Solid Oxide Electrolysis Cells Operation Research Articles

Understanding the phase stability of yttria stabilized zirconia electrolyte under solid oxide electrolysis cell operation conditions

The degradation mechanism of yttria-stabilized zirconia (YSZ) with varying Y2O3 compositions under an applied electric potential.

(Invited) Materials Stability in Anode-Supported Solid Oxide Cells

Professor Anil Virkar has been a pioneer in the development and characterization of high-performance anode-supported solid oxide fuel cells, and in understanding degradation processes in such cells operated in electrolysis mode. This talk reviews methods for improving anode-supported cells, particularly for yielding high performance at reduced operating temperature. The understanding of degradation mechanisms occurring during solid oxide electrolysis cell operation will be discussed.

Performance optimization of solid oxide electrolysis cell for syngas production by high temperature co-electrolysis via differential evolution algorithm with practical constraints

Co-electrolysis of water and carbon dioxide using solid oxide electrolysis cell combined with the catalytic reactor to produce renewable synthesis fuel is considered as a promising strategy for carbon dioxide utilization. However, in order to achieve a target product with a remarkable system-level efficiency, a certain proportion of carbon monoxide and hydrogen from solid oxide electrolysis cell must be sent to the catalytic reactor directly, which imposes practical constraints on the solid oxide electrolysis cell performance. Take this constraint into consideration, we proposed a performance design methodology that enables a rapid and effective optimization of the solid oxide electrolysis cell operating conditions. Firstly, a physical model is developed by integrating chemical equilibrium, electrochemistry, and energy conservation involved in the solid oxide electrolysis cell. Secondly, a physical model is utilized to generate a dataset for the development of neural network model and optimization algorithm. This approach allows for the efficient determination of optimal conditions while considering practical constraints. In the case study, under the constraint that the desired fuel is a long carbon chain fuel with a carbon dioxide conversion rate ranging from 80 % to 90 %, the highest efficiency achieved by the solid oxide electrolysis cell is optimized to 66.75 %. Moreover, this approach is extended to other two target fuels (dimethyl ether and methane) under different carbon dioxide conversion rate, and the optimal performance is designed. According to the results, there is a trade-off between high efficiency and high carbon dioxide conversion rate, as the low carbon dioxide conversion rate often leads to higher efficiency. This method allows for flexible design of operating conditions to meet different fuel requirements and guide practical applications.

Control strategy of solid oxide electrolysis cell operating temperature under real fluctuating renewable power

Power-to-liquid (PtL) technology through solid oxide electrolysis cells (SOECs) can efficiently convert renewable energy sources into synthetic fuels for long-time energy storage. However, due to its fluctuation and intermittence, renewable energy sources only produce time-varying currents, which directly impact the operating temperature of SOEC, generating high thermal stresses for SOEC and reducing its durability. To deal with this issue, a control-oriented SOEC dynamic model containing an electrochemical part, a reversible water gas shift (RWGS) part, and a thermodynamic part, was developed in this paper. Afterward, main control parameters, able to adjust SOEC operating temperature effectively, were determined through the Extended Fourier Amplitude Sensitivity Test (EFAST). The corresponding results show that the air flow rate was the best choice to control SOEC temperature when it works in a low current density case, while the fuel flow rate becomes optimal when SOEC operates in a high current case. Given this knowledge, an adaptive feedback control strategy was proposed to stabilize the operating temperature of SOEC in practice. In the end, the effectiveness of the proposed control was demonstrated through step signals of current in both low- and high-current density cases and an actual fluctuating current from real solar cells.

Role of phosphorus impurities in decomposition of La2NiO4–La0.5Ce0.5O2-δ oxygen electrode in a solid oxide electrolysis cell

This study explored the decomposition mechanism of a La2NiO4 (LNO) phase in the La2NiO4–La0.5Ce0.5O2-δ (LNO-LDC) oxygen electrode in a solid oxide electrolysis cell (SOEC) after testing at 800 °C. Scanning electron microscopy and scanning transmission electron microscopy examinations of the LNO-LDC oxygen electrode before and after testing were undertaken. Other than phosphorus contamination in the form of P-rich grains and P-rich deposits along all grain boundaries (GBs), LNO and LDC phases were intact without degradation in the as-fabricated electrode. However, mild to aggressive LNO phase decomposition triggered by the phosphorus poisoned GBs was observed after testing at 800 °C for 900 h. The evolution of the LNO phase decomposition was noted beginning with the exsolution of Ni into the surrounding LNO matrix and GBs, forming La-rich and Ni-rich phases correspondingly, in the LNO. This study illustrates a detailed decomposition progress of the LNO phase at the atomic level under an SOEC operation condition, and sheds light on how to ameliorate the fabrication process of SOECs to enhance their performance and durability.

Phase Evolution in (La0.6Sr0.4)0.95Co0.2Fe0.8O3-Δ Oxygen Electrodes after SOEC Operation

This work aims to evaluate the microstructural and compositional changes in (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ (LSCF) electrodes in solid oxide electrolysis cells (SOEC) before and after testing at 750°C for 2800 hours at 1.3V. Scanning electron microscopy (SEM) examination of the LSCF electrodes before and after SOEC operation was conducted. In untested LSCF electrodes, LSCF perovskite was a predominant phase with very minor presence of La(Fe,Co) oxide, and Fe,Co spinel phases. The post-tested electrodes exhibited a change where the La(Fe,Co) oxide phase exhibited a broader composition range, most notably with respect to the La/Fe ratio and potential loss of Co, and there was a noticeable increase in the volume fraction of Fe,Co spinel from 1.5% in the untested cell to over 3% after testing. To better understand the phases and their compositions, a transmission electron microscopy (TEM) study of tested and untested electrodes was undertaken using an aberration (Cs) corrected JEM-ARM200CF microscope operated at 200 kV. TEM confirmed the presence of cubic spinel particles with a composition of 26 at.% Fe and 13-15 at.% Co, with little to no Sr or La present. The composition of the La(Fe,Co) phase was more variable in agreement with the SEM analysis, with the Co/Fe ratio higher in the untested cell versus the cell tested at 2800 hours.

Understanding La2NiO4-La0.5Ce0.5O2 Oxygen Electrode Phase Evolution in a Solid Oxide Electrolysis Cell

This work aims to understand the decomposition mechanism of a La2NiO4 phase in La2NiO4-La0.5Ce0.5O2 (LNO-LDC) oxygen electrodes after testing in a solid oxide electrolysis cell (SOEC) at 800oC. Scanning/transmission electron microscopy (S/TEM) examination of LNO-LDC electrode at atomic level before and after testing was undertaken. Pure LNO and LDC phases as well as a low density of lanthanum phosphate (LPO) grains were detected in the as prepared (untested) oxygen electrode. P enrichment, a potential constituent in some of the synthesis additives, was found as small, nanometer scale, deposits along all grain boundaries which is suggested to poison the LNO phase during SOEC operation. Mild to aggressive decomposition of La2NiO4-La0.5Ce0.5O2 (LNO-LDC) oxygen electrode was observed after SOEC operating at 800 °C at 1.3V for 920 hours. Detailed microstructural and microchemical characterization of the decomposed regions was performed using an aberration (CS) corrected JEM-ARM200CF transmission electron microscope operated at 200 kV. The evolution of the LNO phase decomposition was noted beginning with the expulsion of Ni into the surrounding LNO matrix and grain boundaries, forming La-rich and Ni-rich phases in the LNO correspondingly. An acicular La2O3 phase was always noted initiating at either the LNO/LDC or LNO/LNO interfaces, growing and eventually intersecting one another, especially in aggressively decomposed regions. Initial decomposition exhibited single layers of Ni atoms along (004) plane of LNO, followed by gradual formation of La3Ni2O7, LaNiO3, and fine NiO (several nanometer in size) phases during moderate to aggressive decomposition. In addition, NiO was noted at LDC/LDC grain boundaries ranging from a few to several tens of nanometers thick. This presentation will demonstrate how P contamination may affect the stability of SOEC cells.

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

In-Situ Observation of Temperature Distribution on a Planar Type SOEC During Start-Stop Cycle Operation

Introduction It is important to establish various analytical methods for solid oxide electrolysis cells (SOECs) under practical operating conditions in order to improve their performance and to analyze their degradation phenomena. In-situ observation of temperature distribution on a planar type SOEC enables to reveal the distributions of electrochemical reactions and the cell degradation. While there have been several reports on the internal visualization of solid oxide fuel cells (SOFCs)1, 2, there have been only a few reports on that of SOECs. Since the endothermic electrolysis reaction occurs in SOECs, comparing with SOFCs, a different temperature distribution will appear. Therefore, it should be necessary to evaluate the phenomena on the planar SOECs for analyzing the temperature distribution taking gas flow and current distribution into consideration.The purpose of this study is to evaluate the effect of cell degradation caused by the start-stop cycle operation on the temperature distribution using a 5 cm × 5 cm planar cell observed by infrared camera. Experimental In this study, a planar cell was fabricated using a scandia-stabilized zirconia electrolyte (ScSZ, 200 µm thick, 5 cm × 5 cm in area) as a substrate. LSM ((La0.8Sr0.2)0.98MnO3) and LSM-ScSZ composite materials were used for the air electrode, and nickel and ScSZ cermet were used for the fuel electrode. The electrode area was 4 cm × 4 cm (16 cm2). Each electrode material was screen-printed and sintered. A platinum mesh was used as the current collector.In this test, the cell was operated at 800 °C and 50%-humidified hydrogen (200 ml min-1) was supplied to the fuel electrode. The durability test was conducted under the accelerated degradation conditions of a start-stop cycle repeated in every 1 hour with 0.2 A cm-2 for 500 cycles (1000 hours). Electrochemical characteristics were measured by connecting Pt wires, spot-welded every 1 mm on one side of both electrode surfaces, to the voltage terminals of the electrochemical measurement setup and to the current terminals of the external power supply unit.Figure 1 shows a schematic view of the experimental setup and temperature change compared with the initial condition. The temperature distribution was observed by infrared camera (FLIR SC2500 - NIR, uncertainty ±0.02 oC) from the air electrode side open to the electric furnace. The difference between the temperatures before and during current loading was derived as the distribution of temperature changes. Results and discussion First, the electrochemical characteristics of the planar type SOECs during the start-stop cycle test were obtained. The cell voltage at 0.2 A cm-2 increased from 1.19 V to 1.29 V during the test up to 500 cycles. The cell voltage was 1.27 V even after the initial 50 cycles. This suggests that SOEC electrode degradation3,4 occurred in the early stage of the start-stop cycling.The temperature changes after 50 cycles showed that each temperature change was relatively small because the cell was operated near the thermoneutral potential (around 1.29 V at 800 °C). It was confirmed that the endothermic electrolysis reaction was predominant near the fuel inlet. In addition, heat generation due to Joule heating was also appeared near the current collecting Pt wire connected to the power supply. Therefore, in SOEC operation, it is suggested that the temperature distribution is affected by the shape of current collector, gas supply condition, and electrochemical reaction distribution. The detailed analysis on the SOEC electrochemical properties and temperature distribution observed by in-situ measurements during cycling tests is also presented. Acknowledgements A part of this research was supported by New Energy and Industrial Technology Development Organization (NEDO), project No. JPNP14026. References (1) S. Shinichi and H. Iwai, J. Power Sources, 482, 229070 (2021).(2) Y. Shiratori, T. Ogura, H. Nakajima, M. Sakamoto, Y. Takahashi, Y. Wakita, T. Kitaoka, and K. Sasaki, Int. J. Hydrogen Energy, 38 (25), 10542 (2013).(3) M. Keane, M. K. Mahapatra, A. Verma, and P. Singh, Int. J. Hydrogen Energy, 37 (22), 16776 (2012).(4) M. Hubert, J. Laurencin, P. Cloetens, B. Morel, D. Montinaro, and F. Lefebvre, J. Power Sources, 397, 240 (2018). Figure 1

Benchmarking and Simulation of Solid-Oxide Steam Electrolysis for Propellant Production in a Vacuum Chamber-Simulated Lunar Environment

NASA and other space agencies have prioritized the development of in-situ resource utilization (ISRU) technology for producing critical materials such as propellants in space. Electrolysis to split water found in lunar, asteroid, and Martian environments provides a pathway for liquid H2 and O2 production, but the challenges of deploying such technology in extraterrestrial environments put an extreme emphasis on minimizing specific energy (measured in kWhelec/kgH2) and system specific mass (system kg/(kgH2/h)). High-temperature, solid oxide electrolysis cell (SOEC) systems have the potential to achieve lower specific energy than conventional liquid-phase alkaline and polymer electrolyte membrane (PEM) electrolysis systems, but this requires efficient thermal integration and minimization of balance of plant loads in order to take advantage of the lower electrolysis voltages for steam (vs. liquid H2O). The high operating temperatures of SOECs, however, require significant system mass for steam generators, heat exchangers for recuperation, and insulation of the electrolysis stack, particularly for space applications where ambient temperatures may be extremely low.This NASA-sponsored collaboration between OxEon Energy and Colorado School of Mines explored the feasibility of a lunar-based SOEC system, which was modeled and demonstrated at the lab-scale by operating a breadboard system, consisting of a SOEC stack and associated balance of plant BOP) in a cryogenic vacuum chamber in order to simulate a lunar environment. The combined »2.5-kWelec SOEC stack and BOP achieved a specific energy < 50 kWhelec/kgH2 at an H2 production rate of > 0.075 kgH2/h operating in the cryo-vacuum chamber.This study has benchmarked system-level models of the lab-scale SOEC stack based on doped-zirconia electrolytes fabricated at OxEon and designed to operate near thermoneutral voltage (1.28 V/cell) at 800°C. The stack BOP includes a steam generator, an oil-free scroll compressor, cathode and anode exhaust recuperators, and a H2 dryer, which uses cold incoming liquid water feeds to cool the cathode exhaust and condense most of the excess H2O in the H2 product stream out. Operation of this demonstration system provides a basis for designing scaled-up lunar fuel production systems of 40 kgH2/h. The success of the lab-scale tests in terms of thermal integration and the benchmarked models suggest a pathway to such a full-scale SOEC system that can achieve < 46 kWhelec/kgH2 with much lower specific mass.Both the modeling and the lab-scale demonstration tests revealed that system specific energy was most sensitive to stack steam utilization, and that achieving < 50 kWhelec/kgH2 required stack steam utilization > 75%. In addition, effective thermal integration that limited the electrical heating primarily to steam generation is critical to minimizing the SOEC-system BOP loads. The stack was shown to run reliably at steam utilizations > 90% with a single pass feed. Efforts will be presented on how to simulate such high utilization at larger scales for full lunar deployment. Additional testing of the lab-scale system in larger cryo-vacuum chambers at NASA during 2023 will allow for further confirmation of thermal management strategies to ensure reliable SOEC operation for lunar production of H2/O2 propellants.

Dynamic response and safety performance of an anode-supported solid oxide electrolysis cell operating under electrical transients

Solid oxide electrolysis cells have emerged as a promising technology for renewable energy storage. The intermittent operation, however, could jeopardize durability. Understanding the transient response characteristics of the solid oxide electrolysis cell under diverse electrical inputs is crucial to comprehend the dynamic durability of the electrolysis cell. Yet, the electrochemical and temperature dynamic responses are challenging to be in-situ measured. In this study, a three-dimensional electrochemical-mass-heat coupled dynamic model of the solid oxide electrolysis cell is established and validated by the experimental I–V curve and electrochemical impedance spectroscopy data. The current and voltage manipulations on the transient responses are evaluated and the cause of voltage overshoot during current control is analyzed. The effects of amplitude and switching time of current upward manipulation on the electrochemical and thermal responses are also investigated. It is discovered that 40% current upward (60% power upward) slightly exceeds the upper limit of the temperature gradient of 8 K/cm and approaches the FU limit of 90%. On the other hand, the influence of switching time on safety performance is negligible.

Towards strategies to mitigate degradation and regenerate performance of a solid oxide electrolyzer during co-electrolysis operation

One of the main objectives towards the broad commercialization of solid oxide electrolysis cells (SOEC) is to reduce the effects of degradation and extend the lifetime of the technology. Special industrially relevant operating conditions can, however, amplify and accelerate the degradation of SOECs, thus significantly reducing the lifetime of the technology. Nevertheless, not all degradation processes are completely irreversible, especially if they are detected at an early stage. In order to develop strategies to mitigate degradation and to regenerate performance, several experiments were performed on a five-layer SOEC stack operating in steam and co-electrolysis mode. The experiments were performed at different industrially relevant gas mixtures, reactant conversion rates and operating temperatures. The experimental results revealed different types and degrees of effects of the degradation, mitigation and regeneration strategies on the performance of the stack, which were analyzed in detail and linked to the underlying mechanisms and processes. As a result, promising strategies for mitigating degradation and regenerating the performance of SOECs could be derived and optimized. In addition, this study provides a set of recommendations for safe and stable operation of SOECs under various operating conditions.

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

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

Fuel electrode materials are important for achieving higher performance and durability in solid oxide fuel cells (SOFCs), solid oxide electrolysis cells (SOECs), and reversible solid oxide cells (r-SOCs). On the other hand, the air electrode also faces performance and durability issues. For air electrodes, studies have been conducted on their performance and durability in SOFC operation, but the performance and durability of air electrodes in SOEC and r-SOC operation needs to be investigated in more detail. The electrochemical performance and durability of SOEC and r-SOC are evaluated by conducting electrolysis performance tests of LSCF-based air electrodes with different preparation conditions, electrolysis durability tests at the thermoneutral potential, and a 1000-cycle test in r-SOC mode.

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

Solid oxide electrolysis cells (SOECs) can be used to perform steam electrolysis in a process which is the reverse of power generation using a solid oxide fuel cell (SOFCs). They are capable of highly efficient hydrogen production. In this study, various model fuel electrode materials were compared and evaluated over a wide range of current densities in both SOFC and SOEC modes. Here, we prepared three types of cells: (i) with a conventional Ni-scandia-stabilized-zirconia (Ni-ScSZ) cermet fuel electrode, (ii) with a Ni-gadolinia-doped ceria (Gd0.1Ce0.9O2, Ni-GDC) cermet fuel electrode, and (iii) with a Ni-GDC co-impregnated fuel electrode fabricated by the co-impregnation method. The electrode reactions are characterized through measurements of electrochemical impedance spectra (EIS) and subsequent analysis of distribution of relaxation times (DRT). The results suggest that the use of mixed ionic and electronic conductor GDC as a fuel electrode material is advantageous especially in SOEC operation mode.

Performance evaluation and multi-objective optimization of an innovative solar-assisted multigeneration energy storage system for freshwater/O2/H2 generation

A novel multigeneration energy system encompassing a two-stage water purification unit, high-temperature solid oxide electrolysis cell (SOEC), heliostat solar field, and power generation cycles including organic and two-stage steam Rankine cycles is introduced in this study. The two-stage desalination system comprises humidification, dehumidification, and reverse osmosis units. A thermoelectric generator is also utilized to convert dissipated heat into electricity. The proposed system is comprehensively analyzed from energy, exergy, economic, and sustainability standpoints. The effect of design parameters such as heliostat concentration ratio, view factor, desalination mass flow ratio, SOEC operating temperature, and current density on the hydrogen, oxygen and freshwater generation rate, sustainability index, total cost rate, and overall performance of the energy system are discussed. Further, multi-objective optimization based on non-dominated sorting genetic algorithm (NSGA-II) is implemented to identify the optimal operating state of the system. Comparative assessment of the system in the actual condition in the Middle East region showed that the plant has higher performance in June compared to December. Also, optimization revealed that by choosing C = 988, P7 = 1500 kPa, P8 = 300 kPa, MFD = 2.5 and Tcell = 1250 K, the highest obtainable exergy efficiency and minimum total cost rate in the optimal operating state would be 11.32 % and 78.18 $/h, respectively.

A zero carbon route to the supply of high-temperature heat through the integration of solid oxide electrolysis cells and H2–O2 combustion

Previously suggested options to achieve carbon neutrality involve the use of fossil fuels with carbon capture or exploiting biomass as sources of energy. Industrial high-temperature heating could possibly exploit electrical heating or combustion using hydrogen. However, it is difficult to replace all the current coal or natural gas furnaces with these options for chemical industry. In this work, a method that integrates solid oxide electrolysis cells (SOEC) and H2–O2 combustion is proposed, and the related parameters are modelled to analyze their impacts. There is no waste heat and waste emissions in the proposed option, and all substances are recycled. Unlike previous research, the heat required for SOEC operation is generated from H2 combustion. The best working condition is under thermoneutral voltage, and the highest electricity-to-thermal efficiency that can be achieved is 86.88% under a current density of 12000 A/m2 and operating temperature of 750 °C. Ohmic overpotential has the greatest effect on electricity consumption, and the anode activation overpotential is the second most important option. Increasing combustion product temperature cannot significantly improve thermal efficiency, but can raise the available maximum thermal energy.

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.

Long-term performance prediction of solid oxide electrolysis cell (SOEC) for CO2/H2O co-electrolysis considering structural degradation through modelling and simulation

Solid oxide electrolysis cells (SOECs) are promising devices for CO2 and H2O co-electrolysis into syngas. Degradation occurring within SOECs is still of great concern for their commercial deployment. Therefore, it is important to understand the degradation mechanisms and optimize SOEC long-term operation. A 1-dimensional (1D) pseudo-dynamic model was developed for SOEC operating under CO2/H2O co-electrolysis and implemented in Aspen Plus® using Fortran® routines. The structural degradation of a typical SOEC material set: Ni-YSZ cathode, YSZ electrolyte and LSM-YSZ anode were also accounted for. The model was dynamically validated for different current densities (0.5, 0.75 and 1.0 A/cm2). The effects of operating conditions and structural degradation on SOEC long-term electrochemical performance and syngas production are assessed. Performance prediction indicated that increasing the current density and operating temperature initially enhance SOEC performance. However, the operating voltage increases at faster rates at higher current densities and temperatures causing the syngas production efficiency to decrease due to higher power utilisation. Long-term SOEC degradation is mostly due to LSM-YSZ anode delamination with a degradation rate of 3.96 %/1000hrs at 750 °C, 1 bar and 1.5 A/cm2. Applying an anti-oxidant layer on the interconnect surface and adjusting La/Sr ratio in the A-site of LSM considerably reduce the degradation rate by roughly 78% at the anode.