Catalytic Proton–Hydroxide Recombination for Forward-Bias Bipolar Membranes

Bipolar membranes (BPMs) can generate steady-state pH gradients in electrochemical cells, enabling half-reactions to occur in different pH environments, and are thus of broad interest. Forward-bias BPMs further enable new approaches to fuel cells, redox-flow batteries, and CO2 electrolyzers. 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 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. This work thus elucidates materials-design strategies for recombination catalysts to advance forward-bias BPM technologies.