Controllable Ni/NiO interface engineering on N-doped carbon spheres for boosted alkaline water-to-hydrogen conversion by urea electrolysis

Interface engineering has gradually attracted substantial research interest in constructing active bifunctional catalysts toward urea electrolysis. The fundamental understanding of the crystallinity transition of the components on both sides of the interface is extremely significant for realizing controllable construction of catalysts through interface engineering, but it still remains a challenge. Herein, the Ni/NiO heterogenous nanoparticles are successfully fabricated on the porous N-doped carbon spheres by a facile hydrothermal and subsequent pyrolysis strategy. And for the first time we show the experimental observation that the Ni/NiO interface can be fine-tuned via simply tailoring the heating rate during pyrolysis process, in which the crystalline/amorphous or crystalline/crystalline Ni/NiO heterostructure is deliberately constructed on the porous N-doped carbon spheres (named as CA−Ni/NiO@NCS or CC−Ni/NiO@NCS, respectively). By taking advantage of the unique porous architecture and the synergistic effect between crystalline Ni and amorphous NiO, the well-designed CA−Ni/NiO@NCS displays more remarkable urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) activity than its crystalline/crystalline counterpart of CC−Ni/NiO@NCS. Particularly, the whole assembled two-electrode electrolytic cell using the elaborate CA−Ni/NiO@NCS both as the anode and cathode can realize the current density of 10 mA·cm−2 at a super low voltage of 1.475 V (264 mV less than that of pure water electrolysis), as well as remarkable prolonged stability over 63 h. Besides, the H2 evolution driven by an AA battery and a commercial solar cell is also studied to enlighten practical applications for the future.