Amorphous chromium oxide confined Ni/NiO nanoparticles-assembled nanosheets for highly efficient and stable overall urea splitting

Abstract

Applications of urea oxidation reaction (UOR) in various sustainable energy-conversion systems are greatly hindered by its slow kinetics. Herein, we demonstrate an in-situ confined synthesis method that produces amorphous chromium oxide confined Ni/NiO nanoparticles-assembled nanosheets (Ni/NiO@CrOx) with fast reaction kinetics towards UOR. The confinement effect of the in-situ generated CrOx overlay contributes to ultrafine Ni/NiO nanoparticles, bringing about rich Ni/NiO and NiO/CrOx interfaces. In-situ Raman and electrochemical characterization show that both CrOx and metallic Ni can promote the formation of the NiOOH species and the electron transfer, leading to high intrinsic activity and fast reaction kinetics. At 1.40 V vs. reversible hydrogen electrode, the Ni/NiO@CrOx delivers a current density of 275 mA cm−2, which is about 2.6 and 6.1 times as large as those of the NiO@CrOx and NiO, respectively. In addition, the protective effect of the CrOx overlay leads to robust working stability towards UOR. Further, the Ni/NiO@CrOx nanosheets are used as bifunctional catalysts for overall urea splitting, and a small electrolysis cell voltage of 1.44 V is needed to reach the benchmark current density of 10 mA cm−2.