Solid oxide cells (SOC) are a key technology for large-scale green hydrogen production due to their high efficiency. However, widespread commercial application faces considerable challenges. In particular, these are challenges in achieving simultaneously long-term stability and high performance, which affects cost efficiency. The key component of the technology that links both properties is the air electrode. High performance materials often suffer from chemical instability and are prone to react with impurities in the air stream or other cell components. There are also morphological challenges, particularly in electrolysis mode, with delamination of the air electrode layer due to increased interfacial oxygen partial pressures. The search for more stable electrodes that exhibit fast oxygen exchange kinetics and high electronic conductivity remains one of the key priorities towards technology industrialization.
This thesis investigates high entropy perovskites (HEP) as air electrode materials for SOC, focusing primarily on the solid oxide electrolysis cell (SOEC) mode of operation. The air electrode is tailored to replace the critical element cobalt with the sustainable and cost effective element iron, while maintaining chemical stability and high power densities. Structural analysis provides insights into phase purity and crystal structure using X-ray powder diffraction and Rietveld refinement. Oxygen exchange kinetics and electrical dc-conductivity are analyzed by 4-point dc-conductivity (relaxation) experiments. Electrochemical performance is assessed by cell tests including current density-voltage curves and electrochemical impedance spectroscopy. Results are correlated with advanced analyses of the cell components’ morphology and elemental distribution by field emission scanning electron microscope (FESEM) and energy dispersive X-ray (EDX) imaging. Long-term tests with 5x5 cm² cells are carried out to assess the stability of the air electrode under application-relevant conditions.
The results of the thesis confirm the long-term stability of HEP as air electrodes in SOEC mode and its potential to outperform state of the art equivalents. It is successfully demonstrated that full cells with La0.2Pr0.2Nd0.2Sm0.2Sr0.2CoO3-δ (LPNSSC) and La0.2Pr0.2Nd0.2Sm0.2Sr0.2FeO3-δ (LPNSSF) air electrodes can be maintained in electrolysis mode for several hundred hours at constant, high electrolysis currents. After a run-in phase, LPNSSF shows a continuous degradation of 0.8% 1,000 h-1, while LPNSSC exhibits constant cell voltage during the individual test phases. Compared to the state of the art air electrode material La0.6Sr0.4CoO3−δ, LPNSSC shows less formation of secondary phases, no delamination and higher current densities at same test conditions.
Trends in oxygen exchange kinetics and electronic conductivity are discussed with respect to the influence of temperature, oxygen partial pressure, and cobalt-to-iron substitution. These fundamental insights on mass- and charge transport properties are used to improve electrode performance of LPNSSF by an innovative composite/current-collector approach. The results of this thesis highlight the potential of HEP as promising air electrode materials for SOC and are an incentive for further research into this class of materials to advance production of green hydrogen.