Fine-Tuning of Air Electrode Microstructure and Its Composition as a Way to Enhance the Performance and Durability of Solid Oxide Electrolyzer - preliminary results

Abstract

Among actions undertaken to reach a net-zero economy by 2050, implementation of the hydrogen technologies into various sectors e.g. energy and transport seems to be crucial. The significant increase of the installed capacity of electrolyzer in the last few years has been observed, which will accelerate in next decades to meet declared by many countries goals of their national hydrogen strategies. Nowadays, a dominant role on the electrolyze markets possess low temperature solution, namely, alkaline and PEM electrolyzes. However, due to higher efficiency – lower energy demand for hydrogen production, it is forecast that solid oxide electrolyzers (SOE) will take part of the market. The development of SOE, which are on the final R&D phase, is mainly focused on the extension of their lifespan and minimizing their manufacturing costs. The La 1-x Sr x CoO 3-δ (LSC) and La 1-x Sr x Co y Fe 1-y O 3-δ (LSCF) oxides due to their good catalytic activity and high mixed ionic-electronic conductivity are recognized as state-of-the-art air electrodes for SOC. However, Co-based perovskites are characterized by high thermal and chemical expansion, which might cause a mechanical mismatch with electrolyte, resulting in intensified SOC degradation. To mitigate mentioned issues different strategies have been proposed in the literature. Through the combined approach focused on modification of the bulk properties, simultaneously tailoring the microstructure of the electrodes and electrode/solid electrolyte interface, it is possible to overcome the kinetic limitations of operation at decreased temperatures. To maximize cell performance, and prevent the potential electrode degradation (i.e. its delamination) composite GDC-LSC/LSFC electrodes with gradual changes of the composition from electrolyte-electrode interphase to the electrode surface, were proposed. Furthermore, the impact of modification of electrode microstructure by an increase of its porosity and infiltration of the electrode surface with catalytically active oxides (e.g. Pr x O y ) was investigated. Fine-tuning of electrode porosity was achieved by the addition of the pore-forming agent, and the selection of its type (graphite or PMMA), amount, and size of its grains. Moreover, the work presents an approach to optimize the buffer layer, inter alia by its densifying, to mitigate Sr diffusion to the electrolyte and prevent air electrode delamination. The developed composite air electrodes were screen-printed (with an active area of 16 cm 2 ) on the fuel electrode-supported cell and evaluated in the SOE mode at the 650-750 °C temperature range. Tests included measurements of j-V dependences and EIS spectra (at different temperatures, current densities, and for different gas flow delivered at the air side of the cell). In order to assess the impact of the added amount and type of pore-forming agent on the microstructure of the electrode layer, as well as to investigate possible microstructural changes of the cell after testing SEM, SEM-EDS, and FIB-SEM analysis were performed. The proposed modification of the composition and microstructure resulted in higher current densities and reduced cell polarization compared to standard cells with LSC and LSCF as the air electrode. Acknowledgments The presented research was financially supported by: the National Centre for Research and Development, Poland, within project no. LIDER/1/0003/L-12/20/NCBR/2021 (research related to composite GDC-LSC/LSFC electrodes), and Ministry of Science and Higher Education through the statutory grant, within grant no. CPE.4000.001.2023 (research related to the improvement of the buffer layer-electrode interphase).