Active sites and mechanism on nitrogen-doped carbon catalyst for hydrogen evolution reaction

Abstract

The nature of active sites and mechanism of hydrogen evolution reaction (HER) on the nitrogen-doped carbon catalyst is extensively investigated, by combining physicochemical and electrochemical methods. Two carbon catalysts, with the same chemical nature but different nitrogen content, are employed in this investigation. Electrochemical methods are applied to investigate the electrochemical behavior at different pH values (1.0–2.0, 12.0–13.0). It is found that increasing nitrogen content has a positive effect on the electrocatalytic activity, and therefore, the doped nitrogen atoms should be the active sites. The kinetic current, normalized by the surface nitrogen content, is found to be the same for the two catalysts, confirming the above claim. As such, HER is proposed to proceed on these active sites by the Volmer-Heyrovsky mechanism. The electrochemical tests reveal that the electrocatalytic activity closely relies on the solution pH, which is due to the chemical evolution of the active site in different solutions. In acid media, the electrocatalytic activity increases with the concentration of proton, and the Tafel slope is ca. 120 mV dec−1. It is proposed that the electrochemical desorption of proton on the doped nitrogen atoms is the rate determining step (r.d.s.). In alkaline media, the electrocatalytic activity increases with pH, because the increase in pH dramatically enhances the basicity or the surface charge density, thereby facilitating charge transfer and improving activity. In alkaline media, Tafel analysis shows that Heyrovsky step is the rate determining. © 2017 Elsevier Inc.