The Synthesis Of Nickel-cobalt Nanocomposites And Their Enhanced Electrocatalytic Properties

In recent years,with the development of industrial level and the improvement of human living standard,the demand of energy is increasing day by day.The excessive consumption of non-renewable energy fossil fuels,which are the main energy sources,makes their reserves less and less.A series of environmental problems caused by the combustion of fossil fuels have gradually restricted the sustainable development of human and society.Therefore,people have realized that the current energy structure urgently needs to change,and have begun to explore and develop renewable energy sources.Hydrogen with the advantages of environment-friendly and high energy density has attracted great attention.Hydrogen can be produced by electrolysis of water.In the process of electrolysis,the process of mutual transformation between water and hydrogen can realize the storage and transportation of energy,which does not release greenhouse gas CO₂ and so on.It is an efficient,carbon-free and promising energy storage method.Water decomposition is composed of two half reactions:cathodic hydrogen evolution reaction?HER?and anodic oxygen evolution reaction?OER?.The HER reaction is hindered by the slow formation of rigid O=O bonds in the OER reaction.Exploring excellent electrocatalysts to improve the efficiency of the water splitting process is the most efficient way to solve the above problem.At present,the most effective catalyst is the noble metal catalyst,the application of which is limited by their high cost and scarcity.Therefore,the development and preparation of electrocatalytic water splitting catalysts with high natural reserves,low price and high catalytic efficiency have gradually become the research focus in recent years.Transition metal materials?including oxides,phosphides,sulfides,selenides,nitrides,etc.?have been applied to electrocatalytic water splitting reactions for their excellent catalytic activity and stability for HER and OER.Among the above metal-based compounds,transition metal phosphides?TMPs?have attracted much attention due to their unique hydrogenase-like mechanisms.Especially,the transition metal phosphides not only present near 100%faraday efficiency in alkaline environment,but also exhibit high reactivity and good stability.Among them,the unique high stability corrosion resistance of nickel phosphide and the high conductivity of cobalt phosphide make them become excellent electrocatalysts.Based on the above considerations,a variety of Ni-Co bimetallic composites with different composition and morphology were designed,prepared and applied as the electrocatalysts in water splitting.The detail content of the dissertation is listed as follows:1.The synthesis of conductive Ni supported nickel-cobalt oxide nanosheets as active electrocatalyst toward water splitting reaction.Several nickel-cobalt oxide composite materials with different metallic Ni/Co ratio were prepared by dealloying method with the advantages of high efficiency,convenience and controllable structure.The electrochemical water splitting properties of the nickel-cobalt composite oxide catalysts were tested systematically.The characterization shows that the surface of the nickel-cobalt oxide composite possesses two-dimensional nanosheet structure.The morphology and quantity of the nanosheets on the surface of the nickel-cobalt oxide varies with different Ni/Co ratio,which leads to various catalytic surface areas.As the result from strong synergistic effect between Ni and Co components,the as-prepared Ni-Co bimetallic oxide composite generally show much better catalytic performance than mono-metal oxide materials.Moreover,the differences in catalytic performance between the catalysts with different Ni/Co ratio reveal the catalytic tendency of nickel and cobalt in two half reaction processes.The simple controllable fabrication and improved catalytic performance of nickel-cobalt oxide composite offer a promising strategy to develop highly efficient single reaction catalysts.2.The synthesis of conductive Ni supported Ni-Co phosphorus oxide composite and their much improved electrocatalytic performance toward water splitting reaction.The nickel-cobalt oxide composite was partially phosphorized to prepare the nickel-cobalt phosphorus oxide catalyst.The electrocatalytic water splitting performance of the Ni-Co phosphorus oxide composite was tested to evaluate the feasibility of the phosphorization strategy.The Ni-Co phosphorus oxide inherited the morphology of the original Ni-Co oxide,indicating that the morphology remained well during the partially phosphorization process.In terms of catalytic properties,the as-obtained Ni-Co phosphorus oxide composite present much improved catalytic performance in comparison with the Ni-Co oxide composite and mono-metallic phosphates,illustrating that the improvement of the catalytic performance via partial phosphorization is achieved successfully.Besides,the bimetallic phosphates shows obvious catalytic tendency in the water splitting reaction,which is similar to the original Ni-Co oxide.3.The synthesis of novel nickel-cobalt phosphides nanowires with commercial Ni mesh as matrix and their outstanding electrocatalytic performance toward water splitting reaction.Nickel-cobalt phosphides composite with commercial Ni mesh as matrix were prepared to achieve the large-scale development of the catalyst.In this section,the Ni meshes after the dealloying was employed as the matrix,which greatly enhance the conductivity of the composite.The dealloying treatment enhances the diameter of a single Ni mesh wire in the Ni mesh,increases the specific surface area,and provides more growth sites for the Ni-Co composite.The rich nickel-cobalt phosphides nanowires were obtained after the facile solvothermal treatment,which provide abundant catalytic active sites for the water splitting.The electrocatalytic tests reveal that the as-obtained nickel-cobalt phosphides nanowires have excellent catalytic activity and catalytic stability.The synergistic effect between Ni,Co and the conductive Ni mesh in the nickel-cobalt phosphides composite is believed as the origin of the superior catalytic performance.