Electrochemical impedance spectroscopy as a performance indicator of water dissociation in bipolar membranes

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A bipolar membrane (BPM) can be used to maintain a pH difference in an electrolysis cell, which provides freedom to independently optimize the environments and catalysts used for paired redox reactions. A BPM consists of two physical layers, of which one is selective for the exchange of cations and the other for anions. The water dissociation reaction (WDR) splits water into protons and hydroxide ions under an electric field that concentrates at the interface of the two membrane layers. However, salt ions in commonly used electrolytes influence this WDR when they are present at the interface. Using electrochemical impedance spectroscopy (EIS), we observed the rate of water dissociation decrease in the presence of salt ions while also observing the diffusion and migration of these salt ions, showing a clear link between the peaks observed in EIS and ion crossover. In addition, we show how EIS can be used to in situ monitor the stability and ageing of a BPM, revealing that degradation of the BPM is more prominent in extreme pH electrolyte pairs compared to non-extreme electrolyte pairs. The in situ monitoring of the WDR and stability of a BPM are vital methods for adequate and consistent comparison of novel designs of BPM-based systems, where EIS allows for discriminating BPM characteristics from other components even during operation.