High‐Valent Nickel Species in Urea Electrooxidation: Phase Transitions, Mechanistic Debates, and Active Site Modulation

ABSTRACT The electrochemical urea oxidation reaction (UOR) stands at the forefront of sustainable energy research, offering a compelling pathway for energy‐saving hydrogen production while simultaneously remediating nitrogen‐rich wastewater. In alkaline media, nickel‐based electrocatalysts have established undisputed dominance. A critical consensus in the field is that pristine nickel acts merely as a precatalyst; the true catalytic activity is strictly governed by an electrochemically driven phase transition to transient, high‐valent nickel species (Ni 3+ /Ni 4+ ). However, despite their universally acknowledged necessity, the precise structural identity, dynamic evolution, and mechanistic role of these high‐valent active sites remain a subject of intense debate. This perspective provides a comprehensive critical analysis of the high‐valent nickel species in UOR electrocatalysis. We systematically unravel the origin of these highly electrophilic sites and dissect current mechanistic controversies, particularly the debate between Ni 3+ and Ni 4+ active centers, as well as the competing pathways. Furthermore, we evaluate state‐of‐the‐art strategies to rationally modulate these sites, including heteroatom doping, defect engineering, and heterostructure construction, to overcome the kinetic bottleneck of intermediate poisoning.