Hydrogen Binding Energy Is Insufficient for Describing Hydrogen Evolution on Single-Atom Catalysts

Abstract

The design principles for metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) in the hydrogen evolution reaction (HER) have been extensively studied. While hydrogen binding energy ([[EQUATION]]) has long been used as a HER descriptor during the past decades, its applicability to HER SACs has been met with significant controversy. Herein, we investigate the effects of HO*/O* poisoning and H* coverage on SACs with varied metal centers and coordination environments using pH-dependent surface Pourbaix diagrams at the reversible hydrogen electrode (RHE) scale and microkinetic modeling. Our findings reveal that HO* poisoning, realistic H* adsorption strengths at active metal sites, and the potential HER activity at the coordinating N-sites are crucial factors that should be considered for accurate descriptor development. Experimental validation using a series of M-phthalocyanine/CNT catalysts confirms the theoretical predictions, with excellent agreement in exchange current densities and the role of N-sites in Ni/Cu-phthalocyanine/CNT catalysts. More importantly, the controversy surrounding HER SAC design principles is resolved through a novel 2D microkinetic volcano model that incorporates active sites, H* coverage, and HO* poisoning. This work provides answers to a long-lasting debate on HER descriptors by establishing [[EQUATION]] and [[EQUATION]] as a combined HER descriptor for SACs, offering new guidelines for catalyst design.