Preparation Of Nickel/Carbon Catalysts For Electro-oxidation Of Urea

Urea,as a milestone molecule in modern chemistry,has made great contributions to the prosperity of industry and agriculture.However,if a large amount of urea wastewater directly flows into the environment without treatment,it will decompose and lead to the release of toxic ammonia,Which can cause serious pollution to groundwater and air.Urea electrocatalytic oxidation（UOR）can be used to treat urea wastewater due to its advantages of distributed,cold start electrochemical device and high compatibility with renewable energy.At the same time,it can be used for hydrogen production,which has double benefits.However,UOR involves complex 6e^-transfer,resulting in slow reaction kinetics and high overpotential,which seriously restricts the application of UOR in production and life.Therefore,it is necessary to develop high-performance UOR catalysts.In this paper,Ni-based catalysts were taken as the research objects to study their performance in electrocatalytic oxidation of urea by synthesizing catalysts with different morphologies,structures and surface properties.The details are as follows:1.Preparation of Carbon-coated Ni-Ni S catalyst for electro-oxidation performance of ureaNi-Ni S@C catalyst with spherical structure was synthesized by hydrothermal-high temperature calcination.It is found that the catalyst with this structure has good catalystic performance of UOR.For the catalyst with appropriate carbon-nickel ratio,the current density of its catalytic UOR is as high as 743 m A·cm^-2·mg^-1（0.6 V vs.Ag/Ag Cl）and the stability is good.The existence of carbon shell is very important for the performance of catalyst.Although the carbon shell has no catalytic activity in UOR,it can effectively increase the conductivity of the catalyst,regulate the electronic structure of the catalytic active center and prevent the agglomeration of the catalytic active center in the UOR process and effectively enrich the reactants in the catalytic reaction process.2.Preparation of Ni S-Fe S₂@C catalyst for electro-oxidation performance of ureaNi S-Fe S₂@C composite catalyst with spherical structure was successfully synthesized by hydrothermal and high-temperature calcination.Compared with Ni S@C and Fe S₂@C,Ni S-Fe S₂@C has better catalytic performance of UOR.The introduction of Fe can effectively increase the content of Ni³⁺on the catalyst surface,and improve the electrocatalytic oxidation capacity of active Ni species to urea molecules.When the Ni/Fe mole ratio is 7:1,The current density of UOR is up to 1073 m A·cm^-2·mg^-1（0.6 V vs.Ag/Ag Cl）.It is much higher than the oxidation current density of Ni S@C(404 m A·cm^-2·mg^-1)and Fe S₂@C(17 m A·cm^-2·mg^-1)catalysts at the same oxidation voltage.3.Preparation of Carbon doped X-Ni-O catalyst for electro-oxidation performance of ureaFour kinds of carbon doped Ca-Ni-O,carbon doped Mg-Ni-O,carbon doped Ce-Ni-O and carbon doped La-Ni-O catalysts were constructed by hydrothermal and high-temperature calcination reaction,respectively.It was found that the type of cations in the catalyst had a great influence on the morphology and structure of the nickel-based catalyst.Among them,carbon-doped Mg-Ni-O,carbon-doped Ce-Ni-O and carbon-doped La-Ni-O have flake structure,while carbon-doped Ca-Ni-O has honeycomb structure.The carbon doped Ca-Ni-O catalyst with honeycomb structure has larger surface area and can expose more active sites.Meanwhile,its special honeycomb structure can accelerate the mass transfer rate in the UOR reaction process.Therefore,compared with the other three catalysts,the carbon doped Ca-Ni-O catalyst has better UOR catalytic performance.The current density of Ca-Ni-O catalyst can reach 645 m A·cm·^-2·mg^-1（0.6 V vs.Ag/Ag Cl）by optimizing the doping amount of carbon in Ca-Ni-O catalyst,and the catalyst has good stability.