Fig. 6: Reaction pathways for hydrogen evolution reaction (HER) on nickel single site.
a The minimum energy reaction path for proton adsorption (Volmer step) from solution to the nickel single site at −0.8 V applied potential. We use total seven images (00–06) for this adsorption step. The hydrogen near the single nickel at point 00 gradually moves toward the nickel site leading to a transition state (TS) at 03 before converting into the adsorbed final state at 06. b The adsorption free energies and TS barrier for the Volmer step of HER on different nickel sites at −0.8 V vs RHE applied potential. Among all sites, Ni–C2N2 shows a lower activation barrier for proton transfer at the active site. c The Heyrovsky reaction path for hydrogen gas evolution from the nickel surface at −0.8 V applied potential. We use seven images (00–06) to describe the H2 desorption step. The adsorbed hydrogen at point 00 reacts with a proton from solution to form H2 gas at 06, leading to a transition state at point 03. d Desorption reaction barrier for the Heyrovsky step of HER on different nickel sites at −0.8 V vs RHE applied potential. The H2 evolution is difficult from Ni–C2N2 site because its strong adsorption energy for the Volmer step. The arrow indicates the forward reaction pathways for adsorption and desorption steps, respectively. (Gray color: entire surface represents implicit solvation along the whole reaction pathways, brown: carbon, blue: nitrogen, green: nickel, red: oxygen, off white: hydrogen atom).
