Making and Breaking Bipolar Membrane Protonic Diodes

Abstract

Historically, bipolar membranes have been used for application in electrodialysis at moderate current densities (<100 mA/cm2). In the past decade bipolar membranes have experienced a renaissance due to interest in driving higher current density processes with intentional pH gradients between the anode and cathode, such as CO2/H2O electrolysis and electrochemical generation of acid and base for direct oceanic carbon capture, two applications that my group actively studies. My group’s main contribution to this field is that we recently showed that bipolar membranes are in fact high-quality protonic diodes, where water serves as a (protonic) semiconductor. Given this fact, we leveraged techniques from the semiconductor physics and electrocatalysis communities, i.e. Mott–Schottky analysis and Butler–Volmer analysis, to further characterize bipolar membrane performance. Using this platform, we have also demonstrated photovoltaic action from photoacid-dye-sensitized bipolar membranes. More recently, we have studied how intentional placement of proton-donating and proton-accepting groups in the bipolar membrane space–charge region forms a recombination/generation junction, which are the non-tunneling variants of tunnel junctions that are important in tandem solar cells. This allows for rapid protonic conduction across originally rather insulating junctions, as desired for ionic conductors in most electrochemical devices. Collectively, our efforts form the foundational framework for new functions and resulting applications that benefit from protonic transfer and transport.