In situ construction of porous hierarchical (Ni_3-xFe_x)FeN/Ni heterojunctions toward efficient electrocatalytic oxygen evolution

As a choke point in water electrolysis, the oxygen evolution reaction (OER) suffers from the severe electrode polarization and large overpotential. Herein, the porous hierarchical hetero-(Ni_3-xFe_x)FeN/Ni catalysts are in situ constructed for the efficient electrocatalytic OER. X-ray absorption fine structure characterizations reveal the strong Ni-Fe bimetallic interaction in (Ni_3-xFe_x)FeN/Ni. Theoretical study indicates the heterojunction and bimetallic interaction decrease the free-energy change for the rate-limiting step of the OER and the overpotential thereof. In addition, the high conductivity and porous hierarchical morphology favor the electron transfer, electrolyte access and O₂ release. Consequently, the optimized catalyst achieves a low overpotential of 223 mV at 10 mA·cm^-2, a small Tafel slope of 68 mV·dec^-1, and a high stability. The excellent performance of the optimized catalyst is also demonstrated by the overall water electrolysis with a low working voltage and high Faradaic efficiency. Moreover, the correlation between the structure and performance is well established by the experimental characterizations and theoretical calculations, which confirms the origin of the OER activity from the surface metal oxyhydroxide in situ generated upon applying the current. This study suggests a promising approach to the advanced OER electrocatalysts for practical applications by constructing the porous hierarchical metal-compound/metal heterojunctions.