Synergistic Effect of Ni ₂ P/Co ₂ P Heterojunctions and Phosphorus Vacancies for High-Performance Bifunctional Oxygen Catalysis in Zinc–Air Batteries

The activity and durability of bifunctional oxygen electrocatalysts are critical determinants for the energy efficiency and economics of fuel cells and metal-air batteries. Herein, we introduce a heterojunction-vacancy coengineering strategy that couples Ni₂P/Co₂P heterojunctions with phosphorus vacancies (V_P) within mesoporous N,P-doped carbon nanospheres (Ni₂P/Co₂P-V_P-NPCs). This catalyst is synthesized by converting mesoporous polydopamine nanospheres via high-temperature phosphidation and H₂ reduction. The resulting structure exhibits outstanding bifunctional oxygen catalytic performance. For the oxygen reduction reaction (ORR), the half-wave potential (<i>E</i>_1/2) is 0.85 V (vs RHE) with a limiting diffusion current density (<i>j</i>_d) of 5.87 mA cm^-2 and 96.67% current retention over 24 h in 0.1 M KOH. For the oxygen evolution reaction (OER), an overpotential of 277 mV (@10 mA cm^-2) is achieved with 94.57% current retention over 24 h in 1.0 M KOH. In a rechargeable zinc-air battery, Ni₂P/Co₂P-V_p-NPCs delivers a peak power density of 141.9 mW cm^-2, excellent specific capacity (797.3 mAh g^-1), and high energy density (1004.6 Wh·kg_Zn^-1), sustaining 600 h cycling at 10 mA cm^-2. Density functional theory (DFT) calculations reveal that the Ni₂P/Co₂P heterojunction downshifts the d-band center of Co cites, lowering the Gibbs free energy (Δ<i>G</i>) for the potential-determining steps (*OH → OH^- in ORR and *O → *OOH in OER), while phosphorus vacancies increase charge density of adjacent Co sites, facilitating interfacial charge transfer and boosting durability. This cooperative modulation precisely tunes the local electronic structure, providing a practical route to high-performance transition-metal catalysts for rechargeable zinc-air batteries.