Solid Oxide Electrolysis Cell Mode Research Articles

Boosting the electrocatalytic activity of NdBaCo2O5+δ via calcium co-doping as bifunctional oxygen electrodes for reversible solid oxide cells

The oxygen electrode of the solid oxide cells (SOCs) operates under an oxidizing atmosphere. Lattice shrinkage in a high oxygen partial pressure environment results in cation misalignment, leading to a decline in performance. Addressing the drawbacks associated with A-site cation mismatch in LnBaCo2O5+δ double perovskite oxides for reversible solid oxide cells (RSOCs) oxygen electrodes, a novel calcium ion co-doping strategy is proposed. This approach effectively mitigates A-site cation segregation and enhances stability. The glycine-nitrate method was employed to synthesize Nd0.8Ca0.2Ba1−xCaxCo2O5+δ (x = 0–0.2) layered double perovskites oxides co-doped with calcium ions, denoted as NCBCC. NCBCC exhibits excellent compatibility with commonly used electrolytes. The doping of calcium co-doped in NdBaCo2O5+δ decreases the thermal expansion coefficient and improves the electron transfer characteristics. The sample with x = 0.1 exhibited an area-specific resistance (ASR) of 0.024 Ω cm2 when operated at a temperature of 800 °C in air. In the solid oxide fuel cell mode, employing x = 0.1 as an oxygen electrode, the maximum power density achieved was 766 mW cm−2 at 800 °C. In the solid oxide electrolysis cell mode, operating with CO2 and steam at an electrolysis voltage of 1.5 V and a temperature of 800 °C, the corresponding current densities were −1.51 and −1.85 A cm−2, respectively. The incorporation of calcium ions into layered perovskite oxides offers a promising strategy to enhance the performance of oxygen electrodes in RSOCs.

Distribution of Relaxation Times of Fuel Electrodes for Reversible Solid Oxide Cells Fabricated Under Various Conditions

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.

Exceptionally stable double perovskite Sr2-xFeTaO6-δ as fuel electrode for solid oxide cells

Developing fuel electrodes with excellent structural stability and high catalytic activity under various fuel conditions is highly desirable for large-scale applications of solid oxide cells (SOCs). In this study, double perovskite Sr2-xFeTaO6-δ (x = 0, 0.1, 0.2, 0.3) materials were designed as fuel electrodes for SOCs. Sr1.8FeTaO6-δ (S18FT) was synthesized in air, and its structural stability could be maintained in both 5 % H2/Ar and pure CO2 after calcination at 850 ℃ for 10 h. The thermal expansion coefficient of S18FT was 11.8 × 10−6 K−1 at 50–850 ℃. To further improve the electrochemical performance of the S18FT electrode, it was impregnated with Pd(NO3)2 to form a composite electrode (Pd-SFT–SDC). When hydrogen, propane, ethyl ethanol, and biochar were used as fuels, the maximum power densities of the fuel cells at 850 ℃ were 1114, 370, 474, and 291 mW cm−2, respectively. The fuel cell operated stably for 300 h when using propane. When the solid oxide electrolysis cell mode was used for the co-electrolysis of CO2/H2O, the current density was −0.835 A cm−2 at 1.3 V and 800 ℃. These results indicated the potential for the use of the Pd-SFT–SDC composite material as a fuel electrode for SOCs.

Reversible Solid Oxide Cells: Cycling and Long-Term Durability of Air Electrodes

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: Selection of Fuel Electrode Materials for Improved Performance and Durability

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

Highly Active Layered Cobaltite Air-Electrode Materials for Reversible Solid Oxide Cells

The use of fossil fuels, which have increased rapidly since the Industrial Revolution, is causing serious climate change. To overcome this, interest in clean and sustainable energy sources to replace fossil fuels is increasing worldwide. Among alternative energies, the electricity generation from solar and wind resources is difficult to achieve sustainable energy production regardless of the weather and season. Hence, hydrogen is considered as the most promising energy source to replace fossil fuels because of its clean, storable, sustainable, and transportable characteristics.Recently, reversible fuel cells that can both produce hydrogen with surplus power and generate electricity using the stored hydrogen have attracting attentions as an electrochemical device to promote reliable and economical hydrogen society [1]. Among them, reversible solid oxide cells show high efficiency in a fuel cell mode due to their high operating temperature (700-1000 ℃). However, reversible solid oxide cells exhibit relatively low performance in water-electrolysis cell mode [2]. The main reason of low performance in the solid oxide electrolysis cell mode is the sluggish kinetics of oxygen evolution reactions in air-electrode. Therefore, the electrolysis performance of reversible solid oxide cells can be greatly improved if the air-electrode with high catalytic activity for oxygen evolution reactions is developed.Among many candidate materials for air-electrodes, cobalt-based layered materials are known to have good electrocatalytic properties in oxygen reduction and oxygen evolution reactions with high electronic conductivity (σ = 100–240 S·cm-1 at 450–650 °C) [3, 4]. Herein, we introduce a novel layered structure material, Ba2Co9O14 (BCO), as air-electrode materials for highly efficient and durable reversible solid oxide cells. In order to further enhance the catalytic property of BCO, Nd and Ca are doped into Ba-site. The Nd0.1Ca0.1Ba1.8Co9O14-cell shows a maximum power density of 2.6 W·cm-2 in fuel cell mode, current density of –4.36 A/cm2 under applied voltage of 1.6 V in electrolysis cell mode, at 800 ℃, surpassing previous reported results of air-electrode catalysts for reversible solid oxide cells.

Performances of Ni−SDC Hydrogen Electrodes in Reversible Operation Between SOEC and SOFC-Modes

A reversible solid oxide cell (R-SOC), which can be operated as solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC), is a highly efficient direct energy converter between hydrogen and electricity (1). We have engaged in the R&D of high-performance, durable electrodes for the R-SOC (2-13). Recently, it was found that the durability of a double-layer H2 (DL) electrode, consisting of a SDC scaffold [samaria-doped ceria (CeO2)0.8(SmO1.5)0.2] with highly dispersed Ni0.9Co0.1 nanoparticles as the catalyst layer and a thin current collecting layer of Ni–YSZ cermet, was improved greatly by a reversible cycling operation between the SOEC and SOFC modes (12). The microstructure was observed to be stabilized by the cycling operation, i.e., lower parts of many Ni‒Co particles were anchored tightly on the SDC support and some portions were coated with SDC film, with certainty due to a strong interaction between Ni‒Co and SDC (13). However, the procedure for dispersing Ni‒Co nanoparticles on SDC (via an impregnation method) is not always suitable for large-scale fabrication. Aiming to develop a durable, practical hydrogen electrode with such a stabilized microstructure, we prepared new Ni–SDC hydrogen electrodes by a convenient protocol and examined the performances in the reversible operation.We prepared a coin-size cell with an air reference electrode (ARE) (6, 9-12):Ni–SDC H2 electrode│YSZ (0.5 mm)│SDC interlayer│LSCF–SDC O2 electrodeFor the H2 electrode, a paste was prepared by mixing Ni nanoparticles with uniform size (prepared by Noritake Co., Limited), SDC powder, and pore former. The paste was painted on YSZ electrolyte, followed by sintering. The LSCF–SDC (40 vol.% SDC) oxygen electrode was prepared on the SDC interlayer in the same manner as that described in our work (7). Test cells were operated at 800ºC by supplying humidified H2 (p[H2O] = 0.4 atm) to the H2 electrode compartment and dry O2 to the O2 electrode, irrespective of operation modes (SOEC or SOFC). The steady-state IR-free polarization curves (I–E curves) of the electrodes with area-specific ohmic resistances were measured by the current-interruption method in a three-electrode configuration. Figure 1 shows a SEM image acquired with a back-scattered electron (BSE) detector and Ni mapping by an energy-dispersive X-ray spectrometer (EDX) for the cross section of Ni–SDC electrode in pristine condition. In the BSE image, SDC is observed as light gray, Ni as dark gray, and pores as black. The distribution of Ni particles are fairly uniform in the electrode. The Ni–SDC electrode thus prepared exhibited a comparable initial performance to that of our DL-H2 electrode with Ni‒Co dispersed SDC prepared by the impregnation method (12). The durability tests of the electrodes are under progress in reversible cycling between −0.50 A cm−2 (SOEC-mode for 11 h) and 0.50 A cm−2 (SOFC-mode for 11 h). This work was supported by funds for the “Collaborative Industry-Academia-Government R&D Project for Solving Common Challenges toward Dramatically Expanded Use of Fuel Cells” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References S. D. Ebbesen, S. H. Jensen, A. Hauch, and M. B. Mogensen, Chem. Rev., 114, 10697 (2014). H. Uchida, N. Osada, and M. Watanabe, Electrochem. Solid-State Lett., 7, A500 (2004). N. Osada, H. Uchida, and M. Watanabe, J. Electrochem. Soc., 153, A816 (2006). Y. Tao, H. Nishino, S. Ashidate, H. Kokubo, M. Watanabe, and H. Uchida, Electrochim. Acta, 54, 3309 (2009). R. Nishida, P. Puengjinda, H. Nishino, K. Kakinuma, M. E. Brito, M. Watanabe, and H. Uchida, RSC Adv., 4, 16260 (2014). H. Uchida, P. Puengjinda, K. Miyano, K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and M. Watanabe, ECS Trans., 68 (1), 3307 (2015). K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, Electrochim. Acta, 225, 114 (2017). K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, J. Ceram. Soc. Jon., 125, 218 (2017). P. Puengjinda, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, J. Electrochem. Soc., 164, F889 (2017). H. Uchida, P. Puengjinda, K. Shimura, H. Nishino, K. Kakinuma, and M. E. Brito, ECS Trans., 78 (1), 3189 (2017). H. Uchida, H. Nishino, K. Kakinuma, and M. E. Brito, ECS Trans., 91 (1), 2379 (2019). H. Uchida, H. Nishino, P. Puengjinda, and K. Kakinuma, J. Electrochem. Soc., 167, 134516 (2020). H. Uchida, H. Nishino, and E. Da’as, ECS Trans., 103 (1), 6119 (2021). Figure 1

Strontium Free Cu-Doped La2NiO4 Oxides as Promising Oxygen Electrodes for Solid Oxide Electrolysis Cells

In this work, strontium-free copper-doped nickelates, La2Ni1-xCuxO4 (x=0.1-0.3), were synthesized using solid state synthesis method and tested as potential oxygen electrodes in solid oxide electrolysis cells (SOEC). Similar to the parent compound, the Ruddlesden-Popper (RP) La2NiO4 oxide, the Cu-doped compositions were confirmed to crystallize with tetragonal crystal structure with I4/mmm space group. The electrochemical performance of La2Ni1-xCuxO4 electrodes was studied using Ni-YSZ electrode-supported button cells in SOEC mode and compared to the state-of-the-art (La0.6Sr0.4)0.95Co0.0.8O3-δ (LSCF)-doped ceria composite electrodes. Electrochemical tests were conducted at 750oC with the Ni-YSZ hydrogen electrode exposed to 50% steam in hydrogen. The polarization resistance of SOEC with the La2Ni0.8Cu0.2O4 (LNCuO-20) electrode was lower than that with LSCF. LNCuO-20 showed stable current density output over 800 hours tested, while cell with LSCF showed more degradation.

Long-Term Tests and Advanced Post-Test Characterizations of the Oxygen Electrode in Solid Oxide Electrolysis Cells

Solid oxide electrolysis cells (SOEC) are a promising energy conversion technology for the production of green hydrogen via steam electrolysis. However, performance loss remains a bottleneck for large-scale commercialization. Here, we investigate the different types of degradation occurring in a state-of-the-art cell composed of La0.6Sr0.4Co0.2Fe0.8O3-Ce0.8Gd0.2O2 (LSCF-GDC) composite for the oxygen electrode, GDC for the barrier layer, Y0.16Zr0.84O1.92 (8YSZ) for the electrolyte and Ni-YSZ for the fuel electrode. Electrochemical impedance spectra measured before and after operation at 750 °C for 2000 hours at -1 A cm-2 in SOEC mode revealed no significant evolution for the oxygen electrode contribution. To investigate potential degradation of this electrode, we performed multi-modal chemical imaging based on synchrotron X-ray diffraction (µ-XRD) with a resolution at the micrometer-scale. In addition, we monitored the evolution of the chemical composition and chemical environment before and after ageing using laboratory X-ray photoelectron spectroscopy (XPS). All these analyses revealed a slight evolution of the crystal lattice parameter in the oxygen electrode and the inter-diffusion layer. It was also found that Sr segregation accelerates with ageing. However, more severe ageing conditions and/or longer ageing durations are required in order to observe an effect on the performance.

PrBa0.5Sr0.5Co1.5Fe0.5O5+δ as air electrode for proton-conducting solid oxide cells

Proton-conducting solid oxide cell (H–SOC) is suitable for operation at lower temperatures. However, its commercialization still faces the problem of insufficient catalytic activity of air electrode. Here a triple ionic-electronic conductor PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) is synthesized by glycine-nitrate method, and the PBSCF slurry was screen printed onto the surface of half-cell which is fabricated by aqueous co-tape casting to obtain the single cell with the structure of PBSCF ǀ BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) ǀ NiO-BZCYYb (active layer) ǀ NiO-BZCYYb (support layer). The peak power density of the single cell with dry air as oxidant and hydrogen as fuel at 750, 700, and 650 °C are 958.75, 872.78, and 709.22 mW cm−2, respectively. When the atmosphere is 20%H2O+80%air at air electrode side and hydrogen at fuel electrode side, the single cell operating as a proton-conducting solid oxide electrolysis cell shows high electrolysis current densities of 4299.4, 2744.6, 1711.8, and 974.6 mA cm−2 under 1.3 V at 750, 700, 650, and 600 °C respectively. Additionally, during the constant voltage electrolysis, the cell shows a good stability in 78 h at 650 °C. The results show that PBSCF is potential as air electrode of H–SOCs, especially in solid oxide electrolysis cell mode.

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

To promote the electrocatalytic activity and stability of traditional La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3– δ (LSCF) oxygen electrodes in reversible solid oxide cells (RSOCs), conventional physical mixed method was used to prepare the Pd-LSCF composite oxygen electrode. The cell with Pd-LSCF|GDC|YSZ|Ni-YSZ configuration shows perfect electrochemical performance in both solid oxide fuel cell (SOFC) mode and solid oxide electrolysis cell (SOEC) mode. In the SOFC mode, the cell achieves a power density of 1.73 W/cm 2 at 800 °C, higher than that of the LSCF oxygen electrode with 1.38 W/cm 2 . In the SOEC mode, the current density at 1.5 V is 1.67 A/cm 2 at 800 °C under 50 vol% steam concentration. Moreover, the reversibility and stability of the RSOCs is tested during 192 h long-term reversible operation. The degradation rate of the cell is only 2.2%/100 h and 2.5%/100 h in the SOEC and the SOFC mode, respectively. These results confirm that compositing Pd with the LSCF oxygen electrode can considerably boost the electrochemical performance of LSCF electrode in RSOCs field. The cell with configuration of Pd-LSCF|GDC|YSZ|Ni-YSZ shows well performance in RSOCs mode. In the SOFC mode, it can reach a maximum power density of 1.73 W/cm 2 at 800 °C; In the SOEC mode, the current density at 1.5 V can achieve 1.67 A/cm 2 at 800 °C with 50 vol% H 2 O. In addition, the cell also shows well reversibility and stability during 192 h long-term test. • Pd-La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3– δ .|GDC|YSZ|Ni-YSZ cell was tested as reversible solid oxide cells (RSOCs). • In the SOFC mode, the cell achieves a higher power density of 1.73 W/cm 2 at 800 °C. • In the SOEC mode, the current density achieves 1.67 A/cm 2 at 800 °C with 50 vol% steam concentration at 1.5V. • The cell shows superior reversibility and stability during 192 h long-term test.

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

Due to its high conversion efficiency in fuel-to-power and power-to-fuel modes, reversible solid oxide cell (RSOC) is widely regarded as a promising device. In this study, La0.6Sr0.4Co0.2Fe0.7M0.1O3 (M=Fe, Mo, Nb) oxides are prepared and composited with Sm0.2Ce0.8O2-(Li0.67Na0.33)2CO3 to form reversible single component cell (RSCC). Among them, La0.6Sr0.4Co0.2Fe0.7Mo0.1O3 (LSCFM) shows the highest oxygen vacancy concentration, further promoting catalytic activity towards oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR). Besides, Mo or Nb doping enhances the surface reaction rate constant (kchem) and oxygen chemical bulk diffusion coefficient (Dchem) at the temperature range of 550–700 °C. LSCFM shows the highest kchem of 39.3 × 10−5 cm s−1, approximate 3.5 times as 11.1 × 10−5 cm s−1 for La0.6Sr0.4Co0.2Fe0.8O3 (LSCF), and Dchem for LSCFM is 31.1 × 10−6 cm2 s−1, approximate 2.5 times as 12.4 × 10−6 cm2 s−1 for LSCF at 700 °C. Furthermore, the rate determining steps (RDS) of ORR and HOR for LSCFM-based cell are the reduction of oxygen atoms to O− and the charge transfer reaction, respectively. Under SOFC/SOEC reversible operation, the RSCC composed of LSCFM exhibits the best cell performance. In the solid oxide fuel cell mode, the maximum power density reaches 236.2 mW cm−2 and in the solid oxide electrolysis cell mode the current density at 1.3 V is -312.4 mA cm−2.

Solid Oxide Cells with Phase-Inversion Tape-Casted Hydrogen Electrode and SrSc0.175Nb0.025Co0.8O3−δ Oxygen Electrode for High-Performance Reversible Power Generation and Hydrogen Production

Solid oxide cells (SOCs) have been considered as a promising energy conversion and storage device. However, state-of-the-art cells' practical application with conventionally fabricated Ni-(Y2O3)0.08(ZrO2)0.92 (YSZ) cermet hydrogen electrode and La0.8Sr0.2MnO3 perovskite oxygen electrode is strongly limited by the unsatisfactory performance. Instead, new advances in cell materials and fabrication techniques that can lead to significant performance enhancements are urgently demanded. Here, we report a high-performance reversible SOC that consisted of a combination of SrSc0.175Nb0.025Co0.8O3-δ (SSNC) and phase-inversion tape-casted Ni-YSZ, which served as the oxygen and hydrogen electrode, respectively. The hydrogen electrode synthesized from phase-inversion tape-casting showed a high porosity of 60.8%, providing sufficient active sites for hydrogen oxidation in the solid oxide fuel cell (SOFC) mode and H2O electrolysis in the solid oxide electrolysis cell (SOEC) mode. Accordingly, it was observed that the maximum power density of 2.3 W cm-2 was attained at 750 °C in SOFC mode and a current density of -1.59 A cm-2 was obtained at 1.3 V in SOEC mode. Hence, these results reveal that the simultaneous optimization of oxygen and hydrogen electrodes is a pragmatic strategy that improves the performance of SOCs, which may significantly accelerate the commercialization of such an attractive technology.

Polarization Resistance of Ceria-Containing Fuel Electrodes in Solid Oxide Cells Studied By Impedance and DRT Analysis

Introduction Solid oxide electrolysis cell (SOEC), which enables steam electrolysis by reversely operating solid oxide fuel cell (SOFC), is capable of highly efficient hydrogen production. Whilst SOFC and SOEC are using similar materials and cell structures, electrode reactions in SOEC operation have to be studied in more details. In our previous study [1], Ni-ScSZ cermet, widely used as SOFC fuel electrodes, exhibited a tendency where electrode resistance increases at a low absolute current density in SOEC operation, while Ni-GDC co-impregnated cells fabricated by the impregnation method exhibited sufficient performance in a wide current density range. Here in this study, various model fuel electrode materials were evaluated and compared in a wide current density range in the SOFC and SOEC modes under the same measurement conditions. Electrode reactions are evaluated through measurements of electrochemical impedance spectra (EIS) and subsequent analysis of distribution of relaxation times (DRT). Experimental Three types of cells with different fuel electrodes were prepared: Ni-ScSZ cermet fuel electrode; Ni-GDC cermet fuel electrode; and Ni-GDC co-impregnated fuel electrode. The Ni-GDC cermet fuel electrodes were fabricated using gadolinia-doped ceria (GDC), widely known as a mixed ionic electronic conductor, besides a pure ionic conductor ScSZ. The Ni-GDC co-impregnated cell was fabricated by preparing a composite electrode backbone of GDC and La-doped SrTiO3 (LST), followed by impregnation of the catalyst particles, Ni and GDC. La-Sr-Ti oxides are known to exhibit sufficient stability in both oxidizing and reducing atmospheres [2, 3]. Moreover, co-impregnated fuel electrodes have shown high tolerance against highly-humidified hydrogen fuel and redox durability [4].Electrode resistances were separated into the ohmic resistance and the nonohmic polarization resistance through electrochemical impedance spectroscopy. Impedance measurements were made by using an impedance analyzer (1255WB, Solartron, UK) in a frequency range between 0.1 Hz and 1 MHz. The fuel, 50%-humidified H2, was supplied to the fuel electrode. The operating temperature was 800 °C. To characterize the electrode processes in both SOFC and SOEC modes, impedance measurements were performed by applying stepwise changes in current density up to ±1.2 A cm-2, for every 0.2 A cm-2. The DRT analysis of the EIS data was performed using a numerical calculation program based on Tikhonov regularization (Z-Assist, Toyo Corporation, Japan) to separate electrochemical processes involved in the electrode reactions. Results and discussion The polarization resistances obtained by the EIS measurements are shown in Fig. 1. The Ni-ScSZ cermet fuel electrode (Fig. 1 (a)) exhibited unique polarization resistance in the SOEC mode. With increasing the absolute value of current density in the SOEC mode, polarization resistance increased significantly at first, reaching a maximum value at -0.4 A cm-2, then decreased. In contrast, for the Ni-GDC cermet fuel electrode (Fig. 1 (b)), polarization resistance increased only slightly with increasing current density in the SOEC mode, and overall polarization resistance remained low. This tendency was also seen for the Ni-GDC co-impregnated fuel electrode (Fig. 1 (c)), where such increase in polarization resistance was further suppressed.Such increase and decrease in polarization resistance observed for the Ni-ScSZ cermet have to be explained by different mechanisms (at least two opposite factors). One possible explanation could be that the reduced and recovered catalytic activity due to e.g. oxidation and reduction of the Ni catalyst surface. The presence of GDC in the fuel electrode has a positive effect reducing the polarization resistance in the SOEC mode. The electrode reaction area may be extended beyond the Ni surface and/or the triple phase boundaries. Furthermore, the Ce ion in ceria undergoes a valence change, which may promote flexible surface exchange reactions, and surface reactions on the Ni catalysts. For the SOEC operation, it will be desirable to clarify the effects of GDC in the r-SOC electrode reactions. Acknowledgments 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) (Contract No.20001460-0). 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. References N. Endo, T. Fukumoto, R. Ushijima, K. Natsukoshi, Y. Tachikawa, J. Matsuda, S. Taniguchi, and K. Sasaki, ECS Trans., 103(1), 1981 (2021).Q. Ma and F. Tietz, Solid State Ionics, 225, 108 (2012).G. Chen, H. Kishimoto, K. Yamaji, K. Kuramoto, and T. Horita, J. Electrochem. Soc., 162(3), F223 (2014).S. Futamura, A. Muramoto, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, Int. J. Hydrogen Energy, 44(16), 8502 (2019). Figure 1

Performance and Durability of Solid Oxide Electrolysis Cell Air Electrodes Prepared By Various Conditions

Introduction Fuel electrode materials are important for achieving higher performance and durability of Solid Oxide Fuel Cell (SOFC), Solid Oxide Electrolysis Cell (SOEC), and Reversible Solid Oxide Cells (r-SOCs). On the other hand, the air electrode also faces performance and durability issues. For example, the diffusion of Sr ions from the air electrode materials has been reported.1, 2 In SOECs, if electrolysis can be performed at a thermoneutral potential balancing heat absorption by electrolysis and heat generation by the internal resistance of the electrolysis cell, hydrogen can be generated theoretically without heat supply. The purpose of this study is to evaluate the electrochemical performance and durability of the SOECs by conducting electrolysis performance tests of LSCF-based air electrodes with different preparation conditions, and electrolysis durability tests at the thermoneutral potential. Experimental Each test was conducted using a cell with a scandia-stabilized zirconia electrolyte (ScSZ, 200 µm thick) as a substrate. For the fuel electrode, Ni-GDC co-impregnated fuel electrode was used3. A mixture of Ni(NO3)2･6H2O, Gd(NO3)3･6H2O, and Ce(NO3)3･6H2O was impregnated into the porous LST-GDC framework, a mixture of LST powder (La0.1Sr0.9TiO3) and GDC powder (Gd0.9Ce0.1O₃). LSCF, (La0.6Sr0.4)(Co0.2Fe0.8)O3, was used for the air electrodes. GDC was inserted between the LSCF layer and the electrolyte plate as a buffer layer. In fabricating air electrodes, four types of air electrodes were applied, at shown in Table 1. For the electrochemical measurements of the air electrodes, Pt reference electrode was attached to the electrolyte on the fuel electrode side, and the voltage terminals of the electrochemical measurement system were connected between the reference electrode and the air electrode to measure air electrode voltage.In the performance tests, the voltage (potential) and impedance of the air electrode were measured at operating temperatures of 800°C, 750°C, and 700°C. In the durability tests, the air electrode was maintained at thermoneutral potentials at 800°C and 700°C (1.286 V and 1.283 V, respectively), and the voltage and impedance of the air electrodes were measured before and after the durability tests to evaluate the degradation of the air electrodes. Results and discussion Figure 1 shows the air electrode voltage obtained from the performance tests at each operating temperature for the four types of air electrodes shown in Table 1. First, regarding the dependence of the electrolysis performance on operating temperature, an increase in air electrode voltage (degradation of the air electrode) was found at 800°C.The increase in air electrode voltage with current density was almost linear, but at lower operating temperatures, the increase in air electrode voltage at lower current densities becomes larger. In other words, the I-V curve was rather ohmic at 800°C, but non-ohmic at 700°C.Figure 2 shows the results of an 80-hour durability test at different temperatures. The current density of the cell corresponding to the thermoneutral potential was -0.298 Acm-2 when operated at 800°C and -0.018 Acm-2 when operated at 700°C.The degradation rate was calculated from the difference in air electrode voltage before and after the performance and durability test at these current densities: 0.172% at 800°C and 0.792% at 700℃. The degradation rate increased as the operating temperature decreased in the SOEC mode, despite the low current density. Microstructural observation of the air electrode after the durability test showed that Sr diffused to the LSCF/GDC interface similar to SOFCs mode4, and that the diffusion of Sr was accelerated at 700°C. In addition, Co diffusion was also observed. Such diffusion may occur during LSCF firing process and/or during SOEC operation. More detailed investigation is in progress. Acknowledgements This research was supported in part by NEDO (New Energy and Industrial Technology Development Organization). We thank all parties involved. References (1) V. Subotić, S. Futamura, G. F. Harrington, J. Matsuda, K. Natsukoshi, and K. Sasaki, J. Power Sources, 492, 229600 (2021).(2) S. P. Simner, M. D. Anderson, M. H. Engelhard, and J. W. Stevenson, Electrochem. Solid-State Lett., 9 A478 (2006).(3) K. Natsukoshi, K. Miyara, Y. Tachikawa, J. Matsuda, S. Taniguchi, G. F. Harrington, and K. Sasaki, ECS Trans., 03, 203 (2021).(4) S. Kanae, Y. Toyofuku, T. Kawabata, Y. Inoue, T. Daio, J. Matsuda, J,-T. Chou, Y. Shiratori, S. Taniguchi, and K. Sasaki, ECS Trans., 68, 2463 (2015). Figure 1

Layered barium cobaltite structure materials containing perovskite and CdI2-based layers for reversible solid oxide cells with exceptionally high performance

To improve the sluggish kinetics of oxygen electrode reactions in reversible solid oxide cells (RSOCs), a novel layered structure Nd0.1Ca0.1Ba1.8Co9O14 (NCBCO) having alternating perovskite and CdI2-based layers is introduced in this field for the first time as a highly active and durable catalyst. Maximum power densities of NCBCO with zirconia-based cells are high, reaching 2.6 W·cm−2 at 800 °C in solid oxide fuel cell (SOFC) mode. Under a solid oxide electrolysis cell (SOEC) mode, the zirconia-based cell with NCBCO achieves an outstanding current density of − 4.36 A·cm−2 at 800 °C with an applied voltage of 1.6 V, surpassing many results of the reported oxygen evolution reaction catalysts with the high activities. Furthermore, RSOCs with NCBCO presents no degradation over entire 400 h in SOFC and SOEC modes. Such outstanding electrocatalytic performances of NCBCO for oxygen electrode reactions might be attributed to its high oxygen vacancy content with a high electrical conductivity. This is confirmed by X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption of oxygen, and electron paramagnetic resonance measurements. These results validate that the layered structure NCBCO can be a potential oxygen electrode material for RSOCs.

Transition Metal Doping of La0.3Ca0.7Fe0.7Cr0.3O3-δ for Nanoparticle-Enhanced Reversible CO2-CO Electrocatalysis

Fluctuations in supply and demand result in wasted energy during off-peak electric grid hours, with Reversible Solid Oxide Cells (RSOCs) potentially providing an efficient means by which such energy can be stored to create a highly sustainable grid.1 A RSOC can catalyze reactions such as the CO2 Reduction Reaction (CO2RR) by operating in the Solid Oxide Electrolysis Cell (SOEC) mode when energy supply is high, or in the Solid Oxide Fuel Cell (SOFC) mode when demand is high while oxidizing CO.1 The estimated yearly energy storage requirement is about 2-3 months per year, with RSOCs capable of achieving this goal.1 Single phase perovskite materials with the composition La0.3M0.7Fe0.7Cr0.3O3-δ (M = Sr, Ca) (LMFCr) have emerged as promising electrocatalysts for both fuel and oxygen electrodes in symmetrical RSOCs due to their phase stability in air and fuels (pO2 ~ 0.21 – 10-21 atm) and their compatibility with doped ceria-buffered yttria- and scandia-stabilized zirconia (YSZ/SSZ) electrolytes.2,3 The Ca analogue of LMFCr (LCFCr) has generally shown better performance and compatibility with other cell components,3 with significant efforts directed towards further enhancing its CO2RR activity by employing various approaches, including nanoscale modifications.4 One such technique is nanoparticle (NP) exsolution, which can greatly increase the catalytic surface area.5 This method employs B-site doping with Ni or Co followed by exposure to reducing conditions to decorate the catalyst surface with B-site metal alloy NPs.4 These are strongly anchored to the bulk material, highly resistant to coking, and stable under the harsh conditions of RSOCs.4 In our initial work, the exsolution characteristics of Fe-Ni NPs from 5% Ni-doped LCFCr (LCFCrN) were studied in H2:N2 and CO2:CO atmospheres using ex situ XRD, SEM, and STEM-EDS.4 Exsolution kinetics were rapid in H2:N2 (pO2 ~ 10-23 atm) at 800 °C with NPs assuming an average size of 45 nm and an Fe-rich Fe0.64Ni0.36 composition within 1 h exposure to 5H2:95N2. On the other hand, 70CO:30CO2 atmospheres (pO2 ~ 10-20 atm) gave sluggish kinetics with the NPs achieving an average size of 40 nm and a Ni-rich FeNi3 composition even after more than 25 hours of treatment. This suggests stability to coarsening in highly reducing atmospheres.4 In more recent work, 5% Co-doped LCFCr (LCFCrCo) perovskites show visible Fe-Co NP formation at 800 °C upon exposure to 5H2:95N2 for at least 1 h or to 70CO:30CO2 for at least 5 h.Electrochemical characterization of Fe-Ni NP decorated LCFCrN (Fe-Ni@LCFCrN) was conducted on 2.5 cm diameter cells using ceria-buffered SSZ electrolyte. A LCFCrN ink was screen-printed onto a ~0.5 cm2 area on both sides of the cell followed by sintering at 1100 °C for 2 h. Au was painted onto each electrode, and the cell was sintered again at 825 °C for 1 h. Electrical connections were made via Au gauzes and wires. NP exsolution was induced by exposing the fuel electrode to 5H2:95N2 for 2 h at 800 °C. Various CO2:CO mixtures (100:0, 90:10, 70:30, 50:50) were supplied to the fuel electrode while air was supplied to the oxygen electrode with flow rates of 50 mL/min. Electrochemical performance was tested via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronoamperometry. CVs showed that the NPs enhanced LCFCrN activity for CO2RR (~ 15%) and more notably for CO oxidation (~ 75%) at the same overpotential (~ 0.7 V), making the catalyst equally active for the two reactions. 15-minute potentiostatic tests indicated stable current densities of about –0.65, –0.634, and –0.618 A/cm2 in 100% CO2, 90CO2:10CO, and 70CO2:30CO respectively at a cell potential of 1.6 V. Medium-term (10 h) potentiostatic tests for CO2RR indicated excellent stability with a current density of –0.28 A/cm2 at a cell potential of 1.3 V in 70CO2:30CO (pO2 ~ 10-18 atm). The excellent electrochemical performance in both the SOFC and SOEC modes makes Fe-Ni@LCFCrN a very promising electrode material for RSOCs. Further work on LCFCrCo is underway, with comparisons being made between LCFCrN and LCFCrCo on CO2-CO electrocatalysis and NP characteristics. References Jensen, S. H.; Graves, C.; Mogensen, M.; Wendel, C.; Braun, R.; Hughes, G.; Gao, Z.; Barnett, S. A. Energy Environ. Sci. 8, 2471 (2015).Molero-Sánchez, B.; Addo, P.; Buyukaksoy, A.; Paulson, S.; Birss, V. I. Faraday Discuss. 182, 159 (2015).Molero-Sánchez, B.; Prado-Gonjal, J.; Avila-Brande, D.; Chen, M.; Moran, E.; Birss, V. I. J. Hydrog. Energy. 40, 1902 (2015).Ansari, H. M.; Bass, A. S.; Ahmad, N.; Birss, V. I. Mater. Chem. A (2022).Zhu, Y.; Dai, J.; Zhou, W.; Zhong, Y.; Wang, H.; Shao, Z. Mater. Chem. A. 6, 13582 (2018).

Exchange current density of reversible solid oxide cell electrodes

Reversible solid oxide cells (r-SOCs) can be operated in either solid oxide fuel cell or solid oxide electrolysis cell mode. They are expected to become important in the support of renewable energy due to their high efficiency for both power generation and hydrogen generation. The exchange current density is one of the most important parameters in the quantification of electrode performance in solid oxide cells. In this study, four different fuel electrodes and two different air electrodes are fabricated using different materials and the microstructures are compared. The temperature, fuel humidification, and oxygen concentration at the air electrode are varied to obtain the apparent exchange current density for the different electrode materials. In contrast to ruthenium-and-gadolinia-doped ceria (Rh-GDC) as well as nickel-and-gadolinia-doped ceria (Ni-GDC) electrodes, significant differences in the apparent exchange current density were observed between electrolysis and fuel cell modes for the nickel-scandia-stabilized zirconia (Ni-ScSZ) cermet. Variation of gas concentration revealed that surface adsorption sites were almost completely vacant for all these electrodes. The apparent exchange current densities obtained in this study are useful as a parameter for simulation of the internal properties of r-SOCs.

Operando observations of active three phase boundary of patterned nickel - Yttria stabilized zirconia electrode in solid oxide cell

In solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs), the substantial performance degradation of nickel (Ni) - yttria-stabilized zirconia (YSZ) fuel electrode has been a growing challenge for commercialization. The microstructure of Ni-YSZ electrode is not stable during long-term operation due to Ni phase redistribution. In the present study, operando observations of patterned Ni film electrodes under SOFC, SOEC and reversible solid oxide cell (RSOC) operation modes are conducted. Confocal laser scanning microscopy is applied to acquire real-time microscopy images of the three phase boundary (TPB) under three operation modes to study the Ni mobility on the YSZ surface. The operando observation showed that the Ni phase dynamically spreads and splits up only under the SOFC mode, while this phenomenon is not observed under the SOEC mode. Ni coarsening occurs regardless of the operation mode. The dynamic motion of active TPB qualitatively correlates with the variation in the current density during the operation. The contact angles of Ni on flat YSZ surface after operation are the smallest for SOFC, followed by RSOC and the largest for SOEC. Competition between the altered Ni wettability and the Ni agglomeration finally determines the complicated Ni morphological changes.

Mutual Conversion of CO-CO2 on a Perovskite Fuel Electrode with Endogenous Alloy Nanoparticles for Reversible Solid Oxide Cells.

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.