Formation of 6H-Ba(3)Ce(0.75)Mn(2.25)O(9 )during Thermochemical Reduction of 12R-Ba4CeMn3O12: Identification of a Polytype in the Ba(Ce,Mn)O-3 Family

The resurgence of interest in a hydrogen economy and the development of hydrogen-related technologies has initiatednumerous research and development efforts aimed at making thegeneration, storage, and transportation of hydrogen more efficientand affordable. Solar thermochemical hydrogen production(STCH) is a process that potentially exhibits numerous benefitssuch as high reaction efficiencies, tunable thermodynamics, andcontinued performance over extended cycling. Although CeO2hasbeen the de facto standard STCH material for many years, morerecently 12R-Ba4CeMn3O12(BCM) has demonstrated enhancedhydrogen productionat intermediate H2/H2O conditionscompared to CeO2, making it a contender for large-scale hydrogenproduction. However, the thermo-reduction stability of 12R-BCMdictates the oxygen partial pressure (pO2) and temperature conditions optimal for cycling. In this study, we identify the formation ofa 6H-BCM polytype at high temperature and reducing conditions, experimentally and computationally, as a mechanism and pathwayfor 12R-BCM decomposition. 12R-BCM was synthesized with high purity and then controllably reduced using thermogravimetricanalysis (TGA). Synchrotron X-ray diffraction (XRD) data is used to identify the formation of a 6H-Ba3Ce0.75Mn2.25O9(6H-BCM)polytype that is formed at 1350 degrees C under strongly reducingpO2. Density functional theory (DFT) total energy and defectcalculations show a window of thermodynamic stability for the 6H-polytype consistent with the XRD results. These data provide thefirst evidence of the 6H-BCM polytype and could provide a mechanistic explanation for the superior water-splitting behaviors of12R-BCM