Preparation Of And Performance Of Nickel-based Electrocatalysts For Urea Electrooxidation And Electrolysis

Urea electrooxidation as an anode reaction has widely applications in direct urea fuel cells and urea electrolysis,which enable not only generate electricity and product hydrogen,but also effectively remediation the environmental pollution caused by naturally decomposition of urea-rich wastewater,so as to achieve the dual purpose of producing clean energy and environmental protection.However,the high onset potential,low current density and poor stability of anodic urea electrooxidation reaction?UOR?have greatly obstacles their applications in direct urea fuel cells and urea electrolysis.Usually,noble metal-based catalysts?Pt and Rh?are exploited as a high-efficient electrocatalyst to catalyze the UOR.Nevertheless,the limited reserves of these expensive catalysts have greatly restricted their large-scale commercialization application.Therefore,finding a high-efficiency,economical and stable anodic catalysts is activity perused.Nickle-based electrocatalyst have been intensively explored as an abundant reserves,high efficiency,superior durability and low-cost non-precious metal based electrocatalysts.Therefore,based on the transition metal nickel-based catalyst,a series of efficient catalysts for urea electrooxidation were prepared by designing and regulating the structure of the catalyst.The Ni Co-carbonates hydroxide precursor nanowire arrays uniformly grown on the Ni foam is obtained through a simple hydrothermal treatment.Then,the Ni Co₂O₄/NF nanowire arrays were obtained after followed thermal treatment.The results of scanning electron microscopy and transmission electron microscopy reveal that the nanowire structure was composed of a large number of small nanoparticles and the interconnected mesoporous structure exists in each nanowire.The self-supported 3D porous nanowire arrays structure provided a large specific surface area,which enable affords a large number of active sites for urea electrooxidation.The electrocatalytic properties of Ni Co₂O₄/NF were directly characterized by cyclic voltammetry and chronoamperometry technology.The as-obtained Ni Co₂O₄/NF catalysts shows remarkable UOR performance with a low onset potential of 0.19V and a remarkable current density of 570 m A cm^-2 in 5 mol L^-1 KOH and 0.33 mol L^-1 urea.Meanwhile,Ni Co₂S₄/NF nanowire arrays were prepared by two-step simple hydrothermal processes.Under the same test conditions,the Ni Co₂S₄/NF electrode delivered a high current density of 720 m A cm^-2 at low onset potential of 0.18 V.After long-term chronoamperometry test,the current density remained nearly constant without any reduction,indicating the superior catalytic stability of the Ni Co₂S₄/NF electrode.Mn Co₂O_4.5 nanosheet arrays uniformly grown on the Ni foam is obtained through a simple hydrothermal method and low-temperature calcination treatment.Then,Ni?OH?₂nanosheets layer were coated on the per-grown Mn Co₂O_4.5 arrays through further hydrothermal treatment to obtained the self-supported Mn Co₂O_4.5@Ni?OH?₂/NF nanosheet arrays.The results of scanning electron microscopy and transmission electron microscopy show that the wavy-like Ni?OH?₂ nanosheets are distributed on the surface of Mn Co₂O_4.5nanosheet arrays,forming a sandwich-like hierarchically structure.The electrochemical performance of Mn Co₂O_4.5/NF,Ni?OH?₂/NF and Mn Co₂O_4.5@Ni?OH?₂/NF electrode were evaluated by cyclic voltammetry and chronoamperometry tests.The Mn Co₂O_4.5@Ni?OH?₂/NF electrode exhibits much advanced UOR activity with a much higher current density(650 m A cm^-2)and smaller onset potential?0.19 V?than that of individual Mn Co₂O_4.5/NF and Ni?OH?₂/NF catalysts in 5 mol L^-1 KOH and 0.33 mol L^-1 urea.The current density retention keeps 89.8%after 7200s chronoamperometry tests.Thorny leaf-like Ni Co P/CC was fabricated via a simple hydrothermal reaction,followed by phosphating treatment.The results of scanning electron microscopy and transmission electron microscopy show that the thorny leaf-like Ni Co P/CC comprises both 1D nanowires and 2D nanosheets,which consist of numerous small nanoparticles.The electrochemical performance of bimetallic phosphide electrode with different Ni/Co ratios were evaluated by cyclic voltammetry and chronopotentiometry tests.Ni Co P/CC electrode with Ni:Co=1:1exhibited the excellent electrocatalytic performance with the requirement of 107 m V and 1.30V to reach 10 m A cm^-2 for hydrogen evolution reaction?HER?and UOR,respectively.Assembling an urea electrolytic cell using Ni Co P/CC as the anode and cathode can provide a current density of 10 m A cm^-2 at a cell voltage of 1.42 V?160 m V less than that for its urea-free counterpart?as well as remarkable durability over 30 h.Ni S@Ni₃S₂/Ni Mo O₄ with core/shell structure was obtained by loading Ni Mo O₄nanosheets on Ni S@Ni₃S₂ skeleton via a simple two-step strategy with Ni foam as precursors and templates.Based on results of scanning electron microscopy and transmission electron microscopy,the diameter of Ni S@Ni₃S₂ nanorod is about 280 nm.For Ni S@Ni₃S₂/Ni Mo O₄composite,the nanosheets attach to the nanorods and they bond closely together.The HER and UOR activities of Ni S@Ni₃S₂/Ni Mo O₄ were directly characterized by cyclic voltammetry and chronopotentiometry measurements.The results showed that the overpotential and Tafel slope required by Ni S@Ni₃S₂/Ni Mo O₄ to reach the current density of 10 m A cm^-2 under alkaline conditions were 80 m V and 75 m V dec^-1,respectively.In addition,Ni S@Ni₃S₂/Ni Mo O₄ only needs 1.30 V potential to reach 10 m A cm^-2 current density and maintain the electrocatalytic stability up to 40 h in 1.0 mol L^-1 KOH and 0.5 mol L^-1 urea.When act as dual-functional electrocatalysts for urea electrolysis,a lower voltage of 1.40 V is needed to deliver 10 m A cm^-2,which is 200 m V less than the voltage of traditional water splitting system.The electrode can continuously operate for 30 h with negligible rise in operating potential.