Hydrogen peroxide (H 2 O 2) synthesis by electrochemical oxygen reduction reaction has attracted great attention as a green substitute for anthraquinone process. However, low oxygen utilization efficiency (<1%) and high energy consumption remain obstacles. Herein we propose a superhydrophobic natural air diffusion electrode (NADE) to greatly improve the oxygen diffusion coefficient at the cathode about 5.7 times as compared to the normal gas diffusion electrode (GDE) system. NADE allows the oxygen to be naturally diffused to the reaction interface, eliminating the need to pump oxygen/air to overcome the resistance of the gas diffusion layer, resulting in fast H 2 O 2 production (101.67 mg h-1 cm-2) with a high oxygen utilization efficiency (44.5%-64.9%). Long-term operation stability of NADE and its high current efficiency under high current density indicate great potential to replace normal GDE for H 2 O 2 electrosynthesis and environmental remediation on an industrial scale.

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As nitrate contamination causes serious environmental problems, it is necessary to develop stable and efficient electrocatalysts for efficient electrochemical nitrate reduction reaction (ENRR). Here, a nonprecious Co 3 O 4 /carbon felt (CF) electrode with a 3D structure was prepared by integrating electrodeposition with calcination methods. This 3D structured Co 3 O 4 /CF electrode exhibits a high-rate constant of 1.18 × 10 −4 s −1 cm −2 for the ENRR, surpassing other Co 3 O 4 electrodes in previous literature. Moreover, it also has an excellent stability with a decrease of 6.4% after 10 cycles. Density functional theory calculations, electron spin resonance analysis, and cyclic voltammetry were performed to study the mechanism of the ENRR on the Co 3 O 4 /CF electrode, proving that atomic H* (indirect pathway) plays a prominent role in NO 3 − reduction and clarifying the synergistic effect of Co(III) and Co(II) in the Co(II)−Co(III)−Co(II) redox cycle for the ENRR: Co(III) prefers the adsorption of NO 3 − and Co(II) favors the production of H*. Based on this synergy, a relatively large amounts of Co(II) on the surface of the Co 3 O 4 /CF electrode (1.3 Co(II)/Co(III) ratio) was maintained by controlling the temperature of calcination to 200 °C with a lower energy barrier of H* formation of 0.46 eV than other ratios, which is beneficial for forming H* and enhancing the performance of the ENRR. Thus, this study suggests that building 3D structure and optimizing Co(II)/Co(III) ratio are important for designing efficient Co 3 O 4 electrocatalyst for ENRR. KEYWORDS: electrochemical nitrate reduction reaction, 3D structure, Co 3 O 4 /carbon felt electrode, high-rate nitrate removal, atomic H*, synergistic effect, Co(II)−Co(III)−Co(II) redox cycle