Solid oxide proton conductors beyond perovskites

Solid oxide proton conductors are crucially emerging as key materials for enabling hydrogen-based energy conversion, storage, and electrochemical technologies. Oxides crystallising in the ideal ABO₃perovskite structure, such as barium cerates and zirconates, are widely investigated thanks to their excellent proton conducting properties. Nevertheless, alternative structure-type solid oxide systems (hexagonal perovskite derivatives, brownmillerite, scheelite,etc.) can efficiently incorporate and enable the transport of protonic defects, with recent reports of materials exhibiting high ionic conductivity comparable to the conventional perovskite conductors. This perspective provides an overview of these alternative and less established proton conducting materials, with particular attention to the relationship between the structural and ionic conduction features and the mechanistic aspects. The goals are to highlight the differences between these materials and the traditional perovskites and to point out new potential crystal routes for the discovery of innovative solid oxide proton conductors.