Role of Structural\nand Compositional Changes of Cu<sub>2</sub>O Nanocubes in Nitrate\nElectroreduction to Ammonia

Nitrate electroreduction reaction (NO<sub>3</sub>RR)\nto ammonia\n(NH<sub>3</sub>) still faces fundamental and technological challenges.\nWhile Cu-based catalysts have been widely explored, their activity\nand stability relationship are still not fully understood. Here, we\nsystematically monitored the dynamic alterations in the chemical and\nmorphological characteristics of Cu<sub>2</sub>O nanocubes (NCs) during\nNO<sub>3</sub>RR in an alkaline electrolyte. In 1 h of electrolysis\nfrom −0.10 to −0.60 V vs RHE, the electrocatalyst achieved\nthe maximum NH<sub>3</sub> faradaic efficiency (FE) and yield rate\nat −0.3 V (94% and 149 μmol h<sup>–1</sup> cm<sup>–2</sup>, respectively). Similar efficiency could be found\nat a lower overpotential (−0.20 V vs RHE) in long-term electrolysis.\nAt −0.20 V vs RHE, the catalyst FE increased from 73% in the\nfirst 2 h to ∼90% in 10 h of electrolysis. Electron microscopy\nrevealed the loss of the cubic shape with the formation of sintered\ndomains. <i>In situ</i> Raman, X-ray diffraction (XRD),\nand <i>in situ</i> Cu K-edge X-ray absorption near-edge\nspectroscopy (XANES) indicated the reduction of Cu<sub>2</sub>O to\noxide-derived Cu<sup>0</sup> (OD-Cu). Nevertheless, a remaining Cu<sub>2</sub>O phase was noticed after 1 h of electrolysis at −0.3\nV vs RHE. This observation indicates that the activity and selectivity\nof the initially well-defined Cu<sub>2</sub>O NCs are not solely dependent\non the initial structure. Instead, it underscores the emergence of\nan OD-Cu-rich surface, evolving from near-surface to underlying layers\nover time and playing a crucial role in the reaction pathways. By\nemploying <i>online</i> differential electrochemical mass\nspectrometry (DEMS) and <i>in situ</i> Fourier transform\ninfrared spectroscopy (FTIR), we experimentally probed the presence\nof key intermediates (NO and NH<sub>2</sub>OH) and byproducts of NO<sub>3</sub>RR (N<sub>2</sub> and N<sub>2</sub>H<sub><i>x</i></sub>) for NH<sub>3</sub> formation. These results show a complex\nrelationship between activity and stability of the nanostructured\nCu<sub>2</sub>O oxide catalyst for NO<sub>3</sub>RR.