Interface effect of Fe doped NiSe/Ni3Se2 heterojunction as highly efficient electrocatalysts for overall water splitting

It matters to design and explore transition metal bifunctional electrocatalysts which can efficiently catalyze overall water splitting. Herein, we report a series of Fe doped NiSe/Ni3Se2 (NiSe/Ni3Se2-Fe-X@t, X is Fe dosage, and t is electrodeposition time) heterojunctions assembled on nickel foam (NF) by one-step electrodeposition. Under alkaline medium (1.0 M KOH), the optimized NiSe/Ni3Se2-Fe-5@5min catalyst shows ultra-low overpotentials of 144 and 200 mV respectively for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) to afford the current density of 10 mA·cm−2, and a small cell voltage of 1.57 V at 10 mA·cm−2 is needed for overall water splitting with extraordinary high stability (≥120 h). Density functional theory (DFT) calculations demonstrate the heterojunction keeps metallic state, and Fe3+ doping reduces electron density of Ni2+, effectively accelerating Ni → Fe electron transfer. For HER, Se serves as the active site in the materials with and without Fe, and Gibbs free energy of hydrogen adsorption (ΔGH*) at Se site decreases from 0.149 eV in NiSe/Ni3Se2 to −0.051 eV in NiSe/Ni3Se2-Fe, whose the much smaller value near to zero indicates significantly enhanced HER activity. For OER, in NiSe/Ni3Se2, Ni serves as active site, while in NiSe/Ni3Se2-Fe, Fe becomes the active site. In the both materials, rate-determining step (RDS) is the formation of *O→*OOH intermediates and the much lower energy barrier (1.589 eV) after Fe doping contributes to superior OER activity. The calculated TDOS shows that Fe doping increases carrier density and intrinsic conductivity of the catalyst. This work provides important reference for regulating interface effect of electrocatalysts through elemental doping.