Synthesis And Electrochemical Properties Of MoFs-derived Transition Metal Phosphides

In order to alleviate the energy shortage and environmental pollution caused by rapid economic development,it is imperative to develop and utilize clean,efficient and sustainable energy storage/conversion technologies.Supercapacitors and electrolytic water devices are the most popular electrochemical energy storage/conversion technologies,which have been widely used in aerospace,transportation,industry and so on.However,the commercial application of these technologies is still limited by high cost,low energy density,undesirable efficiency and poor performance.Electrode materials/electrocatalysts play a key role in affecting energy conversion efficiency of the above equipment.Therefore,designing and constructing functional materials with novel and high-efficient merits is the focus and difficulty of research.In this paper,metal organic framework or composite precursors with CNTs were used as sacrificial templates to prepare transition metal phosphides.In order to improve the electrochemical properties of the materials,we tried three strategies including building composite materials,adjusting morphology and structure and doping ion to synthesize transition metal phosphides with high activity and high stability,and analyzed the effects of the composition,structure and morphology of the materials on the electrochemical properties of the materials.The main contents of this paper were as follows:1.The CNTs were firstly introduced into the cobalt based formic acid framework([CH₃NH₃][Co(HCOO)₃]/CNTs)by liquid precipitation method,and then the CNTs intercalated Co P/Co P₂composite electrocatalyst(Co P/Co P₂/CNTs)was obtained by low temperature gas-solid phosphating process.Meanwhile,Co P and Co P/Co P₂ were prepared by controlling the experimental conditions.The morphology,phase and electrocatalytic properties of as-prepared samples were characterized.The results of electrochemical tests displayed that compared with single component Co P and binary composite Co P/Co P₂,porous Co P/Co P₂/CNTs composite showed excellent bifunctional electrocatalytic activity for both HER and OER.At a current density of10 m A cm^-2,the HER overpotential of Co P/Co P₂/CNTs was 126 m V and the OER over potential of Co P/Co P₂/CNTs was 270 m V,respectively.Additionally,Co P/Co P₂/CNTs exhibited outstanding catalytic stability under acid-base conditions,which indicates that based on the synergistic effect between porous structure and different components,the electrochemical performance of transition metal phosphides has been successfully improved by fabricating composite materials.2.On the basis of the above work,two kinds of[CH₃NH₃][Ni(HCOO)₃]/CNTs composite templates with different morphologies were constructed by changing the type of reaction solvent at room temperature.Based on the inheritance of precursor morphology,flower-like and rod-like Ni₂P/CNTs composite were constructed by further low-temperature phosphating.As electrode materials of supercapacitor,the flower-like Ni₂P/CNTs composite showed superior electrochemical performance.Under the current density of at 1 A g^-1,its specific capacitance was as high as 1480 F g^-1.Additionally,the asymmetric capacitors(Ni₂P/CNTs//AC,ASCs),assembled by flower-like Ni₂P/CNTs and activated carbon AC(YP-80F)presented the energy density of 25.2 Wh kg^-1at the power density of 749.8 W kg^-1 and excellent cycling stability(retained 83.5%of the initial capacity after 5000 cycles).This suggested that the flower-like Ni₂P/CNTs,prepared by adjusting the morphology,displayed the potential practical application value.3.The low stability and capacity rate of Ni based phosphides were further improved by doping Co ion.Firstly,the precursors of[CH₃NH₃][Ni_xCo_1-x(HCOO)₃](x=0,0.33,0.5,0.66,1)was prepared by liquid-phase precipitation at room temperature.Then,on the basis of kirkendall effect,hollow Nix Co1-x microblocks with different components and sizes can be obtained after gas-solid phosphating method.The electrochemical properties of phosphides with different nickel and cobalt contents were evaluated.Among them,Ni_0.66Co_0.33P(Ni Co P-2)displayed the best electrochemical performance.The specific capacitance was 1258 F g^-1 at 1 A g^-1,and the capacity retention rate was 81.3%after 8000 cycles of continuous charging and discharging at 6 A g^-1.In addition,the specific capacitance of the asymmetric supercapacitor(Ni Co P-2//AC,ASCs)was 121.5 F g^-1 at 1 A g^-1,and the energy density can reach 43.2 Wh kg^-1 at corresponding power density of 801.6 W kg^-1,which indicates that doping ions and constructing hollow structure can the efficiently improve whole electrochemical performance and practical application value of the electrode materials...