Photo/electrochemical splitting of water to hydrogen (H2) fuel is a sustainable way of meeting our energy demands at no environmental cost, but significant challenges remain: for example, the sluggish anodic reaction imposes a considerable overpotential requirement. By contrast, urea electrolysis offers the prospect of energy-saving H2 production together with urea-rich wastewater purification, whereas the lack of inexpensive and efficient urea oxidation reaction (UOR) catalysts places constraints on the development of this technique. Here we report a porous rod-like NiMoO4 with high oxidation states of the metal elements enabling highly efficient UOR electrocatalysis, which can be readily produced through annealing solid NiMoO4·xH2O as a starting precursor in Ar. This precursor gives the derived Ni/NiO/MoOx nanocomposite when switching the shielding gas from Ar to H2/Ar, exhibiting platinum-like activity for the hydrogen evolution reaction (HER) in alkaline electrolytes. Assembling an electrolytic cell using our developed UOR and HER catalysts as the anode and cathode can provide a current density of 10 milliamperes per square centimeter at a cell voltage of mere 1.38 volts, as well as remarkable operational stability, representing the best yet reported noble-metal-free urea electrolyser. Our results demonstrate the potential of nickel–molybdenum-based materials as efficient electrode catalysts for urea electrolysers that promises cost-effective and energy-saving H2 production.

Ni–Mo–O nanorod-derived composite catalysts for efficient alkaline water-to-hydrogen conversion via urea electrolysis

The replacement of precious-metal-based catalysts with earth-abundant alternatives for a diverse range of electrochemical applications is of great importance for next-generation energy technologies. Many self-supported earth-abundant nanoarrays have emerged as state-of-the-art electrocatalysts due to their fascinating structures and electrochemical properties. This Review presents recent advances made toward developing self-supported earth-abundant nanoarrays for a wide range of energy-conversion processes. We summarize the different synthetic methods used to construct nanoarrays and tune their compositions, morphologies, and structures. Then, we highlight their application and performance as catalysts for various energy-related reactions. We also discuss their ability to perform as bifunctional electrocatalysts in energy devices. Finally, we conclude with the challenges and prospects in this promising field, where further exploration into these materials will facilitate the development of next-generation...

Self-Supported Earth-Abundant Nanoarrays as Efficient and Robust Electrocatalysts for Energy-Related Reactions

Ceria (CeO2) nanoparticles were grown on reduced graphene oxide (RGO) via the in situ reduction of graphene oxide (GO) in the presence of cerium nitrate and CTAB, followed by a one step hydrothermal treatment. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible spectroscopy (UV-Vis), Raman spectroscopy (RS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the samples. The characterization suggests that the ammonia-assisted hydrothermal method is a facile and advantageous route to synthesize CeO2–RGO nanocomposites compared to the widely used method utilising hydrazine hydrate as the reducing reagent. TEM investigations revealed that the CeO2 nanoparticles with an average size of ∼14 nm were dispersed on the layers of RGO. The catalytic activity of the CeO2–RGO nanocomposites towards the electrooxidation of hydrazine was further investigated by cyclic voltammetry measurements. The results obtained suggest that compared to bare CeO2 nanoparticles, the CeO2–RGO nanocomposite exhibits remarkably enhanced electrocatalytic activity, due to the synergistic effects between the CeO2 nanoparticles and RGO.

Characterizations of in situ grown ceria nanoparticles on reduced graphene oxide as a catalyst for the electrooxidation of hydrazine

Direct urea fuel cells: Challenges and opportunities

Bifunctional cobalt oxide (Co3O4) nanowire catalysts grown on carbon cloth (CC) fibers and their modification with nickel oxide (NiO) and manganese dioxide (MnO2) to produce core-shell nanoarchitectures are explored as catalysts for urea oxidation reaction and oxygen reduction reaction in direct urea fuel cells (DUFC). Based on a systematic electrochemical characterization of the catalyst, the as-developed core-shell nanoarchitectures are optimized toward DUFC performance. Under alkaline conditions with an anion exchange membrane, the DUFC with a cell configuration of Co3O4@NiO(1:2)/CC(a vertical bar c)Co3O4@MnO2(1:2)/CC exhibits a maximum power density of 33.8 mW cm(-2) with excellent durability for 120 h without any performance loss. Furthermore, the DUFC exhibits a maximum power density of 23.2 mW cm(-2) with human urine as a fuel. These findings offer an approach to convert human waste into treasure.

3D Hierarchical Core–Shell Nanostructured Arrays on Carbon Fibers as Catalysts for Direct Urea Fuel Cells

Preparation of nickel nanowire arrays electrode for urea electro-oxidation in alkaline medium

Electrochemical impedance analysis of urea electro-oxidation mechanism on nickel catalyst in alkaline medium

Highly porous nickel@carbon sponge as a novel type of three-dimensional anode with low cost for high catalytic performance of urea electro-oxidation in alkaline medium

Fen Guo

Papers

Preparation of nickel nanowire arrays electrode for urea electro-oxidation in alkaline medium

Electrochemical impedance analysis of urea electro-oxidation mechanism on nickel catalyst in alkaline medium

Highly porous nickel@carbon sponge as a novel type of three-dimensional anode with low cost for high catalytic performance of urea electro-oxidation in alkaline medium

Preparation of nickel-cobalt nanowire arrays anode electro-catalyst and its application in direct urea/hydrogen peroxide fuel cell

Enhancement of direct urea-hydrogen peroxide fuel cell performance by three-dimensional porous nickel-cobalt anode