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Abstract Developing non‐precious‐metal electrocatalysts that can operate with a low overpotential at a high current density for industrial application is challenging. Heterogeneous bimetallic phosphides have attracted much interest. Despite high hydrogen evolution reaction (HER) performance, the ordinary oxygen evolution reaction (OER) performance hinders their practical use. Herein, it is shown that Fe‐doping reverses and enlarges the interfacial electrical field at the heterojunction, turning the H intermediate favorable binding sites for HER into O intermediate favorable sites for OER. Specifically, the self‐supported heterojunction catalysts on nickel foam (CoP@Ni2P/NF and Fe‐CoP@Fe‐Ni2P/NF) are readily synthesized. They only require the overpotentials of 266 and 274 mV to drive a large current density of 1000 mA cm−2 (j1000) for HER and OER, respectively. Furthermore, a water splitting cell equipped with these electrodes only requires a voltage of 1.724 V to drive j1000 with excellent durability, demonstrating the potential of industrial application. This work offers new insights on interfacial engineering for heterojunction catalysts.