Solid oxide cells (SOCs) are likely to play crucial role in the green energy transition, but their widespread adoption is hindered by degradation issues, particularly catalyst agglomeration. Nanoparticle exsolution in double-perovskite materials offers a promising solution by creating electrode materials with stable metallic nanocatalysts strongly bonded to the parent oxide, mitigating high-temperature agglomeration issues. Thus, understanding the dynamic evolution of microstructure and catalytic behavior in such materials is vital for developing high-performing SOC catalysts. This study utilized a multimodal approach to investigate the dynamics of exsolution in Sr2FeMo0.65Ni0.35O6-delta (SFM-Ni) and its effect on cell performance. In situ environmental transmission electron microscopy (ETEM), in situ transmission electron microscopy (TEM) coupled with mass spectrometry visualized the formation and the stability of exsolved particles especially at the concave faces of the parent material during chemical conversion of CO from CO2. Simultaneously, macro-scale cell experiments coupled with electrochemical impedance spectroscopy, and focused ion beam-scanning electron microscopy (FIB-SEM) tomography, apart from verifying the nanoscale observations, provided crucial insights into the correlation between the exsolution process observed at the micro-scale and the overall cell performance. These findings offers valuable insights into the design and optimization of improved electrode materials for SOCs. Understanding the dynamic behavior of exsolved catalysts would help in enhancing the electrochemical performance at both the nano and macro levels, ultimately advancing the field of sustainable energy technologies.

The present study focuses on the highly catalytic double-perovskite Sr2FeMo0.65Ni0.35O6-delta (SFMNi) fuel electrode material for Solid Oxide Electrolysis Cells (SOECs). The electrolyte-supported single button cells with the highly active SFMNi fuel electrode were electrochemically characterized between 900 degrees C down to 750 degrees C in steam and co-electrolysis conditions using DC- and AC-techniques. The cells achieved current densities of -1.62 A cm(-2) and -1.74 A cm(-2) at 900 degrees C under steam and co-electrolysis conditions, respectively, exceeding the performance of cells with Ni-8YSZ fuel electrodes by similar to 65-79 % and Ni-GDC fuel electrodes by 24-28 %. The post-test SEM-EDX analyses of the as-prepared and tested cells' cross-section showed increased pore formation and particle growth of the SFMNi fuel electrode after testing in the humidified atmosphere for 500 h.

Solid oxide electrolysis cells (SOECs) have proven to be a highly efficient key technology to produce valuable gases (H2, CO). SOECs utilize renewably generated electricity at temperatures between 600 - 900 °C, thereby providing a carbon-neutral method for energy storage. However, the successful industrial implementation of this technology requires long-term stability of all system components and is mainly delayed by the degradation of the electrodes over time. The state-of-the-art Ni-YSZ fuel electrode has been extensively studied and exhibits severe performance loss due to Ni particle agglomeration and Ni migration away from the active sites at the electrolyte/electrode interface under real operating conditions [1]. To mitigate this issue, we have investigated the Ni-free perovskite Sr2Fe2-xMoxO6-δ + 30% GDC as fuel electrode material. As mixed ionic and electronic (MIEC) perovskite structured oxides, these materials have shown excellent short-term redox stability in oxidizing and reducing atmospheres in addition to high conductivity and outstanding coking resistance [2]. These characteristics meet exactly the targeted requirements for new solid oxide electrolyzer materials. We have compared the long-term degradation of an SFM fuel electrode to an electrode made of SFM-GDC (Figure 1). The degradation was less severe for the mixed electrode of SFM-GDC. Impedance and post-test SEM-EDX analysis clarified the main degradation mechanisms of SFM as well as SFM-GDC in steam and CO2 electrolysis. The tested button cells showed demixing of the SFM phase and particle aggomeration in the fuel electrode. [1] S. E. Wolf, V. Vibhu, E. Tröster, I. C. Vinke, R.-A. Eichel and L. G. J. de Haart, Energies, 15(15), 5449 (2022). [2] L. Bernadet, C. Moncasi, M. Torrell and A. Tarancón, Int. J. Hydrog. Energy, 45(28), 14208–14217 (2020). Figure 1

The review article covers all state-of-the art materials related to high-temperature electrolyzers based on oxygen-ion conductors. The focus lies on the cell materials, materials of additional components like interconnects and sealants are briefly described.

Severe performance loss of state-of-the-art Ni-cermet fuel electrodes has been observed due to Ni particle agglomeration and Ni migration away from the triple-phase boundary at the active electrode layer. Investigation of Ni-free perovskite Sr 2 Fe 2-x Mo x O 6-δ (SFM)–30 wt% Ce 0.8 Gd 0.2 O 2-δ (GDC) as fuel electrode material showed good redox stability in oxidizing and reducing atmospheres. These characteristics meet exactly the targeted requirements for new solid oxide electrolyzer materials. A comparison of SFM and SFM-GDC fuel electrodes showed similar performance for H 2 O, CO 2 , and co-electrolysis between 750 °C and 900 °C. Long-term degradation measurements and post-test SEM-EDX analysis clarified that SFM-GDC exhibits good phase stability in comparison to pure SFM fuel electrodes. The composite fuel electrode Sr 2 Fe 1.1 Mo 1.0 O 6-δ –30 wt% Ce 0.8 Gd 0.2 O 2-δ (SFM15-GDC) reached polarization resistances (R P ) in the same range as Sr 2 Fe 1.0 Mo 1.0 O 6-δ with around 127 mΩ∙cm² and 137 mΩ∙cm at 900 °C in steam electrolysis.

High-temperature electrolysis using solid oxide electrolysis cells (SOECs) is an innovative technology to temporarily store unused electrical energy from renewable energy sources. However, they show continuous performance loss during long-term operation, which is the main issue preventing their widespread use. In this work, we have performed the long-term stability tests up to 1000 h under steam and co-electrolysis conditions using commercial NiO-YSZ/YSZ/GDC/LSC single cells in order to understand the degradation process. The electrolysis tests were carried out at different temperatures and fuel gas compositions. Intermittent AC- and DC- measurements were performed to characterize the single cells and to determine the responsible electrode processes for the degradation during long-term operation. An increased degradation rate is observed at 800 °C compared to 750 °C under steam electrolysis conditions. Moreover, a lower degradation rate is noticed under co-electrolysis operation in comparison to steam electrolysis operation. Finally, the post-test analyses using SEM-EDX and XRD were carried out in order to understand the degradation mechanism. The delamination of LSC is observed under steam electrolysis conditions at 800 °C, however, such delamination is not observed during co-electrolysis operation. In addition, Ni-depletion and agglomeration are observed on the fuel electrode side for all the cells.

In the temperature range of high temperature co-electrolysis of both steam and carbon dioxide, the reverse water-gas shift reaction (RWGS) takes place. Prior studies were conducted with a narrow gas composition range to investigate the role of RWGS during co-electrolysis. The results for steam electrolysis, CO 2 electrolysis, and co-electrolysis caused different conclusions regarding the role of electrochemical CO 2 and H 2 O conversion compared to RWGS during co-electrolysis. This work aims to resolve the role of CO 2 conversion as part of RWGS in co-electrolysis. The boundary is characterized by AC and DC measurements over a broad gas composition range from CO 2 electrolysis towards co-electrolysis with nearly 50% eq H 2 O. Especially, the electrochemical CO 2 reduction and CO 2 conversion in the RWGS are compared to clarify their role during co-electrolysis. The results revealed that gas composition determined the predominant reaction (H 2 O or CO 2 reduction). The cell performance of co-electrolysis in the boundary region up to 5% eq H 2 O was similar to the performance of CO 2 electrolysis. Up to 30% eq H 2 O, the performance increases with H 2 O concentration. Here, both CO 2 and H 2 O electrolysis occur. Above 30% eq H 2 O, steam electrolysis and the RWGS reaction both dominate the co-electrolysis process.

This contribution highlights selected current activities of the SOC development at Forschungszentrum Jülich. Continued efforts are being made to gain a better understanding of degradation process in our cells and stacks. New materials are being developed to mitigate known degradation phenomena. Systems development was directed at the improvement of reversible operation, especially in electrolysis mode. On cell and stack level investigation of electrolysis operation was also intensified, focusing on CO 2 -valorization.

High-temperature co-electrolysis of CO2 and H2O at elevated temperatures between 700 °C and 900 °C valorises CO2 to produce a mixture of carbon monoxide (CO) and hydrogen (H2), called syngas. Co-electrolysis has the great advantage over conventional processes, that the desired syngas ratios of downstream processes can be realized by varying process parameters such as temperature and feed gas composition accordingly in a one step process. Co-electrolysis can also play a vital role in counteracting power fluctuations of renewable energy sources by storing temporarily unused electricity through conversion to other energy resources like chemicals or heat for later use. The underlying processes in co-electrolysis for CO production are direct electrochemical CO2 reduction and reverse water gas shift equilibrium (RWGS). Their specific significance has not been clarified in detail yet and was controversially discussed in literature up to this day [1,2]. The impact of the equilibrium partial pressure of H2O on the physical processes in the transition boundary of co-electrolysis towards direct CO2-electrolysis was investigated by AC and DC measurements for various gas compositions. The analysis led to identifying the role of the underlying electrochemical processes during co-electrolysis, in particular the electrochemical CO2 reduction compared to the conversion of CO2 in the reverse water-gas shift reaction and the electrochemical H2O reduction. The area specific resistance (ASR) was, amongst others, taken as an indicator to determine, which of the reduction reactions (H2O or CO2 reduction) is dominant depending on the gas composition. The experiments were conducted using commercially available cathode-supported full cells (Elcogen) made of Ni-8YSZ/8YSZ/CGO/LSC. Results as seen in Figure 1 show that the ASR for an equilibrium concentration of 5 % H2O is considerably larger than for higher H2O contents. Above 15 % H2O, the ASR shows no dependency on the gas composition and is comparable to pure H2O-electrolysis [3]. These observations underline the hypothesis that CO2-electrolysis becomes pre-dominant compared to H2O-electrolysis for low H2O content during co-electrolysis. With increasing H2O content, CO2-electrolysis becomes less significant and carbon dioxide is converted in the reverse water gas shift equilibrium. The origin of the discrepancy in literature was found to be the different operating H2O concentrations. A threshold has been established for the perception of CO2-electrolysis during co-electrolysis experiments. figure caption: Arrhenius plot of ASROCV for different steam concentrations at 6 l·h-1. [1] C. Stoots, J. O'Brien, J. Hartvigsen, Int. J. Hydrogen Energy 2009, 34, 4208. [2] S. D. Ebbesen, R. Knibbe, M. Mogensen, J. Electrochem. Soc. 2012, 159, F482-F489. [3] L. Dittrich, M. Nohl, E. E. Jaekel, S. Foit, L.G.J. (Bert) de Haart, R.-A. Eichel J. Electrochem. Soc. 2019, 166, F971-F975. Figure 1

The Distribution of Relaxation Times (DRT) is an important analytical tool that is capable of giving initial information from Electrochemical Impedance Spectra (EIS) with respect to the number of relaxation processes occurring in the system and their corresponding relaxation frequencies. The DRT transformation with the Tikhonov regularization is used for analysis of EIS data obtained from the characterization Solid Oxide Fuel and Electrolysis Cells (SOFC/SOEC) operating at high temperatures. The effects of this transformation together with occurring pitfalls on the most commonly implemented circuit elements used to describe EIS data was investigated to gain a better understanding. The behavior of the DRT transformation as a function of the individual circuit elements is reported, the regularization parameter λ is taken as a sweep parameter to investigate its influence and optimal ranges for the selection of λ are presented.