Water electrolysis using a catholyte and anolyte at different pH values requires a bipolar membrane to sustain the pH difference and 1.23 V to electrolyze water. Bipolar membranes that separated concentrated aqueous acid and base exhibited an open-circuit potential consistent with the Nernst equation and rapid transport of protons and hydroxide ions. When excess supporting electrolyte was added to both solutions the membrane potential was measured to be ∼0 V, which suggested that water electrolysis occurred at ≪1.23 V and therefore, protons and hydroxide ions were not the majority ionic charge carriers.
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Reiter et al. (2015) studied this question.