Dual‐Molten‐Salt Etching Strategy Enables Built‐in Electric Field in Mo‐ <scp>MX</scp> ene/ <scp>N</scp> i <scp>F</scp> e <scp>S</scp> e Heterostructure for Superior <scp>OER</scp> Performance

Transition metal selenides have emerged as potential catalysts for the oxygen evolution reaction, yet their practical implementation faces two fundamental challenges: irreversible nanoparticle aggregation under operational conditions and unfavorable adsorption energetics for critical oxygen intermediates. Herein, a strategy that integrates dual‐molten‐salt etching with hydrothermal selenization has been developed to tightly anchor FeNiSe nanoparticles onto a porous Mo‐based MXene substrate (Mo‐MXene/NiFeSe). The hierarchical porous architecture of Mo‐MXene/NiFeSe facilitates rapid mass and charge transport. The NiFeSe nanoparticles are chemically anchored within the conductive Mo‐MXene matrix via in‐situ formed Mo–O–Fe/Ni bonds, effectively preventing agglomeration during the catalytic process. Additionally, the work function gradient between Mo‐MXene and NiFeSe induces charge redistribution, creating a built‐in electric field that optimizes intermediate adsorption kinetics and enhances charge transport efficiency. Therefore, Mo‐MXene/NiFeSe achieves an ultralow overpotential of 231 mV to reach current density of 10 mA cm −2 and sustain 91% of its initial current density over 55 h in alkaline electrolyte. Density functional theory (DFT) calculations reveal that the adsorption free energy of *OOH for Mo‐MXene/NiFeSe is significantly reduced, effectively lowering the kinetic barrier of the rate‐determining step. This work provides both fundamental insights into built‐in electric field‐enhanced catalysis and a practical strategy for developing high‐performance and durable oxygen evolution reaction electrocatalysts.