$Operando$ Tracking of the Structural Evolution of Copper Carbodiimide during Electrocatalytic Nitrate Reduction

Heterogeneous electrocatalysts can undergo reconstruction under operando operational conditions to modify their microstructure and form a different active phase, even if their structural motifs are generally perceived as stable. Real-time tracking of these changes is necessary to provide an in-depth understanding of their structure–function relationships for further knowledge-driven catalyst design. In this study, copper carbodiimide (CuNCN) nanoparticles were prepared for catalyzing a waste-to-wealth process, namely, nitrate electroreduction to ammonia. Operando spectroscopic studies revealed that while CuNCN evolved into metallic Cu, Cu2O was formed as an intermediate during electrocatalysis. The metallic Cu phase provided the real active sites for ammonia synthesis and showed considerable Faradaic efficiency (>80%) and robustness over 20 recycling tests. Our results highlight the significance of operando spectroscopy methods for revealing the transformations of metal carbodiimides under electrochemical conditions, which will facilitate their efficient applications in electrochemical syntheses of small molecules.