Ceramic proton conductors and their diversity of transport properties

Ceramic proton conductors have been used to develop high-temperature electrochemical cells, including fuel cells, steam electrolysers, gas sensors and hydrogen membrane reactors. The protonic charge carriers derive from incorporated water vapour, resulting in these materials seldom being pure proton conductors. Rather, they present a diversity of transport properties: protonic, oxide ion and electronic (p-type or, less commonly, n-type). While measurements of individual ionic transport numbers are relatively widespread, comprehensive studies of the oxide ion, protonic and electronic transport numbers in the same material are less common. In this article, published data on the transport numbers of perovskite proton conductors are reviewed, and a convenient graphical representation of these is proposed, based on the ternary plot.