Preparation And Performance Of Nickel-phosphorus Based Catalysts For Electrooxidation Of Urea

Direct urea fuel cell（DUFC）is a device using urea as fuel to generate electricity.However,the slow kinetics of urea electrooxidation at the anode and the catalysts are mostly precious metals limit the commercial application of DUFC.The preparation of efficient,inexpensive,stable anode catalysts is the key to solving these problems.It is found that nickel-phosphorus based materials exhibit high activity and stability in many fields,with simple preparation and low cost.In this paper,the highly conductive nickel foam was used as the substrate,and a variety of nickel-phosphorus based catalysts with high activity were prepared by hydrothermal method,electrodeposition,and phosphating method.A series of electrochemical tests were conducted to investigate catalytic performance for urea electrooxidation.The main work is as follows:Ni Fe-hydrotalcite nanosheets were grown on the nickel foam skeleton by hydrothermal method,then the precursor was treated by phosphating reaction in a tube furnace to obtain Fe₂P-Ni₂P@NF.The nanosheets are grown vertically on the surface of nickel foam and interleave with each other,which is conducive to the transmission of electrolyte and the diffusion of generated gases on the catalyst surface.Because of the three-dimensional structure,the electrochemically active surface area of Fe₂P-Ni₂P@NF is 69.67 cm².The nonmetallic P can modulate the electronic structure of nickel to produce reactive sites,and there is a synergistic effect between Ni₂P and Fe₂P.The Fe₂P-Ni₂P@NF has good catalytic activity,cyclic voltammetry curves show the maximum current density is 844.73 m A·cm^-2 in the electrolyte with 5 mol·L^-1 KOH and 0.33 mol·L^-1 urea.In order to improve the problem of partial flaking of nickel-phosphorus based catalysts prepared by hydrothermal method during the activation process,Ni₂P coated nickel foam skeleton（Ni₂P@NF）was prepared by electrodeposition and phosphating method.Then,the Ni（OH）₂ nanosheets were prepared by the hydrothermal reaction to modify the Ni₂P@NF substrate to obtain the composite catalyst Ni（OH）₂-Ni₂P@NF.By cyclic voltammetry,the optimal concentration of KOH is 5 mol·L^-1 and urea is 0.22 mol·L^-1 for Ni（OH）₂-Ni₂P@NF,in which the current density of 0.6 V is 933.14 m A·cm^-2.The electrochemical performance is higher than that of Ni₂P@NF and Ni（OH）₂@NF loaded with a single catalyst.The Ni₂P@NF prepared by electrodeposition effectively improves the stability of the nickel-phosphorus catalyst,the chronoamperometry curve tested under 0.5 V decreases only 15.6 m A·cm^-2 after1800 s.In addition,the modification of Ni₂P@NF by Ni（OH）₂ nanosheets further improves the kinetic process of catalytic urea electrooxidation.Based on nickel phosphide materials,a nickel-phosphorus-derived catalyst with special morphology was developed to further improve the catalytic activity for urea electrooxidation.The Ni Mo O₄-Ni₂P₄O₁₂@NF nanowires were prepared by hydrothermal and phosphating methods.Optimizing the preparation conditions,when the hydrothermal temperature is 150℃,the time is 6 h,the reactants of Ni and Mo are in the ratio of 1:1,the most uniform nanowire structure and the highest catalytic performance are obtained.By analysis of XPS,the Ni Mo O₄-Ni₂P₄O₁₂@NF catalyst contains Ni³⁺that directly generates Ni OOH in alkaline medium.In addition,the Mo⁶⁺in Mo O₄^2-and P₄O₁₂^4-promote the oxidation of Ni²⁺,which increase the generation of Ni OOH and the catalytic rate of urea electrooxidation.Compared with Ni Mo-O@NF and Ni Mo-precursor@NF,the Ni Mo O₄-Ni₂P₄O₁₂@NF shows excellent electrocatalytic activity.The electrochemical performance of Ni Mo O₄-Ni₂P₄O₁₂@NF catalyst is studied by cyclic voltammetry test,the maximum current density is 1025.52 m A·cm^-2 in 5 mol·L^-1 KOH and 0.33 mol·L^-1 urea.