This invention discloses a three-dimensional Fe-mainly electrocatalysts as bifunctional electrocatalysts for hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode in overall water splitting. The iron-mainly electrocatalysts were directly grown on Ni foam or other conductive scaffolds (carbon cloth paper, Cu foam, Co foam, Fe foam, Ti foam, etc.) by a two-time thermal phosphidation in a tube furnace using iron nitrate and red phosphorus as the Fe and P sources, respectively. From this route, we can prepare a hybrid catalyst FeP/Ni2P with high porosity on Ni foam surface. The strong synergistic effects between FeP and its support Ni2P, good electrical conductivity of Ni foam and metal phosphides, and high porosity of the electrocatalysts contribute greatly to the outstanding HER and OER activities of the final catalysts FeP/Ni2P, which requires relatively low overpotentials of 14 mV and 154 mV to deliver a current density of 10 mA cm-2 for the HER and OER in base, respectively, leading to robust overall-water-splitting activity at 10 mA cm-2 with 1.42 V. This activity outperforms the integrated IrO2 and Pt couple (1.57 V), demonstrating that Fe compounds are promising materials for water splitting, especially for catalyzing the OER process. Especially, this kind of bifunctional electrocatalysts exhibit excellent durability without decay operated at high current densities above 500 mA cm-2, providing great potential for large-scale applications. Considering the low cost and earth abundance of these compounds, they are promising alternatives to the noble catalysts for the OER like IrO2 catalysts and Ni alloy-based electrocatalysts currently used in commercial alkaline water electrolysis. In particular, the commercially purchased Ni foam as well as other conductive scaffolds can be supplied in large scale, so the as-prepared hybrid catalysts can be compatible with sizable electrodes for potential applications in water electrolysis.