Ni3+-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 Ni²⁺ to Ni³⁺, the actual active species, requiring high anodic potentials and resulting in substantial overpotentials. Here, we introduce a simple yet effective strategy to intrinsically enrich Ni³⁺ content in nickel sulfide (Ni₃S₂) by adjusting sulfur precursor concentration during hydrothermal synthesis. This approach yields a Ni₃S₂ catalyst with an increased Ni³⁺/Ni²⁺ 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 Ni₃S₂ surface provides a more energetically accessible pathway for C─N bond cleavage compared to sulfur-free Ni(OH)₂, and that increasing the Ni³⁺/Ni²⁺ 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.