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

The design principles for metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) in the hydrogen evolution reaction (HER) have been extensively studied. Yet, consensus remains elusive, hindering advancements in hydrogen energy technologies. Although the hydrogen binding energy (ΔG_H*) has long been used as a key HER descriptor during the past two decades, originating from the activity volcano of metallic surfaces, 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 (M = Co, Ni, Cu) confirms the theoretical predictions, with excellent agreement in exchange current densities and the role of N-sites in Ni/Cu-phthalocyanine/CNT catalysts. This work provides answers to a long-lasting debate on HER descriptors by establishing ΔG_H* and ΔG_HO* as a combined HER descriptor for SACs, offering new guidelines for catalyst design.