Introduction and Overview: Protons, the Nonconformist Ions

Abstract

“The Nonconformist Ion” is the title of a review article on proton-conducting solids by Ernsberger in 1983 [1]. Indeed, many proton properties are peculiar. First of all, the very particular electronic structure is unique: its only valence electron lost, the proton is exceptionally small and light and polarizes its surroundings very strongly. In condensed matter, this will lead to strong interactions with the immediate environment and very strong solvation in solution. Second, two very particular proton migration mechanisms are well established. In “vehicular” motion, a protonated solvent molecule is used as a vehicle. This mechanism is typically characterized by higher activation energy and lower proton mobility. In structuralmotion, the so-calledGrotthussmechanism involves site-to-site hopping between proton donor and proton acceptor sites with local reconstruction of the environment around the moving proton. This mechanism is related to lower values of activation energy and higher proton mobility. Proton conduction can be found in many very different solid materials, from soft organic polymers at room temperature to hard inorganic oxides at high temperature. The importance of atmospheric humidity for the existence and stability of proton conduction is another common point, which goeswith experimental difficulties formeasuring proton conductivity in solids. Proton-conducting solids are the core of many technological innovations, including hydrogen and humidity sensors, hydrogen permeation membranes, membranes for water electrolyzers, and most importantly high-efficiency electrochemical energy conversion in fuel cells working at low temperature (polymer electrolyte membrane or proton exchange