Formation of protonic defects in complex oxides: Fundamentals and methodological considerations of hydration

High-temperature proton-conducting oxides are extremely important materials for next-generation electrochemical energy technologies. Although the hydration chemistry of conventional perovskites has recently been well established, contemporary materials science has revealed proton transport in various non-classical systems, including oxides with cation ordering, coherent intergrowth motifs, and redox-active cations. This review covers newly emerged information and discusses existing data on water uptake across the broad spectrum of complex oxides. The analysis emphasizes the use of thermogravimetry as the principal experimental tool and critically evaluates its methodological nuances that govern measurement accuracy. Common experimental limitations (deviated chemical stoichiometry, surface-limited reactions, and specific instrumental artifacts) have been shown to systematically lead to lower water uptake values than expected. By consolidating these insights, this work provides a rigorous foundation for defect-chemical analysis in terms of proton measurements in diverse oxide compositions. Consequently, this review is an important resource for specialists engaged in the applied development and theoretical modeling of high-temperature proton-conducting oxides and related electrochemical cells.