(Invited) High-Quality Protonic Diodes and Their Transformation into Solar Cells and Conductors

Ionically conductive materials and diodes have been actively studied since the pioneering work of John Bockris in 1959. The subset of purely protonic diodes based on liquid water are far less common, yet are more analogous to electronic semiconductors in that generation, recombination, and transport each occur and junctions fully equilibrate. In my presentation, I will explain how water is a protonic semiconductor and show that it can be doped through addition of salts of H+ and/or OH– that when fixed in place using polymeric bipolar membrane scaffolds enable purely protonic diodes. Electrochemical evaluation is achieved using membrane–electrode–assemblies that drive reversible H2 redox chemistry to transduce changes in electronic electrochemical potential into protonic electrochemical potential, and vice versa. Characterization of the semiconducting properties of these protonic diodes allowed for determination of flatband potentials using Mott–Schottky analysis, and mobilities via analysis of space-charge-limited conduction. Further, via covalent modification of polymers with photoacid dye sensitizers we were able to demonstrate solar cell function, and modification with generation/recombination buffering groups we were able to demonstrate ohmic conduction. The latter is relevant for application of bipolar membranes in CO2 electrolyzers. Collectively, these efforts form the foundational framework for new devices and functions that benefit from purely protonic transport.