Role of Structural and Compositional Changes of CuO Nanocubes in Nitrate Electroreduction to Ammonia.

Nitrate electroreduction reaction (NORR) to ammonia (NH) still faces fundamental and technological challenges. While Cu-based catalysts have been widely explored, their activity and stability relationship are still not fully understood. Here, we systematically monitored the dynamic alterations in the chemical and morphological characteristics of CuO nanocubes (NCs) during NORR in an alkaline electrolyte. In 1 h of electrolysis from -0.10 to -0.60 V vs RHE, the electrocatalyst achieved the maximum NH faradaic efficiency (FE) and yield rate at -0.3 V (94% and 149 μmol h cm, respectively). Similar efficiency could be found at a lower overpotential (-0.20 V vs RHE) in long-term electrolysis. At -0.20 V vs RHE, the catalyst FE increased from 73% in the first 2 h to ∼90% in 10 h of electrolysis. Electron microscopy revealed the loss of the cubic shape with the formation of sintered domains. Raman, X-ray diffraction (XRD), and Cu K-edge X-ray absorption near-edge spectroscopy (XANES) indicated the reduction of CuO to oxide-derived Cu (OD-Cu). Nevertheless, a remaining CuO phase was noticed after 1 h of electrolysis at -0.3 V vs RHE. This observation indicates that the activity and selectivity of the initially well-defined CuO NCs are not solely dependent on the initial structure. Instead, it underscores the emergence of an OD-Cu-rich surface, evolving from near-surface to underlying layers over time and playing a crucial role in the reaction pathways. By employing differential electrochemical mass spectrometry (DEMS) and Fourier transform infrared spectroscopy (FTIR), we experimentally probed the presence of key intermediates (NO and NHOH) and byproducts of NORR (N and NH ) for NH formation. These results show a complex relationship between activity and stability of the nanostructured CuO oxide catalyst for NORR.