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• Freeze-drying/annealing approach induced phase structure tuning of Ni-NiO system. • Annealing temperature was significant to influence the Ni-NiO crystal structure. • Efficient Ni-NiO synergistic effect promoted the catalytic performance for urea oxidation. • Efficient kinetics and rapid charge transfer-ability were found for urea oxidation. The sluggish kinetics of urea electro-oxidation seriously limits its application in hydrogen production and wastewater treatment. Herein, we demonstrated an easy freeze-drying/annealing approach induced phase structure tuning of graphene supported Ni-NiO nanoparticles system for efficiently boosting urea oxidation performance. The annealing temperature was found significant to influence the Ni precursors decomposition, crystal structure formation and catalytic performance for urea oxidation. By increasing the temperature from 350 °C to 550 °C, more metallic Ni was formed in the Ni-NiO system, and because of the efficient Ni-NiO synergistic effect and conductivity improvement, the sample obtained at 450 °C exhibited the highest catalytic activity and stability for urea oxidation. Specifically, the current density at 0.5 V was 38.24 mA cm−2, about 5.6 and 30 times of the control sample of graphene supported NiO and Ni respectively. Efficient kinetics and rapid charge transfer-ability were also found for urea oxidation by a series of electro-kinetics analysis. The current work showed an easy way for Ni-based catalyst structure tuning with enhanced kinetics for urea oxidation, which is helpful for the sustainable utilization of urea molecular in electrochemical energy. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Urea oxidation catalyzed by NiSe 2 largely improved by a facile Fe doping strategy. • Doping amount of Fe would greatly impact the catalytic ability. • NiSe 2 with 1.68 at.% Fe-doping exhibited the best catalytic performance. • Current density at 1.54 V was 4.4 times higher than that of pristine NiSe 2 electrode. • The cell voltage of 1.45 V to reach 10 mA cm−2 was 170 mV lower than water electrolysis. The non-precious Ni-based catalysts are promising in the electrooxidation of urea while the low intrinsic activity still needs to be increased for practical application. Herein, we demonstrated the urea oxidation catalyzed by NiSe 2 nanorod could be greatly improved by Fe doping strategy. The Fe-doping effect was probed by density functional theory which revealed that doping could enhance the interface electric field inducing the charge redistribution near the Ni atom, and enhancing the adsorption capacity of Ni to urea molecules. As a result, the balanced interaction of Fe/Ni synergism and the Ni active site exposure can be realized by tuning the Fe doped amount in the system, which would greatly impact the catalytic ability and intrinsic performance. The NiSe 2 with 1.68 at.% iron doping demonstrated the highest catalytic performance for urea oxidation, displaying the largest surface-active area, highly efficient kinetics, fast charge transfer rate, and outstanding stability. The current density of 125.8 mA cm−2 for urea oxidation catalyzed by Fe doped NiSe 2 electrode was obtained at 1.54 V vs. RHE when supported on the glassy carbon electrode, almost 4.4 times that of the pristine NiSe 2 electrode. When constructed in the two-electrode system combined with the commercial Pt/C cathode, the cell voltage used for urea electrolysis was 1.45 V to provide the specific current density of 10 mA cm−2, approximately 170 mV lower than that of pure water electrolysis, indicating the potential application of hydrogen generation for urea-assisted water splitting. The urea oxidation catalyzed by NiSe 2 nanorod can be largely improved by a facile Fe doping strategy, and the doping amount of Fe would greatly impact the catalytic ability and intrinsic performance. [Display omitted] [ABSTRACT FROM AUTHOR]

The sluggish kinetics of urea elecroooxidation restricts the application of the electrochemical approach in dealing with urea-rich wastewater. Herein, urea-assisted water electrolysis was demonstrated efficiently on the catalyst system of iron oxide promoted nickel/nickel oxide rough nanorods. With the help of a series of physical characterization and electrochemical measurements, the content of iron oxide in the system was found to largely influence the crystal structure, roughness and electrochemically active surface area as well as the electrochemical mechanism. The sample with the Fe/Ni ratio of 0.5:1 showed the best water and urea oxidation catalytic performance because of the largest surface area and rapid charge transfer rate and efficient kinetics in the rate-determining step. As a urea-assisted water electrolysis catalyst, the current density can reach 30 mA cm−2 at 1.45 V, about 20 times higher than that of pure water oxidation; the cell voltage as low as 1.49 V can afford 10 mA cm−2 for urea electrolysis, about 120 mV lower than that of pure water electrolysis in the two-electrode catalytic systems. Considering the low price of Ni and Fe and anti-corrosion ability in the electrolyte, the current work is instructive for the efficient catalyst fabrication and urea-containing wastewater purifying. Image 1 • Electrochemical approach in dealing with urea-rich wastewater. • Urea-assisted water electrolysis efficiently catalyzed on FeNi oxide electrode. • Iron oxide content influenced the catalytic performance of urea oxidation. • 20 times higher of urea electrolysis than that of pure water oxidation. • Cell voltage about 120 mV lower for urea electrolysis than pure water electrolysis. [ABSTRACT FROM AUTHOR]