Accelerating Water Dissociation in Bipolar Membranes and Electrocatalysis

Abstract

Catalyzing water dissociation (WD), H2O -> H+ + OH-, is practically important for fabricating bipolar membranes (BPMs) that couple different pH environments into a single electrochemical device and for accelerating (electro)catalytic reactions that consume water. We use a BPM electrolyzer for quantitative measurement of WD kinetics in the absence of soluble electrolyte. We discover that the rate of WD on metal-oxide nanoparticle surfaces is affected by the local pH, consistent with a proton-transfer mechanism. We combine WD catalysts efficient near the acidic proton-exchange layer with those efficient near the alkaline hydroxide-exchange layer and yield WD overpotentials < 10 mV across the BPM at 20 mA cm-2. We demonstrate pure-water BPM electrolyzers that maintain a gradient of ~ 14 pH units between the basic anode and acidic cathode that operate continuously at 500 mA cm-2 with a total electrolysis voltage of ~ 2.2 V. We further investigate the WD activity of metal electrocatalysts in the BPM junction and find a direct correlation with their electrocatalytic activity for the hydrogen-evolution reaction (HER) in alkaline media. This result is direct evidence of the rate-limiting role of WD in alkaline HER and guides advanced (electro)catalyst design for (electrochemical) reactions where water is a reactant.