Ni ³⁺ ‐Enriched Nickel Sulfide Catalysts for Urea Oxidation

ABSTRACT The urea oxidation reaction (UOR) offers a low‐energy alternative to the oxygen evolution reaction (OER) for hydrogen generation while simultaneously enabling urea‐rich wastewater remediation. However, most nickel (Ni)‐based catalysts rely on in situ oxidation of Ni ²⁺ to Ni ³⁺ , the actual active species, requiring high anodic potentials and resulting in substantial overpotentials. Here, we introduce a simple yet effective strategy to intrinsically enrich Ni ³⁺ content in nickel sulfide (Ni ₃ S ₂ ) by adjusting sulfur precursor concentration during hydrothermal synthesis. This approach yields a Ni ₃ S ₂ catalyst with an increased Ni ³⁺ /Ni ²⁺ ratio, achieving a UOR current density of 100 mA cm ⁻² at only 1.361 V vs. RHE. Density functional theory (DFT) calculations reveal that sulfur‐modulated Ni ₃ S ₂ surface provides a more energetically accessible pathway for C─N bond cleavage compared to sulfur‐free Ni(OH) ₂ , and that increasing the Ni ³⁺ /Ni ²⁺ ratio within the sulfide framework further lowers the energy barrier of the rate‐determining step. These results highlight the critical role of sulfur in defining the catalytic scaffold and modulating Ni oxidation states. Furthermore, integrating the catalyst with a 3D‐printed interpenetrating device architecture and flow‐cell system markedly enhances mass transport and overall performance. This work highlights sulfur‐enabled Ni valence modulation as an effective route to advance UOR kinetics and demonstrates the synergy between catalyst design and device engineering for sustainable hydrogen generation coupled with wastewater remediation.