La0.8Sr0.2FeO3−δ as Fuel Electrode for Solid Oxide Reversible Cells

Article pubs.acs.org/JPCC

La0.8Sr0.2FeO3−δ as Fuel Electrode for Solid Oxide Reversible Cells Using LaGaO3‑Based Oxide Electrolyte Kohei Hosoi,† Hidehisa Hagiwara,†,‡ Shintaro Ida,†,‡ and Tatsumi Ishihara*,†,‡ †

Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan

‡

ABSTRACT: Activity of La0.8Sr0.2FeO3 (LSF) to the fuel electrode reaction in solid oxide reversible cells (SORCs) was investigated by using La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) and Ba0.6La0.4CoO3 (BLC) as electrolyte and air electrode, respectively. In electrolysis mode (SOEC), the LSF electrode exhibited small overpotential under the atmosphere without H2 cofeeding; the current densities reached −1.42, −0.92, and −0.36 A/cm2 at 1.4 V at 900, 800, and 700 °C, respectively, and the H2 formation rate is well agreed with that estimated by Faraday’s law. On the other hand, in the SOEC−SOFC reversible mode with the gas composition of 20% steam/20%H2/60% Ar, the maximum power densities of 0.42, 0.28, and 0.11 W/cm2 were achieved at 900, 800, and 700 °C, respectively. In addition, the cyclic reversible operation was also investigated at 800 °C, and it was found that the cell showed high stability over 30 cycles. DC polarization measurement suggests that the exchange current density of LSF is 14 mA/cm2 at 700 °C, which is almost the same as that of NiYSZ reported. X-ray diffraction (XRD) measurement and scanning electron microscopy (SEM) observation after the reversible measurement suggest that LSF is highly stable under SOEC−SOFC cyclic operation conditions. Therefore, LSF is promising as the fuel electrode for SORCs, although the conductivity is not sufficiently high as an electrode.

1. INTRODUCTION Solid oxide fuel cells (SOFCs) have been attracting much attention as a power generator recently because of their high energy conversion efficiency.1 Another advantage of SOFCs is a flexibility of fuel; i.e., various hydrocarbons can be directly used as fuel. However, practically, it is difficult to use dry hydrocarbons directly on the present SOFC systems due to the serious formation of carbon.2,3 Therefore, for the present SOFC systems, the fuel is limited to hydrogen or CH4, which is produced from various hydrocarbons by using an external prereformer. On the other hand, the reverse operation of SOFCs, so-called solid oxide electrolysis cells (SOECs), can efficiently produce hydrogen by using heat energy from hightemperature steam;4,5 the H2O molecule at the fuel electrode side is electrolyzed into a H2 molecule and O2− ion, which transports to the oxygen electrode through the electrolyte and is oxidized into O2. The material configurations of SOEC are almost the same as those of SOFC. Therefore, if SOFC and SOEC can be reversibly operated by using the same cell, then the cell can reversibly convert the energy between hydrogen and electricity; i.e., the cell can be used as an energy storage system like a battery.6 This energy storage system using SORCs is suitable for large scale, and the high efficiency is expected. Ni-based composites, which have been widely used in the conventional anode of SOFC, can be also used as the cathode © XXXX American Chemical Society

of SOEC because of its excellent catalytic activity and high electronic conductivity. However, the conventional Ni-based cermet has disadvantages such as easy reoxidation and low stability, in particular, the rapid degradation under SOEC condition due to the poor tolerance against oxidation with steam.7 In SOEC mode, it is necessary to introduce a high partial pressure of steam; that is, the cathode is constantly exposed to a high oxygen partial pressure, resulting in the oxidation and also aggregation of Ni-based anode. As a result, the supply of a certain amount of hydrogen to the anode is required to avoid the reoxidation of Ni, which requires a complicated and high cost refeeding system. Moreover, the stable reversible operation of SOFC and SOEC mode has limitations on steam partial pressure when Ni is used for the fuel electrode.8 For resolving the stability issues, application of oxide for the electrode has been investigated as the alternative cathode of SOEC.9−14 The oxide cathodes have several advantages such as good stability against the redox cycles and can also be used without cofeeding H2. However, the oxide electrodes also have several disadvantages such as the low catalytic activity and the Special Issue: Kohei Uosaki Festschrift Received: December 31, 2015 Revised: April 15, 2016

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Under the steam electrolysis mode, steam diluted with Ar (steam:Ar = 20:80) was fed to the cathode side, and air (100 mL/min) was fed to the anode side. The total gas flow rate was kept at 100 mL/min in both sides. After the steam electrolysis measurement, the cathode gas was changed to 20% steam/20% H2/60% Ar (100 mL/min) for the reversible operation. Additionally, the cyclic reversible measurements were also operated at 800 °C under the same gas condition. In this case, the negative current of −0.82 A/cm2 or positive current of 0.51 A/cm2 applied for SOEC and SOFC operation, and the cell voltage was monitored. For the comparison, a Ni-SDC (60/40 wt %) hydrogen electrode cell was also prepared. The Ni/SDC cathode was reduced by H2 at 900 °C before the measurements, and steam with 1% H2−Ar (steam:H2:Ar = 20:1:79) was fed to the cathode side. Air was also fed to the anode side, and the total gas flow rate was always kept at 100 mL/min in both sides. The current across the cell was controlled by using a Potentiostat/galvanostat (HAL-3001, Hokuto Denko Corp.), and the terminal voltage was measured with a digital multimeter (R6451A, Advantest). Steam was generated by evaporating water which is fed with a micropump (LC-20AT, Shimadzu) and evaporated at 150 °C. The formation rate of H2 was analyzed with a gas chromatograph with a thermal conductive detector (GC-8A, Shimadzu). Impedance measurement was performed with the impedance/gain-phase analyzer (Solartron type 1260) combined with the electrochemical interface (Solartron type 1287). The microstructures of the LSF electrode and cross sectional of the cathode/electrolyte were observed by a field emission scanning electron microscope (FE-SEM, Versa 3D, FEI). X-ray photoelectron spectroscopy (XPS) measurement of LSF was performed after electrolysis (Shimazu, AXIS-165, Al line with monoclometer (5 mA, 12 kV)) at 1.6 V and 600 °C for 1 h in 20% steam/80% Ar atmosphere. The binding energy of the peaks was compensated by adjusting a C 1s peak to 284.6 eV.

low electronic conductivity. For these reasons, the electrolysis cell using the oxide cathodes usually achieved quite low value up to now. Although some perovskite-based materials such as La1−xSrxCr1−yMnyO3−δ (LSCM)9−11 and Sr2Fe1.5Mn0.5O6−δ (SFM)12 showed relatively high performance among the oxide cathodes reported, there are no reports on reversible operation. Typically, ferrite-based perovskite oxides such as LSF and La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) are widely used as the SOFC cathode because of their high mixed electronic−ionic conductivity and catalytic property.15,16 On the other hand, recently, Fleig et al. reported that La0.6Sr0.4FeO3−δ (LSF) can be used as an SOEC cathode,17,18 and the superior activity of LSF was explained by the formation of metallic Fe on the LSF surface under cathodic polarization conditions, which is active to water splitting. However, it is still not revealed whether such a ferrite-based oxide can be used as a hydrogen electrode for SOEC−SOFC reversible operation, especially the durability of the cyclic mode. In this paper, we have chosen Sr-doped LaFeO 3 (La0.8Sr0.2FeO3−δ, LSF) as a new fuel cathode of SORCs. Steam electrolysis and reversible operation with LSF hydrogen electrode were investigated by using LSGM electrolyte. Furthermore, the stability and activity in SOFC−SOEC cyclic operation were also investigated.

2. EXPERIMENTAL SECTION La0.8Sr0.2FeO3−δ (LSF) powder was prepared by the following method: La(NO3)3·6H2O, Sr(NO3), and Fe(NO3)3·9H2O were dissolved in deionized water. After drying the nitrate solution under constant stirring, the resultant powder was precalcined in a ventilated furnace at 400 °C for 2 h and was calcined again in a furnace at 900 °C for 6 h. After cooling, the obtained powder was ground using Al2O3 mortar and was further calcined at 1200 °C for 6 h. Ba0.6La0.4CoO3−δ (BLC) powder was prepared using a conventional solid-state reaction method. The stoichiometric amounts of BaCO3, La2O3, and Co3O4 were mixed using Al2O3 mortar and precalcined at 900 °C for 3 h, and after mixing again by using Al2O3 mortar, the powder mixture was further calcined at 1200 °C for 6 h. La0.9Sr0.1Ga0.8Mg0.2O3−δ (LSGM) electrolyte was also prepared by the conventional solid-state reaction.19 The stoichiometric amounts of La2O3, SrCO3, Ga2O3, and MgO were mixed using Al2O3 mortar. The powder mixture was precalcined at 1000 °C for 6 h and then isostatically pressed into a disk (2.0 cm in diameter) at 275 MPa for 30 min. The disk was sintered at 1500 °C for 6 h in air and polished to 300 μm in thickness. LSF or Ni-SDC and BLC powder were grinded again with a planetary ball-mill mixer (Pulverisette 7, Fritsch), and the obtained powder was further mixed with ethyl cellulose and hexane for making electrode ink. The LSF and BLC ink was screen-printed on each face of the LSGM disk and fired at 1100 °C for 1 h (the effective electrode area: 0.2 cm2). Pt electrode was used as a reference electrode and set close to the air electrode. The details of a single cell were reported in our previous study.20 To conform the stability of LSF in reduction atmosphere, LSF powder was reduced at 900 °C for 10 h under 20% steam/20% H2/60% Ar, and the crystal structure was identified by XRD measurement using a Cu Kα line (Rigaku, RINT type 2500). Power generation and electrolysis performances were measured by using a four-probe method. Pt mesh was used for a current collector, and a Pt line was used for a lead line.

3. RESULTS AND DISCUSSIONS Figure 1 shows the I−V curves of the cell using LSF for cathode in the electrolysis mode when 20% steam/80% Ar was fed. In this figure, electrolysis curves of the cell using a conventional Ni-SDC cathode under 20% steam/1% H2/79% Ar atmosphere were also shown for comparison. In the case of LSF cathode,

Figure 1. I−V curves of LSF/LSGM/BLC and Ni-SDC/LSGM/BLC cell in electrolysis mode at different temperatures. Feed gas is 20% steam/80% Ar for LSF and 20% steam/1% H2/79% Ar for Ni-SDC. B

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low frequency RL relates with the mass transfer processes;’ therefore, this indicates that the diffusion resistance is decreased with increasing DC potential, and this tendency is similar to the behavior of the impedance results using oxide electrode in SOEC mode reported by Yue et al.21 Figure 5 shows the I−V curves of the cell with LSF in the reversible operation mode (i.e., cycle operation of SOFC and SOEC) under the 20% steam/20% H2/60% Ar atmosphere. The theoretical OCV values from the Nernst equation are 909, 942, and 974 mV at 900, 800, and 700 °C, respectively. On the other hand, the observed OCVs were 899, 935, and 969 mV at 900, 800, and 700 °C, respectively. Therefore, the observed OCV values are close to the theoretical ones. In SOEC mode, the current densities of −1.45 A/cm2, −0.87 A/cm2, and −0.34 A/cm2 were obtained at 900, 800, and 700 °C, respectively, when 1.4 V was applied to the cell. These currents are almost the same as the results in Figure 1, even though the gas composition was different. Therefore, it seems that the H2 cofeed has a small influence on the SOEC performance, at least, if its hydrogen amount was smaller than 20%. In other words, LSF can be used for the cathode of SOEC even under the condition without H2 cofeeding. On the other hand, in the case of SOFC mode, it can be seen that the cell voltage linearly decreases with the current density, and the maximum power densities were 0.42, 0.28, and 0.11 W/cm2 at 900, 800, and 700 °C, respectively. In our previous study, we investigated Ce0.6Mn0.3Fe0.1O2 (denoted as CMF) for an active anode of SOFC and also cathode of SOEC, and it was found that CMF shows reasonably small fuel electrode overpotential in both operating modes.22 However, in the case of Mn- and Fe-doped ceria (CMF), the maximum power density of 0.31 W/cm2 was achieved at 900 °C;22 namely, the LSF anode showed smaller anodic overpotential than that of CMF in spite of smaller H2 partial pressure in SOFC operation mode. Therefore, LSF is active for cathodic reaction of SOEC and also anodic reaction of SOFC. Additionally, compared with the slope of I−V curves, the slopes of SOFC and SOEC mode were almost the same values at each temperature. This indicates that the overpotential of the cell was small, and the slope of I−V curves was dominated with IR loss for the present cell. This is reasonable because the electrolyte thickness was as thick as 0.3 mm in this study. Figure 6 shows the complex impedance plots of the cell under open-circuit condition when the gas composition of 20% steam/20% H2/60% Ar was fed. The polarization resistance values are similar to that of SOEC mode under applied DC potentials as shown in Figure 4. This result suggests that cathodic activity of LSF is negligibly influenced by cofeeding of H2 and also highly stable even under reducing conditions. Additionally, it can be seen that the main part of the electrode reaction overpotential is the overpotential of LSF fuel electrode, and the resistance of BLC air electrode is always quite small under the experimental conditions. The activity of LSF electrode for SOFC and SOEC was estimated by using static DC polarization method. Figure 7 shows overpotential as a function of current density, the socalled “Tafel plot”, in 3% steam/20% H2/77% Ar at temperature from 900 to 700 °C. Typical polarization curves in Tafel plots were observed for LSF suggesting LSF working as a fuel electrode like a conventional Ni-based cermet electrode. At 900 °C, the slope of the plots (αc transfer coefficient) in electrolysis and fuel cell mode (αa) is almost the same; however, with decreasing temperature, it is seen that αa became

the open-circuit potential of the cell was smaller than 50 mV at each temperature, and the potential was rapidly increased when the current started to flow because of no H2 cofeed. This change in potential was explained by hydrogen formation, and PO2 drastically decreased at the cathode side when electrolysis proceeded. As can be seen, the current densities reached −1.42, −0.92, and −0.36 A/cm2 at 1.4 V at 900, 800, and 700 °C, respectively. On the other hand, the current densities of a NiSDC cell were −1.50, −0.61, and −0.25 A/cm2 at 1.4 V at 900, 800, and 700 °C, respectively. Thus, the cell using LSF cathode showed similar or higher electrolysis current density compared with that of the Ni-based conventional cathode, suggesting that LSF is highly active to SOEC cathodic reaction. H2 formation rate of the cell with LSF cathode was shown in Figure 2 as a function of current density at temperature from

Figure 2. H2 formation rate on the cell using LSF cathode as a function of current density. The straight line represents the H2 formation rate from Faraday’s law.

900 to 700 °C. It can be seen that the observed H2 formation rate at each temperature agreed well with the theoretical one. For example, the observed H2 formation amount at 1.53 A/cm2 was 470.2 μmol/(cm2·min) at 900 °C, whereas that calculated from Faraday’s law is 475.8 μmol/(cm2·min). So the Coulomb efficiency was estimated to be 98.8%. Considering the experimental analytical error by gas chromatography, the LSGM cell using the LSF cathode generates the theoretical amount of H2, suggesting that oxide ion transport number in LSGM under electrolysis is unity and LSF works as cathode. Figure 3 shows the AC impedance plots of LSF cathode without applied potential. As can be seen, the impedance arc of the cathode is significantly larger than that of the anode; in other words, the anode polarization resistance is much smaller than that of the cathode under open-circuit condition. Figure 4 shows the AC impedance plots of LSF cathode at 900 °C under polarized conditions, i.e., DC potential applied between fuel and air electrodes. It can be seen that the polarization resistance was significantly decreased with increasing potential. In order to analyze this impedance change, fitting a simple equivalent circuit (shown in Figure 4) was carried out, and resistances, RH and RL, were estimated from high and low frequency reason semicircles, respectively. Without applied potential, RH and RL are estimated to be 0.38 and 3.58 Ω cm2 (RH:RL = 9.6%:91.4%), respectively. On the other hand, at 1.0 V, the values of RH and RL are 0.18 and 0.08 Ω cm2 (RH:RL = 69.2%:30.8%), respectively. Generally, the C

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Figure 3. AC impedance plots of LSF cell under open-circuit condition in 20% steam/80% Ar atmosphere at 900 (a) and 700 °C (b). (a-2) and (b2) show the total resistance divided into cathode and anode side by using Pt reference electrode at 900 and 700 °C.

Figure 4. AC impedance spectra of LSF cell (total resistance) under open-circuit condition and DC potentials of 1.0 and 1.2 V at 900 °C. (b) is magnified plots of (a). Feed gas composition: 20% steam/80% Ar.

smaller than αc and this means that the LSF is more active to the fuel cell mode than the electrolysis mode. This may be explained by the decreased conductivity of LSF in electrolysis mode because of p-type semiconductivity. However, comparing with Ni-YSZ conventional anode,23 a similar value was observed for αa and αc on LSF suggesting the superior reversibility for hydrogen oxidation and steam reduction. The exchange current density was estimated by the Y axis intercept of a straight line in SOFC and SOEC mode of Tafel plots in Figure 7, and the temperature dependence of the exchange current density was shown in Figure 8 in SOFC and SOEC

operation mode. The activation energies of LSF electrode reaction in SOFC and SOEC were estimated to be 0.07 and 0.15 eV, respectively. It is seen that LSF shows much smaller activation energy to electrochemical oxidation of H2 than that to electrolysis of H2O. In Figure 8, similar plots for Ni-SDC in SOFC mode were also shown under the same condition. The estimated activation energy was 0.71 eV, and the similar value was also reported for the conventional Ni-YSZ (0.91 eV reported by Stimming et al.23 or 0.67 eV by Uchida et al.24) for SOFC operation, although PH2O and PH2 are different. Therefore, the activation energy for fuel electrode reaction on D

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Figure 5. I−V curves of the LSF/LSGM/BLC cell in both the electrolysis (negative current density) and fuel cell (positive current density) mode. The feed gas composition is 20% steam/20% H2/60% Ar.

Figure 7. Overpotential as a function of current density, so-called “Tafel plots”, at temperature from 900 to 700 °C. 3% H2O/20% H2/ 77% Ar was fed for the fuel side.

LSF is much smaller than that of Ni-based cermet electrode. This might suggest that the surface activity to anodic and/or cathodic reaction of LSF is much higher than that of the conventional Ni-based cermet. The reason why LSF shows small activation energy for the fuel electrode reaction in SOFC and SOEC mode could be explained by the large amount of oxygen vacancy which is an active site, and the activity of one

site for the electrochemical oxidation of H2 and reduction of H2O is much higher than that of Ni-SDC. On the other hand, a pre-exponential term in Arrhenius plots of LSF in Figure 8 is much smaller than that of Ni-SDC, and this suggests that the reaction area might be smaller on LSF because of smaller surface area compared with that of Ni-SDC.

Figure 6. AC impedance plots of LSF/LSGM/BLC cell under open-circuit condition in 20% steam/20% H2/60% Ar at 900 (a), 800 (b), and 700 °C (c). The total resistance was divided into hydrogen and air electrode component with Pt reference electrode. E

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Figure 8. Temperature dependence of the exchange current density in SOFC and SOEC mode of LSF and Ni-SDC in 3% H2O/20% H2/77% Ar.

Figure 9 shows the cyclic reversible operation of the cell using LSF fuel electrode at 800 °C. In this case, constant

Figure 10. AC impedance plots of LSF cell (total resistance) without applied potential in 20% steam/20% H2/60% Ar measured before and after 40 cycles at 800 °C.

particles may be aggregated during SOFC or SOEC single operation resulting in the increased overpotential during longterm operation; however, under reversible operation, redox of LSF may have occurred, and small particle size could be sustained by changing PO2. The microstructures of the LSF electrode and cathode/ electrolyte interface after the reversible operation are shown in Figure 11. As expected, no significant aggregation and delamination were observed before and after measurement. Therefore, LSF shows high compatibility with LSGM electrolyte and is stable during cyclic operation of SOFC/SOEC. Figure 12 shows the XRD patterns of the LSF powder before and after reduction in 20% H2O/20% H2/60% Ar atmosphere. The LSF phase still remained after the reduction treatment since no secondary phases were detected by XRD measurement. The oxygen partial pressure in 20% H2O/20% H2/60% Ar feed gas is estimated to be log(PO2/atm) = −16.3, −18.4, and −20.9 from the emf under open-circuit conditions at 900, 800, and 700 °C, respectively. The oxygen nonstoichiometry and stability of La1−xSrxFeO3−δ have been reported by Mizusaki et al.,26,27 and LSF is decomposed significantly under the oxygen partial pressure lower than log(PO2/atm) = ca. −17 at 900 °C since with decreasing temperature the oxide phase became more stable. The oxidation state of LSF after steam electrolysis at 1.6 V, 600 °C, was further studied with XPS. Figure 13 shows the XPS spectra before and after steam electrolysis. Before steam electrolysis, the XPS spectrum mainly consisted of the oxide phase because binding energy (BE) of the Fe 2p3/2 peak was observed at 710 eV. On the other hand, after electrolysis, the Fe 2P3/2 peak was shifted to a lower BE value suggesting that iron was reduced under electrolysis conditions. By using the reported BE of Fe0, Fe2+, and Fe3+ peaks,28 separation of the Fe 2p3/2 peak was performed by using Gaussian curves. The ratio of Fe, Fe2+, and Fe3+ was estimated from the peak area of the fitted results shown in Figure 13 with a dotted line, and the peak area and its ratio were summarized in Table 1. Obviously, before electrolysis, Fe3+ is the major oxidation state of Fe; however, after electrolysis, Fe2+ amount increased to 42% suggesting that a large amount of oxygen vacancy was introduced. Although a small amount of metallic Fe was recognized (less than 3%), XPS measurements suggest that the

Figure 9. Cyclic reversible operation of the cell using LSF fuel electrode at 800 °C. The cell voltages which were recorded at −0.82 and 0.51 A/cm2 constant current applied were plotted as a function of cyclic numbers.

positive and negative current was applied to the cell alternatively and recorded the response potential. The cell voltages were plotted as a function of cyclic numbers. In SOEC mode, the LSF cathode is exposed to more reducing atmosphere compared with that in SOFC anode. In other words, this reversible operation of SOFC/SOEC is the same meaning with a redox durability test. As can be seen, the cell voltage was stable, and the degradation behavior was not observed in both SOEC and SOFC mode up to 40 cycles. Figure 10 shows the complex impedance plots of LSF fuel electrode before and after the cyclic operation under opencircuit condition. Compared with the impedance plots before the measurement, the X-axis intercept at high frequency, which represents ohmic resistance, was negligibly changed by 40 cycle reversible operations. Therefore, it is concluded that the degradation of electrode activity by reversible operation does not occur during the cyclic measurement. Additionally, the overpotential of the LSF fuel electrode is slightly decreased after the cyclic operation measurement. This suggests that the activity of LSF to fuel electrode reaction is slightly increased by reversible operation of cathode and anode. Similar phenomena are also reported for the Ni-YSZ cermet electrode25 and explained by keeping small particles. It is considered that LSF F

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Figure 11. SEM observation results of LSF electrode surface and cross section before (a) and after (b) SOEC and SOFC reversible operation for 40 cycles.

Figure 13. XPS spectra of Fe 2p3/2 of LSF before and after steam electrolysis at 1.6 V, 1 h, and 600 °C. Dashed line is results of peak separation.

Figure 12. XRD patterns of LSF electrode after SOEC and SOFC reversible operation for 40 cycles.

Table 1. Results of Peak Separation of Fe Peaks before and after Electrolysisa

oxidation state of Fe is +2 and +3. These reports well correspond to the results of XRD and SEM observation in this study, and thus it can be concluded that LSF is reasonably active and stable as a fuel electrode of SORCs.

after before a

4. CONCLUSIONS LSF was studied as the oxide hydrogen electrode for the SOEC/SOFC reversible operation cell. In SOEC mode, the LSF electrode showed a small overpotential and also IR loss which was similar to that of the conventional Ni-based electrode. It was found that the H2 formation rate of the LSF electrode cell corresponded well to the Faraday’s law, indicating almost 100% Coulomb efficiency. From impedance analysis, the polarization resistance was largely decreased, especially the low frequency part, with an increase in the DC bias potential. On

Fe3+

Fe2+

Fe0

4479 (55.7%) 7203 (70.7%)

3378 (42.0%) 2991 (29.3%)

181 (2.3%) 0 (0%)

Fe3+: 710.9 eV, Fe2+: 709.4 eV, Fe0: 707.0 eV.28 Peak area (ratio).

the other hand, in SOEC/SOFC reversible operation with the gas composition of 20% steam/20% H 2/60% Ar, the experimental OCV values were close to the theoretical ones, and it showed the maximum power densities of 0.42, 0.28, and 0.11 W/cm2 at 900, 800, and 700 °C, respectively. In addition, the cyclic reversible measurement was operated at 800 °C, and the cell showed good stability. From the XRD pattern and SEM observation, the LSF phase is highly stably, and no serious G

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(16) Sun, C.; Hui, R.; Roller, J. Cathode materials for solid oxide fuel cells: a review. J. Solid State Electrochem. 2010, 14, 1125−44. (17) Walch, G.; Opitz, A. K.; Kogler, S.; Fleig, J. Correlation between hydrogen production rate, current, and electrode overpotential in a solid oxide electrolysis cell with La0.6Sr0.4FeO3‑δ thin-film cathode. Monatsh. Chem. 2014, 145, 1055−61. (18) Opitz, A. K.; Nenning, A.; Rameshan, C.; Rameshan, R.; Blume, Rd.; Hävecker, M.; Knop-Gericke, A.; Rupprechter, G.; Fleig, J.; Klötzer, B. Enhancing electrochemical water-splitting kinetics by polarization-driven formation of near-surface iron(0): An in situ XPS study on perovskite-type electrodes. Angew. Chem., Int. Ed. 2015, 54, 2628−32. (19) Ishihara, T.; Matsuda, H.; Takita, Y. Doped LaGaO3 perovskite type oxide as a new oxide ionic conductor. J. Am. Chem. Soc. 1994, 116, 3801−3803. (20) Ishihara, T.; Jirathiwathanakul, N.; Zhong, H. Intermediate temperature solid oxide electrolysis cell using LaGaO3 based perovskite electrolyte. Energy Environ. Sci. 2010, 3, 665−72. (21) Yue, X.; Irvine, J. T. S. (La,Sr) (Cr,Mn)O3/GDC cathode for high temperature steam electrolysis and steam-carbon dioxide coelectrolysis. Solid State Ionics 2012, 225, 131−35. (22) Ishihara, T.; Shin, T. H.; Vanalabhpatana, P.; Yonemoto, K.; Matsuka, M. Ce0.6(Mn0.3Fe0.1)O2 as an oxidation-tolerant ceramic anode for SOFCs using LaGaO3-based oxide electrolyte. Electrochem. Solid-State Lett. 2010, 13, B95−97. (23) Holtappels, P.; de Haart, L. G. J.; Stimming, U. Reaction of hydrogen/water mixtures on nickel-zirconia cermet electrodes I. DC polarization characteristics. J. Electrochem. Soc. 1999, 146, 1620−1625. (24) Uchida, H.; Yoshida, M.; Watanabe, M. Effects of ionic conductivities of zirconia electrolytes on polarization properties of platinum anodes in solid oxide fuel cells. J. Phys. Chem. 1995, 99, 3282−3287. (25) Graves, C.; Ebbesen, S. D.; Jensen, S. D.; Simonsen, S. B.; Mogensen, M. B. Eliminating degradation in solid oxide electrochemical cells by reversible operation. Nat. Mater. 2015, 14, 239−244. (26) Mizusaki, J.; Yoshihiro, M.; Yamauchi, S.; Fueki, K. Nonstoichiometry and defect structure of the perovskite-type oxides La1‑xSrxFeO3‑δ. J. Solid State Chem. 1985, 58, 257−66. (27) Kuhn, M.; Hashimoto, S.; Sato, K.; Yashiro, K.; Mizusaki, J. Oxygen nonstoichiometry, thermo-chemical stability and lattice expansion of La0.6Sr0.4FeO3‑δ. Solid State Ionics 2011, 195, 7−15. (28) Graat, P. C. J.; Somers, M. A. J. Simultaneous determination of composition and thickness of thin iron-oxide films from XPS Fe 2p spectra. Appl. Surf. Sci. 1996, 100, 36−40.

degradation was observed during the cyclic reversible operation. Therefore, this study reveals that LSF is a promising fuel electrode for solid oxide reversible cell.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81-92-802-2870. Fax: +81-92-802-2871. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported in part by a Grant-in-Aid for Scientific Research(S), No. 24226016 from Ministry of Education, Sports, Culture, Science, and Technology (MEXT), Japan, and New Energy and Industrial Technology Development Organization (NEDO).

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DOI: 10.1021/acs.jpcc.5b12755 J. Phys. Chem. C XXXX, XXX, XXX−XXX