Two-dimensional nickel hydroxide nanosheets with high-content of nickel(III) species towards superior urea electro-oxidation

Development of high-efficient and stable electrocatalysts for urea oxidation reaction (UOR) is of a great challenge due to the sluggish kinetics of 6e⁻ transfer process. Here, we have developed a facile and easy-to-scale approach to fabricate two-dimensional Ni(III)-rich Ni(OH)₂ nanosheets on amine-functionalized carbon (Ni³⁺-rich Ni(OH)₂/C-NH₂). Morphological characterizations confirm the existence of nanosheets, and XPS spectra indicate that the content of Ni³⁺ species in Ni³⁺-rich Ni(OH)₂/C-NH₂ (ca. 57.6%) is significantly higher than that of in Ni(OH)₂/C-NH₂ (ca. 43.1%) and Ni(OH)₂/C (ca. 20.7%). Electrochemical analyses illustrate that the as-prepared Ni³⁺-rich Ni(OH)₂/C-NH₂ catalyst exhibits the highest current density (91.72 mA cm⁻²) at a potential of 0.61 V, which is 2.06-, 2.08- and 3.47-fold higher than that of Ni(OH)₂/C-NH₂, Ni(OH)₂/C and Pt/C, respectively. Moreover, the Ni³⁺-rich Ni(OH)₂/C-NH₂ catalyst also demonstrates an outstanding voltammetric cycles and long-term chronoamperometric stability. The superior electrocatalytic activity and stability could be ascribed to the synergistic effect of Ni³⁺ doping as well as the amine-functionalized carbon, where higher concentration Ni³⁺ species in Ni(OH)₂ sheets could provide more active sites for adsorption and transformation of urea molecules, while fluffy C-NH₂ support could enhance the ability of solute diffusion, electron transport and gas emissions, thereby dramatically improve the catalytic activity.