Understanding the Role of Proton and Hydroxide Transport in Forward-Bias Bipolar Membrane for Electrochemical Applications

A forward-bias bipolar membrane (BPM) provides an alkaline cathode condition, which can be beneficial to some electrochemical reactions, such as the CO₂ reduction reaction (CO₂RR), but the water association (WA) in forward-bias BPM is not well understood at all. In this study, BPMs are designed with different interfacial polymeric catalysts to investigate the WA reaction under forward-bias for electrochemical applications. An enhanced current density is observed with added polymeric catalysts (−OH, −O−, −N−, and graphene oxide) compared to the blank control. Temperature-dependent measurements indicated that the WA in BPM is not kinetically controlled. The in-plane and through-plane ions diffusion is investigated, which showed that the WA in BPM is limited mostly by the transport of OH⁻ and, to a lesser degree, H⁺ at the interface. Molecular dynamic studies presented that the migration rate of OH⁻ at the interface is approximately one order of magnitude lower than that of H⁺, indicating that the WA is mainly governed by the transport of OH⁻. Finally, a forward-bias CO₂RR electrolyzer is demonstrated with an Faradaic effeiciency CO (FE_CO) of 92.2 ± 2.7%. This work provides important fundamental insights into the WA reaction that would enable the use of forward-bias BPM electrolyzers in future electrochemical applications.