Uniting Experiment and Computational Modeling to Elucidate Structure-Property Relationships of Water Splitting Materials

Abstract

The layered perovskite BaCe0.25Mn0.75O3 (BCM) is a proven solar thermochemical water splitting material demonstrating higher hydrogen generation and improved redox kinetics relative to current state of the art materials. However, fundamental understanding of the underlying mechanisms of BCM’s efficacy is necessary for solar thermochemical water splitting in related perovskite-type materials to become a viable pathway for green hydrogen generation. To this end, a family of BXM (X = Ce, Nb, Pr) materials has been synthesized and undergone in-depth, multi-faceted characterization. Despite the three materials having identical structures, and differing only in 1/8 of their cation species, their water splitting performances are markedly different. X-ray absorption spectroscopy (XAS) analysis reveals substantial differences in the electronic structure of oxygen between the three samples, both when fully oxidized and at quantified degrees of reduction. However, interpreting the changes in XAS spectra of these complex materials via conventional means has proven to be challenging. The Molecular Foundry has successfully simulated the XAS spectra of the oxidized and defected structures allowing specific peaks in the XAS spectra to be correlated to molecular orbitals and providing invaluable insights into the underlying electronic changes observed experimentally. Additionally, both in situ and ex situ electron energy loss spectroscopy (EELS) analysis of the BXM materials allowed changes in the O K-edge and the Mn L-edge to be monitored with increasing extent of reduction and directly compared to XAS. The unique advantage of performing STEM-EELS analysis of these materials during in situ thermal reduction is the ability to observe underlying structural modifications, such as phase changes or defect formations, in tandem with the electronic changes in the material. Taken together, the results reported here provide a key component towards elucidating structure-property relationships for this class of solar thermochemical hydrogen production (STCH) materials.