Heterojunction nanocomposite electrocatalysts with porous structures and large specific surface areas show great potential in improving their intrinsic activity and the number of accessible active sites for oxygen evolution reaction (OER). Herein, we describe an “exchanging sulfur for oxygen” protocol to fabricate a porous molybdate-based heterojunction electrocatalyst, Fe₂(MoO₄)₃/CoMoO₄, utilizing a sulfur-rich reagent, ammonium tetrathiomolybdate ((NH₄)₂MoS₄). During the calcination of the solid product formed from (NH₄)₂MoS₄ and CoCl₂/FeCl₃, the sulfur atoms of MoS₄²⁻ are oxidized into the acidic SO₂ gas plus HCl and NH₃ gases evolved in the system, which greatly facilitates the formation of macro/mesopores of the molybdate-based nanomaterial. It exhibits excellent electrocatalytic OER performance in alkaline media and only requires a low overpotential of 244 mV at a current density of 10 mA·cm⁻² with outstanding durability. Experimental examination and theoretical calculations reveal that its uniform interparticle porous structure enhances spatial connectivity and electrode–electrolyte contact, while strong electronic interactions at the heterointerface boost electrocatalytic activity. The phase combination increases interface electron concentration, accelerates charge transfer, and lowers free energy. This work provides a new strategy to construct the porous molybdate-based heterostructure electrocatalyst for remarkably boosting the OER performance.

The synergistic catalysis of heterojunction electrocatalysts for the multi-step process in hydrogen evolution reaction (HER) is a promising approach to enhance the kinetics of alkaline HER. Herein, we proposed a strategy to form nanoscale Ni/NiO heterojunction porous graphitic carbon composites (Ni/NiO-PGC) by reduction-pyrolysis of the preformed Ni-metal-organic framework (MOF) under H₂/N₂ atmosphere. Benefiting from low electron transfer resistance, increased number of active sites, and unique hierarchical micro-mesoporous structure, the optimized Ni/NiO-PGC_10-1-400 exhibited excellent electrocatalytic performance and robust stability for alkaline HER (η₁₀ = 30 mV, 65 h). Density functional theory (DFT) studies revealed that the redistribution of electrons at the Ni/NiO interface enables the NiO phase to easily initiate the dissociation of alkaline H₂O, and shifts down the d-band center of Ni and optimizes the H* adsorption–desorption process of Ni, thereby leading to extremely high HER activity. This work contributes to a further understanding of the synergistic promotion of the multi-step HER processes by heterojunction electrocatalysts.