Abstract: Deciphering how lattice mismatch-induced vacancies enhance heterointerface catalytic activity remains a critical yet challenging frontier in heterojunction catalyst design. Herein, we constructed a defect-rich Mo₂N/MoO₃ heterostructure embedded in a nitrogen-doped carbon matrix (Mo₂N/MoO₃@NC-30) via the in situ controllable oxidation of Mo₂N@NC. In the Mo₂N/MoO₃ heterojunction, interfacial defects are introduced by utilizing the lattice mismatch between the two materials to enhance interface polarization, thereby triggering strong charge transfer from Mo₂N to MoO₃ and optimizing the interfacial charge distribution. Systematic experimental and theoretical investigations reveal that interfacial vacancy in Mo₂N/MoO₃ heterojunctions could induce interfacial electron delocalization and accumulation and optimize water/intermediate adsorption/desorption, boosting catalytic activity and stability. The fabricated Mo₂N/MoO₃@NC-30 heterojunction catalyst exhibits exceptional hydrogen evolution reaction performance in 1.0 M KOH, maintaining remarkable stability exceeding 1000 h at -500 mA cm^-2, surpassing commercial 20 wt% Pt/C in high-current-density regimes. Additionally, a Zn-H₂O battery incorporating Mo₂N/MoO₃@NC-30 as the cathode is developed for simultaneous hydrogen generation and electricity production. The system delivers a maximum power density of 10.9 mW cm^-2 and maintains stable discharge performance over 70 h. This study not only advances the mechanistic understanding of vacancy-mediated interface enhancement in heterostructural catalysts, but also provides a way to the design of decoupled water electrolysis devices.