Mechanistic Un​derstanding of Weak Electrolyte-Containing Bipolar Membranes at Open Circuit and Under Forward Bias

Bipolar membranes (BPMs) are critical components of a variety of electrochemical energy technologies. Many electrochemical applications require the use of buffers to maintain stable, non-extreme pH environments, yet the impact of buffers or weak acids/bases on the electrochemical behavior of BPMs remains poorly understood. We have investigated the operation of BPMs containing weak electrolytes at open-circuit and under forward bias polarization. At open circuit, we demonstrate that ionic short-circuiting processes occur, severely attenuating the membrane voltage and eroding the efficiency of reverse bias operation. Under forward bias polarization, ionic current passage is gated by the thermochemistry of acid-base recombination for given species present in the BPM, but is also subject to transport limitations from the slow place-exchange of reactive ions with unreactive ions within the BPM. These findings establish a mechanistic basis for the operation of BPM cells in the presence of weak acid/base electrolytes, and reveal important implications for the design of next-generation devices incorporating BPMs.