Mo-doped δ-MnO2 nanoflowers enable efficient nitrogen oxidation to nitrate under mild conditions

Abstract

The development of efficient electrocatalysts for the nitrogen oxidation reaction (NOR) under mild conditions is crucial for sustainable nitrate synthesis. Mo-doped δ-MnO₂ electrocatalysts with varying Mo concentrations were successfully prepared for the NOR. Structural and electrochemical analyses revealed that Mo doping simultaneously enhanced the conductivity and electrochemically active surface area (ECSA) while promoting N₂ adsorption and activation through electronic structure modulation. The optimized 2.5% Mo-doped δ-MnO₂ (denoted as MM2.5) exhibited superior NOR performance in 0.1 M KOH, delivering a NO₃⁻ production rate of 116.75 μg h⁻¹ mg_cat⁻¹ with a faradaic efficiency (FE) of 7.04% and excellent long-term stability. In addition, a Zn–N₂ device was formed with MM2.5 as the anode and a Zn plate as the cathode, and the NO₃⁻ yield obtained in this device was even higher than 144.5 μg h⁻¹ mg_cat⁻¹. However, structural characterization revealed that excessive Mo doping disrupted the δ-MnO₂ crystal structure, reducing specific surface area and active site density. Density functional theory (DFT) calculations demonstrated that Mo doping lowered the Gibbs free energy of the rate-determining step (*N₂ → *NNOH) from 2.41 eV to 1.94 eV by facilitating electron transfer, thereby optimizing the reaction pathway. This study provides a new strategy for the design of transition metal oxide-based electrocatalysts, as well as the application in artificial nitrogen fixation.