Materials design, processing and application of proton-conducting oxides for electrochemical energy conversion

The growing demand for renewable energy and the need for efficient energy conversion technologies have spurred significant interest in solid oxide cells (SOCs). Recently, proton-conducting ceramic cells (PCCs) have gained considerable attention due to their potential to operate at lower temperatures than oxygen-ion-conducting SOCs. However, their widespread adoption faces challenges related to material stability, mechanical integrity, electrolyte densification, and fabrication compatibility. This dissertation seeks to overcome these challenges through a systematic investigation, from material design to fabrication, aimed at developing mechanically robust and high-performance PCCs. The results are presented in four main sections: First, a systematic study was conducted on BaZr0.8-xCe0.2YxO3-δ (BZCY, x = 0.1–0.3) proton conductors, targeting high Zr/Ce ratios for enhanced thermochemical stability. The effect of Y substitution on phase formation, grain boundary characteristics, proton conductivity, and hydration behavior was evaluated. Electrochemical impedance spectroscopy revealed that proton mobility in the bulk remained nearly constant for Y ≤ 25%, indicating a complex interplay between Y-trapping and percolation effects. Meanwhile, grain boundary conductivity was enhanced through Y segregation, supporting the space charge layer model. A substitution level of 20 at% Y was found to be optimal, maximizing conductivity by promoting both bulk and grain boundary proton transport while avoiding secondary phase formation due to solubility saturation. Second, the mechanical performance of BZCY/NiO fuel electrode supports was analyzed across different sintering and reduction conditions. Ring-on-ring testing, SEM-based fractography, and Weibull statistics demonstrated that sintering temperature and NiO-to-Ni reduction strongly influenced the porosity, flaw population, and ultimately the mechanical strength. Compared to YSZ/NiO composites, BZCY-based supports showed lower strength, underscoring the need for improved structural design for large-area PCCs applications. Third, to address the fabrication of thin electrolyte layers, a binder-free wet powder spraying (WPS) method was developed using ethanol-based suspensions without organic additives. By reducing the solid content and spraying rate, individual wet layers were dried between successive passes, minimizing tensile stress and preventing crack formation. Systematic optimization of solid content, particle size distribution, and spray distance led to the successful deposition of crack-free electrolyte films. This work proved that WPS can be a scalable and cost-effective method for effectively fabricating thin electrolyte for large scale PCCs. Finally, co-sintering of the thin electrolyte with different fuel electrode substrates was investigated, revealing critical correlations between substrate shrinkage behavior, Ba evaporation, and electrolyte densification. Zr-rich Ba-based substrates with poor shrinkage failed to retain Ba during sintering, leading to secondary phase formation and incomplete densification. While Ce rich Ba-based substrate demonstrated more substantial shrinkage before Ba evaporation occurred, greatly facilitating electrolyte densification. Meanwhile, Sr-based fuel electrodes introduced Sr diffusion that partially compensated for Ba loss and promoted sintering at lower temperatures. By leveraging an optimized wet powder spraying process to deposit an uniform, ultra-thin electrolyte layer and applying insights from half-cell co-sintering studies, gas-tight half-cells with an electrolyte thickness below 3 µm were successfully fabricated. The best fabricated full cell with Ce-rich fuel electrode exhibited power densities exceeding 1000 mW·cm⁻² at 600 °C and 0.7 V in fuel cell mode, along with a current density of 2911 mA/cm² at an applied voltage of 1.3 V in electrolysis mode. In summary, this work not only contributes new insights into the structure-processing-property relationships of Ba-based proton conductors but also establishes scalable fabrication routes and innovative design strategies for next-generation protonic ceramic devices. The demonstrated combination of advanced processing techniques with fundamental materials understanding paves the way for large-scale deployment of these technologies in renewable energy conversion and storage systems.