Well-Defined Cu Precatalysts Indicate Design Rules for Reactivity in Nitrate Electroreduction

The electrochemical nitrate reduction reaction (NO₃RR) to ammonia (NH₃) is a promising route for sustainable NH₃ synthesis. Cu-based materials are the most used and promising catalysts for this reaction. However, Cu undergoes uncontrollable compositional and structural changes under NO₃RR conditions, necessitating a deeper understanding of the relationship between precatalyst features, structural evolution, and catalytic performance for the advancement of current catalyst design rules. Here, we exploit well-defined Cu and Cu oxide nanocrystals (NCs) as precatalysts to elucidate these correlations. We find that the size, shape, and oxide content of the Cu precatalysts all play a role in driving structural evolution and, thus, the catalytic behavior during NO₃RR. In particular, a higher oxide content, an optimized {111}/{100} facet ratio, and the spatial proximity of these facets forming grain boundaries within the active catalysts emerge as key factors to enhance NH₃ selectivity. Among the studied Cu precatalysts, 10 nm Cu spheres integrate these key features, achieving a competitive NH₃ production rate compared to the state of the art. This work links pre- and in situ-formed catalyst features to catalytic performance, offering insights into the morphological dynamics of Cu catalysts under NO₃RR conditions.