Electric-Double-Layer Origin of the Kinetic pH Effect of Hydrogen Electrocatalysis Revealed by a Universal Hydroxide Adsorption-Dependent Inflection-Point Behavior

The mechanism of the kinetic pH effect in hydrogen electrocatalysis, that is, the order-of-magnitude kinetic gap between the hydrogen oxidation and evolution reactions (HOR/HER) in acidic and alkaline electrolytes, has been drastically explored but still intractable to reach a consensus, which severely limits the catalyst advance for alkaline-based hydrogen energy technologies. Herein, the HOR/HER kinetics on a number of precious metal-based electrocatalysts are evaluated in electrolytes with pHs spanning a wide range from 1 to 13. Instead of a monotonous decrease with pH as generally believed, we surprisingly find a universal inflection-point behavior in the pH dependence of HOR/HER kinetics on these catalysts, with both the inflection-point pH and the acid-alkaline activity gap depending on the hydroxide binding energy of the catalyst. Based on a triple-path microkinetic model, in which hydronium (H3O+) and water (H2O) with and without formation of adsorbed hydroxide (OHad), respectively, act as hydrogen donors participating in HOR/HER in various pHs, we reveal that the formation of OHad should promote the HOR/HER kinetics mainly by improving the hydrogen-bond network in the electric double layer (EDL), rather than merely through modulating the energetics of surface reaction steps such as disassociation/formation of water. The present results and conclusions indicate that it is the interfacial EDL that dominates the substantial kinetic pH effects of hydrogen electrocatalysis.