(Invited) Water Dissociation Catalysis in Bipolar Membranes and in Electrocatalysis

Abstract

Water is arguably humanity’s most-important molecule due to its ubiquitous role in biological, industrial, and environmental processes. Reactions of water typically involve breaking H-O bonds. The simplest reaction is heterolytic water dissociation (WD), H2O -> H+ + OH-, the understanding of which has been one focus of experiment and theory for decades. WD is also a fundamental elementary step in many electrochemical processes. For example, the WD step is thought rate-limiting for the hydrogen-evolution reaction (HER) under alkaline conditions and modification of Pt surfaces with metal hydroxides, which presumably accelerate WD, increase HER activity. While measurements of dissociative water adsorption are often made using surface-science tools in vacuum, the WD reaction is remarkably poorly understood under electrochemical conditions.We use a bipolar-membrane (BPM) electrolyzer, where WD is driven in the region between a hydroxide-exchange and proton-exchange membrane by an applied potential, to study WD kinetics across a range of materials.1 We find that the local pH is a critical, but previously unrecognized, variable affecting WD kinetics and propose a pH-dependent proton-transfer WD mechanism. Combining WD catalysts efficient in acidic and basic conditions nearly eliminates the WD overpotential in BPM electrolyzers operating at 20 mA cm-2 and enables BPM operation at 0.5 A cm-2 with a total applied electrolysis potential of ~ 2 V. These values are substantial improvements over the state of the art and enable new applications for BPMs. We further discovered that the WD kinetics measured in the BPM correlate with HER kinetics under conditions where WD is an important elementary step and illustrate the design of ‘bifunctional’ electrocatalysts based on insight into the underlying WD steps. 1Oener, S. Z.; Foster, M. J.; Boettcher, S. W., Accelerating water dissociation in bipolar membranes and for electrocatalysis. Science 2020, DOI: 10.1126/science.aaz1487 Figure 1