Catalytic Proton-Hydroxide Recombination for Forward-Bias Bipolar Membranes

Abstract

Bipolar membranes (BPMs) uniquely generate steady state pH gradients in electrochemical cells, enabling half reactions to occur efficiently in different pH environments, and are thus of broad interest. Forward-bias BPMs further enable new fuel-cell, redox-flow batteries, and CO2-electrolyzer concepts. In forward bias, the gradient in electrochemical potential drives ionic charge carriers toward the bipolar junction where they can recombine. We use a H2-pump electrochemical cell to study H+ / OH− recombination at the bipolar junction. We discover that metal oxide nanoparticles substantially catalyze the recombination reaction in the bipolar junction under forward bias and find evidence that H+ / OH− recombination occurs via a surface mechanism on the oxide catalyst. We propose a rate equation to describe the catalytic H+ / OH− recombination mechanism, supported by numerical simulations. Membrane sensing experiments further reveal the nature of the oxide catalyst layers and provide insight into the potential and temperature dependence of H+ / OH− recombination at the bipolar junction. This work thus elucidates materials-design strategies for recombination catalysts to advance forward-bias BPM technologies.