Revealing Evolution of Cobalt Oxide Thermochemical Materials During Thermal Redox Cycles via Operando Synchrotron X-Ray Nano-Tomography and Spectroscopic Imaging

Thermal energy storage (TES) is crucial for enhancing energy efficiency, integrating renewable energy sources, and supporting sustainable energy systems. Among all thermal energy storage materials, thermochemical material (TCM), particularly Co₃O₄, is one of the most promising due to its high reaction enthalpy, fast reaction kinetics, and lower cost. However, in long-term applications, the energy efficiency of Co₃O₄ decreases due to degradation. The detailed degradation mechanism remains unclear, in part because the correlation between its morphological, structural and chemical evolution is not fully understood. In this work, the chemical and three-dimensional (3D) morphological evolution of Co₃O₄ at the micro and nanoscale during thermal reduction and oxidation cycles between 800 to 900 °C in air were studied using <i>in situ</i> synchrotron transmission X-ray microscopy (TXM), transmission electron microscopy (TEM) and thermal analysis techniques. Results indicate that the conversion rate of Co₃O₄/CoO was faster in nanoparticles compared to microparticles, especially during the reverse oxidation reaction. By further comparing different structural and morphological evolutions between microparticles and nanoparticles, the conversion kinetics is related to the morphological characters such as specific surface area, edge gradient, and the formation of porous structures within particles. These findings provide insights into the distinct thermochemical behaviors of Co₃O₄ particles of different scales, which is essential for their applications in TES systems.