An N-defective graphitic carbon nitride-supported Ir/Fe catalyst with low Ir-content as highly efficient oxygen evolution reaction catalyst for acidic water splitting

•A catalyst with highly dispersed Ir/Fe complex sites (∼ 5 wt% Ir) was synthesized.•N-defective g-C3N4 (NCN) was introduced to enhance the support effect.•The overpotential and the mass activity of Ir/Fe@NCN were better than those of IrO2.•The Gibbs free energy of Ir/Fe@NCN was reduced in the rate-determining step O*→OOH*.•The formed interaction coordination helped maintain high OER property and stability.