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 (FECO) 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.