Eli Nygren1,Robert Bell2,Sarah Shulda2,Dan Plattenberger2,Eric Coker3,Nicholas Strange2,4,Karen Heinselman2,Philip Parilla2,Anthony McDaniel3,Michael Toney4,Sai Gautam Gopalakrishnan5,Emily Carter6,Ellen Stechel7,Sue Carter1,David Ginley2
University of California, Santa Cruz1,National Renewable Energy Laboratory2,Sandia National Laboratories3,SLAC National Accelerator Laboratory4,Indian Institute of Science5,Princeton University6,Arizona State University7
Eli Nygren1,Robert Bell2,Sarah Shulda2,Dan Plattenberger2,Eric Coker3,Nicholas Strange2,4,Karen Heinselman2,Philip Parilla2,Anthony McDaniel3,Michael Toney4,Sai Gautam Gopalakrishnan5,Emily Carter6,Ellen Stechel7,Sue Carter1,David Ginley2
University of California, Santa Cruz1,National Renewable Energy Laboratory2,Sandia National Laboratories3,SLAC National Accelerator Laboratory4,Indian Institute of Science5,Princeton University6,Arizona State University7
<br/><br/><b>For H<sub>2</sub> and CO production, redox-active multinary metal oxides are being designed for cycling between high-temperature formation of oxygen vacancies via Schottky defects and low temperature (e.g. ~800 <sup>o</sup>C) annihilation of these defects by reducing either H<sub>2</sub>O or CO<sub>2</sub> respectively. The Gibbs free energy of formation for these Schottky defects can be tuned to change their chemical reactivity and concentration, being dependent on temperature and oxygen chemical potential. Recent computational modeling efforts to increase configurational entropy of Schottky defects (via charge compensation by local cation reduction) have predicted that co-reduction of multiple cations in the same metal oxide, especially when found on different sub-lattices of a crystal structure, could lead to a desirable increase in defect concentration and reactivity, all else equal. Led by these simulations, the quinary oxide perovskite (Ca,Ce)(X,Y)O<sub>3</sub> (CCXY) has recently been identified to have solar thermochemical hydrogen (STCH) water-splitting performance exceeding state of the art material ceria under measured operating conditions. In this material family, vacancies and local cation reduction constitute Schottky defects in the pristine unit cell ABO<sub>3</sub>, yielding the off-stoichiometry product ABO<sub>3-δ</sub>, where δ is the stoichiometric quantity of oxygen vacancies.</b><br/><br/><b>In this work, we present our efforts to understand the relationship between redox-active cation ratios (on both A and B sites of CCXY), and the Schottky defect concentrations and chemical reactivities. Using a combination of in-situ (TGA) and ex-situ (XRD) techniques, we find that in all cases, these intentional defects are reversible, allowing for continuous redox cycling. We have tracked the crystal structure throughout the redox process by “freezing in” oxygen vacancies by cooling under reducing conditions. XRD data show that these materials exhibit changing unit cell dimensions, dependent on their stoichiometry, during cycling. Furthermore, their orthorhombic structure, aside from lattice parameter shifting, remains unchanged, demonstrating remarkable defect tolerance. XAS measurements confirm simultaneous reduction of A and B sites, lending credence to further investigation of CCXY as a water splitter for STCH.</b>