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

High‐temperature electrolysis (HTE) using solid oxide electrolysis cells (SOECs) is a promising hydrogen production technology and has attracted substantial research attention over the last decade. While most studies are conducted on hydrogen electrode‐supported type cells, SOEC operation using metal‐supported cells has received minimal attention. The development of metal‐supported SOECs with performance similar to the best conventional SOECs is reported. These cells have stainless steel supports on both sides, 10Sc1CeSZ electrolyte and electrode backbones, and nano‐structured catalysts infiltrated on both hydrogen and oxygen electrode sides. Samarium‐doped ceria (SDC) mixed with Ni is infiltrated as a hydrogen electrode catalyst, and the effect of ceria:Ni ratio is studied. On the oxygen electrode side, catalysts including lanthanum strontium manganite (LSM), lanthanum strontium cobalt ferrite (LSCF), praseodymium oxide (Pr6O11), and their composite catalysts with SDC (i.e., LSM‐SDC, LSCF‐SDC, and Pr6O11‐SDC) are compared. Using the materials with highest catalytic activity (Pr6O11‐SDC and SDC40‐Ni60) and optimizing the catalyst infiltration processes, excellent electrolysis performance of metal‐supported cells is achieved. Current densities of −5.31, −4.09, −2.64, and −1.62 A cm−2 are achieved at 1.3 V and 50 vol% steam content at 800, 750, 700, and 650 °C, respectively.

Solid Oxide Electrolysis Cells (SOECs) are advancing toward global commercialization, driven by significant improvements in cell performance, extended lifetimes, and effective applications. Leading companies like Bloom Energy and Topsoe are demonstrating the feasibility of industrial-scale SOECs hydrogen production and synthetic fuel generation. Additionally, SOECs are being integrated with renewable energy and industrial heat sources to reduce green hydrogen cost and improving energy efficiency. Despite challenges like degradation and expensive costs, ongoing research are paving the way for SOECs to play a pivotal role in the global energy transition.China has been actively involved in the SOECs development since the early stages of this technology. However, the Chinese SOEC industry has not yet reached the same level of maturity as its international counterparts. Currently, SOEC technology in China remains in the research and small-scale demonstration phase, hindered by challenges such as expensive capital costs, poor lifespan, and unclear applications. Despite these barriers, China boasts the world's largest green hydrogen market, where alkaline electrolyzers (AECs) currently dominate due to their lower costs. However, SOECs known for their superior efficiency, have good potential application prospects in the future if key bottlenecks such as cost, and durability can be effectively addressed. This transition could significantly enhance global green hydrogen production capabilities and contribute to decarbonization goals.The primary objectives for the Chinese SOEC industry are: (1) enhancing durability, and (2) reducing hydrogen production costs. To achieve these goals, specific efforts are being directed at three key areas: (1) Material and cell: develop advancing surface defect theory and novel cell (micro-)structures to achieve higher current densities at lower temperatures; (2) Stack: design advanced stack structures and improve manufacturing procedures to enhance stack consistency, reliability, robustness, and performance; (3) System: develop cost-effective BoPs, enable high-value chemical production via effective integration. These targeted initiatives aim to address critical challenges and position the Chinese SOEC industry as a global leader in green hydrogen area. SOEC development is vital for building a green energy and chemical industry and promoting China's energy structure transformation.

Transition from fossil fuels to renewable energy is critical to achieve climate protection objectives. Fuel production using renewable energy and carbon dioxide is an essential part of reducing fossil dependence and reducing overall carbon footprint. One of the highest value applications in current markets is the production of liquid transportation fuels like sustainable aviation fuels (SAF) and low (fossil) carbon diesel fuel through carbon capture and utilization pathways. Liquid hydrocarbon fuels only achieve decarbonization goals if they replace fossil fuels and are produced from sustainable carbon resources.With the support of EERE BETO contract EE0008917, OxEon designed and fabricated a system to produce high value, energy dense, liquid transportation fuels from biogas. The technology has completed initial subsystem verification, and the fully integrated pre-pilot system demonstration is scheduled to start operation in Q1 2025. Anaerobic digester gas is first converted to synthesis gas (CO + H2, syngas), the bio-CO2 through CO2-steam co-electrolysis in a SOEC system and the bio-CH4 through a low energy, non-thermal plasma reformer.The combined syngas streams are compressed and supplied to a modular fixed bed FT reactor for production of liquid hydrocarbons, which serve as the feedstock for SAF, renewable diesel, and other qualified marine fuels. The combination of technologies offers several advantages: the yield of biofuel nearly doubles by using bio-CO2 compared to bio-CH4 alone. The FT reactor cooling system generates steam needed in the SOEC, which lowers the effective operating voltage (including phase change energy) from ~1.5 V/cell to < 1.3 V/cell, and oxygen by-product from the SOEC is used in the autothermal reformer, as is some of the FT produced water. The resulting product fuel is all bio-carbon. The system concept block diagram and the pre-pilot demonstration hardware are shown in Figure 1.The pre-pilot demonstration system was designed and fabricated by OxEon Energy and installed at Wasatch Resource Recovery (WRR). WRR is a public-private partnership producing renewable natural gas (RNG) from food waste. The OxEon pre-pilot system, converting anaerobic digester gas CO2 and CH4 to FT hydrocarbons with a production rate of 6-8 gallons/day is in the commissioning phase. Operation is scheduled to begin in Q1 2025. The system will demonstrate a cumulative time on stream of 500-1,000 hours, producing 100-250 gallons of FT synthetic crude. Subsystem verification of the SOEC, plasma reformer, and FT technologies were conducted to demonstrate key performance targets. In addition to the verification run specifically for this pre-pilot demonstration, OxEon has operated each subsystem technology in thousands of hours of validation testing. A current program sponsored by the Naval Research Laboratory (NRL), includes scope to upgrade FT produced hydrocarbons to MIL-SPEC compliant JP-5 fuel.There is an emerging market for FT technology aimed at a variety of small, distributed resources including biomass, biogas, and cement kiln CO2, augmented by renewable and nuclear-powered CO2 co-electrolysis to increase the sustainable fuel capacity. FT technology to convert these fuel sources to liquid fuels would require smaller systems that can be transported to the site versus site-built plants. Biomass/biogas has previously been an impractical feedstock for FT technology due to shipping logistics to meet required economies of scale. The challenge in developing these resources is producing a small-scale plant at the same cost per bbl/day capacity as large plants. To accomplish this task, OxEon Energy developed a system that uses larger diameter reactor tubes with a heat transfer insert to manage thermal control of the system. OxEon’s FT design requires fewer reactor tubes than the traditional fixed-bed industry design and uses standard pipe sizes for reduced reactor fabrication costs. These modular systems are designed for a 10-15 bbl/day production increment enabling 100 bbl/day modules.OxEon has demonstrated FT operation with a variety of syngas sources including syngas produced via natural gas reformation, reverse water gas shift (RWGS) of CO2, and co-electrolysis of steam and CO2. Extensive performance mapping with the modular FT reactor design has also informed catalyst characteristics such as pore size, support shape, and precious metal promoter loading to allow for targeted FT product production. OxEon’s FT systems have been built and operated for both government and private customers ranging in production from 100 mL/day to 2 bpd. Figure 1

Integrating high-temperature solid oxide electrolysis cells (SOECs) with exothermic Fischer-Tropsch (FT) synthesis can unlock significant synergies for enhancing process efficiency. In such a coupled system, syngas is generated via co-electrolysis, while the excess heat from the FT reactor is utilized to produce steam for the SOEC which can significantly improve energy utilization and system performance. In industrial FT reactors, single-pass syngas conversion typically ranges from 60–85% to avoid hot spot formation, necessitating recycling to boost conversion. This recycling leads to the accumulation of light hydrocarbons (C1–C6) in the gas phase, requiring a substantial purge stream that reduces product yield and overall carbon efficiency.To address these challenges, this work proposes a long-recycling approach, where FT tail gas is recycled into the SOEC, enabling hydrocarbon reforming directly within the system. This integration reduces the purge gas stream and introduces a cooling effect in the SOEC, facilitating higher current densities under isothermal operating conditions.A combined experimental and simulation study was conducted to explore the transient thermal effects of introducing FT tail gas into a 120 kW SOEC module, which consists of 24 stacks of electrolyte-supported cells (ESCs). Emulated FT tail gas compositions, representative of an industrial FT synthesis reactor, were introduced into the system. Transient simulations, performed using the validated TEMPEST reactor model, evaluated spatial and temporal temperature gradients within the SOEC, identifying operating regimes that could pose risks to system stability.From these analyses, optimized operating strategies were developed to minimize temperature gradients and enhance system reliability. Experimental validation demonstrated the efficacy of these strategies in maintaining stable and safe SOEC operation. This study highlights the potential of the proposed SOEC-FT integration to improve system performance, reduce carbon losses, and advance the development of sustainable, high-efficiency energy solutions.

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

This study is focused on the development of layered perovskites based alternative oxygen electrodes for high temperature Solid Oxide Electrolysis Cells (SOECs). In this respect, rare earth nickelates Ln2NiO4+δ (Ln = La, Pr or Nd) have taken considerable attention due to their good electrochemical properties resulting from high oxide ion diffusivity and a large surface exchange rate. Among them, Pr2NiO4+δ (PNO) shows the best electrochemical properties, however, it displays relatively higher degradation rate under SOEC operation at high current density. Therefore, in this work, we perform further modification by substituting nickel by cobalt in order to enhance the physico-chemical properties, electrochemical performance and most importantly the durability of SOECs. Three compositions (x = 0.0, 0.1 and 0.2) are prepared and characterized using different techniques. The electrochemical measurements are performed with symmetrical as well as single cells using DC- and AC-techniques in the 700–900 °C temperature range. The electrode reaction mechanism is also examined by recording the impedance spectra at different pO2. An improvement in electrochemical performance as well as lower degradation rate is observed with cobalt substitution, during short term SOEC operation at −1 A⋅cm−2 current density at 800 °C with 50% H2 and 50% H2O feed gas mixture.

Due to the random fluctuations in power experienced by high-temperature green electric hydrogen production systems, further deterioration of spatial distribution characteristics such as temperature, voltage/current, and material concentration inside the solid oxide electrolysis cell (SOEC) stack may occur. This has a negative impact on the system’s flexibility and the corresponding control capabilities. In this paper, based on the SOEC electrolytic cell model, a comprehensive optimization method using an adaptive incremental Kriging surrogate model is proposed. The reliability of this method is verified by accurately analyzing the dynamic performance of the SOEC and the spatial characteristics of various physical quantities. Additionally, a thermal dynamic analysis is performed on the SOEC, and an adaptive time-varying LPV-MPC optimization control method is established to ensure the temperature stability of the electrolysis cell stack, aiming to maintain a stable, efficient, and sustainable SOEC operation. The simulation analysis of SOEC hydrogen production adopting a variable load operation has demonstrated the advantages of this method over conventional PID control in stabilizing the temperature of the stack. It allows for a rapid adjustment in the electrolysis voltage and current and improves electrolysis efficiency. The results highlighted that the increase in the electrolysis load increases the current density, while the water vapor, electrolysis voltage, and H2 flow rate significantly decrease. Finally, the SOEC electrolytic hydrogen production module is introduced for optimization scheduling of energy consumption in Xinjiang, China. The findings not only confirmed that the SOEC can transition to the current load operating point at each scheduling period but also demonstrated higher effectiveness in stabilizing the stack temperature and improving electrolysis efficiency.

In solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs), degradation of hydrogen electrode during long–term operation is one of the issues to be solved. Migration and aggregation of Ni particles are major factors of this degradation, and especially after steam electrolysis operation, intense Ni migration has been observed at the hydrogen electrode/electrolyte interface [1]. This phenomenon is assumed to be related to the change in wettability of the Ni/electrolyte interface under polarization [2]. A quantitative measurement of interface wettability is the concept of contact angle. It is known that the contact angle is determined by the balance between the surface tension of contacting materials and the interfacial tension. Contact angle analysis is usually used for liquid–solid interfaces, but it has also been suggested to be useful for the characterization of solid metal–oxide interfaces [3]. Therefore, the aim of this study was to directly measure the contact angle between Ni and YSZ electrolyte under various conditions in order to quantify the changes in wettability at the interface.Ni thin film electrodes were prepared by sputtering Ni on YSZ electrolyte. Using this model electrode, electrochemical measurements were performed under open circuit holding or a potentiostatic operation (SOFC operation or SOEC operation) at high temperatures. Subsequently, postmorterm analysis of the samples was performed using a Focused Ion Beam–Scanning Electron Microscope (FIB–SEM). Using the cross–sectional SEM images of the model electrode, the contact angle was calculated by distinguishing the three phases of Ni, YSZ, and pores based on the difference in contrast of each phase. The analysis revealed that the wettability of the Ni/YSZ electrolyte interface changed depending on the operation mode. Specifically, the contact angle after SOEC operation increased compared to values after open circuit holding or SOFC operation. This phenomenon is thought to be related to changes in oxygen potential.AcknowledgementsThis paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

The effect of the presence of an Fe-Cr alloy metallic interconnect on the performance and stability of La(0.8)Sr(0.2)MnO3 (LSM) oxygen electrodes is studied for the first time under solid oxide electrolysis cell (SOEC) operating conditions at 800 °C. The presence of the Fe-Cr interconnect accelerates the degradation and delamination processes of the LSM oxygen electrodes. The disintegration of LSM particles and the formation of nanoparticles at the electrode/electrolyte interface are much faster as compared to that in the absence of the interconnect. Cr deposition occurs in the bulk of the LSM oxygen electrode with a high intensity on the YSZ electrolyte surface and on the LSM electrode inner surface close to the electrode/electrolyte interface. SIMS, GI-XRD, EDS and XPS analyses clearly identify the deposition and formation of chromium oxides and strontium chromate on both the electrolyte surface and electrode inner surface. The anodic polarization promotes the surface segregation of SrO and depresses the generation of manganese species such as Mn(2+). This is evidently supported by the observation of the deposition of SrCrO4, rather than (Cr,Mn)3O4 spinels as in the case under the operating conditions of solid oxide fuel cells. The present results demonstrate that the Cr deposition is essentially a chemical process, initiated by the nucleation and grain growth reaction between the gaseous Cr species and segregated SrO on LSM oxygen electrodes under SOEC operating conditions.

The depletion of fossil fuels in the current world has been a major concern due to their role as a primary source of energy for many countries. As non-renewable sources continue to deplete, there is a need for more research and initiatives to reduce reliance on these sources and explore better alternatives, such as renewable energy. Hydrogen is one of the most intriguing energy sources for producing power from fuel cells and heat engines without releasing carbon dioxide or other pollutants. The production of hydrogen via the electrolysis of water using renewable energy sources, such as solar energy, is one of the possible uses for solid oxide electrolysis cells (SOECs). SOECs can be classified as either oxygen-ion conducting or proton-conducting, depending on the electrolyte materials used. This article aims to highlight broad and important aspects of the hybrid SOEC-based solar hydrogen-generating technology, which utilizes a mixed-ion conductor capable of transporting both oxygen ions and protons simultaneously. In addition to providing useful information on the technological efficiency of hydrogen production in SOEC, this review aims to make hydrogen production more efficient than any other water electrolysis system.

Introduction With the recent enhancement of renewable energy utilization, large scale energy conversion and storage become more important to control the surplus electric power due to the unsteady power generation. Reversible hydrogen energy storage system via solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC) are expected as a high efficient device because of its high operating temperature (600℃~1000℃). It is possible to unify them to one device as reversible system (RSOFC/EC), but it is necessary to understand the mechanism of FC/EC reactions and design the device with a good balance between FC and EC in order to apply them for an actual energy system. While SOFC reaction models and many electrochemical data of electrodes have been developed and reported by many researchers, there are few reports to investigate the reaction model of SOEC. Thus, it is possible to predict the performance of SOEC based on a comprehensive SOFC/EC models and experimental data of SOFC, because the reactions of SOFC and SOEC are a reversible relationship and the equal value of IR loss can be applied. However, the comprehensive SOFC/EC electrochemical model has not been established. A series of experimental data of anodic and cathodic current density vs. the potential should be investigated at various conditions to develop the SOFC/EC model. In this work, a series of anodic and cathodic electrode performance was taken using a SOFC in various conditions. Furthermore, attempting to predict SOEC performance from SOFC performances, we proposed and discussed a new model of electrolysis based on our developed SOFC model. Experimental The Disks of 8 mol% Y2O3-ZrO2 (YSZ; 0.25 mm thickness, 20 mm diameter) were used as the electrolyte and the anode was a Ni/YSZ or Ni/GDC composite, and the cathode was a La0.8Sr0.2MnO3/ScSZ composite. We measured the dependence of the current density of anodes on oxygen activity with wide range to get both power generation and electrolysis data for various gas composition (PH2=0.3~0.99 atm, PH2O=0.01, 0.03, 0.05 atm). Results and Discussion 1. Method to fit parameters on the SOFC/EC model We have developed the electrochemical reaction model on the fuel electrode (=anode) based on the equilibrium of competitive adsorption and oxygen activity, ao , specified by anode potential (Eq.1, Table.1) [1]. The SOFC model was constructed by assuming competitive adsorption equilibrium of H2, H2O and O on Ni surfaces at three-phase boundary and the rate-determining step to be Langmuir-type reactions of H with O (Table.1). We obtained an equation of the electrical current density, i, as a function of the oxygen activity (ao ) calculated from the anode potential which is simply measured from experiments. To confirm effectiveness of the model, we fitted the model to actual experimental data described by i vs. ao . From the equations of model on Table.1, each current density of SOFC (ia ) and SOEC(ic ) are described as Eq.2. To draw the plot of i vs. ao on the model, the rate constants (ka, kc ) and the equilibrium constants (KO, KH, KH2O ) need to be defined as reliable value. Thus, from the equilibrium potential vs. 1 atm O2 (Eq.3), we defined the rate constant of kc as Eq.4. When the kc became a dependent variable of ka , then we fitted ia to power generation experimental data described by i vs. ao so that the values of KHKO/KH2O were fitted approximately. Finally, i c model could be drawn on the plot of i vs. ao and the maximum value of i c was obtained by Eq.5 which is independent of ao and Eq.6 was obtained using Eq.4. 2. Experimental results and parameter fitting on the SOFC/EC model As the expreimental results, the maximum power density of SOFC with Ni/YSZ was 588.5 mW/cm2 for FC mode at PH2=0.99 atm, PH2O=0.01 atm, and ic(max) was 0.285 A/cm2 for EC mode at PH2O=0.03 atm. However, ic(max) calculated by the model (Eq.6) was 0.050 A/cm2. Also, in case of SOFC with Ni/GDC, ic(max) was 0.400 A/cm2 for EC mode at PH2O=0.03 atm, while ic(max) calculated by the model (Eq.6) was 0.055 A/cm2. The electrolysis current density measured by our RSOFC/EC showed 6 times higher than model values approximately. This result suggested the possibility that additional parallel reaction path was existed. In this presentation, we will discuss the possible electrochemical paths to explain the disagreement between the model and the experimental data. Finally, the comprehensive SOFC/EC model will be proposed to predict EC performance using SOFC data.

The global consensus holds that water electrolysis from renewable energy sources is the primary source of lowcarbon hydrogen supply in the future. Solid oxide electrolyzer cell (SOEC) with high efficiency, simple structure, flexible operation, and environmental friendliness has become a hot topic in green hydrogen research. This paper focuses on the thermal balance characteristics of the SOEC. An efficiency-power model that considers the thermal balance of the SOEC system with renewable energy sources is established. Then, the optimization method of SOEC operation by controlling the electrolysis current and water flow is proposed to maximize operating profit. The proposed model and method are verified by an example of wind plant data in the Gonghe basin of Qinghai Province. Moreover, the optimal electrolysis current and water mass flow values under the SOEC economic operation strategy are analyzed. The results show that the operation optimization method obtained by considering the thermal balance characteristic of the SOEC efficiency-power model can ensure the economical of SOEC while flexibly controlling the efficiency, which provides a helpful reference for the economic operation strategy of SOEC in practical engineering.

Different solutions adopted in producing Solid Oxide Cells (SOCs) have proven to be scalable and the technology is now approaching commercialization. The high energy conversion efficiency, coupled with the fuel flexibility and the reduced use of expensive catalyst materials promoted by the favorable thermodynamics at the operative temperatures, are making the technology increasingly attractive and stacks are finding applications in more fields. Large scale power-to-X plants, particularly the ones devoted to hydrogen production (Solid Oxide Electrolysis Cells – SOEC), are among the solutions that are gaining most momentum in the energy market. X-to-power (Solid Oxide Fuel Cells – SOFC) solutions such as decentralized heat and electricity production, auxiliary power systems and heavy duty transport propulsion are spreading as well, facing challenges such as power density and dynamic load operation.Advancement in the SOC technology is opting at substituting ceramic materials with mechanical support function, in favor of metal. A lower amount of ceramic materials not only reduces the raw material costs, but also allows rapid thermal cycling without compromising the mechanical integrity of the cells – hence allowing faster start-ups and load regulations. Furthermore, our recent studies demonstrated a scalable and innovative layer integration process that allows to fabricate complex 3D multilayer structures with internal gas channels and cavities of different geometries, using conventional tape casting. The monolithic SOC stacks rising from this process, combined with the integration of metallic parts in the great majority of their volume, may achieve groundbreaking power densities and the dynamic load operation desired for the mentioned applications, while decreasing CapEX and resources necessary to produce high performing SOCs of the future.The monolithic SOC stacks obtained within this work are made of three standard repetitive units (SRUs). Each SRU consists of a SOC backbone, composed by two porous zirconia based electrode backbones to be infiltrated with appropriate electro-catalysts, a dense thin zirconia based electrolyte, two sets of ferritic steel gas channel structures – one at the fuel side and one at the oxygen side – conferring enhanced gas diffusion to the triple phase boundaries, mechanical support and electronic conductivity and composite metal/ceramic interconnect plates connecting in series the SRUs. The total number of layers co-fired to produce monoliths within this work is 19 and the monoliths are denoted as “MNx3” (nomenclature defined for a monolith stack with 3x SRUs). The cross section is showed in Figure, where scanning electron microscopy is complemented by qualitative energy dispersive x-ray spectroscopy (EDS) to highlight the material composition of the three SRUs. This work presents the challenges and a solid framework for the solutions adopted to fabricate monolithic stacks aimed at hydrogen production, using the conventional tape casting shaping technique integrated with the innovative assembly process developed. Among the challenges, focus is given to i) the shaping of internal gas channels, ii) structural integrity during fabrication and operation and iii) the use of non-critical raw materials. Detailed electrochemical characterization is currently in progress and the results will be presented at the conference. Besides the real initial performance of the stacks at this development stage, it is speculated that SRU cells performance could get close to the ones of state of the art metal supported cells, also relying on electrocatalyst infiltration. Figure 1

This study investigates thermo fluid coupling mechanisms governing temperature distribution and thermal stress evolution in Solid Oxide Electrolyzer Cells (SOEC) under varying operating conditions. A multiphysics model integrating electrochemistry, gas flow and diffusion, heat transfer, and thermo mechanical response is developed to examine the effects of operating temperature, steam ratio, air flow rate, and flow configuration on electrolysis performance, internal temperature fields, and stress distributions. The numerical model is validated against the experimental data of Tu et al., showing a maximum polarization curve deviation of 9.84%. Results show that increasing operating temperature from 973 K to 1073 K raises current density by approximately 31.7%, while increasing peak temperature by about 11.5% and electrolyte tensile stress from 439.9 MPa to 518.6 MPa. Increasing the cathode side steam fraction from 60% to 90% improves current density by about 11.7%, with minor increases in peak temperature below 1% and stress near 1%. Increasing the anode side air flow rate from 10 sccm to 50 sccm reduces temperature spread and decreases maximum electrolyte tensile stress by approximately 7%. These results highlight the role of thermo fluid interactions in temperature gradient development and thermal stress evolution in SOECs.

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