Proton conduction in ionic crystals based on polyoxometalates

Polyoxometalates (POMs) have attracted considerable attention as an important class of proton-conductive materials since POMs can efficiently transport protons due to their small effective negative surface charge density. However, the acid salts of POMs suffer from low structural stability and limit the practical application for fuel cells. A promising approach to overcome the drawback is hybridizing POMs with appropriate counter cations to construct POM-based ionic crystals. In this review, we will classify POM-based ionic crystals into four groups by the type of counterpart of POMs: 1) inorganic cations, 2) organic cations, 3) organic polymers, and 4) cationic complexes, and present an overview of the recent advances including the design strategies, the proton conductivity, and the mechanism for proton conduction.