The significant change of nitrogen cycle as a result of growing production and industrial use of nitrogen fertilizers has led to great environmental concern. The nitrate (NO₃^-) in groundwater and industrial wastewater pose a serious threat to the nitrogen cycle and human health, which creates a need for efficient methods that can convert nitrate species. Although many techniques such as biological denitrification, ion exchange, reverse osmosis, and electrodialysis have been developed for NO₃^--rich water remediation, electrocatalytic nitrate reduction reaction (NO₃RR) is a promising technology that utilize electricity from renewable solar/wind energies to selectively reduce NO₃^- to nitrogen (N₂) or ammonia (NH₃). Of note that NH₃ is a fertilizer source and is also considered as a green hydrogen-rich fuels. Therefore, converting NO₃^- in wastewater to recyclable NH₃ is attractive with regard to environmental protection and energy saving.

Metal oxide catalysts with well-controlled shape and size are currently developed to explore catalytic performance trends and intrinsic mechanism for the NO₃RR. For example, Co₃O₄-based nanoparticles and nanosheet array exhibit improved performance because of the optimized exposed surface and size distribution. TiO₂ nanotube arrays with anatase structure displayed better electrocatalytic activity than those with rutile structure. These findings have demonstrated that metal oxide catalysts with subtle composition and crystal structure play an important role in enhanced performance for the NO₃RR. However, the redox potential for metal oxide reduction is more positive than the potential required for the NO₃RR, so metal oxides can be further reduced to metal state under NO₃RR.

During the reduction process, the loss of structural oxygen ions in metal oxide catalyst would lead to its nonstoichiometry. Accordingly, the electrochemical reduction conditions may drive the generation of defects to accommodate the nonstoichiometry. Therefore, we hypothesized that the substantial structural perturbations such as dislocations could induce lattice strains in the defect region. Such lattice strains will modulate the local surface electronic structure of the catalysts, tuning the interaction between the reaction species and catalyst surface. As a result, the catalytic activity of metal oxides could be optimized.

In this study, we selected Cu-based oxide electrodes to fully investigate the structural change and its effects on catalytic performance towards NO₃RR. We found that Cu oxide catalysts have a distinct structural change during the NO₃RR. The Cu oxide catalysts were reduced to metallic Cu under the negative potential of NO₃RR, which serves as the actual active species. More importantly, we found that abundant stacking faults were formed on the oxide-derived Cu surfaces due to the applied negative potential during the NO₃RR or the electroreduction pretreatment. The in situ electrochemical reduction-generated stacking faults can be utilized to increase NO₃^--N removal, NH₄⁺-N selectivity, and NH₃ Faradaic efficiency up to 93, 94, and 80%, respectively. On the basis of theoretical and experimental investigations, it is concluded that the tensile strain resulting from the stacking faults facilitates NO₃^- adsorption and suppresses hydrogen evolution reaction. This work not only helps explain the improved reactivity of related metal oxide for the NO₃RR, but also provides a general strategy to develop active and stable electrocatalysts with rational designed surface structure.