Nickel nanocrystal/sulfur-doped carbon composites as efficient and stable electrocatalysts for urea oxidation reaction

Abstract

Urea oxidation reaction (UOR) is an anodic reaction in the electrochemical generation hydrogen and also a crucial half-reaction of direct urea fuel cell (DUFC). However, the scarcity and high cost of noble metals severely limited their application as electrocatalysts for UOR. Herein, we have prepared the composite (Ni@S-C) of nickel nanocrystal embedded in sulfur-doped carbon through facile impregnation method followed by calcination method. X-ray photoelectron spectroscopy (XPS) study indicates the Ni(III) species in Ni@S-C are more positive than typical Ni(III), which serve as the active sites for UOR. Meanwhile, the active Ni(III) is coordinated with S, which facilitates charge transfer and chemisorption of active oxygen species. Under optimal experimental conditions, the Ni@S-C-500 catalyst indicates an excellent UOR activity of 177 mA cm−2 in 1 M KOH + 0.33 M urea at 1.656 V, which was 3.75 times as much as that of the commercial Pt/C catalyst. Meanwhile, the optimal Ni@S-C-500 catalyst also presents a current density (j) retention rate of 80% after 12 h UOR test, which is much higher than that of the commercial Pt/C catalyst (<15% retention). This work provides a strategy for preparing stable and active Ni-based electrocatalyst for the UOR.