A novel solid oxide electrolytic cell with reduced endothermic load for CO2 electrolysis using (La0.80Sr0.20)0.95MnO3-δ cathode

The electrochemical reduction of CO2 was studied via solid oxide electrolysis cell (SOEC), a type of device that can work reversely into a solid oxide fuel cell (SOFC) to generate electricity. Application of SOEC for CO2 electrolysis possesses potential rewards both in energy and environmental aspects, as it offers a way to recycle CO2 into chemicals and value-added fuels, which helps to reduce the accumulation of atmospheric CO2 and realize the carbon neutral cycling of fuels. Secondly, SOEC techniques provide a means to utilise the intermittent renewable sources, such as wind, tide, etc., as energy input to store excess electricity in the form of H2, CO and hydrocarbons and use these chemicals when necessary. Yet it is a challenging task to realize an efficient reduction of CO2 by SOEC due to the non-polar nature of CO2 fuels which are hard to be chemically absorbed and activated in high temperature range. The CO product from CO2 reduction is also demanding for the choice of fuel electrode (i.e. cathode in SOEC and anode in SOFC) materials, which has been an issue for the CO/hydrocarbon-fuelled SOFCs. To date, the CO2 electrolysis by SOEC is still at the starting point, and the mechanisms on the electrochemical reduction of CO2through SOECs are not fully understood. Extensive efforts need to be dedicated to the material developments, mechanism study, and system designs etc. Effort was made in our lab to find a highly performed, long-term durable cathode material for electrochemical reduction of CO2 by SOEC and to obtain more understandings of the mechanism of CO2 electrochemical reduction process. Different cathode materials were employed, including Ni-8 mol% yttria stabilised zirconia (YSZ) cermet, (La,Sr)(Cr,Mn)O3 (LSCM)-YSZ composite, and LSCM-(Gd, Ce)O2 (GDC) composite. Focus was casted on LSCM based cathodes which were found to be carbon-resistive, and the microstructure of LSCM-based cathodes was tuned to high performance and efficient CO2 electrolysis by the strategy of applying a gradient composite cathode and of adopting wet impregnation as cell fabrication procedures. The electrochemical performance of CO2 electrolysis was characterized in various CO2-CO mixtures and applied potentials in 900-750oC with the aid of impedance spectroscopy, on YSZ electrolyte supported three-electrode SOECs. In this paper, impedance behaviour of the electrochemical reduction reaction of CO2 from different cathode SOECs was correlated with the variations in gas composition, operating temperature and loading potential. As both charge transfer and surface adsorption/desorption equilibration and surface diffusion of activated species were found to be dominant processes taking place in the scope of cathodes under examination, with the later being significant in CO2-rich fuels, areas of discussion will be the effects of operational conditions on the impedance arcs associated with these steps, and how the cathode microstructure impacts the impedance behaviour and the corresponding elementary steps from CO2electrochemical reduction. By introducing a LSCM-YSZ 30-70/LSCM-YSZ 60-40 graded cathode and by incorporating extra catalyst into LSCM-GDC composite, the cathode performance was greatly improved, and the surface activity was profoundly accelerated. However, the most effective way to promote cathode performance was to introduce the cathode components in separate steps via wet impregnation. A competitive CO2 electrolysis performance to Ni-YSZ cermet was obtained from the GDC impregnated LSCM-YSZ cathode with 0.5% Pd extra catalyst, which also showed a comparable performance between SOEC and SOFC when operating in CO2-CO 50-50 mixture.

One of the greatest challenges for any manned space mission is the amount of oxygen that is required, both for life support and as a chemical oxidant. It is not feasible to carry all the oxygen needed for an extended mission to any planet. In the case of Mars, an oxygen generator may be used to separate and collect oxygen from a planet’s atmosphere. The Martian atmosphere is composed of ~95% CO2, which may be electrolyzed to produce oxygen (O2) and carbon monoxide (CO) (and possibly water via reaction with H2) for human consumption or use as a chemical oxidant for power generation systems. There are a few established methods for generating oxygen from oxygen-containing gases, but these methods have significant power requirements, and the systems produce lower purity oxygen (<97%). A few authors have reported work on the fabrication and testing of high purity oxygen generators using various selective solid-oxide electrolysis (SOE) membranes. Solid-oxide electrolysis membranes function by the movement of oxygen ions through a solid electrolyte membrane due to an applied field or pressure driving force. Many recent works have centered on the co-electrolysis of CO2/H2O, but the study of direct electrolysis of pure CO2 to O2 and CO is still limited. The objective of this work was to investigate cermet electrodes for use in solid-oxide electrolysis cells (SOECs) for the efficient reduction of CO2. This work includes the investigation of the optimal electrode microstructure that is required for performance stability. In this work, palladium (Pd)- and platinum (Pt)-based compositions were mixed with yttrium-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC) to form cermet composite compositions. The compositions were synthesized by a mixed-oxide synthesis process in which the precious metal component is mixed at various ratios with the electrolyte powders. The particle size distribution of the electrolyte powders were varied (by combining different powder lots with different particle size distributions) in order to alter the final pore size and electrode continuity throughout the final sintered film. In addition to the particle size distribution effects on the microstructure, various levels of Pd levels within the electrolytes were investigated. Screen-printing inks were prepared for the powders produced. The electrode compositions were screen-printed and sintered on YSZ electrolyte substrates (~150 mm thick) to produce symmetrical cells for electrolysis testing. Platinum electrode mesh were attached to each side of the sample with small amount of the similar ink. Electrode interfacial polarization measurements were completed on the compositions by impedance spectroscopy (at temperatures 650-850°C) using a Solartron 1260 in air and CO2. Four probe current-voltage (I-V) measurements were completed in dry CO2 on the symmetrically printed cells. The current density versus the applied voltage were related to the calculated oxygen separation efficiency. The optimal Pd-based electrodes demonstrated O2 production from dry CO2 at higher level than previous reported SOE cells, with production rates as high as 9.8´10-5 mol×cm-2×min-1 (for 1.5 DCV) at 800°C. Acknowledgements: A portion of this work was sponsored by the NASA West Virginia Space Grant Consortium (WVSGC) and by the WVU Statler College of Engineering and Mineral Resources. The authors would also like to acknowledge West Virginia University Shared Research Facilities for support through materials characterization.

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.

High temperature electrolysis is a promising option for carbon-free hydrogen production and huge energy storage with high energy conversion efficiencies from renewable and nuclear resources. Over the past few decades, yttria-stabilized zirconia (YSZ) based ion conductor has been widely used as a solid electrolyte in solid oxide electrolysis cells (SOECs). However, its high operation temperature and lower conductivity in the appropriate temperature range for solid electrochemical devices were major drawbacks. Regarding improving ionic-conducting electrolytes, several groups have contributed significantly to developing and applying LaGaO3 based perovskite as a superior ionic conductor. La(Sr)Ga(Mg)O3 (LSGM) electrolyte was successfully validated for intermediate-temperature solid oxide fuel cells (SOFCs) but was rarely conducted on SOECs for its high efficient electrolysis performance. Their lower mechanical strengths or higher reactivity with electrode compared with the YSZ electrolysis cells, which make it difficult to choose compatible materials, remain major challenges. In this field, SOECs have attracted a great attention in the last few years, as they offer significant power and higher efficiencies compared to conventional YSZ based electrolysers. Herein, SOECs using LSGM based electrolyte, their applications, high performance, and their issues will be reviewed.

As renewable energy sources are inherently intermittent, their increasing contributions to electricity grids will necessitate large-scale energy storage to balance supply and demand. Storing electrical energy in chemical bonds by electrolysis of CO2 and / or H2O is one option to achieve this. We chose solid oxide reactors for this application, inter alia, because in principle they can be operated cyclically in electrolyser (SOE) and fuel cell (SOFC) modes. Results will be presented of measurements of ohmic and electrode polarization resistances and global performances for CO2 splitting to CO and O2 in micro-tubular solid oxide electrolysers (SOEs), with different types of current collectors; electrolysis of steam and (CO2 + steam) exhibited similar behaviour. Silver or nickel wire current collectors, with and without silver or nickel paste, were used to contact the 220 μm thick inner lumen Ni-YSZ electrode (SOE cathode) of micro-tubular solid oxide electrolysers [1] (SOEs: Ni-YSZ|YSZ|YSZ-LSM|LSM) incorporating 34 μm thick yttria-stabilized zirconia (YSZ) electrolyte, and 40 μm thick lanthanum strontium manganite (LSM, SOE anode), as described previously for CO2 reduction [2]. Silver wire and paste was used as the current collector for the LSM electrode. As shown in Fig. 1, the best performance (ca. 1 A cm-2 at 1.5 V (thermo-neutral potential difference) at 800 °C) was achieved for reactors with nickel wire current collectors alone, implying metallic pastes had no net beneficial effects on area-specific ohmic resistances. Furthermore, applying silver paste had a detrimental effect on the Ni cathode polarization resistance, for reasons that will be presented. Fig. 2 shows polarization and ohmic resistance data at open circuit; similar behaviour was exhibited with potential differences applied to reactors. Ohmic resistances were expected to decrease by using silver wire, due to its higher electronic conductivity compared to nickel, but no stable cell performance was achieved.

In recent years, solid oxide electrolysis cells (SOEC) have attracted considerable attention due to its high efficiency, stability, and environmental friendliness. The primary motivation of this chapter was to provide an overview of the current status of CO2 electrolysis in SOECs. In this chapter, the basic working principles of SOEC technology were briefly introduced, the development of key materials involved in SOEC were comprehensively reviewed, and the main applications of SOEC to CO2 electrolysis were summarized. In addition, the challenges faced by SOEC technology in CO2 electrolysis were systematically investigated. It is envisioned that the information collected in this chapter will be a valuable information source for researchers and provide direction and guidance for future research in SOEC technology.

Massive carbon dioxide (CO2) emission from recent human industrialization has affected the global ecosystem and raised great concern for environmental sustainability. The solid oxide electrolysis cell (SOEC) is a promising energy conversion device capable of efficiently converting CO2 into valuable chemicals using renewable energy sources. However, Sr-containing cathode materials face the challenge of Sr carbonation during CO2 electrolysis, which greatly affects the energy conversion efficiency and long-term stability. Thus, A-site Ca-doped La1-xCaxCo0.2Fe0.8O3-δ (0.2 ≤ x ≤ 0.6) oxides are developed for direct CO2 conversion to carbon monoxide (CO) in an intermediate-temperature SOEC (IT-SOEC). With a polarization resistance as low as 0.18 Ω cm2 in pure CO2 atmosphere, a remarkable current density of 2.24 A cm-2 was achieved at 1.5 V with La0.6Ca0.4Co0.2Fe0.8O3-δ (LCCF64) as the cathode in La0.8Sr0.2Ga0.83Mg0.17O3-δ (LSGM) electrolyte (300 μm) supported electrolysis cells using La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) as the air electrode at 800 °C. Furthermore, symmetrical cells with LCCF64 as the electrodes also show promising electrolysis performance of 1.78 A cm-2 at 1.5 V at 800 °C. In addition, stable cell performance has been achieved on direct CO2 electrolysis at an applied constant current of 0.5 A cm-2 at 800 °C. The easily removable carbonate intermediate produced during direct CO2 electrolysis makes LCCF64 a promising regenerable cathode. The outstanding electrocatalytic performance of the LCCF64 cathode is ascribed to the highly active and stable metal/perovskite interfaces that resulted from the in situ exsolved Co/CoFe nanoparticles and the additional oxygen vacancies originated from the Ca2Fe2O5 phase synergistically providing active sites for CO2 adsorption and electrolysis. This study offers a novel approach to design catalysts with high performance for direct CO2 electrolysis.

Electrochemical impedance spectroscopy (EIS) is a powerful diagnostic tool used in a wide range of applications, including solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs). While EIS measurements on single cell level are commonplace, implementation of EIS on stack level remains limited, primarily due to the extremely low impedance of large-footprint cells, which imposes stringent demands on the impedance spectrometer and experimental setup (1). Furthermore, compared to single-cell EIS data, stack-level data reported in the literature are rarely subjected to the same degree of equivalent circuit modeling, making a detailed analysis of underlying electrochemical processes difficult.Here, we report a framework for analyzing stack-level impedance spectroscopy data. The quality of the acquired data is first verified using Kramers-Kronig transformations (2), followed by the application of distribution of relaxation times (DRT) and analysis of the difference of impedance spectra (ADIS) methods to experimental data collected under a set of pre-determined operating conditions (electrochemical test procedure) (3-4). DRT and ADIS results were incorporated into the complex non-linear least squares (CNLS) fitting procedure and the validity of the chosen equivalent circuit model was confirmed by reconstruction of Nyquist, Bode, the individual electrochemical contributions, and DRT plots of the fitted data.The developed framework was successfully implemented on a commercial 75-cell SOEC stack (TSP-1) developed by Haldor Topsoe (5). An electrochemical impedance spectrum is recorded simultaneously on all 13 cell groups in the stack using Zahner instrumentation. In this work, the electrochemical impedance data obtained from a state-of-the-art TSP-1 stack, containing three different types of Ni/YSZ based cells, is reported. The stack was initially investigated by EIS, using an electrochemical test procedure including CO2 electrolysis under constant conversion, H2O electrolysis under constant conversion, variations in the pO2 on the oxygen electrode, and finally temperature variations in CO2 electrolysis operation. This electrochemical test procedure was performed to develop an equivalent circuit model for the stack. Afterward, the stack was subjected to a degradation in steam electrolysis under 580 mA/cm2 (70% conversion), with an impedance spectrum being recorded every 24 hours. The operational point for this degradation test was chosen to match the real-life operation point of these stacks and hence to give a realistic image of any potential degradation. The developed framework and the equivalent circuit model developed for the stack was finally used to model the EIS data obtained during the degradation test. The modeling of these EIS data utilizes the full potential of electrochemical impedance spectroscopy and help identify any degradation mechanisms taking place in the stack, an analysis not previously reported on stack level.(1) R. R. Mosbæk, J. Hjelm, R. Barfod, and P. V. Hendriksen. ECS transactions, 35, 1 (2011).(2) B. Boukamp, J. Electrochem. Soc., 142, 1885 – 1894 (1995).(3) S. H. Jensen, A. Hauch, P.V. Hendriksen, M. Mogensen, N. Bonanos, and T. Jacobsen. Journal of the Electrochemical Society, 154(12), B1325-B1330, (2007)(4) H. Schichlein, and E. Ivers-Tiffee. Journal of Applied Electrochemistry, 32, 875-882, (2002).(5) J. Rass-Hansen, P. Blennow, T. Heiredal-Clausen, R. Küngas, T. Holt Nørby, and S. Primdahl, Proceedings of the 12th European SOFC & SOE Forum, 318-325 (2016). Figure 1

Hydrogen energy has been hailed as a versatile energy of the future that could aid the transition to net-zero emissions. It has large potential as an alternate fuel source for mobility and power, heating and feedstock for the industry. The energy carrier is produced from water electrolysis technology using different types of electrolyzer (i.e., Proton Exchange Membrane (PEM) or Solid Oxide Electrolysis Cells (SOECs)) for hydrogen separation process. In this energy research journey, PETRONAS has jointly worked with SLB to evaluate the feasibility of using SOECs for green hydrogen production. The study's focus is to explore integrating SOECs into existing industrial facilities creating hybrid systems, specifically with available flue gas from combustion process e.g. power generation. This allows for the utilization of waste heat from existing processes to enhance overall energy efficiency in hydrogen production. To understand hydrogen production and its utilization in the existing facility, the full system including existing gas turbines, solid oxide electrolyzer cells (SOECs), a hydrogen compression train, and a temperate cooling water system were modeled in a process simulation software. The model also included a heat recovery steam generator (HRSG) to recover the energy from gas turbine exhaust gases to heat water feed to produce saturated steam which will be the feed to SOEC. Several configurations considering heat integrations were explored, including heat recovery from SOEC product stream. Various sensitivities were run for different configurations of the process to study hydrogen purity, hydrogen blending, overall power required, water required in the overall process, and power produced by the hydrogen generated from SOEC and CO2 emission values in all the configuration. The results were compared and evaluated to help assess how variation in parameters affect the performance or outcome. The results were evaluated for blending the produced hydrogen with the existing export gas before and after export gas compression and how it influences the hydrogen purity. Hydrogen blending with the fuel gas to the gas turbine also helped in overall reduction of CO2 emission. It was observed that additional cooling of the hydrogen stream to achieve better hydrogen purity did not significantly help in reducing CO2 emissions when hydrogen is used as fuel gas; therefore, cooling with temperate water system can be considered. All the cases were compared and evaluated for expanding the application of green hydrogen technologies in the facility considering viability and reduction in emissions. The study was a part of a collaborative research initiative by the industry players to accelerate the development and deployment of SOEC technologies in existing facilities. The results from the study provided an overall understanding of adding SOEC in the facility and valuable insights to the stakeholders to make well-informed choices on the configuration.

Composite electrodes comprising a rare-earth nickelate phase and a doped rare-earth ceria phase with a high level of rare-earth doping have been studied as oxygen electrodes. While rare-earth nickelates have shown great promise as oxygen electrodes in both solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOECs), they become unstable in contact with rare-earth doped ceria. In this work we describe a strategy to eliminate this instability and enable the use of these electrodes as stable high performance oxygen electrodes in both SOFC and SOEC applications. The performance of such composite electrodes is explored using a symmetrical cell approach, and through the electrical conductivity relaxation (ECR) method and compare it to state-of-the-art oxygen electrodes comprising strontium doped lanthanum manganite (LSM) – yttria stabilized zirconia (YSZ) composite electrodes. The symmetrical cell tests which reveal the stabilized nickelate-ceria electrodes to be superior, are also well-correlated with the results of testing complete cells in both SOFC and SOEC modes. Based on these results we make the case for deploying these stabilized nickelate-ceria composite electrodes as oxygen electrodes in reversible SOFC-SOEC systems.

A novel system for the production of pure hydrogen from natural gas based on solid oxide fuel cell–solid oxide electrolyzer

Experiments and modeling of solid oxide co-electrolysis: Occurrence of CO2 electrolysis and safe operating conditions

Electrochemical performance and long-term durability of a 200 W-class solid oxide regenerative fuel cell stack

In order to address the issue of the anthropogenic greenhouse effect, the supply with renewable energy will play a major role in the future energy systems. A particularly promising approach to store or even use renewable energy in sectors where electricity cannot be used effectively yet (for example in aviation) are synthetic fuels. Within the synthetic fuel generation process, known as Power-to-X, syngas (H2+CO) can be synthesized into liquid hydrocarbons, such as gasoline or jet fuel, via the Fischer-Tropsch process. In comparison with alternative processes, such as a steam electrolyzer in combination with a reverse water gas shift reactor, solid oxide electrolysis cells (SOEC) offer great potential for producing the required syngas in a single, less complex, and more efficient step within Power-to-X utilizing only steam (H2O), carbon dioxide (CO2), and electricity from renewable sources.This so-called co-electrolysis, which involves the simultaneous reduction of H2O and CO2 in SOECs, is of particular importance. It enables direct and efficient syngas production with a tailored H2/CO ratio, thereby reducing the need for additional downstream processing steps. This integrated approach enhances overall system efficiency, minimizes energy losses, and supports carbon utilization efforts, making it a crucial technology for sustainable fuel and chemical production. However, different syngas applications and associated SOE operating conditions can cause performance deterioration of SOECs. Consequently, a comprehensive understanding of losses incurred by diverse inlet mixtures, in conjunction with other operational conditions, such as temperature or the total flow rate, during electrolysis is essential.To the best of the authors’ knowledge, there is currently a lack of literature on operating cells with CO at the inlet, especially in CO2-electrolysis, where H2 is often added to avoid oxidation of Nickel in the fuel electrode. In order to investigate the influence of different gas components (H2O, H2, CO2, CO) of inlet compositions on performance and losses of SOECs, a series of experiments was performed on a commercial fuel electrode-supported cell with an active area of 81 cm2. To maintain a total flow rate of 2.2 slpm nitrogen was added, and the amount of H2O, H2, CO2, CO was varied, depending on the desired mixture. It is noteworthy that the air flow rate applied to the air electrode remained constant at 2.2 slpm throughout the experimental series. Initially, H2O electrolysis was conducted as a reference, followed by various mixtures for co-electrolysis and CO2 electrolysis, each with distinct H2O/CO2-, H2/CO-, and product/reactant-ratios. In the final experiment, pure CO2 was utilized as the feedstock for CO2-electrolysis, aiming to determine the losses in electrolysis in the absence of product species (CO).During the experiments polarization curves and electrochemical impedance spectroscopy (EIS) in 50 mA/cm2 steps were used for characterizing the performance of the cell at current densities up to 350 mA/cm2. The monitoring of the temperature distribution over the active area was realized through thermocouples integrated in the cell-housing near the gas flow channels on both the anode and cathode side. This approach enables further analysis of the reaction mechanism over the cell area, correlation of impedance data with thermal effects, and detection of hotspots. The outlet gas composition was monitored via a gas analyzer to determine the dry composition.This study investigates the impact of gas components at the inlet in co- and CO2-electrolysis on the performance of fuel electrode-supported SOECs, emphasizing losses in specific operating conditions. Furthermore, it offers a distinctive perspective on CO2 electrolysis with pure CO2 in an industrially sized cell, emphasizing substantial losses with no products at the inlet. The results show that the resistance in co-electrolysis is close to H2O-electolysis and way lower than in CO2-electrolysis which indicates that the main electrochemical reaction is the H2O-electolysis and CO2 is mostly converted to CO via the reverse water gas shift reaction. Figure 1

High-performance composite cathode for electrolysis of CO2 in tubular solid oxide electrolysis cells: A pathway for efficient CO2 utilization