Understanding Transport at the Acid-Alkaline Interface of Bipolar Membranes

Abstract

The combination of cation exchange membrane (CEM) and anion exchange membrane (AEM) electrolytes to form of a hybrid, or bipolar membrane (BPM) electrolyte, can have unique advantages for electrochemical systems including fuel cells, electrolyzers, electrodialysis, and photovoltaic solar-to-fuel devices. However, a major challenge for this approach is the development of a stable and active interfacial region (i.e., junction) that adjoins the CEM and AEM layers. Moreover, a fundamental understanding of transport at the CEM-AEM junction is lacking. Therefore, the present study focuses on the theoretical development and analysis of the nature of the BPM interface. A Poisson-Nernst-Planck (PNP) theory is formalized and applied to a representative BPM interface. The findings are reported in terms of bias (i.e., overpotential) in a galvanic device with respect to CEM and AEM material requirements. Specific attention is paid to our interests in the application of the BPM to a fuel cell device with an acidic (CEM) anode and alkaline (AEM) cathode. We demonstrate that a BPM with an acidic CEM anode and alkaline AEM cathode must promote a trap-assisted type of recombination mechanism under forward bias. Without such a mechanism, large overpotentials are needed to drive ionic recombination processes.