Advanced air electrode materials for high-performance reversible protonic ceramic cells

Abstract

The development of efficient and environmentally friendly energy conversion and storage devices is crucial for advancing renewable energy. Solid oxide cells (SOCs) demonstrate significant potential due to their high energy conversion efficiency and environmental benefits. However, SOCs typically operate at temperatures exceeding 700 °C, which accelerates component degradation and increases system costs. Reversible protonic ceramic cells (RePCCs), a type of proton-conducting SOC, operate at intermediate temperatures (350-650 °C), extending component lifespan and reducing costs. However, the slow kinetics of the oxygen reduction reaction (ORR) and water oxidation reaction (WOR) at the air electrode limit the performance of these cells. Developing advanced air electrode materials is essential to enhance efficiency. Currently, most air electrode materials are Co-based perovskites, which exhibit excellent performance due to the catalytic activity of Co. However, Co-based electrodes generally have a high thermal expansion coefficient (TEC), leading to electrode delamination during long-term operation. Fe-based air electrodes, with relatively lower TEC and significantly lower costs compared to Co-based electrodes, are promising candidates for RePCC air electrodes. This study focuses on the modification of Fe-based Ruddlesden-Popper (RP) layered perovskite oxides. RP layered perovskites are promising candidates for RePCC air electrodes due to their excellent oxygen transport properties and hydration characteristics, which make them H+/O²⁻/e⁻ triple-conductive oxides. However, RP materials tend to decompose under humid atmospheres during long-term operation and exhibit lower activity compared to Co-based electrodes. In this thesis, we employ strategies such as oxygen-proton balancing, elemental doping, and self-assembly to develop a series of high-performance, high-stability Fe-based RP-type RePCC air electrodes, overcoming the low electrocatalytic activity and poor stability of conventional RP materials in steam environments. Firstly, a high-valent ion doping strategy is used to regulate the oxygen and proton properties of RP materials under air electrode working conditions. Moderate doping effectively optimizes oxygen/water surface exchange, thereby enhancing electrochemical activity and providing guidance for the development of RP materials as RePCC air electrodes. Secondly, a co-substitution strategy involving A-site vacancies and B-site elements is employed to improve the stability and electrochemical performance of RP perovskites, enabling the preparation of efficient RePCC air electrodes. High-valent Nb doping enhances phase and thermal stability, while appropriate A-site Sr deficiencies create oxygen vacancies to compensate for performance losses caused by Nb doping while maintaining the stability of the parent phase. Furthermore, a self-assembly strategy is utilized to prepare a series of multiphase nanocomposite materials. The complementary properties of the individual phases collectively enhance electrode performance and stability. Additionally, phase regulation of the self-assembled composites is achieved by adjusting elemental composition. Optimized phase proportions lead to RePCC air electrode materials with excellent electrochemical activity and durability. This study provides a comprehensive investigation of the reaction mechanisms, current research progress, and modification methods for air electrodes. Strategies including oxygen-proton balancing, the dual-modification approach, and controllable self-assembly material design are identified as crucial for further improving RePCC performance.