Solid-state Protonic Conductors: Principles, Properties, Progress and Prospects

A brief overview is given of the types and principles of solid state protonic conductors. Their properties are summarised in terms of protonic conductivity and operation temperature. A few trends and highlights of the current scientific advances are given for some of the main classes of protonic conductors. Despite our progress, a crucial point still seems to be the ability and computer power required to understand and model the complex quantum mechanical proton transfer process. Many of the commercial prospects are closely connected with the need for cleaner energy technologies and the use of hydrogen as an energy carrier. In particular, polymer-based proton conductors are quickly approaching the market for small fuel cells and electrolysers: from bench-scale demonstration units for hydrogen energy to automobiles. There is also emerging interest in mixed protonic–electronic conductors for use as hydrogen permeable membranes in hydrogen separation technologies. This will also be helpful in the much needed development of electrodes for high- and intermediate-temperature proton conductors.