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 oxidationand 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 hydrogenenergy technologies. Herein, the HOR/HER kinetics on a number of preciousmetal-based electrocatalysts are evaluated in electrolytes with pHsspanning a wide range from 1 to 13. Instead of a monotonous decreasewith pH as generally believed, we surprisingly find a universal inflection-pointbehavior in the pH dependence of HOR/HER kinetics on these catalysts,with both the inflection-point pH and the acid-alkaline activity gapdepending on the hydroxide binding energy of the catalyst. Based ona triple-path microkinetic model, in which hydronium (H3O+) and water (H2O) with and without formationof adsorbed hydroxide (OHad), respectively, act as hydrogendonors participating in HOR/HER in various pHs, we reveal that theformation of OHad should promote the HOR/HER kinetics mainlyby improving the hydrogen-bond network in the electric double layer(EDL), rather than merely through modulating the energetics of surfacereaction steps such as disassociation/formation of water. The presentresults and conclusions indicate that it is the interfacial EDL thatdominates the substantial kinetic pH effects of hydrogen electrocatalysis.

Automated summary

The kinetic pH effect in hydrogen electrocatalysis, particularly the difference in kinetics between hydrogen oxidation and evolution reactions in acidic and alkaline electrolytes, remains controversial. Evaluating the kinetics of these reactions on precious metal-based electrocatalysts across a wide pH range, we discover a universal inflection-point behavior, where the inflection point pH and the acid-alkaline activity gap depend on the catalyst's hydroxide binding energy. Our results suggest that the formation of adsorbed hydroxide improves the hydrogen-bond network in the electric double layer, thus promoting the kinetics of hydrogen oxidation and evolution.