Steering Two-Electron Oxygen Reduction Pathway on NiO via Induced Oxygen Vacancies for Enhanced H2O2 Electrosynthesis

The electrochemical two-electron oxygen reduction reaction (2e- ORR) offers a sustainable route for H2O2 synthesis, yet developing nonprecious-metal catalysts with high activity and selectivity remains challenging. Herein, Ar plasma etching is employed to controllably introduce oxygen vacancies into NiO. Structural characterizations reveal lattice expansion, decreased crystallinity, enhanced oxygen vacancy concentration, and a higher Ni2+ /Ni3+ ratio after plasma treatment. The optimized NiO-Ov-20 catalyst achieves a 15.5% enhancement in H2O2 selectivity with an electron-transfer number of approximately 2.5 in 0.1 M KOH. The accelerated aging and long-term stability tests demonstrate the satisfactory durability of NiO-Ov-20. In an H-cell, NiO-Ov-20 delivers a H2O2 yield of 12.16 mmol L-1 h-1 (972.8 mmol g-1 h-1) and good stability. Moreover, in situ degradation experiments demonstrate that more than 80% of Rh B is degraded within 1 h, holding practical potential for applications. In situ spectroscopy and electrochemical analyses reveal that oxygen vacancies stabilize the *OOH intermediate, accelerate charge transfer, and suppress the competing 4e- pathway, effectively steering the ORR toward the 2e- pathway. As verification, DFT calculations corroborate that NiO-Ov with oxygen vacancies optimizes the adsorption free energy of the key *OOH intermediates (ΔG*OOH) during the 2e- ORR process.