The Impacts of Synthesis Routes and Phase-Purity on Water-Splitting Behavior in the Barium-Cerium-Manganese System

Solar thermochemical hydrogen production (STCH) is a promising technology for solar fuels with high theoretical solar-thermal water-splitting efficiency. However, many candidate STCH oxides are complex ternary oxides, and they have the potential for secondary phase formation during synthesis and subsequent redox cycling. During synthesis, pervasive refractory impurity phases can occur, which impede the ability to develop a basic understanding of the STCH potential and complicate structural refinement of chemical expansion during isothermal reduction/oxidation. This work focuses on high phase-purity synthetic routes for known water-splitting oxides; in particular, BaCe_0.25Mn_0.75O₃ (BCM). These are approached by synthesizing both bulk ceramic and thin films, which are analyzed via in-operando synchrotron-based X-ray diffraction (XRD) to provide the highest quality structural refinements on BCM and other complex STCH oxides to date. The relationship between chemical shift and oxygen vacancy concentration is determined using a combination of flow-cell measurements and in-operando XRD during isothermal reduction and oxidation. In addition, key data for training computational simulations of vacancy formation and the effect of vacancies on structures is presented.