Abstract:: Water is arguably the most-important molecule to humanity because of its ubiquitous role in biological, industrial, and environmental processes. Reactions of water typically involve breaking the H-O bond. The simplest reaction is heterolytic water dissociation (WD), H2O → H+ + OH-, the understanding of which has been a focal point of experiment and theory for decades. Related dissociative adsorption reactions occur on surfaces and are important when water is used as a reactant for thermochemical processes, such as the water-gas-shift reaction. In biological systems, metalloenzymes such as carbonic anhydrase dissociate water to catalyze, for example, CO2 equilibria.
WD is also a fundamental elementary step in many electrochemical processes. During the hydrogen evolution reaction (HER) in alkaline media, the WD step is thought to be rate-limiting and thus modification Pt HER catalysts with metal hydroxides, that presumably accelerate WD, lead to large increases in HER activity. Driving the oxygen evolution reaction (OER) under acidic or neutral conditions likewise requires WD to generate absorbed hydroxide species that can be further oxidized. While measurements of dissociative water adsorption are often made using the tools of surface science under vacuum conditions, the WD reaction has not been systematically studied under electrochemical conditions. Here we report the use of a bipolar-membrane (BPM) electrolyzer architecture (where WD is driven in the region between a hydroxide exchange membrane and a proton exchange membrane by an applied potential) to study WD kinetics across a range of materials. We find that the local pH is a critical but previously unrecognized variable in describing WD kinetics in under electrochemical conditions. Combining WD catalysts efficient in locally acidic conditions with those efficient in basic conditions, nearly eliminates the WD overpotential in BPM electrolyzers operating at 20 mA cm-2 and enables continuous operation at 0.5 A cm-2 with a total applied electrolysis potential near 2 V – substantial improvements over the state of the art and suggesting new applications for BPMs.
We further discovered that the WD kinetics measured in the BPM correlate well with kinetics for electrocatalytic reactions under conditions where WD may be an important elementary step. We discuss the design of bifunctional electrocatalysts based on the insight into the underlying WD steps.