Fig. 5

DFT calculations of NiCo2S4 and N-NiCo2S4. a The top-view structures of N-NiCo2S4 (100) with labeled bond lengths (Å) of Co–S and Co–N bonds. The top-view electron density difference of b NiCo2S4 (100), and c N–NiCo2S4 (100), ranging from −0.1 to 0.1. d Relative energy profiles and the simplified surface structures of the various reaction species along the reaction pathway, including the H* formation process (left panel) and H2 formation process (right panel) in alkaline on the NiCo2S4 (100) and N–NiCo2S4 (100) at electrode potential U = 0 V and USHE = 0.83 V, respectively. R Reactant, RC Reactant Complex, TS Transition State, IM Intermediate, P Product