Surface Structure Reformulation from CuO to Cu/Cu(OH)₂ for Highly Efficient Nitrate Reduction to Ammonia

Electrochemical conversion of nitrate (NO₃ ^-) to ammonia (NH₃) is a potential way to produce green NH₃ and remediate the nitrogen cycle. In this paper, an efficient catalyst of spherical CuO made by stacking small particles with oxygen-rich vacancies is reported. The NH₃ yield and Faraday efficiency are 15.53 mg h^-1 mg_cat ^-1 and 90.69%, respectively, in a neutral electrolyte at a voltage of -0.80 V (vs. reversible hydrogen electrode). The high activity of the electrodes results from changes in the phase and structure during electrochemical reduction. Structurally, there is a shift from a spherical structure with dense accumulation of small particles to a layered network structure with uniform distribution of small particles stacked on top of each other, thus exposing more active sites. Furthermore, in terms of phase, the electrode transitions from CuO to Cu/Cu(OH)₂. Density functional theory calculations showed that Cu(OH)₂ formation enhances NO₃- adsorption. Meanwhile, the Cu(OH)₂ can inhibit the competing hydrogen evolution reaction, while the formation of Cu (111) crystal surfaces facilitates the hydrogenation reaction. The synergistic effect between the two promotes the NO₃- to NH₃. Therefore, this study provides a new idea and direction for Cu-based oxides in electrocatalytic NH₃ production.