The deep exploration of real reaction sites in composite catalysts for electrocatalytic reactions has profound significance, especially in low energy cost hydrogen production coupled with the urea electrocatalytic oxidation reaction (UOR). Herein, we combined non-UOR-active nano MnO_x with hierarchical NiS₂, which achieved high catalytic performance for UOR (over 200 mA cm⁻² at 1.9 V vs. RHE and approximately 100% UOR selectivity at 2.0 V vs. RHE). A series of rigorous experiments, including ex situ XPS, selective inactivation experiment, in situ electrochemical Raman spectroscopy, in situ electrochemical mass spectrometry isotope tracing experiment and DFT calculations proved that the UOR takes place at the Mn sites rather than Ni sites, and the role of Ni changes from the active site to an electrons pump. The electron pump effect proposed here provides a new idea for the design of highly efficient catalysts for most small molecular electrocatalysis.