Design And Synthesis Of Transition Metal(Fe,Co And Ni) Phosphides And Investigation Of Their Electrocatalytic Performance For Water Splitting

The burning of fossil fuels leads to the problems of environmental pollution and energy scarcity.It is urgent to research and develop sustainable and renewable energy to reduce the dependence on fossil fuels.Hydrogen is recognized as a kind of high-efficiency and clean energy because of its advantages of high energy,environmental friendly and renewable.Hydrogen production by electrolysis of water,which has high purity of hydrogen,simple preparation method and carbon-free,has attracted widespread attention.To improve the efficiency of hydrogen production,scientists are devoted to developing efficient catalysts for electrochemical water decomposition.So far,precious metal-based materials are still the best electrocatalysts,but their further large-scale commercial application are limited due to low reserves,high cost and poor stability.Therefore,the design of efficient and low-cost non-precious metal based materials is the main research direction.Transition metal phosphides have a wide sources,adjustable structure and composition,good conductivity,and unique physical and chemical properties.Therefore,the thesis mainly aims at the problems of less active sites and slow electron transfer rate of transition metal phosphates.The synthesis of transition metal phosphate catalysts are used for efficient electrocatalysis of water decomposition by adjusting the composition,morphology and electronic structure of the materials.The specific research contents are as follows:（1）Nickel cobalt phosphide（NiCoP）has excellent conductivity,but its surface active sites are few,which leads to low catalytic activity.We further improve the oxygen evolution performance by introducing FeCo-layered double hydroxide（FeCo LDH）to construct heterojunction.FeCo LDH nanosheets are uniformly grown on NiCoP/NF nanowires（denoted as FeCo LDH@NiCoP/NF）.The catalyst has good conductivity and intrinsic catalytic activity for oxygen evolution reaction（OER）mainly due to the strong electronic coupling effect of the heterogeneous interface formed by NiCoP/NF nanowire core and FeCo LDH nanosheet shell.The catalyst exhibits an outstanding OER activity,at the overpotential of 206 m V and 240 m V,it can reach the current density of 20 mA cm^-2 and 50 mA cm^-2,respectively.Furthermore,we fabricated a two-electrode system for overall water splitting by using FeCo LDH@NiCoP/NF for both anode and cathode,which achieved a potential of 1.48 V at the current density of 10 mA cm^-2.（2）In the first work,the influence of the interaction between FeCo LDH and NiCoP for the electrocatalytic performance was mainly explored,and the content of metal and phosphoru for the catalytic activity was not considered.Therefore,a novel metal-rich Fe₂P and phosphorus-rich CoP₂ heterojunction catalysts（denoted as Fe₂P/CoP₂）was constructed by adjusting the ratio of phosphorus to metal.The catalyst retains the characteristics of a single metal phosphide,and the synergistic effect of the heterostructure interface can effectively regulate the electronic structure,so that Fe₂P/CoP₂ exhibits excellent bifunctional catalytic activity and stability.The as-prepared Fe₂P/CoP₂ exhibited excellent catalytic properties with a HER overpotential of 88 m V at 10 mA cm^-2 and an OER overpotential of 218 m V at 50 mA cm^-2.Furthermore,the Fe₂P/CoP₂ electrode for overall water splitting can reach a current density of 10 mA cm^-2 at a cell voltage of 1.62 V,along with outstanding stability for more than 100 h.The experimental results showed that Fe₂P/CoP₂ heterojunction nanosheet was a highly efficient and stable bifunctional catalyst.（3）In the previous work,the Fe₂P/CoP₂ heterostructure catalyst shown excellent electrocatalytic performance,and we also found that the Fe₂P has OER activity.In order to further improve the OER activity and stability of the Fe₂P catalyst,the FeMoP/NF nanorod composites composed of Fe₂P and FeMoO₄ were successfully prepared by hydrothermal and partial phosphating methods.The synergistic effect between Fe₂P and FeMoO₄ enhances its intrinsic catalytic activity,leading to remarkable electrocatalytic activities and stability.FeMoP/NF demonstrated a superior OER performance with the small overpotential of 206 m V at 10 mA cm^-2,and a remarkable long-term stability of120 h.