A focused review on structures and ionic conduction mechanisms in inorganic solid-state proton and hydride anion conductors

Solid-state proton and hydride anion conductors are an important family of materials as electrolytes for solid state electrochemical cells such as fuel cells, batteries, sensors, and gas separation membranes. Searching for new proton and hydride-anion conductors has been an active research area for many decades. The focus of this article is on reviewing the types and mechanisms of each proton/hydride-anion conductor developed and their pros and cons. This review starts off with the most studied and most promising perovskite structured oxides as proton conductors, followed by other types of perovskite-related structures such as the Ruddlesden-Popper phase, pyrochlores and rare earth orthoniobates/orthotantalates. This review then moves to solid polyanionic compounds as proton conductors, including sulfates, nitrates, and phosphates, which is followed by hydrates and nanocomposites. This review finally discusses the types and conduction mechanisms of new hydride-anion conductors that emerged recently.