSCF-YSZ at ±600 mA cm¡2 and 750 C in fuel cell/steam electrolysis operation.
By infiltrating LSCF into porous YSZ backbone, composite oxygen electrodes were developed for an RSOFC. The electrochemical performance of the cell was investigated in both fuel cell and steam electrolysis modes using polarization curves and electrochemical impedance spectroscopy. Galvanostatic charge/discharge polarization at ±600 mA cm2 and 750 C showed the cell has voltage degradation rates of 3.4% and 4.9% for fuel cell mode and steam electrolysis mode, respectively. After 8 repeated discharge/charge cycles, microstructure analysis of the post-test cell demonstrated that the degradation was probably due to the agglomeration of infiltrated LSCF particles.
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
Acknowledgments [16]
We are grateful to National Basic Research Program of China (973 Program No. 2012CB215404, No. 2012CB215404), and China Scholarship Council (No. 201306430012) for financial support of this work.
references
[1] Laguna-Bercero MA, Kinadjan N, Sayers R, Shinawi HE, Greaves C, Skinner SJ. Performance of La2xSrxCo0.5Ni0.5O4±d as an oxygen electrode for solid oxide reversible cells. Fuel Cells 2011;11:102e7. [2] Nguyen VN, Fang QP, Packbier U, Blum L. Long-term tests of a Ju¨lich planar short stack with reversible solid oxide cells in both fuel cell and electrolysis modes. Int J Hydrogen Energy 2013;38(11):4281e90. [3] Ibrahim H, Ilinca A, Perron J. Energy storage systemsdcharacteristics and comparisons. Renew Sustain Energy Rev 2008;12:1221e50. [4] Chen KF, Ai N, Jiang SP. Reasons for the high stability of nano-structured (La, Sr)MnO3 infiltrated Y2O3eZrO2 composite oxygen electrodes of solid oxide electrolysis cells. Electrochem Commun 2012;19:119e22. [5] Liang MD, Yu B, Wen MF, Chen J, Xu JM, Zhai YC. Preparation of LSM-YSZ composite powder for anode of solid oxide electrolysis cell and its activation mechanism. J Power Sources 2009;190:341e5. € nstrup A, Graule T, Menzler NH, Sitte W, [6] Ried P, Lorenz C, Bro et al. Processing of YSZ screen printing pastes and the characterization of the electrolyte layers for anode supported SOFC. J Eur Ceram Soc 2008;28:1801e8. [7] Hauch A, Jensen SH, Ramousse S, Mogensen M. Performance and durability of solid oxide electrolysis cells. J Electrochem Soc 2006;153:A1741e7. [8] Chen KF, Ai N, Jiang SP. Performance and stability of (La, Sr) MnO3eY2O3eZrO2 composite oxygen electrodes under solid oxide electrolysis cell operation conditions. Int J Hydrogen Energy 2012;37:10517e25. [9] Kim-Lohsoontorn P, Brett DJL, Laosiripojana N, Kime YM, Bae JM. Performance of solid oxide electrolysis cells based on composite La0.8Sr0.2MnO3d-yttria stabilized zirconia and Ba0.5Sr0.5Co0.8Fe0.2O3d oxygen electrodes. Int J Hydrogen Energy 2010;35:3958e66. [10] Virkar AV. Mechanism of oxygen electrode delamination in solid oxide electrolyzer cells. Int J Hydrogen Energy 2010;35:9527e43. [11] Li N, Keane M, Mahapatra MK, Singh P. Mitigation of the delamination of LSM anode in solid oxide electrolysis cells using manganese-modified YSZ. Int J Hydrogen Energy 2013;38:6298e303. [12] Keane M, Mahapatra MK, Verma A, Singh P. LSM-YSZ interactions and anode delamination in solid oxide electrolysis cells. Int J Hydrogen Energy 2012;37:16776e85. [13] Kostogloudis GC, Tsiniarakis G, Ftikos C. Chemical reactivity of perovskite oxide SOFC cathodes and yttria stabilized zirconia. Solid State Ionics 2000;135:529e35. [14] Esquirol A, Brandon NP, Kilner JA, Mogensen M. Electrochemical characterization of La0.6Sr0.4Co0.2Fe0.8O3 cathodes for intermediate-temperature SOFCs. J Electrochem Soc 2004;151:A1847e55. [15] Qiu L, Ichikawa T, Hirano A, Imanishi N, Takeda Y. Ln1xSrxCo1yFeyO3d (Ln ¼ Pr, Nd, Gd; x ¼ 0.2, 0.3) for the
[17] [18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
14077
electrodes of solid oxide fuel cells. Solid State Ionics 2003;158:55e65. Jiang SP. Nanoscale and nano-structured electrodes of solid oxide fuel cells by infiltration: advances and challenges. Int J Hydrogen Energy 2012;37:449e70. Vohs JM, Gorte RJ. High-performance SOFC cathodes prepared by infiltration. Adv Mater 2009;21:943e56. Jiang ZY, Xia CR, Chen FL. Nano-structured composite cathodes for intermediate-temperature solid oxide fuel cells via an infiltration/impregnation technique. Electrochim Acta 2010;55:3595e605. Chen J, Liang FL, Liu LN, Jiang SP, Chi B, Pu J, et al. Nanostructured (La, Sr)(Co, Fe)O3 þ YSZ composite cathodes for intermediate temperature solid oxide fuel cells. J Power Sources 2008;183:586e9. Shah M, Barnett S. Solid oxide fuel cell cathodes by infiltration of La0.6Sr0.4Co0.2Fe0.8O3d into Gd-doped ceria. Solid State Ionics 2008;179:2059e64. Nie LF, Liu MF, Zhang YJ, Liu ML. La0.6Sr0.4Co0.2Fe0.8O3d cathodes infiltrated with samarium-doped cerium oxide for solid oxide fuel cells. J Power Sources 2010;195:4704e8. Ding D, Li XX, Lai SYX, Geides K, Liu ML. Enhancing SOFC cathode performance by surface modification through infiltration. Energy Environ Sci 2014;7:552e75. Lou XY, Liu Z, Wang SZ, Xiu YH, Wong CP, Liu ML. Controlling the morphology and uniformity of a catalystinfiltrated cathode for solid oxide fuel cells by tuning wetting property. J Power Sources 2010;195:419e24. Lynch ME, Yang L, Qin WT, Choi J-J, Liu MF, Blinn K, et al. Enhancement of La0.6Sr0.4Co0.2Fe0.8O3d durability and surface electrocatalytic activity by La0.85Sr0.15MnO3±d investigated using a new test electrode platform. Energy Environ Sci 2011;4:2249e58. Ding D, Liu MF, Liu ZB, Li XX, Blinn K, Zhu ZB, et al. Efficient electro-catalysts for enhancing surface activity and stability of SOFC cathodes. Adv Energy Mater 2013;3:1149e54. Yoon BY, Bae J. Characteristics of nano La0.6Sr0.4Co0.2Fe0.8O3d-infiltrated La0.8Sr0.2Ga0.8Mg0.2O3d scaffold cathode for enhanced oxygen reduction. Int J Hydrogen Energy 2013;38:13399e407. Tao Y, Nishino H, Ashidate S, Kokubo H, Watanabe M, Uchida H. Polarization properties of La0.6Sr0.4Co0.2Fe0.8O3based double layer-type oxygen electrodes for reversible SOFCs. Electrochim Acta 2009;54:3309e15. Hjalmarsson P, Sun XF, Liu YL, Chen M. Influence of the oxygen electrode and inter-diffusion barrier on the degradation of solid oxide electrolysis cells. J Power Sources 2013;223:349e57. Schiller G, Anser A, Lang M, Patz O. High temperature water electrolysis using metal supported solid oxide electrolyser cells (SOEC). J Appl Electrochem 2008;39:293e301. Kim-Lohsoontorn P, Bae J. Electrochemical performance of solid oxide electrolysis cell electrodes under hightemperature coelectrolysis of steam and carbon dioxide. J Power Sources 2011;196:7161e8. Liu Z, Zheng ZW, Han MF, Liu ML. High performance solid oxide fuel cells based on tri-layer yttria-stabilized zirconia by low temperature sintering process. J Power Sources 2010;195:7230e3. Zhu XB, Ding D, Li YQ, Lu¨ Z, Su WH, Zhen L. Development of La0.6Sr0.4Co0.2Fe0.8O3d cathode with an improved stability via La0.8Sr0.2MnO3-film impregnation. Int J Hydrogen Energy 2013;38:5375e82. Liu MF, Ding D, Blinn K, Li XX, Nie LF, Liu ML. Enhanced performance of LSCF cathode through surface modification. Int J Hydrogen Energy 2012;37:8613e20. Adijanto L, Ku¨ngas R, Bidrawn F, Gorte RJ, Vohs JM. Stability and performance of infiltrated La0.8Sr0.2CoxFe1xO3
14078
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
electrodes with and without Sm0.2Ce0.8O1.9 interlayers. J Power Sources 2011;196:5797e802. Lee SI, Kim J, Son JW, Lee JH, Kim BK, Je HJ, et al. High performance air electrode for solid oxide regenerative fuel cells fabricated by infiltration of nano-catalysts. J Power Sources 2014;250:15e20. Kim G, Lee S, Shin JY, Corre G, Irvine JTS, Vohs JM, et al. Investigation of the structural and catalytic requirements for high-performance SOFC anodes formed by infiltration of LSCM. Electrochem Solid State Lett 2009;12:B48e52. Lou XY, Wang SZ, Liu Z, Yang L, Liu ML. Improving La0.6Sr0.4Co0.2Fe0.8O3d cathode performance by infiltration of a Sm0.5Sr0.5CoO3d coating. Solid State Ionics 2009;80:1285e9. Laguna-Bercero MA, Kilner JA, Skinner SJ. Development of oxygen electrodes for reversible solid oxide fuel cells with scandia stabilized zirconia electrolytes. Solid State Ionics 2011;192:501e4. Chang YC, Lee MC, Kao WX, Wang CH, Lin TN, Chang JC. Fabrication and evaluation of electrochemical characteristics of the composite cathode layers for the anode-supported solid-oxide fuel cells. J Taiwan Inst Chem Eng 2011;42:775e82. Murray EP, Sever MJ, Barnett SA. Electrochemical performance of (La, Sr)(Co, Fe)O3e(Ce, Gd)O3 composite cathodes. Solid State Ionics 2002;48:27e34. Serra JM, Vert VB, Bu¨chler O, Meulenberg WA, Buchkremer HP. IT-SOFC supported on mixed oxygen ionicelectronic conducting composites. Chem Mater 2008;20:3867e75. Jun SH, Uhm YR, Song RH, Rhee CK. Synthesis and properties of Ag nanoparticles attached La0.6Sr0.4Co0.3Fe0.7O3d. Curr Appl Phys 2011;11:S305e8.
[43] Brisse A, Schefold J, Zahid M. High temperature water electrolysis in solid oxide cells. Int J Hydrogen Energy 2008;33:5357e82. [44] Song HS, Kim WH, Hyun SH, Moon J, Kim J, Lee HW. Effect of starting particulate materials on microstructure and cathodic performance of nanoporous LSM-YSZ composite cathodes. J Power Sources 2007;167:258e64. [45] Adler SB, Lane JA, Steele BCH. Electrode kinetics of porous mixed-conducting oxygen electrodes. J Electrochem Soc 1996;143:3554e64. [46] Zhan ZL, Zhao L. Electrochemical reduction of CO2 in solid oxide electrolysis cells. J Power Sources 2010;195:7250e4. [47] Marina OA, Pederson LR, Williams MC, Coffey GW, Meinhardt KD, Nguyen CD, et al. Electrode performance in reversible solid oxide fuel cells. J Electrochem Soc 2007;154:452e9. [48] Wang WS, Gross MD, Vohs JM, Gorte RJ. The stability of LSFYSZ electrodes prepared by infiltration. J Electrochem Soc 2007;154:B439e45. [49] Shah M, Voorhees PW, Barnett SA. Time-dependent performance changes in LSCF-infiltrated SOFC cathodes: the role of nano-particle coarsening. Solid State Ionics 2011;187:64e7. [50] Liu YH, Wang FZ, Chi B, Pu J, Li J, Jiang SP. A stability study of impregnated LSCF-GDC composite cathodes of solid oxide fuel cells. J Alloy Compd 2013;578:37e43. [51] Liang FL, Chen J, Chi B, Pu J, Jiang SP, Li J. Redox behavior of supported Pd particles and its effect on oxygen reduction reaction in intermediate temperature solid oxide fuel cells. J Power Sources 2011;196:153e8.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Electrochemical stability of La0.6Sr0.4Co0.2Fe0.8O3¡dinfiltrated YSZ oxygen electrode for reversible solid oxide fuel cells Hui Fan a,b, Michael Keane b, Na Li b, Dan Tang a, Prabhakar Singh b, Minfang Han a,* a
Union Research Center of Fuel Cell, School of Chemical & Environment Engineering, China University of Mining & Technology, Beijing 100083, China b University of Connecticut, Center for Clean Energy Engineering, 44 Weaver Rd., Storrs, CT 06269-5233, United States
article info
abstract
Article history:
La0.6Sr0.4Co0.2Fe0.8O3d (LSCF)-YSZ (yttria stabilized zirconia) oxygen electrodes were
Received 31 March 2014
developed by an infiltration process for reversible solid oxide fuel cells (RSOFCs). Electro-
Received in revised form
chemical performance of the LSCF-YSZ composite oxygen electrode was investigated in
22 May 2014
both fuel cell and steam electrolysis modes. Galvanostatic polarization operated at
Accepted 23 May 2014
±600 mA cm2 and 750 C showed that the cell has a voltage degradation rate of 3.4% and
Available online 30 July 2014
4.9% for fuel cell mode and steam electrolysis mode, respectively. Post-test SEM (scanning electronic microscopy) analysis of the electrodes indicates that the agglomeration of
Keywords:
infiltrated LSCF particles is possibly responsible for the performance degradation of the
Lanthanum strontium cobalt ferrite
cell.
Infiltration
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Reversible solid oxide fuel cell Solid oxide electrolysis cell
Introduction Reversible solid oxide fuel cells (RSOFCs) can work as solid oxide fuel cells (SOFCs) to release electric energy using hydrogen gas and as solid oxide electrolysis cells (SOECs) to store excess electrical energy into chemical energy (such as hydrogen gas) [1e3]. Materials employed into RSOFCs are similar to those used for SOFCs. Ni-based cermet and yttria stabilized zirconia (YSZ) are the most widely used hydrogen electrode and electrolyte materials, respectively [4e7]. In
terms of oxygen electrodes, composite lanthanum strontium manganite (LSM) has been proposed to operate reversibly under SOFC and SOEC modes [8,9]. However, significant degradation has been reported on the LSM oxygen electrode under SOEC operation condition [10e12]. Mixed ionic and electronic compound (MIEC) La0.6Sr0.4Co0.2Fe0.8O3d (LSCF) has shown high catalytic activity toward the oxygen reduction. Nevertheless, high resistance zirconate phases such as SrZrO3 and La2Zr2O7 are often formed at the LSCF-YSZ interface above 1000 C, which deteriorates the cell performance [13e15]. A new approach of
* Corresponding author. Tel./fax: þ86 010 62331098. E-mail address: [email protected] (M. Han). http://dx.doi.org/10.1016/j.ijhydene.2014.05.149 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
14072
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
solution infiltration has recently attracted more interests for the development of nano-structured electrodes, which can bypass the high processing temperature via the deposition of catalytically and/or electrochemically active nanoparticles into a rigid and prepared electrolyte scaffold [16e18]. Low calcination temperature results in formation of nano-sized particles and minimizes possible reactions and/or interdiffusion between the infiltrated and scaffold phases. Infiltrated oxygen electrodes have been investigated to yield low polarization resistance in solid oxide fuel cells by achieving nanoscale structures. Nano-structured LSCF-YSZ cathode has been reported by infiltrating LSCF into YSZ backbone [19]. LSCF perovskite phase can be formed at temperature as low as 700 C. However, the stability of the infiltrated LSCF-YSZ cathode was not reported. Study by Shah et al. [20] has investigated infiltration of LSCF into porous Gd-doped Ceria (GDC) scaffold to prepare nano-scale solid oxide fuel cell cathodes. It was found that decreasing the LSCF firing temperature and increasing the infiltrated LSCF loading reduced the polarization resistance of the cell. Nie et al. [21] have demonstrated that porous LSCF electrodes were coated with nano-sized Sm0.2Ce0.8O1.95d (SDC) particles by a one-step infiltration process. It indicated that the polarization resistance of LSCF electrode was significantly reduced due to the SDC infiltration. Furthermore, SDC infiltration enhanced cell stability under SOFC operating conditions. In addition, SOFC performance was significantly enhanced by surface modification through the solution-based infiltration process [22]. An SSC solution was infiltrated into an LSCF backbone to considerably improve the performance of LSCF electrode [23]. It has been reported that both electro-catalytic activity and stability of LSCF cathode have been increased by modification using active Mn-based catalyst coatings [24,25]. In an investigation by Yoon et al. [26], an infiltration technique was utilized to uniformly coat nano-sized LSCF particles into La0.8Sr0.2Ga0.8Mg0.2O3d (LSGM) scaffold. Low firing temperature prevented the formation of undesired secondary phases such as LaSrGaO4. Moreover, the nano LSCF-infiltrated LSGM electrode had improved oxygen reduction, which was attributed to an increase in the triple phase boundary (TPB) regions. As well as for SOFCs, LSCF is also the most commonly used materials for SOEC application. LSCF þ SDC/LSCF double layer-type oxygen electrode has been developed for reversible SOFCs, to exhibit high performance [27]. An SOEC using a CGO inter-diffusion barrier between YSZ electrolyte layer and LSCF:CGO oxygen electrode has been tested in galvanostatic H2O/CO2 co-electrolysis condition [28]. Compared to LSM:YSZ based cells, a lower overall degradation was shown for the cell with the LSCF:CGO oxygen electrode. Schiller et al. [29] have investigated an LSCF oxygen electrode for metal supported SOECs. A low degradation rate was shown for high temperature steam electrolysis after long-term test. The hydrogen electrode was mainly responsible to enhanced polarization resistance during the electrolysis operation. The overpotential of an LSCF oxygen electrode was studied under SOEC and SOFC operating modes [30]. It was shown that the oxygen electrode has higher activity as an SOFC cathode than that as an SOEC anode. Relative to LSF and LSM-YSZ oxygen electrodes, the LSCF oxygen electrode exhibited higher electrochemical performance. However, no studies were focused on
electrochemical stability of the LSCF composite oxygen electrodes for both reversible SOFC and SOEC operation under respective inlet gas conditions. In this study, nano-structured LSCF-YSZ oxygen electrode was prepared by an infiltration process for RSOFCs. Electrochemical performance of the cell was investigated under both fuel cell and steam electrolysis modes. Low steam content fuel gas (3% H2O) and high steam content mixture gas (50% H2O) were introduced for fuel cell and steam electrolysis operation, respectively. Galvanostatic charge/discharge cycles using respective inlet gas composition were performed to evaluate stability of the LSCF-infiltrated YSZ oxygen electrode for reversible fuel cell/steam electrolysis operation. Failure mechanism of the RSOFC was proposed based on post-test SEM analysis.
Experimental Fabrication of porous RSOFC substrates Porous RSOFC substrates consisted of Ni-YSZ hydrogen electrode supports, thin YSZ electrolytes, and porous YSZ layer. The substrates were fabricated by tape casting and hot isostatic pressing techniques, as mentioned before [31]. For fabricating the NiO-YSZ electrode, commercial powders of NiO, YSZ (8 mol% yttria stabilized zirconia, with a median particle size of 0.114 mm) and graphite (Furunda Zirconium Material Co. Ltd., China) pore former were mixed and ballmilled in a weight ratio of 50:50:10 with appropriate amounts of ethanol-butanone solvent, caster oil dispersant, dibutyl phthalate (DBP) plasticizer, and polyvinyl butyral (PVB) binder. A two-stage milling process [31] was adopted. Firstly, NiO and YSZ powders were dispersed in mixed solvent and dispersant via ball-milling for 15e20 h. Secondly, a homogenous mixture including remaining solvent, plasticizer, and binder formed by thermal dissolution was added, and the resultant slurry was then ball milled for another 24 h before adding the last constituent of graphite. For preparing the YSZ electrolyte and the YSZ layer, the same way of tape casting as in the case of NiO-YSZ substrates was used excepted for no addition of NiO and graphite powders. Porous YSZ layers were prepared with higher ratio of pore formers. Through study on different weight ratios of graphite to YSZ, 50 wt% was thought as an optimal proportion. A tape casting machine (DR-150, made in Japan) was used for separately casting the NiO-YSZ substrate, the electrolyte, and the porous YSZ layer. These tapes were dried in air at room temperature for 12 h. One sheet of NiO-YSZ support (~300 mm), YSZ electrolyte layer (20e30 mm), and porous YSZ layer (~70 mm) were stacked together under a vacuum condition and laminated at 20 MPa for 10 min using a hot isostatic press (30Tm Shanxi, China) to form a NiO-YSZ supported tri-layer structure. The resulting tri-layer tape was punched to discs, and then co-sintered at 1300 C for 10 h in order to densify the electrolyte layer, during which graphite was burned out, leaving a well-formed porous YSZ network. The microstructural characteristics of the fired tri-layer tapes were performed using a scanning electron microscope, and the estimated porosity of the porous YSZ layer was 40e50%.
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
Single-cell preparation An infiltration solution of LSCF (La0.6Sr0.4Co0.2Fe0.8O3d) precursor was prepared by dissolving stoichiometric amounts of La(NO3)3$6H2O, Sr(NO3)2, Co(NO3)2$6H2O and Fe(NO3)3$9H2O into deionized water and ethanol [4]. Ethanol has been proved to be an effective additive to lower surface tension of the infiltration solution on backbone [23]. Glycol was used as chelating agent to form LSCF perovskite phase with small average grain size at low firing temperature [32]. Infiltration was carried out by inserting the LSCF precursor solution into the pre-treated porous YSZ layer. The infiltrated samples were dried at room temperature, and then fired at 450 C for 1 h to decompose the nitrate. In order to increase the LSCF loading into the porous layer, repeated infiltration was performed, followed by firing at 450 C for 1 h after each infiltration. The LSCF infiltrated YSZ electrode was eventually sintered at 900 C for 2 h to prepare the NiO-YSZ/YSZ/LSCF-YSZ RSOFCs with an LSCF loading amount of ~40%. Study by Liu et al. [33] has investigated the effect of different catalyst (Sm0.5Sr0.5CoO3d, SSC) loading into LSCF backbone on electrode polarization resistances. Generally, high electrode performance was achieved with high catalyst loading weight ratio (~40e50%) in the infiltration process [31].
14073
Results and discussion XRD characterization of LSCF-infiltrated oxygen electrode Fig. 1 shows XRD patterns of LSCF-infiltrated YSZ oxygen electrode sintered at 900 C. For reference, the XRD diffractions of porous YSZ matrix and pure perovskite LSCF powders are also displayed in Fig. 1. In comparison with XRD patterns of YSZ matrix and pure perovskite LSCF, the XRD pattern of LSCF-infiltrated YSZ indicates that LSCF perovskite phase can be formed in porous YSZ layer from the infiltration precursor solution after firing at 900 C. No impurity phases were detected, which indicated that reactions between LSCF and YSZ can be avoided at this temperature. No reactions between LSCF and YSZ at a calcination temperature of 900 C have been demonstrated by Adijanto et al., and lanthanum and strontium zirconates were produced at the interface and over 1100 C [34]. In infiltration process, the processing temperature required for formation of the desired crystal structure plays a key role in the size and morphology of the nanoparticles [35]. The phase formation of LSCF nano-particles and their chemical compatibility with YSZ backbone are examined at 900 C via XRD analysis. The LSCF-YSZ oxygen electrode is therefore of high phase-purity.
Characterization of RSOFCs Microscopy of RSOFCs Silver paste (Ag ink, Beijing, China) was applied as a current collector onto electrode surfaces. The RSOFCs were mounted on an alumina tube, sealed using ceramic paste (Aramco-552, USA) as sealant, and tested in a temperature-controlled tube furnace. The area of electrolyte and hydrogen electrode was about 1.5 cm2 and the external area of oxygen electrode was 0.2 cm2. Electrochemical performance of the cell was measured from 700 C to 800 C under both fuel cell and steam electrolysis modes. For fuel cell testing, 80 sccm (standard cubic centimeters per minute) flow rate of humidified hydrogen gas (3% H2O þ 97% H2, volume fraction) was fed to the hydrogen electrode, while the oxygen electrode was exposed to ambient air. For steam electrolysis operation, 50% H2O, 25% H2 and 25% Ar were introduced into the Ni-based hydrogen electrode. Similarly, the oxygen electrode was left open to atmospheric environment. Polarization curves and electrochemical impedance spectra (EIS) under open circuit were conducted using an IM6 Electrochemical Workstation (ZAHNER, Germany). For EIS test, the frequency range was from 100 mHz to 100 kHz, and the AC amplitude was 20 mV. In addition, the crystallographic phase of the infiltrated LSCF was verified by XRD analysis using a high power X-ray diffractometer (XRD, PANalytical X0 Pert PRO, Netherlands) with CuKa radiation. To understand the operational stability of the RSOFC under SOFC and SOEC modes, repeated fuel cell and steam electrolysis cycles were galvanostatically measured via an Arbin test system (Arbin MSTAT4) at 750 C, meanwhile the variation of charge/discharge voltage with time at a constant current density of ±600 mA cm2 was recorded. The morphology of the RSOFCs and LSCF infiltrated oxygen electrode as prepared and after galvanostatic polarization was characterized using a scanning electron microscope (SEM, JEOL, JSM 6700F).
Fig. 2 shows the SEM micrographs of the fractured crosssection of a tri-layer NiO-YSZ/YSZ/LSCF-YSZ cell. The cell was composed of a porous NiO-YSZ hydrogen electrode support (~300 mm), a dense YSZ electrolyte (~20 mm), and a porous LSCF-infiltrated YSZ oxygen electrode (~70 mm). It indicated that the co-sintering at 1300 C for 10 h forms dense YSZ electrolyte and porous YSZ electrode backbone. Estimated porosity of the porous YSZ layer was 40e50% in our previous study [31]. Both the highly porous hydrogen and oxygen electrodes are in good contact with the dense electrolyte. SEM
Fig. 1 e X-ray diffraction patterns of LSCF-YSZ oxygen electrode prepared by LSCF infiltration method after sintering at 900 C, porous YSZ matrix, and pure perovskite LSCF powders.
14074
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
the nano-sized LSCF particles were uniformly distributed on the surface of the porous YSZ structure via in-situ calcination at 900 C. Fine microstructure of formed particles is beneficial for increasing the active area of TPB (triple phase boundary). To control the morphology and microstructure of infiltrationformed phase, surface tension of the precursor solution was modified by wetting properties [23]. Water was an excellent solvent for most metal nitrate precursors due to its high polarity [36], however the aqueous solution exhibits inferior wetting properties on perovskite electrodes [37], due to the high surface tension. Ethanol has been proven to be an effective additive to nitrate precursor solution to lower its surface tension, resulted into uniform and continuous coating formation [23]. The addition of ethanol into the aqueous solution remarkably improves the wetting characteristic.
Fuel cell evaluation Fig. 3 shows initial electrochemical characterization of LSCFinfiltrated Ni-YSZ/YSZ/LSCF-YSZ button cell under fuel cell mode. The typical voltage and power density as a function of current density at 800 Ce700 C are shown in Fig. 3 (a). For the fuel cell operation, 97% H2 and 3% H2O at a flow rate of 80 sccm
Fig. 2 e SEM micrographs of fractured cross-sections of (a) LSCF-infiltrated NiO-YSZ/YSZ/LSCF-YSZ RSOFC, (b) porous YSZ backbone, and (c) LSCF-infiltrated YSZ oxygen electrode.
micrographs of the porous YSZ structure before and after LSCF-infiltration treatment are displayed in Fig. 2 (a) and (b). Prior to the infiltration, the porous YSZ matrix was well sintered, forming a rigid three-dimensional network. The unmodified YSZ backbone has clean surface and cleanly visible grain boundaries. After infiltration of the precursor solution,
Fig. 3 e (a) Voltage and power density vs current density curves, and (b) Nyquist electrochemical impedance spectra (EIS) plots at open circuit for Ni-YSZ/YSZ/LSCF-YSZ cell under fuel cell mode.
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
were used as fuels for a single cell. The open circuit voltage (OCV) is 1.072 V, 1.099 V, and 1.108 V at the temperature from 800 C to 700 C, respectively, indicating a good sealing was achieved for the test of the cells. These measured values are slightly lower than theoretical OCVs of 1.103 V (800 C), 1.111 V (750 C), and 1.112 V (700 C), which were predicted from Nernst equation [38]. The maximum power density decreases from 900 mW cm2 at 800 C to 640 mW cm2 at 700 C. Chang et al. [39] have investigated that NiO-YSZ anode supported SOFCs with screen-printed LSCF composite cathode exhibited a peak power density of 409 mW cm2 at 800 C. It indicated that SOFCs with LSCF infiltrated cathode possessed a better electrocatalytic activity and electrochemical performance [19]. Nyquist electrochemical impedance spectra (EIS) plots for the fuel cell at OCV are displayed in Fig. 3 (b). The EIS plots were composed of a high-frequency arc and a low-frequency arc. The former is usually attributed to charge transfer in composite electrode and the latter to mass transport [40]. With increasing temperature, a pronounced increase in the high-frequency arc was observed, whereas the low frequency arc has low temperature dependence. The ohmic resistance RU obtained from the intersection of the impedance curve and X-axis at high frequency refers to the sum of the electrolyte resistance of ionic transfer, lead resistance of electronic transfer, and electrode/electrolyte contact resistance [39], wherein the lead resistance can be negligible using silver wire as current collectors. The polarization resistance Rp, determined from the differences between high- and low-frequency intercepts on EIS is attributed to mass transport properties of gas and charge transfer resistance, which includes intrinsically electro-catalytic characteristic of the composite electrode materials, ionic transport, TPB length, and grain sizes of electrolyte and electrocatalyst [41]. It has been shown in Fig. 3 (b) that the RU values are 0.24 U cm2, 0.26 U cm2, and 0.29 U cm2 at 800 Ce700 C, respectively. The Rp values are 0.21 U cm2, 0.26 U cm2, and 0.34 U cm2 at the temperatures from 800 C to 700 C, respectively. The infiltrated LSCF cathodes generally exhibited lower Rp values than LSCF prepared by co-firing of powder and screen printing technique [19,42], due mainly to an increase in surface area from the nanoparticles [37].
14075
electrolysis current density increases, which indicates hydrogen production is improved. During steam electrolysis, oxygen gas is firstly adsorbed on the electrode surface and reduced at the TPB sites to produce oxygen ions and consume electrons. The oxygen ions are then transported through oxygen ion conductor such as YSZ to hydrogen electrodes [44]. It has been reported that the principal electrode polarization losses were usually ascribed to the generation and transport of oxygen ions within the porous oxygen electrode structure [45]. Typical Nyquist electrochemical impedance spectra at open circuits for the cells under the steam electrolysis mode are shown in Fig. 4 (b). The total cell resistance was 0.48 U cm2 and 0.33 U cm2 at 700 C and 800 C, respectively. The RU values are 0.24 U cm2, 0.26 U cm2 and 0.28 U cm2 at 800 Ce700 C, respectively, which are similar to those obtained under fuel cell mode. The Rp values are 0.2 U cm2 at 700 C and 0.08 U cm2 at 800 C. The electrode polarization resistance is mainly attributed to the charge transfer reaction at the electrode/ electrolyte interface, the adsorption/desorption, and diffusion inside the porous electrode [46].
Fuel cell/steam electrolysis cycles To evaluate the performance durability of the LSCF-infiltrated YSZ oxygen electrode under both fuel cell and steam
Steam electrolysis performance After initial evaluation in the fuel cell mode, the current direction was changed for operation in steam electrolysis mode. Fig. 4 shows steam electrolysis performance of the Ni-YSZ/ YSZ/LSCF-YSZ cells at 800 Ce700 C. For steam electrolysis operation, 50% H2O, 25% H2 and 25% Ar at a total flow rate of 150 sccm were introduced into the hydrogen electrode, and the oxygen electrode was left in air atmosphere. Current vs voltage curves taken as function of temperature are displayed in Fig. 4 (a). Measured OCV values of the cell in the steam electrolysis operation are 0.919 V, 0.937 V, and 0.956 V at the temperature of 800 C, 750 C, and 700 C, respectively. The measured OCVs are influenced by steam/hydrogen ratio and temperature, as expected from the Nernst equation [43]. The current densities of the cells at an electrolysis voltage of 1.3 V were 1.14 A cm2, 0.98 A cm2, and 0.72 A cm2 within the temperature range of 800 Ce700 C, respectively. With increasing the temperature at the same voltage, the
Fig. 4 e (a) Voltage vs current density curves, and (b) Nyquist EIS plots at open circuit for Ni-YSZ/YSZ/LSCF-YSZ cells under steam electrolysis mode.
14076
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
electrolysis modes, the Ni-YSZ/YSZ/LSCF-YSZ cells were subjected to galvanostatic polarization at ±600 mA cm2 and 750 C. Repeated charge/discharge cycles (1 h of steam electrolysis and 1 h of fuel cell each) under respective operational condition are shown in Fig. 5. During the 8 cycles, the discharge voltage was gradually decreased from 0.82 V to 0.78 V and the charge voltage increased from 1.19 V to 1.23 V. The corresponding voltage degradation of the cell is 3.4% and 4.9% for fuel cell mode and electrolysis mode, respectively. Marina et al. [47] studied electrode performance in reversible solid oxide fuel cells. They observed that the electrodes exhibited generally higher losses in the SOEC mode as compared to the SOFC mode. Investigations by Hauch et al. [7] showed that cells in SOEC mode had much higher degradation than those in SOFC mode. No reports on RSOFCs in literatures include repeated charge/discharge cycles under respective fuel gas/steam electrolysis atmosphere for evaluating the performance durability. By altering inlet gas mixture to the hydrogen electrode, the fuel cell and steam electrolysis operations were alternatively achieved for reversibility evaluation. Furthermore, it is necessary to find evidence for improving the reversible durability of the cell.
Microstructural characterization of pre/post-test cells The electrochemical performance of the cells is closely related to their microstructure. For investigating degradation mechanism of the RSOFCs after repeated discharge/charge cycles, post-test microstructure of the infiltrated LSCF-YSZ oxygen electrode is shown in Fig. 6 (b). An SEM image of as-prepared LSCF-infiltrated YSZ oxygen electrode is displayed as a reference in Fig. 6 (a). Prior to the polarization, the infiltrated LSCF particles were uniformly distributed on the porous YSZ backbone. After being polarized for 8 discharge/charge cycles, the LSCF particles were dramatically agglomerated. A longterm performance degradation is usually associated with the coarsening of the microstructure [48]. The agglomeration of LSCF particles further leaded to a reduction in both the number of TPBs and electrical conductivity [49].
Fig. 6 e SEM images of LSCF-infiltrated YSZ oxygen electrodes (a) as-prepared, (b) after fuel cell/steam electrolysis cycles.
Agglomeration also decreases the porosity of infiltrated LSCFYSZ electrode, and affects the mass transfer process of the oxygen reduction reaction associated with the adsorption and diffusion of oxygen at the gas/electrode interface and the surface diffusion of the oxygen species [50,51]. These results demonstrate that the agglomeration of LSCF particles was possibly responsible for the performance degradation of the Ni-YSZ/YSZ/LSCF-YSZ RSOFCs.
Conclusions
Fig. 5 e Repeated charge/discharge cycles of Ni-YSZ/YSZ/ LSCF-YSZ at ±600 mA cm¡2 and 750 C in fuel cell/steam electrolysis operation.
By infiltrating LSCF into porous YSZ backbone, composite oxygen electrodes were developed for an RSOFC. The electrochemical performance of the cell was investigated in both fuel cell and steam electrolysis modes using polarization curves and electrochemical impedance spectroscopy. Galvanostatic charge/discharge polarization at ±600 mA cm2 and 750 C showed the cell has voltage degradation rates of 3.4% and 4.9% for fuel cell mode and steam electrolysis mode, respectively. After 8 repeated discharge/charge cycles, microstructure analysis of the post-test cell demonstrated that the degradation was probably due to the agglomeration of infiltrated LSCF particles.
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
Acknowledgments [16]
We are grateful to National Basic Research Program of China (973 Program No. 2012CB215404, No. 2012CB215404), and China Scholarship Council (No. 201306430012) for financial support of this work.
references
[1] Laguna-Bercero MA, Kinadjan N, Sayers R, Shinawi HE, Greaves C, Skinner SJ. Performance of La2xSrxCo0.5Ni0.5O4±d as an oxygen electrode for solid oxide reversible cells. Fuel Cells 2011;11:102e7. [2] Nguyen VN, Fang QP, Packbier U, Blum L. Long-term tests of a Ju¨lich planar short stack with reversible solid oxide cells in both fuel cell and electrolysis modes. Int J Hydrogen Energy 2013;38(11):4281e90. [3] Ibrahim H, Ilinca A, Perron J. Energy storage systemsdcharacteristics and comparisons. Renew Sustain Energy Rev 2008;12:1221e50. [4] Chen KF, Ai N, Jiang SP. Reasons for the high stability of nano-structured (La, Sr)MnO3 infiltrated Y2O3eZrO2 composite oxygen electrodes of solid oxide electrolysis cells. Electrochem Commun 2012;19:119e22. [5] Liang MD, Yu B, Wen MF, Chen J, Xu JM, Zhai YC. Preparation of LSM-YSZ composite powder for anode of solid oxide electrolysis cell and its activation mechanism. J Power Sources 2009;190:341e5. € nstrup A, Graule T, Menzler NH, Sitte W, [6] Ried P, Lorenz C, Bro et al. Processing of YSZ screen printing pastes and the characterization of the electrolyte layers for anode supported SOFC. J Eur Ceram Soc 2008;28:1801e8. [7] Hauch A, Jensen SH, Ramousse S, Mogensen M. Performance and durability of solid oxide electrolysis cells. J Electrochem Soc 2006;153:A1741e7. [8] Chen KF, Ai N, Jiang SP. Performance and stability of (La, Sr) MnO3eY2O3eZrO2 composite oxygen electrodes under solid oxide electrolysis cell operation conditions. Int J Hydrogen Energy 2012;37:10517e25. [9] Kim-Lohsoontorn P, Brett DJL, Laosiripojana N, Kime YM, Bae JM. Performance of solid oxide electrolysis cells based on composite La0.8Sr0.2MnO3d-yttria stabilized zirconia and Ba0.5Sr0.5Co0.8Fe0.2O3d oxygen electrodes. Int J Hydrogen Energy 2010;35:3958e66. [10] Virkar AV. Mechanism of oxygen electrode delamination in solid oxide electrolyzer cells. Int J Hydrogen Energy 2010;35:9527e43. [11] Li N, Keane M, Mahapatra MK, Singh P. Mitigation of the delamination of LSM anode in solid oxide electrolysis cells using manganese-modified YSZ. Int J Hydrogen Energy 2013;38:6298e303. [12] Keane M, Mahapatra MK, Verma A, Singh P. LSM-YSZ interactions and anode delamination in solid oxide electrolysis cells. Int J Hydrogen Energy 2012;37:16776e85. [13] Kostogloudis GC, Tsiniarakis G, Ftikos C. Chemical reactivity of perovskite oxide SOFC cathodes and yttria stabilized zirconia. Solid State Ionics 2000;135:529e35. [14] Esquirol A, Brandon NP, Kilner JA, Mogensen M. Electrochemical characterization of La0.6Sr0.4Co0.2Fe0.8O3 cathodes for intermediate-temperature SOFCs. J Electrochem Soc 2004;151:A1847e55. [15] Qiu L, Ichikawa T, Hirano A, Imanishi N, Takeda Y. Ln1xSrxCo1yFeyO3d (Ln ¼ Pr, Nd, Gd; x ¼ 0.2, 0.3) for the
[17] [18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
14077
electrodes of solid oxide fuel cells. Solid State Ionics 2003;158:55e65. Jiang SP. Nanoscale and nano-structured electrodes of solid oxide fuel cells by infiltration: advances and challenges. Int J Hydrogen Energy 2012;37:449e70. Vohs JM, Gorte RJ. High-performance SOFC cathodes prepared by infiltration. Adv Mater 2009;21:943e56. Jiang ZY, Xia CR, Chen FL. Nano-structured composite cathodes for intermediate-temperature solid oxide fuel cells via an infiltration/impregnation technique. Electrochim Acta 2010;55:3595e605. Chen J, Liang FL, Liu LN, Jiang SP, Chi B, Pu J, et al. Nanostructured (La, Sr)(Co, Fe)O3 þ YSZ composite cathodes for intermediate temperature solid oxide fuel cells. J Power Sources 2008;183:586e9. Shah M, Barnett S. Solid oxide fuel cell cathodes by infiltration of La0.6Sr0.4Co0.2Fe0.8O3d into Gd-doped ceria. Solid State Ionics 2008;179:2059e64. Nie LF, Liu MF, Zhang YJ, Liu ML. La0.6Sr0.4Co0.2Fe0.8O3d cathodes infiltrated with samarium-doped cerium oxide for solid oxide fuel cells. J Power Sources 2010;195:4704e8. Ding D, Li XX, Lai SYX, Geides K, Liu ML. Enhancing SOFC cathode performance by surface modification through infiltration. Energy Environ Sci 2014;7:552e75. Lou XY, Liu Z, Wang SZ, Xiu YH, Wong CP, Liu ML. Controlling the morphology and uniformity of a catalystinfiltrated cathode for solid oxide fuel cells by tuning wetting property. J Power Sources 2010;195:419e24. Lynch ME, Yang L, Qin WT, Choi J-J, Liu MF, Blinn K, et al. Enhancement of La0.6Sr0.4Co0.2Fe0.8O3d durability and surface electrocatalytic activity by La0.85Sr0.15MnO3±d investigated using a new test electrode platform. Energy Environ Sci 2011;4:2249e58. Ding D, Liu MF, Liu ZB, Li XX, Blinn K, Zhu ZB, et al. Efficient electro-catalysts for enhancing surface activity and stability of SOFC cathodes. Adv Energy Mater 2013;3:1149e54. Yoon BY, Bae J. Characteristics of nano La0.6Sr0.4Co0.2Fe0.8O3d-infiltrated La0.8Sr0.2Ga0.8Mg0.2O3d scaffold cathode for enhanced oxygen reduction. Int J Hydrogen Energy 2013;38:13399e407. Tao Y, Nishino H, Ashidate S, Kokubo H, Watanabe M, Uchida H. Polarization properties of La0.6Sr0.4Co0.2Fe0.8O3based double layer-type oxygen electrodes for reversible SOFCs. Electrochim Acta 2009;54:3309e15. Hjalmarsson P, Sun XF, Liu YL, Chen M. Influence of the oxygen electrode and inter-diffusion barrier on the degradation of solid oxide electrolysis cells. J Power Sources 2013;223:349e57. Schiller G, Anser A, Lang M, Patz O. High temperature water electrolysis using metal supported solid oxide electrolyser cells (SOEC). J Appl Electrochem 2008;39:293e301. Kim-Lohsoontorn P, Bae J. Electrochemical performance of solid oxide electrolysis cell electrodes under hightemperature coelectrolysis of steam and carbon dioxide. J Power Sources 2011;196:7161e8. Liu Z, Zheng ZW, Han MF, Liu ML. High performance solid oxide fuel cells based on tri-layer yttria-stabilized zirconia by low temperature sintering process. J Power Sources 2010;195:7230e3. Zhu XB, Ding D, Li YQ, Lu¨ Z, Su WH, Zhen L. Development of La0.6Sr0.4Co0.2Fe0.8O3d cathode with an improved stability via La0.8Sr0.2MnO3-film impregnation. Int J Hydrogen Energy 2013;38:5375e82. Liu MF, Ding D, Blinn K, Li XX, Nie LF, Liu ML. Enhanced performance of LSCF cathode through surface modification. Int J Hydrogen Energy 2012;37:8613e20. Adijanto L, Ku¨ngas R, Bidrawn F, Gorte RJ, Vohs JM. Stability and performance of infiltrated La0.8Sr0.2CoxFe1xO3
14078
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
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 e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 7 1 e1 4 0 7 8
electrodes with and without Sm0.2Ce0.8O1.9 interlayers. J Power Sources 2011;196:5797e802. Lee SI, Kim J, Son JW, Lee JH, Kim BK, Je HJ, et al. High performance air electrode for solid oxide regenerative fuel cells fabricated by infiltration of nano-catalysts. J Power Sources 2014;250:15e20. Kim G, Lee S, Shin JY, Corre G, Irvine JTS, Vohs JM, et al. Investigation of the structural and catalytic requirements for high-performance SOFC anodes formed by infiltration of LSCM. Electrochem Solid State Lett 2009;12:B48e52. Lou XY, Wang SZ, Liu Z, Yang L, Liu ML. Improving La0.6Sr0.4Co0.2Fe0.8O3d cathode performance by infiltration of a Sm0.5Sr0.5CoO3d coating. Solid State Ionics 2009;80:1285e9. Laguna-Bercero MA, Kilner JA, Skinner SJ. Development of oxygen electrodes for reversible solid oxide fuel cells with scandia stabilized zirconia electrolytes. Solid State Ionics 2011;192:501e4. Chang YC, Lee MC, Kao WX, Wang CH, Lin TN, Chang JC. Fabrication and evaluation of electrochemical characteristics of the composite cathode layers for the anode-supported solid-oxide fuel cells. J Taiwan Inst Chem Eng 2011;42:775e82. Murray EP, Sever MJ, Barnett SA. Electrochemical performance of (La, Sr)(Co, Fe)O3e(Ce, Gd)O3 composite cathodes. Solid State Ionics 2002;48:27e34. Serra JM, Vert VB, Bu¨chler O, Meulenberg WA, Buchkremer HP. IT-SOFC supported on mixed oxygen ionicelectronic conducting composites. Chem Mater 2008;20:3867e75. Jun SH, Uhm YR, Song RH, Rhee CK. Synthesis and properties of Ag nanoparticles attached La0.6Sr0.4Co0.3Fe0.7O3d. Curr Appl Phys 2011;11:S305e8.
[43] Brisse A, Schefold J, Zahid M. High temperature water electrolysis in solid oxide cells. Int J Hydrogen Energy 2008;33:5357e82. [44] Song HS, Kim WH, Hyun SH, Moon J, Kim J, Lee HW. Effect of starting particulate materials on microstructure and cathodic performance of nanoporous LSM-YSZ composite cathodes. J Power Sources 2007;167:258e64. [45] Adler SB, Lane JA, Steele BCH. Electrode kinetics of porous mixed-conducting oxygen electrodes. J Electrochem Soc 1996;143:3554e64. [46] Zhan ZL, Zhao L. Electrochemical reduction of CO2 in solid oxide electrolysis cells. J Power Sources 2010;195:7250e4. [47] Marina OA, Pederson LR, Williams MC, Coffey GW, Meinhardt KD, Nguyen CD, et al. Electrode performance in reversible solid oxide fuel cells. J Electrochem Soc 2007;154:452e9. [48] Wang WS, Gross MD, Vohs JM, Gorte RJ. The stability of LSFYSZ electrodes prepared by infiltration. J Electrochem Soc 2007;154:B439e45. [49] Shah M, Voorhees PW, Barnett SA. Time-dependent performance changes in LSCF-infiltrated SOFC cathodes: the role of nano-particle coarsening. Solid State Ionics 2011;187:64e7. [50] Liu YH, Wang FZ, Chi B, Pu J, Li J, Jiang SP. A stability study of impregnated LSCF-GDC composite cathodes of solid oxide fuel cells. J Alloy Compd 2013;578:37e43. [51] Liang FL, Chen J, Chi B, Pu J, Jiang SP, Li J. Redox behavior of supported Pd particles and its effect on oxygen reduction reaction in intermediate temperature solid oxide fuel cells. J Power Sources 2011;196:153e8.