Innocent buffers reveal the intrinsic pH- and coverage-dependent kinetics of the hydrogen evolution reaction on noble metals

The hydrogen evolution reaction (HER) is a critical transformation for a renewable energy economy and an ideal testbed for refining mechanistic models of interfacial electrocatalysis. The HER is less efficient in alkaline than in acidic electrolytes; however, the mechanistic origin of this is widely debated. Activation-controlled kinetics at intermediate pH are non-existent due to pH swings or buffer convolution, impeding mechanistic analysis. We identify formate and borate as buffers that minimally perturb the HER kinetics on Au and Pt and report activation-controlled kinetic data between pH 1 and 12. We find that the reaction order in H3O+ is fractional across the pH range as a result of intermediate metal–H coverage and propose that efficiency loss arises from slow proton-coupled electron transfer (PCET) of H2O versus H3O+. These studies highlight the role of surface coverage in electrocatalytic kinetics and motivate strategies to promote efficient PCET in alkaline media.