Double substituted Co-free perovskite oxides for quasi-symmetric reversible solid oxide cells (rSOCs)

Introduction Reversible Solid Oxide Cells (rSOCs, or solid oxide reversible cells) are an attractive electrochemical energy conversion technology that can act as both a Solid Oxide Fuel Cell (SOFC) for power generation, and a Solid Oxide Electrolysis Cell (SOEC) for steam electrolysis (1). In the near future, renewable energy sources, such as wind and solar, are set to dominate energy production, however, these suffer from intermittency issues based on weather conditions. Therefore, it is desirable to adjust the supply of fluctuating electricity production from renewable energy sources to match demand using rSOCs, for hydrogen storage in the SOEC mode and for power generation using stored hydrogen in the SOFC mode. This concept is schematically described in Figure 1. The purpose of this study is to examine the degradation of rSOCs using conventional fuel electrode materials, for comparison with alternative fuel electrode materials, which are under development. Experimental Reversible voltage cycle tests simulating rSOC operation were conducted using conventional cell materials: Cermet of Ni- with Sc2O3-doped ZrO2 (ScSZ) for the fuel electrode; dense 200 m thick ScSZ membrane for the electrolyte; and La0.6Sr¬0.4Co0.2Fe0.8O0.3 oxide (LSCF) air electrode with Gd-doped CeO2 (Gd0.1Ce0.9O2) acting as a buffer layer between the ScSZ electrolyte. A Pt-based reference electrode was deposited beside the air electrode, such that the fuel electrode potential was measured as the voltage between the fuel electrode and the reference electrode. The durability of the fuel electrodes were evaluated at an operating temperature of 800C. Air was supplied to the air electrode, while 50%-humidified hydrogen was supplied to the fuel electrode. Reversible operation was investigated by repeatedly changing between the SOFC and SOEC modes by switching the direction of current for up to 1,000 cycles, as shown in Figure 2. The current density in the SOFC and SOEC modes are expressed as positive and negative values, respectively. The current sweep rate was 1.5 mA cm-2 s-1. Current density was varied within ± 0.2 A cm-2, and the fuel electrode potential was measured at the peak current of ± 0.2 A cm-2 for each cycle. Here, the performance degradation of the Ni-cermet fuel electrode was examined by switching between the SOEC mode and the SOFC mode, simulating possible rSOC operation conditions. Results and discussion Figure 3 shows the fuel electrode potential measured at ± 0.2 A cm-2, every 50 cycles. Degradation within 1,000 cycles was calculated to be 30.6%, as the average value of voltage degradation between the SOFC and SOEC modes. Such degradation could be associated with microstructural changes, e.g. due to a redox reaction between Ni and NiO (2), during rSOC operation. Such degradation could be reduced by using redox-tolerant fuel electrode materials, in which redox-stable materials are used as the stable backbone in the porous electrode structure. Current-voltage characteristics, rSOC cycle durability, and long-term stability of various fuel electrodes will be presented, including alternative co-impregnated electrodes (3,4). Such results will be compared with those using the conventional Ni-cermet fuel electrode. Acknowledgements A part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO). Collaborative support by Prof. H. L. Tuller, Prof. B. Yildiz, and Prof. J. L. M. Rupp at Massachusetts Institute of Technology (MIT) is gratefully acknowledged.

Introduction Reversible solid oxide cell (r-SOC) enables both power generation and electrolysis, when the cell can be operated as a solid oxide fuel cell (SOFC) and as a solid oxide electrolysis cell (SOEC). Many studies are being conducted to improve their performance and durability (1,2).Lanthanum strontium cobalt ferrite (LSCF) is an air electrode material with a perovskite-type crystal structure which exhibits high electronic and ionic conductivity, oxygen diffusivity, and electrocatalytic activity (3). However, performance and durability of the cells with Sr-containing oxide air electrodes have to be carefully analyzed, as Sr ions tend to easily diffuse during sintering and in long-term operation, reacting with Zr ions from the solid electrolyte to form a highly resistance reaction layer such as SrZrO3 (4-6). While the performance and durability of LSCF-based air electrodes and the Sr diffusion in SOFC operation have been studied, there are still limited number of studies on the SOEC operation, and especially on the r-SOC operation. The diffusion of constituent ions such as Co and Fe may not be negligible in SOCs.Here in this study, we fabricate YSZ electrolyte-supported r-SOCs and conduct durability studies in both SOFC and SOEC modes. STEM-EDS observation of the air electrode materials is also made to investigate the durability and diffusion of various elements in the LSCF-based air electrode of r-SOCs. Experimental R-SOCs with yttria stabilized zirconia electrolytes (YSZ, 200 µm thick) were prepared. Ni-GDC co-impregnated fuel electrode was prepared, where a mixture of La0.1Sr0.9TiO3 (LST) and Gd0.1Ce0.9O2 (GDC) at a volume ratio of 50:50 was used as the porous electrode backbone, onto which Ni-containing solution of 1 µL was impregnated for Ni loading of 0.167 mg cm-2 (7). Gd0.1Ce0.9O2 (GDC) buffer layer was prepared between the electrolyte and the air electrode to prevent elemental diffusion (8). La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) was used for the air electrode. In addition, a Pt reference electrode was deposited onto the electrolyte on the fuel electrode side. The voltage measurement terminals of the electrochemical measurement setup were connected between the reference electrode and the air electrode to evaluate the air electrode potential. 50%-humidified hydrogen (100 ml min-1) was supplied to the fuel electrode, and air (150 ml min-1) was supplied to the air electrode. Negative current density means the value in the SOEC mode, while positive current density in the SOFC mode. Results and discussion As shown in Fig. 2 describing one cycle, current density was varied at 800°C for the 1000-cycle durability tests in the r-SOC operation. The range of current density was between -0.2 A cm-2 and 0.2 A cm-2. The air electrode potential at different current densities and the electrode impedance in every 100 cycles were measured. For the 1000-hour electrolysis durability test in the SOEC mode, current density of -0.2 A cm-2 was applied for 1000 hours at 800°C. The air electrode potential and the impedance in every 100 hours were measured (9). It has been found that a gradual degradation of the r-SOCs associated with an increase in air electrode potential in the SOEC mode and a decrease in the potential in the SOFC mode could be distinguished. However, such degradation tended to be stabilized with the number of cycles.STEM-EDS observations were performed to analyze the elemental distribution around the air electrode and the electrolyte in the cell. It was found that such elemental diffusion could occur during sintering and durability experiments.AcknowledgmentsA part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO) (Project No. JPNP20005). Collaborative support by Prof. H. L. Tuller, and Prof. B. Yildiz at Massachusetts Institute of Technology (MIT) for their continuous support is gratefully acknowledged. Figure 1

Reversible solid oxide cells (rSOCs) are a promising electrochemical technology able to work both as energy storage devices in solid oxide electrolysis (SOEC) mode, and as power generators in solid oxide fuel cell (SOFC) mode. rSOCs implemented for CO2 electroreduction into valuable CO-rich steams through electrolysis would provide opportunities for CO2 utilization, and hence become useful tools for the reduction of greenhouse gases emissions. High-temperature co-electrolysis of CO2/H2O mixtures has recently become of interest since high conversion and energy efficiency are thermodynamically favored in addition to the reduction in cell area-specific resistance when compared to pure CO2 electrolysis [1]. However, reversible operation with CO2-containing feeds requires developing flexible, high-performing, and long-lasting materials for the rSOCsManaging issues such as coking and low redox tolerance in the state-of-the-art (SoA) Ni-Yttria-stabilized-zirconia (Ni-YSZ) cermet fuel electrode in the rSOCs is crucial. Ni-YSZ cermet exhibits high electronic conductivity and electrochemical activity in hydrogen. Nonetheless, Ni is prone to oxidation due to operation under high steam concentrations, and rapid variations in the fuel supply. Additionally, Ni is well-known for its coke-formation tendency which becomes detrimental in CO2-rich mixtures for electrolysis [2]. Thus, ceramics with mixed ionic and electronic conductivity (MIEC) have emerged as alternative fuel electrodes thanks to the larger electrochemically active area compared to standard cermets. Among the MIEC electrodes, the SrTi0.3Fe0.7O3 (STF) perovskite has proven to give low polarization resistances especially when modification techniques such as exsolution of metal nanoparticles are employed [3]. This work focuses on the investigation of the electrocatalytic properties of exsolution-based STF electrodes operating with CO/CO2 and H2O/CO2 mixtures. The mixtures used were 50% CO2/50% CO, 25% H2O/25% CO2/25%H2/25%CO, and 45% H2O/45% CO2/10% H2. A comparison between the performance and microstructure of the studied electrode formulation is provided.Electrolyte-supported cells were manufactured via screen-printing of the electrode inks on scandia-stabilized zirconia (ScSZ) electrolytes. STF perovskite-based electrodes were used, namely: SrTi0.3Fe0.7O3 (STF), Sr0.95(Ti0.3Fe0.63Ni0.07)O3 (STF-Ni), and Sr0.95(Ti0.3Fe0.63Ru0.07)O3 (STF-Ru). The latter two formulations allowed the enhancement of the perovskite structure via exsolution of catalytic nanoparticles (Ni, Co alloyed with Fe) on the oxide surface during cell operation. Each cell configuration included a gadolinium-doped ceria (GDC) buffer layer coupled with an LSCF-GDC (La0.6Sr0.4Co0.2Fe0.8O3/Ce0.9Gd0.1O1.95) oxygen electrode. Baseline performances of cell mounting the Ni-YSZ cermet electrode were also evaluated for comparison with the perovskite alternatives, both in the short and long term. Electrochemical characterization was done via impedance spectroscopy, polarization experiments, and durability tests in potentiostatic mode. Chemical characterization was performed via X-ray diffraction, scanning electron microscopy, and temperature-programmed reduction analyses.Compared to Ni-YSZ, significant improvement in terms of reversibility and maximum current densities of the current-voltage (I/V) curves in both SOFC and SOEC modes is found on the STF-based cells at 750°C under CO/CO2 mixtures (Figure 1B). The initial difference in the performance of the STF cells with respect to the Ni-YSZ cermet in humidified H2 (SOFC mode, Figure 1A) was overcome by using the exsolved electrode formulations. Aging experiments up to 100 h performed in alternating operation modes from SOFC (6 h) to SOEC (6 h) indicated an initial performance degradation within the first 24 h for both the cermet and perovskite-based fuel electrodes which later stabilized in time. Compared to the undoped STF formulation, improvement in performance stability was observed for the exsolved STF-Ni and STF-Ru fuel electrodes while working under the target CO/CO2 mixture. This was attributed to the combination of the STF structure benefits (low polarization resistance towards CO/CO2 and H2O/CO2 mixtures) with the boost that metallic nanoparticles provide to the electrode’s heterogeneous catalytic reactivity and overall stability over time.

Pd–La0.6Sr0.4Co0.2Fe0.8O3– composite as active and stable oxygen electrode for reversible solid oxide cells

Introduction Reversible solid oxide cells (r-SOCs) are electrochemical energy devices that can switch reversibly between power generation as a solid oxide fuel cell (SOFC) and hydrogen production as a solid oxide electrolysis cell (SOEC). It is important to systematically understand the electrode reaction processes and degradation factors in these two modes to tailor high-performance and durable r-SOC electrodes. The dependence of polarization resistance on cell fabrication conditions and operating conditions has been individually investigated for SOFC and SOEC by impedance measurement and distribution of relaxation times (DRT) analysis [1-4], but it is necessary to evaluate r-SOC as a whole [5]. Here in this study, we aim to systematically clarify the similarities and differences in the electrode reaction processes at the fuel electrodes of both SOFC and SOEC modes of r-SOCs, with respect to fuel electrode fabrication conditions. Experimental Several types of cells were fabricated with different fuel electrode constituent materials, thicknesses, and sintering temperatures. Three types of fuel electrode materials were used: Ni-GDC cermet, a composite of Ni and mixed ionic-electronic conductor Gd0.9Ce0.1O3 (GDC); Ni-YSZ cermet, a composite of Ni and pure ionic conductor YSZ (8 mol% Y2O3 - stabilized ZrO2); and Ni-GDC co-impregnated electrode with LST-GDC as a backbone support (LST: La0.1Sr0.9TiO3). Scandia-stabilized zirconia (ScSZ) was used as the electrolyte plate, and (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF) was used as the air electrode. GDC buffer layer was prepared between the electrolyte and the air electrode to suppress the formation of interfacial insulting layers.Electrochemical impedance measurements were performed for the fuel electrode of the fabricated cell under SOFC and SOEC modes by supplying H2-H2O mixture and varying operating temperature, fuel humidification, and current density. The DRT analysis was conducted to separate various polarization resistance components of the fuel electrode with different relaxation times. The area of each DRT peak corresponds to the resistance of each electrode reaction process. In this study, activation energy was derived based on the assumption that the electrical conductance is of an Arrhenius-type thermally activated character. Its dependence on the fuel electrode fabrication conditions was investigated. Furthermore, electrode microstructure was observed by using focused-ion beam scanning electron microscopy (FIB-SEM). Results and Discussion Figure 1 shows typical DRT peaks of the Ni-GDC cermet fuel electrode in (a) SOFC and (b) SOEC modes, measured at different temperatures. In the range of 10-1 to 106 Hz where the frequency response was measured, three DRT peaks appeared, P1: 10-1 to 100 Hz, P2: 100 to 102 Hz, and P3: 102 to 103 Hz, all with a DRT peak area decreasing with increasing operating temperature. This indicates that there are at least three electrode reaction processes with different relaxation times, which are thermally active processes. Activation energies of 0.1-0.3 eV (SOFC mode) and 0.2-0.5 eV (SOEC mode) were obtained for P1, and 0.9-1.0 eV (SOFC mode) and 1.1-1.3 eV (SOEC mode) for P2. The activation energies in SOEC mode were higher than those in SOFC mode for both P1 and P2. In the presentation, the dependence of activation energies on the fuel electrode constituent materials and related electrode reaction processes will be discussed.

Reversible solid oxide cells (RSOCs) can efficiently render the mutual conversion between electricity and chemicals, for example, electrolyzing CO2 to CO under a solid oxide electrolysis cell (SOEC) mode and oxidizing CO to CO2 under a solid oxide fuel cell (SOFC) mode. Nevertheless, the development of RSOCs is still hindered, owing to the lack of catalytically active and carbon-tolerant fuel electrodes. For improving mutual CO-CO2 conversion kinetics in RSOCs, here, we demonstrate a high-performing and durable fuel electrode consisting of redox-stable Sr2(Fe, Mo)2O6-δ perovskite oxide and epitaxially endogenous NiFe alloy nanoparticles. The electrochemical impedance spectrum (EIS) and distribution of relaxation time (DRT) analyses reveal that surface/interface oxygen exchange kinetics and the CO/CO2 activation process are both greatly accelerated. The assembled single cell produces a maximum power density (MPD) of 443 mW cm-2 at 800 °C under the SOFC mode, with the corresponding CO oxidation rate of 5.524 mL min-1 cm-2. On the other hand, a current density of -0.877 A cm-2 is achieved at 1.46 V under the SOEC mode, equivalent to a CO2 reduction rate of 6.108 mL min cm-2. Furthermore, reliable reversible conversion of CO-CO2 is proven with no performance degradation in 20 cycles under SOEC (1.3 V) and SOFC (0.6 V) modes. Therefore, our work provides an alternative way for designing highly active and durable fuel electrodes for RSOC applications.

Reversible solid oxide cells (RSOCs) represent a promising technology for the efficient exploitation of intrinsically intermittent renewable energy sources. RSOCs allow to derive fuel and chemicals from power (power-to-gas technology, P2G) and power from fuel and chemicals (gas-to-power technology, GTP) and can be interchangeably operated either as a solid oxide fuel cell (SOFC) or as a solid oxide electrolyzer cell (SOEC). The key aspect to render these devices competitive on a market scale is the development of multi-tasking, reliable, cost-effective and long-lasting electrodes (1,2). Besides, to overcome the issues related to hydrogen production, storage and distribution (e.g. impractical conversion of the existing grid to hydrogen-based infrastructures), the fuel-electrode of RSOCs should ensure high catalytic activity and coking resistance toward carbon-containing species (3-4). Using a hydrocarbon-tolerant fuel electrode, energy can be obtained by natural gas and biogas (SOFC-mode), with useful recovery of CO2 in the exhausts (carbon capture and storage, CCS). Besides, if the electrode is also active towards CO2 electrolysis (SOEC mode), CO2 is reduced to CO and O2 (carbon capture and utilization, CCU).Ni-YSZ is the reference fuel-electrode material for both H2 fed SOFC and for CO2 electrolysis in SOEC. Nevertheless, Ni-based cermets cannot be used in SOFCs fed with methane-containing fuels, as they suffer from two main drawbacks: mechanical instability upon NiO-Ni redox cycles (5) and passivation due to coking, being Ni a catalyst for methane cracking (3, 4). Moreover, during Ni-YSZ operation in CO2-SOEC mode, ZrO2 reduction can occur at high cathodic potential, resulting in Ni-Zr compounds formation (6).In this work, a recently developed (7, 8) composite material containing La0.6Sr0.4Fe0.8Mn0.2O3-δ (LSFMn) and 5wt% Ni-containing Ce0.58Sm0.15O2-δ (NiSDC) is tested as fuel electrode for LSGM-electrolyte supported cells. In reducing conditions, Fe exsolved from the LSFMn perovskite forms a Fe-Ni alloy with Ni present on SDC. In SOFC mode, the composite is designed to operate in dry methane: Fe-Ni catalytic sites activate CH4, which is successively oxidized on Mn-containing LSFMn, while SDC increases the O2- supply at the anode to get rid of any carbonaceous deposits. As Fe-Ni alloy was reported to be highly active for CO2 reduction (9), the composite was also tested as SOEC cathode in different CO2:CO ratios. LSFMn+NiSDC was tested in SOFC-mode as anode for hydrogen, dry methane and carbon monoxide oxidation and showed power density outputs of 657, 668 and 527 mW/cm2, respectively (Fig. a), a redox stable behavior and coking resistance for over 120 h. LSFMn+NiSDC in SOEC-mode delivered 2.66 A/cm2 at 2 V in 95:5 CO2:CO mixture (Fig. b), keeping 1 A/cm2 of current density output for over 40 h. (1) - M. Mogensen, M. Chen, H. Frandsen, C. Graves, J. Hansen, K. Hansen, A. Hauch, T. Jacobsen, S. Jensen, T. Skafte, Reversible solid-oxide cells for clean and sustainable energy, Clean Energy, 3 (2019) 175-201. (2) - M.B. Mogensen, Materials for Reversible Solid Oxide Cells, Current Opinion in Electrochemistry, (2020). (3) - M. Mogensen, K. Kammer, Conversion of hydrocarbons in solid oxide fuel cells, Annual Review of Materials Research, 33 (2003) 321-331. (4) - S. McIntosh, R.J. Gorte, Direct hydrocarbon solid oxide fuel cells, Chemical reviews, 104 (2004) 4845-4866. (5) - J. Malzbender, R. Steinbrech, Advanced measurement techniques to characterize thermo-mechanical aspects of solid oxide fuel cells, Journal of Power Sources, 173 (2007) 60-67 (6) - A. Hauch, K. Brodersen, M. Chen, M.B. Mogensen, Ni/YSZ electrodes structures optimized for increased electrolysis performance and durability, Solid State Ionics, 293 (2016) 27-36 (7) - L. Duranti, I. Luisetto, S. Licoccia, C. Del Gaudio, E. Di Bartolomeo, Electrochemical performance and stability of LSFMn+ NiSDC anode in dry methane, Electrochimica Acta, (2020) 137116. (8) - L. Duranti, I.N. Sora, F. Zurlo, I. Luisetto, S. Licoccia, E. Di Bartolomeo, The role of manganese substitution on the redox behavior of La0.6Sr0.4Fe0.8Mn0.2O3-δ, Journal of the European Ceramic Society, (2020) ( 9 ) - Liu, S., Liu, Q., & Luo, J. L. (2016). Highly stable and efficient catalyst with in situ exsolved Fe–Ni alloy nanospheres socketed on an oxygen deficient perovskite for direct CO2 electrolysis. ACS Catalysis, 6(9), 6219-6228. Figure 1

Introduction Reversible solid oxide cells (r-SOCs) are attractive electrochemical energy devices that can operate as both solid oxide fuel cells (SOFCs) for power generation, and solid oxide electrolysis cells (SOECs) for steam electrolysis (1). Renewable power generation, such as wind and solar, depends on daily weather conditions and so that it is desirable to adjust the supply of fluctuating electricity to match demand using reversible energy devices such as r-SOCs. The operational concept of r-SOCs is schematically described in Figure 1. The aim of this study is to examine the electrochemical properties and durability of r-SOCs using alternative fuel electrode materials. Experimental Various r-SOCs were prepared using conventional and alternative fuel electrode materials. As the conventional materials, Ni-cermet with Sc2O3-doped ZrO2 (ScSZ) was used for the fuel electrode; dense 200 mm thick ScSZ membrane for the electrolyte; and La0.6Sr0.4Co0.2Fe0.8O0.3 oxide (LSCF) air electrode with Gd-doped CeO2 (Gd0.1Ce0.9O2) acting as a buffer layer between the ScSZ electrolyte and the LSCF air electrode. As alternative fuel electrode materials, GDC and/or La0.1Sr0.9TiO3 (LST) were used as the fuel electrode backbone. By applying the co-impregnation procedure of Ni and GDC, fuel electrodes with highly dispersed nanoparticles of Ni catalysts and GDC, denoted as Ni-GDC/LST-GDC, were prepared (2,3). A Pt-based reference electrode was deposited beside the air electrode, such that the fuel electrode potential was measured as the voltage between the fuel and reference electrodes.Current-voltage characteristics were characterized, including voltage cycle tests performed by switching the direction of current simulating r-SOC operation. Long-term durability tests in SOEC and SOFC modes were conducted at 800°C. Air was supplied to the air electrode, while 50%-humidified hydrogen gas was supplied to the fuel electrode. The current density in the SOFC and SOEC modes is denoted as positive and negative values, respectively. Results and discussion Figure 2 shows fuel electrode potential as a function of current density in both SOFC and SOEC modes. The cells with the Ni-zirconia cermet, GDC, LST, LST-GDC, and Ni-GDC/LST-GDC could be operated in both modes. In the SOFC mode, the conventional Ni-zirconia cermet exhibited the highest fuel electrode voltage among these materials studied. In contrast, in the SOEC mode, the GDC-containing materials exhibited lower (i.e. better) fuel electrode voltage compared to Ni-zirconia cermet even without Ni catalysts, indicating enhanced electrocatalytic activity of mixed-conducting GDC for the electrode reactions with water vapor at the fuel electrodes. Voltage cycle durability up to 1,000 cycles and long-term durability up to 1,000 hours could be measured. Latest results on the durability with various electrode materials will be presented. Acknowledgements A part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO) (Project No. JPNP20005). References Q. Minh and M. B. Mogensen, Electrochem. Soc. Interface, 22, 55 (2013).Futamura, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, J. Electrochem. Soc., 164 (10), F3055 (2017).Futamura, A. Muramoto, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, International J. Hydrogen Energy, 44 (16), 8502 (2019). Figure 1

Energy and environmental issues have always been topics of great concern to the world in recent years. The production of electricity from solar and wind energy has increased dramatically. These significant developments in renewable energy technologies are now driving the development of a method to efficiently store and transport the inherently intermittent electricity from these natural resources [1]. Under this situation, the emergence of reversible solid oxide cells (RSOCs) has been attracted widespread attention. As a clean and efficient electrochemical conversion device, RSOCs can link electrical and chemical energy and convert flexibly, which is of great significance for power generation and energy storage [2]. It can operate in two modes: (i) as solid oxide fuel cells (SOFCs) for power production and (ii) as solid oxide electrolysis cells (SOECs) for hydrogen and oxygen production [3].Herein, we successfully synthesized a novel perovskite Pr0.8Ca0.2Fe0.8Co0.2O3- δ and used it as an air electrode in reversible solid oxide cells. In the SOFCs mode, the peak power density is up to 1.056 W cm-2 at 850 ºC using hydrogen as the fuel and air as the oxidant. It can achieve 1.43 A cm-2 at 850 ºC under an operating voltage of 1.3 V in the SOECs mode with water content of 50%. Moreover, the cell can run stably in both SOFCs and SOECs modes with showing excellent reversibility. During a short-time reversible term, it remains both electrochemical performance and structure stable. These results demonstrate that the perovskite Pr0.8Ca0.2Fe0.8Co0.2O3- δ is promising air electrode for applications in RSOCs.

Introduction In recent years, environmentally-compatible renewable energy becomes increasingly important as major power resources. However, there exist several issues to overcome including energy storage due to the fluctuating nature. Solid oxide reversible cells (SORCs), able to act as both solid oxide fuel cells (SOFCs) and solid oxide electrolyzer cells (SOECs), may enable power generation in an SOFC mode and hydrogen production in an SOEC mode [1-3]. Therefore SORCs are of scientific and technological interest towards low-carbon and carbon-free energy society. However, since SORC operation may be associated with redox cycling of their fuel electrodes, the commonly-used fuel electrode material, Ni-zirconia cermet, has a difficulty in stability against such redox cycling. For SOFCs, alternative catalyst-impregnated fuel electrodes [3-5] are demonstrated to be applicable with long-term durability under high water vapor pressure and against redox cycling. Here, the aim of this study is to investigate the electrochemical properties of such redox-tolerant fuel electrode materials for SOECs and SORCs. Experimental In this study, the electrochemical characteristics of three types of cells were evaluated. First, for comparison, (i) conventional Ni-ScSZ cermet fuel electrode was used as a reference electrode material. As alternative fuel electrodes, (ii) Ni-GDC co-impregnated fuel electrode cell (Ni: 0.167 mg cm-2) and (iii) Rh-GDC co-impregnated fuel electrode (Rh: 0.178 mg cm-2) with electron-conducting backbone (porous composite of La-Sr-Ti oxide (LST) and Gd-doped ceria (GDC)) were applied, for which catalytic metals (Ni or Rh) were co-impregnated with additional GDC, respectively [5]. Electrochemical impedance spectroscopy (EIS, Solatron) was applied to separate and evaluate ohmic and non-ohmic overvoltages. The materials stability against high water vapor pressure was evaluated by durability tests up to 80 h in an SOEC mode, where 80%-humidified hydrogen was supplied to the fuel electrode with the applied current density of -1.2 A cm-2. The durability against reversible SOEC / SOFC cycling was evaluated by varying current density and by switching the current between positive (1.2 A cm-2) and negative (-1.2 A cm-2) at 50%-humidified hydrogen supplied to the fuel electrodes. Results and discussion Figure 1 shows the fuel electrode voltage, measured against the Pt reference electrode on the air-electrode side, of two types of the co-impregnated cells kept at a constant current density during the 80h durability test in the SOEC mode. The co-impregnated cells (ii) and (iii) exhibited a stable fuel electrode voltage. Therefore, performance deterioration was almost negligible. This is probably because the cells of (ii) and (iii) have the redox-stable LST-GDC electrode backbone, which has sufficient durability under high water vapor pressure.Figure 2 shows the fuel electrode voltage measured in the reversible SOEC / SOFC cycling tests. An increase in fuel electrode voltage in the SOEC mode (upper-side) and a decrease in fuel electrode voltage in the SOFC mode (lower-side) correspond to certain performance degradation. However, the increase in fuel electrode voltage was much smaller for the co-impregnated cells of (ii) and (iii), compared to that for the cell (i) using the Ni-cermet electrode. These results reveal that, the co-impregnated fuel electrodes stable in the SOFC operation can also exhibit high durability in the SOEC operation at high water vapor pressure, and thus sufficient durability in the reversible SOEC / SOFC operation. These co-impregnated fuel electrodes are therefore promising for SOFCs, SOECs, and SORCs, with sufficient durability under high water vapor pressure and in reversible operation. References Q. Minh and M. B. Mogensen, Electrochem. Soc. Interface, 22, 55 (2013).N. Q. Minh, MRS Bulletin, 44 (9), 682 (2019).T. S. Irvine, D. Neagu, M. C. Verbraeken, C. Chatzichristodoulou, C. Graves, M.B. Mogensen, Nature Energy, 1, 15014 (2016).Futamura, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, J. Electrochem. Soc., 164 (10), F3055 (2017).Futamura, A. Muramoto, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, International J. Hydrogen Energy, 44 (16), 8502 (2019).P. Jiang, Mater. Sci. Eng. A, 418, 199 (2006). Figure 1

Enhanced electrochemical redox kinetics of La0.6Sr0.4Co0.2Fe0.8O3 in reversible solid oxide cells

An efficient and prospective self-assembled hybrid electrocatalyst for symmetrical and reversible solid oxide cells

Reversible solid oxide cells (RSOCs) are state-of-the-art all-solid-state energy conversion devices that operate as either solid oxide fuel cells (SOFCs) or solid oxide electrolysis cells (SOECs), with the oxygen electrode playing a critical role in both the oxygen reduction (ORR) and oxygen evolution (OER) reactions. Herein, PrBaCo1.5Fe0.5O5+δ (PBCF) is modified via double deficiency modulation of the A-site to improve its performance as an RSOC oxygen electrode. The cation-deficient variant, Pr0.97Ba0.97Co1.5Fe0.5O5+δ (PBCF97), exhibits a balanced oxygen vacancy concentration and ionic conductivity. RSOCs using PBCF97-Gd0.1Ce0.9O2(GDC) as the oxygen electrode achieve a peak power density of 1.685 W/cm2 in SOFC mode and a current density of 2.61 A/cm2 at an electrolytic voltage of 1.5 V in SOEC mode at 800 °C. In addition, the cell demonstrates stable reversible operation for 160 h, highlighting its potential for advanced RSOC applications.

Reversible solid oxide cell (RSOC) is a new technology for energy storage with the advantages of high efficiency and high energy density, combining solid oxide fuel cell (SOFC) mode and solid oxide electrolysis cell (SOEC) mode to achieve two functions in one device. The commercialization of RSOC is limited by the severe degradation of air electrodes during the operation of SOEC. Anode supported cells with yttria-stabilized zirconia (YSZ) electrolytes, Ni/YSZ hydrogen electrodes and perovskite oxygen electrodes with lanthanum strontium cobalt ferrite (LSCF) were tested and the reversible operation has been demonstrated. The electrochemical performance and stability in reversible mode of the cells are demonstrated under 650℃，700℃，750℃ with different composition of fuel gas (30% H2O/70%H2, 50% H2O/50%H2 and 70%H2O/30%H2) and current density (0.5A/cm2 and 1.0 A/cm2) through the experiment research. Post-test is also used to assist in analyzing the degradation mechanism. It is found that the degradation is more severe under SOEC mode at high current density (1.0 A/cm2) and the alternative operation of two modes helps to mitigate the degradation of cells comparing with operating in SOEC mode. Operation under SOFC mode plays a restorative role through reducing the high oxygen partial pressure at the oxygen electrode caused by SOEC mode operation, thus mitigating the degradation of the cell. This study signifies the difference between the SOEC and SOFC mode and provides insight in the operating mechanism of RSOC.

A reversible solid oxide cell (RSOC) can generate electricity and hydrogen, respectively, when operating in solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes. However, at high current densities, serious Sr segregation in the traditional La0.6Sr0.4Co0.2Fe0.8O3-δ-Gd0.1Ce0.9O2-δ (LSCF-GDC) oxygen electrode leads to a decrease in the catalytic activity of the cathode and consequently poor performance stability of RSOC. In this work, Ca doping was proposed to suppress Sr segregation in the LSCF-based composite electrodes. The smaller radius of the Ca atom could reduce the elastic driving force of Sr segregation, while the reasonable Ca-doping concentration could alleviate the steep oxygen vacancy gradient formed under high current density and thereby reduce the electrostatic driving force of Sr segregation. Consequently, after appropriate Ca substitution at the A-site, the prepared La0.6Sr0.2Ca0.2Co0.2Fe0.8O3-δ-Gd0.1Ce0.9O2-δ(LSCCF-GDC) oxygen electrode reported significantly enhanced stability during long-term operation at high current density. At 750 °C and a current density of 1.2 A·cm-2, the degradation rates of the cell with LSCCF-based composite oxygen electrode were 10 mV/100 h in SOFC mode and 17 mV/100 h in SOEC mode, which was much lower compared with the corresponding values of the cell with the traditional LSCF-based composite oxygen electrode (18 mV/100 h in SOFC mode and 82 mV/100 h in SOEC mode).