Preparation Of Carbide Nanomaterials Derived From Polyoxometalates And Their Applications In Dye-sensitized Solar Cells

Among the existing new energy sources,solar energy has the advantages of being safe,reliable,non-polluting,and not restricted by geographical environment,which has attracted special attention from countries all over the world.In addition to the advantages of high photoelectric conversion efficiency,dye-sensitized solar cells(DSSCs)have relatively simple manufacturing processes,low material prices,and are also economical and environmentally friendly.On the whole,they are a good photoelectric conversion equipment,which aroused great research interest of researchers.The counter electrode(CE)is one of the most important components of DSSCs.Metal platinum(Pt)has been widely used as the CE of DSSCs due to its high conductivity and catalytic activity.However,the cost of Pt is high,the reserves on the earth are limited,and Pt may be dissolved in the electrolyte solution during long-term use.Therefore,people hope to replace platinum with cheaper,richer and higher catalytic activity materials.In DSSCs,good CE materials require good electrical conductivity and excellent catalytic performance.Transition metal carbides Molybdenum carbide(Mo₂C)and tungsten carbide(W_xC)have certain conductivity,and(Mo₂C and W_xC have similar catalytic properties to Pt metals because of their d-band electronic structure similar to Pt.It is a good choice to optimize molybdenum carbide/tungsten materials or composite with other materials.Polyoxometalates(POMs)are a class of polyoxometalate anions with abundant constituent elements,diverse structures and chemical properties.It has been widely used as a molecular platform for the design and preparation of new electrocatalysts.POMs can not only assemble metals and non-metals with catalytic properties,but can also be dispersed in various carbon sources,which can be conducive to uniform carbonization and avoid agglomeration.Using POMs as precursors,the development of Mo/W-based carbides is a good choice.In this paper,a simple high-temperature calcination method was used to prepare CE materials with POMs and dicyandiamide(DCA)as precursors.Using different characterization and electrochemical tests,the application of different transition metal-doped Mo₂C and Co₂P/WC@NC nano materials in DSSCs CE were studied.The specific content is as follows:1.The molybdenum-based carbide electrocatalyst X-Mo₂C@NC doped with different transition metals was prepared and used as the CE of DSSCs.We choose Anderson-type polyoxometalate(NH₄)_n[XMo₆O₂₄H₆]·5H₂O(X=Co²⁺,n=4;X=Fe³⁺,Cr³⁺,n=3)as the precursor,carbonized at high temperature to obtain a series of Co,Fe and Cr doped Mo₂C composite nanoparticles(X-Mo₂C@NC).The diffraction peaks in the XRD test correspond to each crystal plane of Mo₂C(JCPDS no.31-0187).SEM and TEM tests show that X-Mo₂C@NC nanoparticles are uniformly distributed on the nitrogen-doped carbon layer.Electrochemical tests show that the material has good catalytic activity for the reduction of I₃^-.The order of photovoltaic performance is as follows:Co-Mo₂C@NC>Fe-Mo₂C@NC>Cr-Mo₂C@NC>Pt,which indicates that Mo₂C doped with transition metal atoms has a catalytic activity equivalent to that of platinum and has great potential as a platinum-free electrode catalyst.2.The Co₂P/WC@NC electrocatalyst was prepared by a one-step high-temperature calcination method and used as the CE of DSSCs.We use sandwich-type POM(Co₂PW₉)and DCA with different mass ratios as precursors,and perform in-situ metal phosphating and carbonization simultaneously during the calcination process to obtain n-Co₂P/WC@NC(n=5,6,7)nanomaterials.The diffraction peaks in the XRD test correspond to the crystal planes of Co₂P and WC respectively.SEM and TEM proved that Co₂P and WC were uniformly distributed on the nitrogen-doped carbon layer.Using it as the counter electrode of dye-sensitized solar cells,it was found that the photoelectric conversion efficiency of DSSCs based on 6-Co₂P/WC@NC CE was increased by 8.5%compared with that of Pt CE.