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.