Synthesis,Structure And Electrochemical Properties Of Nickel-Based Nanomaterials As High-Performance Electrocatalysts For Oxygen Evolution Reaction

The global energy crisis and the aggravation of environmental pollution have made the development of sustainable green energy highly valued.Therefore,the development of new energy sources such as clean and renewable solar energy,tidal energy and hydrogen energy is imminent.Hydrogen,as an energy carrier in the hydrogen economy era,is considered as a promising alternative to fossil fuels.There are many ways to produce hydrogen in industry.The common ones include hydrogen production from fossil fuels,hydrogen production from heavy oil and natural gas reforming,and hydrogen production from water splitting.Among them,electrolytic water splitting is one of the most effective methods for preparing high purity hydrogen to replace heat,wind and solar energy for energy conversion and storage.However,the efficiency of water splitting is largely impeded by the oxygen evolution reaction(OER)with a sluggish reaction kinetics due to more complex four-electron transfer pathway.Although noble-based oxides are effective and considered as the most active OER electrocatalysts,their scarcity and high cost restrict the further wide spread practical production and applications.In recent years,transition metal-based materials have received considerable attention due to their abundant reserves,cost-effectiveness and unfilled valence layer d orbital and become one of the most promising candidates for OER catalysts.However,transition metal-based catalysts still have problems such as poor conductivity,low intrinsic activity and unsatisfied stability.In this thesis,a series of low-cost and high-performance nickel-based nanomaterials with specific composition and unique microscopic morphology have been developed as advanced OER catalysts by the effective strategies of substrate selection,composition optimization,micro-morphology regulation,interface engineering and heteroatom doping.The main contents and results of the thesis are as follows:1.Ni Fe LDH/Co(OH)₂ trimetal hydroxide was first grown on the conductive carbon cloth by a two-step hydrothermal method,and then the three-dimensional nano-arrays nickel-based catalyst(Ni Fe P/Co P/CC)with plate-wire hierarchical structure was synthesized by phosphating Ni Fe LDH/Co(OH)₂ at a relatively low temperature.The unique morphology of the material provides abundant reactive sites and favorable channels for electron transmission to facilitate catalytic performance.In addition,synergistic effect between metals improves charge transfer efficiency of the material,while the self-supported conductive network strengthens the structural stability of the material.As a result,the material presents excellent electrocatalytic properties:at the current density of 20 mA cm^-2,the overpotential for OER is 270 m V,the Tafel slope is51 m V dec^-1 and the turnover frequency is 0.466 molO₂ s^-1;in addition,the initial activity can be maintained for at least 50 h at the current density of 20 mA cm^-2.2.Self-supported and hollow porous MoO₃/NiF₂ heterogeneous nanowires with excellent OER properties have been designed via an advanced top-down strategy.Fluorinating the precursor nanowires of NiMoO₄ successfully activates the Ni sites in the material and constructs porous MoO₃/Ni F₂ heterogeneous nanowires.The hollow nanowires with abundant porous structure provide large specific surface area,while the heterogeneous interfaces cause rich electro-active sites facilitate the electron transfer,which contribute to the significant improvement in the OER performance of the material in alkaline solution.Consequently,the catalysts of hollow porous MoO₃/Ni F₂ nanowires grown on Ni foam deliver a current density of 100 mA cm^-2 with the overpotential of 272 m V in OER and exhibit a good durability of 50 h at high current density of 100 mA cm^-2.3.N/Fe co-doped three-dimensional tremella-like nickel phosphides micro-flowers(N/Fe-Ni₂P/Ni₁₂P₅)OER electrocatalyst has been successfully structured via a facile combination of hydrothermal method to achieve Fe-cation doping and low temperature phosphating method to accomplish phosphating and nitrogen doping synchronously.The N-anion and Fe-cation co-doping synergistically modulate the electronic structure,surface states and microtopography of the sample.The electrochemical surface area and electronic conductivity of the N/Fe-Ni₂P/Ni₁₂P₅ are thus greatly enhanced.In 1 M KOH solution,the as-prepared N/Fe-Ni₂P/Ni₁₂P₅ exhibits splendid OER performances with a low overpotential of 217 m V at the current density of 10 mA cm^-2,as well as a ultra-long durability for over 120 h at high current density of 500 and 1000 mA cm^-2.