Electrochemical Nitrate Reduction with Low-Index Cu Single Crystals: Selectivity Trends in Alkaline Solution.

Electrochemical nitrate (NO 3 - ) reduction is a promising pathway toward the production of ammonia (NH 3 ). While copper (Cu) electrodes are commonly used for this reaction, the influence of Cu surface structure on reaction selectivity is poorly understood. Here, we performed electrochemical NO 3 - reduction in alkaline electrolytes with Cu(100), Cu(110), and Cu(111) single-crystal electrodes and quantified the product distributions across a range of applied potentials. Our systematic study of electrocatalytic NO 3 - reduction demonstrated that the Cu(100) and Cu(110) surfaces exhibited similar selectivity and rates for NH 3 production, but the Cu(100) surface provided >95% Faradaic efficiency toward NH 3 over the broadest range of applied potentials. In contrast, the Cu(111) surface was significantly less selective for NH 3 production. Further comparing the electrochemical behavior of Cu single crystals to their corresponding product distributions revealed that nitrite (NO 2 - ) reduction voltammograms were strong predictors of electrocatalytic performance for all three low-index Cu surfaces in alkaline electrolytes. Quantum mechanical calculations indicate that the Cu(100) surface exhibits the lowest potential-determining step for NO 3 - reduction to NH 3 , supporting our experimental findings. Detailed analysis of metal-adsorbate interactions also revealed the role of Cu surface structure in stabilizing key reaction intermediates to promote selective NH 3 production. Our combined experimental and computational study establishes electrocatalytic selectivity of low-index Cu facets for NH 3 synthesis and offers practical guidelines for the design of Cu-based electrocatalysts capable of selective electrochemical NO 3 - reduction in alkaline solutions.