Boosting the Hydrogen Evolution Activity of a Low-Coordinated Co─N─C Catalyst via Vacancy Defect-Mediated Alteration of the Intermediate Adsorption Configuration.

The cobalt-nitrogen-carbon (Co─N─C) single-atom catalysts (SACs) are promising alternatives to precious metals for catalyzing the hydrogen evolution reaction (HER) and their activity is highly dependent on the coordination environments of the metal centers. Herein, a NaHCO etching strategy is developed to introduce abundant in-plane pores within the carbon substrates that further enable the construction of low-coordinated and asymmetric Co─N sites with nearby vacancy defects in a Co─N─C catalyst. This catalyst exhibits a high HER activity with an overpotential (η) of merely 78 mV to deliver a current density of 10 mA cm, a Tafel slope of 45.2 mV dec, and a turnover frequency of 1.67 s (at η = 100 mV). Experimental investigations and theoretical calculations demonstrate that the vacancy defects neighboring the Co─N sites can modulate the electronic structure of the catalyst and alter the adsorption configuration of the H intermediate from the typical atop mode to the side mode, resulting in weakened H adsorption strength and thus improved HER activity. This work provides an efficient strategy to regulate the coordination environment of SACs for improved catalytic performance and sheds light on the atomic-level understanding of the structure-activity relationships.