Probing Local Environments of Oxygen Vacancies Responsible for Hydration in Sc-Doped Barium Zirconates at Elevated Temperatures: In Situ X-ray Absorption Spectroscopy, Thermogravimetry, and Active Learning Ab Initio Replica Exchange Monte Carlo Simulations

Abstract

Proton-conducting oxides, specifically heavily Sc-doped barium zirconate perovskite, have attracted attention as electrolytes for intermediate-temperature protonic ceramic fuel cells because of their high proton conductivity and high chemical stability against carbon dioxide in that temperature regime. Hydration is a key reaction for incorporating protons by filling oxygen vacancies, VO, with hydroxyl groups and activating proton conduction in the perovskite. However, probing the local environment of oxygen vacancies responsible for hydration is challenging because the behavior depends on the temperature and water partial pressure, which necessitates in situ observations and calculations of the local environments at elevated temperatures. To obtain such information, we combined in situ X-ray absorption spectroscopy (XAS) for both the Sc and Zr K-edges, thermogravimetry, X-ray diffractometry, and active learning ab initio replica exchange Monte Carlo (RXMC) simulations in undoped and 20–40 at % Sc-doped barium zirconates at and below 800 °C. The presence of oxygen vacancies adjacent to Sc and Zr in the dehydrated samples and the hydration of these oxygen vacancies under a wet atmosphere were probed by in situ XAS for Sc and Zr pre-edges at elevated temperatures. Here, the microscopic hydration linearly responds to the macroscopic degree of hydration. RXMC sampling further supports the presence of Sc-VO-Zr and Sc-VO-Sc environments. An initial hydration occurs in the Sc-VO-Zr environment at and above 600 °C, but the Sc-VO-Sc environment contribution is greater at higher degrees of hydration. The Zr-Vo-Zr environment is the least abundant among them for the whole temperature range examined and thus has a negligible impact.