Interfacial Control in Cu–MXene Hybrids Enables Selective NOx-to-NH₃ Electroconversion: A Critical Review

Electrochemical conversion of nitrogen oxides (NO_x, mainly NO and NO₂) to ammonia (NH₃) transforms a pollutant into a valuable chemical, providing a direct link between emissions control and low-carbon NH₃ production under mild conditions. However, NO_x reduction competes with hydrogen evolution (HER) and can diverge to N₂/N₂O, while many catalysts restructure under bias, making results difficult to compare and design rules hard to extract. Cu-MXene hybrids are emerging as a compelling platform because the 2D interface can be engineered to control adsorption, electron (e⁻) transfer, and local proton (H⁺) activity through Cu speciation (single sites, clusters, oxides, reconstructed surfaces) and MXene terminations/defects. This critical review summarizes the interfacial control of these catalysts to achieve selective NO_x-to-NH₃ electroconversion. The review provides a comprehensive analysis of intermediates and performance, quantifying using device-relevant metrics (Faradaic efficiency, NH₃ yield/partial current, energy efficiency, durability, and selectivity against N₂/N₂O and HER). Special emphasis is placed on reactor design, including flow-cell and gas-diffusion electrode (GDE) configurations, which enhance mass transport, stabilize the three-phase boundary, and enable long-term operational stability. Finally, the review emphasizes the use of in situ/operando methods to investigate the reaction mechanisms, outlines design rules and scale-up priorities for practical NO_x upcycling into green ammonia.