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

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@CrO_x) with fast reaction kinetics towards UOR. The confinement effect of the in-situ generated CrO_x overlay contributes to ultrafine Ni/NiO nanoparticles, bringing about rich Ni/NiO and NiO/CrO_x interfaces. In-situ Raman and electrochemical characterization show that both CrO_x 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@CrO_x 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@CrO_x and NiO, respectively. In addition, the protective effect of the CrO_x overlay leads to robust working stability towards UOR. Further, the Ni/NiO@CrO_x 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.