Understanding Active Sites and Kinetic Insights of Co–N/C Electrocatalysts for Hydrogen Evolution Reaction

Abstract

Water electrolysis has emerged as an environmentally benign and sustainable H2 production method which is essential for future hydrogen economy. However, its practical viability is limited by the use of expensive Pt-based catalysts for hydrogen evolution reaction (HER) at the cathode. As a class of non-precious metal electrocatalysts, cobalt- and nitrogen-codoped carbon (Co−N/C) catalysts have shown promising HER activity. Further advancement of Co–N/C catalysts is, however, hindered by poor understanding of the active sites. Typical preparation of Co–N/C catalysts involves high-temperature pyrolysis, which yields catalysts with a heterogeneous distribution of molecular Co−Nx sites and metallic Co nanoparticles (NPs). Moreover, the kinetic insights for the HER over Co−N/C catalysts are lacking. In this work, we prepared a suite of model Co−N/C catalysts with various Co−Nx to Co NPs ratios enabled by active-site-controllable silica-coating synthetic strategy. We found that the HER activities in both acidic and alkaline media linearly increase as the ratio of Co−Nx site increases, suggesting that the Co−Nx sites are major active sites for the HER. Furthermore, insights into HER reaction kinetics on Co−N/C catalysts were acquired by Tafel analysis combined with pH- and temperature-dependent HER activity investigations, which suggest a possible rate-determining step for the HER. Our Co−N/C catalyst exhibited superior durability and stability to a Pt/C catalyst.