Role of Structural and Compositional Changes of Cu₂O Nanocubes in Nitrate Electroreduction to Ammonia

Nitrate electroreduction reaction (NO₃RR) to ammonia (NH₃) 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 Cu₂O nanocubes (NCs) during NO₃RR in an alkaline electrolyte. In 1 h of electrolysis from -0.10 to -0.60 V vs RHE, the electrocatalyst achieved the maximum NH₃ faradaic efficiency (FE) and yield rate at -0.3 V (94% and 149 μmol h^-1 cm^-2, 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. In situ Raman, X-ray diffraction (XRD), and in situ Cu K-edge X-ray absorption near-edge spectroscopy (XANES) indicated the reduction of Cu₂O to oxide-derived Cu⁰ (OD-Cu). Nevertheless, a remaining Cu₂O 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 Cu₂O 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 online differential electrochemical mass spectrometry (DEMS) and in situ Fourier transform infrared spectroscopy (FTIR), we experimentally probed the presence of key intermediates (NO and NH₂OH) and byproducts of NO₃RR (N₂ and N₂H ) for NH₃ formation. These results show a complex relationship between activity and stability of the nanostructured Cu₂O oxide catalyst for NO₃RR.