Preparation And Electrocatalytic Performance Of Transition Metal Phosphides And Its Composites

Because of the high energy density and heat of combustion,hydrogen is regarded as the most promising green energy source in the 21st century,which is expected to become the main pillar of future energy.At present,the main methods for preparing hydrogen are electrolysis of water,coal gasification,steam methane reforming,etc.Among them,water decomposition,which is a green route for hydrogen production,has been extensively studied.During the process of water decomposition,efficient and inexpensive catalysts are the key for mass production.It is well known that platinum-based materials are the most efficient hydrogen-producing catalysts,while ruthenium/iridium-based materials are the most efficient oxygen evolution catalysts.Neverthsless,the scarcity and high cost of noble metals significantly restrict their widespread application.Accordingly,intensive efforts have been devoted to developing alternative electrocatalysts.Due to its abundant reserves,low cost and unique electronic structure,transition metal materials have shown good application potential in catalysis.Taking this into account,in this work,we take transition metal phosphide as the research object,and improve the electrocatalytic performance by micro-nanostructure regulation,doping and recombination and so on.The detailed works are summarized as follows:(1)The nickel-containing compound of the 3D flower-like structure was prepared by hydrothermal method,and the controlled synthesis of the flower-like structure was realized by regulating the reaction time and the kind of the reactant.Nickel phosphide was obtained by low-temperature phosphate treatment.During the heat treatment process,the 3D flower-like structure was completely retained,and the smooth sheet structure exhibits a porous structure after phosphate treatment,which increases the contact area between the catalyst and the electrolyte,provides more reactive sites,and facilitates proton transport and electron transport in the catalytic process and finally improves the electrocatalytic performance.The results show that,the porous 3D flower-like structure nickel phosphide catalyst exhibits enhanced electrocatalytic activity in both acidic and alkaline electrolytes.(2)By improving the traditional Hummers method,ultrathin sheet-graphene oxide was prepared and used as the substrate material.The loading of bimetallic cobalt ions and rhodium ions was successfully achieved by stirring at room temperature.Then,the final composites were obtained by heatment treatment,during which the loading cobalt was phosphate and reduced graphene oxide was obtained.The influence mechanism of the loading on the conductivity and catalytic performance was revealed by studying the effects of different loadings on the morphology of the material.The optimal performance of the compounds was obtained by adjusting the compositions.In this work,reduced graphene oxide with good electrical conductivity and large surface area was used as the substrate material,which is beneficial to the dispersion of nanoparticles,and enlarge the contract area between the active sites and the electrolyte.The overpotential of 72 mV is needed to achieve the current density of 10 mA cm-2 for rGO/CoP-Rh-2.5,which is much lower than that of rGO/CoP(599 mV)and rGO/Rh(205 mV)to achieve the same current density,implying the synergictic effect between the metal Rh introduced in the composite and CoP.(3)Polyhedral copper particles were grown on the surface of nickel foam by in-situ reduction method(Cu/Ni foam).Then,the polyhedral copper particles were successfully converted into copper hydroxide nanowire by surface oxidation,(Cu(OH)2/Ni foam).Finally,Cu@Cu3P/Ni foam composites were obtained by controlling the amount of sodium hypohydrophosphite monohydrate during the phosphating process.In the unique hierarchical architecture of Cu@Cu3P/Ni foam,the Ni foam with 3D network structure can enlarge the contract between the active sites and the electrolyte.Cu@Cu3P grows uniformly on the surface of Ni foam,and the introduction of Cu in the comosits is beneficial to increase the electron transport,improving the electrocatalytic performance.(4)Different proportions of cobalt were doped into the nickel phosphide by coprecipitation and low-temperature treatment method,and the influence of doping on the product phase was studied.The atomic redius of cobalt and nickel are relatively close,resulting in the position of nickel in the crystal lattice replaced by the doped cobalt.The replacement will change the electronic structure of the composite and increase the active sites,improving the electrocatalytic activity.The performance optimization of doping is achieved by comparing the electrocatalytic properties of the bimetallic phosphide obtained by different doping of cobalt,which is of guiding sifnificance for the preparation of bimetal catalytic materials with high performance.