Modulating space charge of FeP/CoP p-n heterojunction for boosting oxygen evolution reaction

• Construction of octagonal 2D nanoplates with large specific surface area exposes more active sites and accelerates mass transfer for improved OER performance. • The formation of p-n heterojunction between FeP and CoP induces the generation of embedded electric field, optimizes the electronic structure, and improves the intrinsic activity of OER. • DFT calculations confirm the existence of charge transfer at the heterojunction interface and effectively optimize the Gibbs free energy of the OER. Surface reconstruction of electrocatalysts is an effective strategy to modulate the space charge distribution to enhance the electrocatalytic activity. The p-n heterostructured FeP/CoP-2D octagonal nanoplates were successfully constructed by cation-exchange method. The space charge effect caused by the p-n heterojunction accelerated the electron transfer, optimized the electronic structure, and improved the activity of the active sites during the oxygen evolution reaction process. As a result, FeP/CoP-2D required only 247 mV overpotential to achieve a current density of 10 mA cm −2 with a Tafel slope as low as 68 mV dec -1 . Density-functional theory calculations confirmed that the construction of p-n heterojunctions can enhance the adsorption of *OH in the active centers and optimize the Gibbs free energy of the OER reaction. This study provides an effective and feasible strategy for constructing p-n heterojunctions to modulate the space charge state for optimizing the OER performance of electrocatalysts.