Many hydrated ABO3-type perovskiles are found to be fast proton conductors in the intermediate temperature range ～200-500 °C. Loading protons in the perovskite structure relies on acceptor-doping to the B site, whereby oxygen vacancies are created, which can be filled with hydroxyl groups in a humid atmosphere at elevated temperatures. On a local scale the proton conduction process is composed of two elementary steps: (i) hydrogen-bond-mediated proton transfer between adjacent oxygens and (ii) rotational motion of the hydroxyl group in between such transfers. The long-range motion of the protons is a series of such transfers and rotations, with an overall rate which depends on the local energy barriers.
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