Pd–La0.6Sr0.4Co0.2Fe0.8O3– composite as active and stable oxygen electrode 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.

The nickel(Ni)-patterned electrode enables researchers to quantify the triple-phase boundary (TPB) length and Ni surface area, exclude the influence of bulk gas diffusion and clearly separate the active regions of the chemical/electrochemical reactions. This tool has been studied for identifying the reaction mechanism in SOFC but rarely in SOEC. In this study, the Ni-patterned electrodes with the stripe width of 100 μm were tested in both the solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes at the atmosphere of H2O/H2 and further H2O/CO2/H2/CO. The experimental test shows the stability of the Ni-patterned electrode in the H2O/H2 atmosphere was much poorer than that in the CO/CO2 atmosphere due to the nickel reacting with steam. Thus, the Ni-patterned electrode needed to operate at a temperature of ≤700oC with the inlet H2/H2O molar ratio of >7 and the current-applied time shouldn’t be over 5 h to keep the accuracy of the TPB length and Ni surface area. The effects of the temperature, partial pressure of H2O and H2 are investigated. The activation energy of the Ni-patterned electrodes was 0.386 eV. The electrochemical performance had a positive correlation with both the partial pressure of H2 and H2O, especially showed higher sensitivity to the partial pressure of H2O. Further, a possible mechanism of the H2O/H2 electrochemical conversion was proposed according to the experimental data and existing literature, which contains two-step charge-transfer reaction: H(Ni)+O2-(YSZ)OH-(YSZ)+(Ni)+e- and H(Ni)+OH-(YSZ)H2O(YSZ)+(Ni)+e-. Based on the experiments, an analytical calculation was performed to investigate the rate-determining steps in the Ni-patterned electrodes by bridging the connection between the kinetic parameters and operating conditions. The results indicate the rate-limiting steps may be different for the SOFC and SOEC modes. In SOFC mode, the H2 electrochemical oxidation could depend more on the charge-transfer reaction H(Ni)+O2-(YSZ)→OH-(YSZ)+(Ni)+e-, however in SOEC mode, the rate-determining step of the H2O electrochemical reduction should be H2O(YSZ)+(Ni)+e-→OH-(YSZ)+H(Ni). In this way, a reversible mechanism could be applied for reversible SOFC (RSOFC) to describe the conversion between hydrogen and steam by different charge-transfer-reaction domination in the SOFC and SOEC mode. 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

Weak electrocatalytic activity of the La0.8Sr0.2MnO3-δ (LSM) oxygen electrode at medium and low temperatures leads to decreasing performance both in the solid oxide fuel cell (SOFC) mode and the solid oxide electrolysis cell (SOEC) mode. Herein, we design an Er0.4Bi1.6O3-δ (ESB) functionalized La0.8Sr0.2MnO3-δ (labeled as LSM/ESB) oxygen electrode via a one-step co-synthesis modified Pechini method. The unique LSM/ESB with polarization resistance of only 0.029 Ω·cm2 at 750 °C enables a highly enhanced rate of oxygen reduction and evolution reaction. The single cell with the LSM/ESB electrode achieves a maximum power density of 1.747 W cm-2 at 750 °C, 2.6 times higher than that of the mechanically mixed LSM-ESB electrode (0.667 W cm-2). In the SOEC mode, it also shows the improved performance of the LSM/ESB electrode. Furthermore, the cell exhibits admirable durability of 90 h in the fuel cell mode and excellent reversibility. The better performance can be concluded as a better surface-active state and a tighter connection between the LSM and ESB particles of LSM/ESB via a co-synthesis process. This work proposes a novel strategy to advance the application of the one-step modified Pechini technology for an efficient and stable oxygen electrode.

Increasing demand for long-term energy storage, reduced carbon emissions, more resilient grid, and the lowest cost grid balancing solutions promotes solid oxide electrolysis cell (SOEC) and reversible solid oxide cell (RSOC) technologies. These systems can achieve high hydrogen production rates with near 100 percent stack electrical efficiency while co-electrolysis of carbon dioxide and water leads to production of syngas. RSOC can serve as a grid storage device (hydrogen production) and power production plant (provide electricity during peak demand). With over 25 years’ experience building and testing solid oxide cell materials, components and stacks, Nexceris has a proven track-record of commercializing products to major OEMs. The established manufacturing platforms at Nexceris enable smooth expansion into new applications.In this talk, development of cells, stack components and stacks will be discussed. Performance and durability improvements from single cell to short stacks will be addressed. Cells with downselected electrode sets were implemented into short stacks with active cell area of 42 cm2 and 96 cm2. Stacks were operated continuously in SOEC mode at ~1.3 V for ≥ 500 hours with <5%/kh degradation and improving. The resulting short stack demonstrations show promising baseline performance and durability platform for future cell and stack development. RSOC tests were recently initiated with dynamic operation. Nexceris’ ChromLok coatings for bipolar plates were also integrated to stacks to enhance durability.

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.

Performance enhancement of Ni-YSZ electrode by impregnation of Mo0.1Ce0.9O2+δ

Electrochemical performance and stability of cobalt-free Ln1.2Sr0.8NiO4 (Ln=La and Pr) air electrodes for proton-conducting reversible solid oxide cells

The nickel (Ni) patterned electrode enables to quantify the triple-phase boundary (TPB) length and Ni surface area as well as exclude the interference of bulk gas diffusion. In this study, an analytical calculation based on the oxygen spillover mechanism is performed by assuming one elementary reaction as the rate-limiting reaction and the others in the equilibrium state to bridge the connection among the polarization resistance, charge transfer coefficient and partial pressure of CO/CO2 based on Butler-Volmer equation. The analytical calculation is compared with the experimental data of the Ni patterned electrode published in previous paper(1) to investigate the rate-limiting steps in both the solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes. In SOFC mode, CO oxidation into CO2 is speculated to be the major electrochemical reaction, besides, adsorbed carbon could also be oxidated into CO. In SOEC mode, the limitation of CO2 adsorption may inhibit CO2 electrochemical reduction, thus, CO reduction into carbon could be more probably be the major electrochemical reaction.

Ceramic high-temperature fuel cells and electrolysers are efficient energy conversion systems for electrical power generation and hydrogen production. Their core components are constituted by a stack of electroactive Solid Oxide Cells (SOCs) in which the electrochemical reactions take place. Thanks to their flexibility, the same stack can be alternatively operated in both fuel cell and electrolysis modes. However, the insufficient durability of SOCs still constitutes a major limitation for the technology. The present study addresses this issue and aims to bring some new insights on the effect of the Solid Oxide Fuel Cell (SOFC) versus Solid Oxide Electrolysis Cell (SOEC) operating modes on degradation of a typical Ni-YSZ//YSZ//CGO//LSCF-CGO cell. The electrochemical degradations are generally attributed to several underlying phenomena such as electrode microstructural evolution, material chemical decomposition or electroactive sites poisoning by contaminants. Among them, it is generally considered that Ni agglomeration in the Ni-YSZ cermet and Lanthanum Strontium Cobalt Ferrite (LSCF) material destabilization are two prevalent mechanisms involved in the cell performance deterioration. Therefore, these two mechanisms have been specifically investigated by a coupled approach of long-term testing in both SOFC and SOEC modes (1000 ≤ t (h) ≤9000) and post-test characterizations. The experimental results have then been analyzed in the frame of an in-house multi-scale model with the purpose to interpret them and to quantify the effect of material ageing on cell performances. The extent of Ni agglomeration has been characterized by three-dimensional electrode reconstructions obtained by X-ray nano-holotomography at European Synchrotron Radiation Facility (ESRF) (on the new Nano-Imaging beamline ID16A-NI). The new set-up and protocol enable the reconstructions of valuable 3D volumes (Fig. 1) with a large field of view (~50 µm) along with a high spatial resolution (~50 nm). The electrode morphological properties, which have been measured on the 3D volumes, have revealed a substantial Ni coarsening over time at 850°C and 750°C, whereas no Ni depletion was detected at the electrolyte interface. The increase of the Ni particle size is found to induce a decrease in both (i) the density of Triple Phase Boundary (TPBs) lines and (ii) the interfacial surface area between Ni and gas. Moreover, it was found that the Ni/YSZ interfacial surface area does not evolve during the experiments. This statement indicates that the ceramic backbone in the cermet prevents a massive Ni agglomeration at the SOFC/SOEC operating temperature. The compilation of all experimental data have allowed fitting the parameters of a physically-based law for Ni coarsening that was introduced in the modelling framework. The simulations have revealed that Ni agglomeration explains around 20-25% of the electrochemical degradation at 850°C after 1000 hrs of operation. However, the electrode microstructural evolution is found not to be affected by the cell polarizations. Therefore, the mechanism cannot explain the higher degradation rates recorded in electrolysis mode compared to the fuel cell ones. To explain the impact of the operating modes (SOFC or SOEC) on the degradation rates, several post-test analyses (i.e. Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray µfluorescence and µdiffraction techniques) have been employed to investigate the phase reactivity in the region of the CGO barrier layer. The characterizations have revealed that Sr diffusion across the barrier layer and formation of SrZrO3 secondary phase occur mainly during electrolysis operation, whereas the process is very limited in fuel cell mode (Fig. 2). As a consequence, LSCF destabilization is found not to be involved in the degradation of cell performances during fuel cell operation while it could explain the highest degradation rates recorded in electrolysis mode. The post-test analyses have also revealed a diffusion and an accumulation of Co in the region of the barrier layer which is concomitant with the formation of SrZrO3. The formation of these Co-rich segregates in contact with SrZrO3 grains have been identified as cobalt-ferrite type compound. The in-house multi-scale model has been used to interpret the role of the cell operating mode on the LSCF destabilization mechanism. The cell polarization curves and the local quantities within the O2 electrode have been computed in both fuel cell and electrolysis modes. The simulations have shown that the electrolysis operation leads to a strong depletion of oxygen vacancies in the LSCF material. It has been proposed that the depletion in oxygen vacancies under electrolysis polarization could drive the Sr release from the structure, and in turn, could explain the experimental results. Based on this proposition, a possible mechanism for the LSCF destabilization and SrZrO3 formation has been detailed. Figure 1

Exchange current density of reversible solid oxide cell electrodes

Increasing demand for long-term energy storage and reduced carbon emissions promotes solid oxide electrolysis cell (SOEC) technology. SOEC systems can achieve high hydrogen production rates with near 100 percent stack electrical efficiency while co-electrolysis of carbon dioxide and water leads to production of syngas. With over 25 years’ experience building and testing solid oxide fuel cell (SOFC) materials, components and stacks, Nexceris has a proven track-record of commercializing SOFC products to major OEMs. The established manufacturing platforms at Nexceris enable smooth expansion into SOEC applications.In this talk, development of new SOEC electrode materials, stack components and stacks will be discussed. Over 50 down-selected air and fuel electrodes were electrochemically screened on 25 cm2 electrolyte-supported single cells and compared to baseline electrodes. Multiple electrode sets provided current density over -1.5 A/cm2 at 1.3 V and 800 °C with 75 percent steam/25% percent hydrogen fuel and air. The promising electrodes were implemented into short stacks with active cell area of 42 cm2. Stacks were operated continuously in SOEC mode at 1.3-1.4 V for ≥ 500 hours. The resulting short stack demonstrations show promising SOEC baseline performance and durability platform for future cell and stack development. Co-electrolysis and water electrolysis tests were also conducted and yielded comparable performance. Nexceris’ ChromLok coatings for bipolar plates were also integrated to stacks and tested under realistic conditions.

High-temperature CO₂ electrolysis in solid oxide electrochemical cells offers one of the most efficient routes for carbon dioxide conversion, enabling the production of highly pure carbon monoxide due to favorable thermodynamics and fast kinetics above 600 °C. The emergence of CO:CO 2 reversible solid oxide cells (rSOCs) further enhances system efficiency promoting integration with CO₂-rich and CO-rich industrial exhaust streams. However, reversible operation imposes stringent requirements on electrode materials, which must combine high catalytic activity, redox stability, and long-term durability under a wide range of oxygen partial pressures. Herein, we report a doubly B-site–substituted perovskite, La₀.₆Sr₀.₄Fe₀.₆Mn₀.₂M₀.₂O₃ −δ (M = Cu, Ni), as a multifunctional electrode platform for rSOCs. Both La₀.₆Sr₀.₄Fe₀.₆Mn₀.₂Cu₀.₂O₃ −δ (LSFMC) and La₀.₆Sr₀.₄Fe₀.₆Mn₀.₂Ni₀.₂O₃ −δ (LSFMN) are synthesized as single-phase perovskites, with rhombohedral symmetry ( R-3c ). When evaluated as oxygen electrodes in symmetric cell configurations, LSFMC and LSFMN exhibit significantly enhanced oxygen electrocatalysis, achieving an area-specific resistance decrease by 51% and 38%, respectively, compared to the unsubstituted material. Under reducing conditions, LSFMN undergoes controlled and homogeneous exsolution of Fe Ni nanoparticles, generating catalytically active metallic domains while preserving structural integrity. A quasi-symmetric electrolyte-supported cell based on La₀.₈Sr₀.₂Ga₀.₈Mg₀.₂O₃ −δ (LSGM) electrolyte, employing LSFMN as fuel electrode and LSFMC as air electrode, demonstrates excellent performance and durability in both CO-fuelled solid oxide fuel cell mode and CO₂ electrolysis mode. Stable and reversible operation is maintained for over 150 h in a 50:50 CO:CO 2 mixture. Targeted B-site substitution of Mn-stabilized ferrites enables the design of high-performance, cobalt-free and reversible electrodes, offering a promising strategy for next-generation rSOCs. • La 0.6 Sr 0.4 Fe 0.6 Mn 0.2 Ni 0.2 O 3-δ and La 0.6 Sr 0.4 Fe 0.6 Mn 0.2 Cu 0.2 O 3-δ are successfully synthesized as electrodes for r-SOCs; • 20 mol% Cu-substitution results in remarkably low air electrode resistance for a Co-free perovskite; • 20 mol% Ni-substitution promotes highly-active and uniform Fe Ni nanoparticles exsolution; • Quasi-symmetric cell with LSFMC at the air side and LSFMN at the fuel sideshows reversible stability up to 150 h in CO:CO 2 at 850 °C; • Post-mortem SEM analysis shows the stability of exsolved nanoparticles after long-term operation;

A three-dimensional computational fluid dynamics (CFD) model has been created to model high-temperature steam electrolysis in a planar solid oxide electrolysis cell (SOEC). The model represents a single cell, as it would exist in an electrolysis stack. Details of the model geometry are specific to a stack that was fabricated by Ceramatec, Inc. and tested at the Idaho National Laboratory. Mass, momentum, energy, and species conservation and transport are provided via the core features of the commercial CFD code FLUENT. A solid-oxide fuel cell (SOFC) model adds the electrochemical reactions and loss mechanisms and computation of the electric field throughout the cell. The FLUENT SOFC user-defined subroutine was modified for this work to allow for operation in the SOEC mode. Model results provide detailed profiles of temperature, Nernst potential, operating potential, anode-side gas composition, cathode-side gas composition, current density and hydrogen production over a range of stack operating conditions. Mean model results are shown to compare favorably with experimental results obtained from an actual ten-cell stack tested at INL.

Nanoscale perovskite oxides with enhanced electrocatalytic activities have been widely used as oxygen electrodes of reversible solid oxide cells (RSOC). Here, La0.6Sr0.4FeO3−δ (LSF) nanoscale powder is synthesized via a novel molten salt method using chlorides as the reaction medium and fired at 850 °C for 5 h after removing the additives. A direct assembly method is employed to fabricate the LSF electrode without a pre-sintering process at high temperature. The microstructure characterization ensures that the direct assembly process will not damage the porosity of LSF. When operating as a solid oxide fuel cell (SOFC), the LSF cell exhibits a peak power density of 1.36, 1.07 and 0.7 W/cm2 at 800, 750 and 700 °C, respectively, while in solid oxide electrolysis cell (SOEC) mode, the electrolysis current density reaches 1.52, 0.98 and 0.53 A/cm2 under an electrolysis voltage of 1.3 V, respectively. Thus, it indicates that the molten salt routine is a promising method for the synthesis of highly active perovskite LSF powders for directly assembled oxygen electrodes of RSOC.