Electric-Double-Layer Origin of the Kinetic pH Effect of Hydrogen Electrocatalysis Revealed by a Universal Hydroxide Adsorption-Dependent Inflection-Point Behavior
Lixin Su, Junxiang Chen, Fulin Yang, Peng Li, Yiming Jin, Wei Luo, Shengli Chen
Abstract
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 (H 3 O + ) and water (H 2 O) with and without formation of adsorbed hydroxide (OH ad ), respectively, act as hydrogen donors participating in HOR/HER in various pHs, we reveal that the formation of OH ad 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.