Heterogeneous Co–N/C Electrocatalysts with Controlled Cobalt Site Densities for the Hydrogen Evolution Reaction: Structure–Activity Correlations and Kinetic Insights

The development of active and stable non-precious-metal electrocatalysts for energy conversion reactions involving hydrogen and oxygen has been of pivotal importance for realizing a clean-energy-based society. As a class of non-precious-metal electrocatalysts, cobalt- and nitrogen-codoped carbon (Co–N/C) catalysts have shown promising activity for the hydrogen evolution reaction (HER). The further advancement of Co–N/C catalysts is, however, hindered by the poor understanding of their active sites; the typical preparation of Co–N/C catalysts involves high-temperature pyrolysis, yielding catalysts with a heterogeneous distribution of atomically dispersed Co–Nx sites and metallic Co nanoparticles encapsulated in graphitic carbon shells (Co@C). Further, kinetic insights into the HER on Co–N/C catalysts are lacking. In this work, we prepared a series of Co–N/C catalysts with controlled Co–Nx and Co@C site densities, which served as model catalysts for identifying the active sites for the HER. We found that the HER activities in both acidic and alkaline media linearly increased with the number of exposed Co–Nx sites, suggesting that the Co–Nx sites are the major active sites for the HER. Density functional theory (DFT) calculations suggested that hydrogen adsorption at Co–Nx sites is closer to the thermoneutral state in comparison to that at Co@C sites, corroborating the HER activity results. Furthermore, pH- and temperature-dependent HER activities combined with in situ X-ray absorption spectroscopy analyses on the Co–N/C catalyst comprising only Co–Nx sites provide insights into HER reaction kinetics, including the rate-determining step and spectator species in alkaline electrolytes. The Co–N/C catalyst with Co–Nx sites exhibited long-term durability and stability. This work may shed light on the design of advanced Co–N/C catalysts as well as other M–N/C catalysts for promoting a diverse set of energy conversion reactions.