Electrocatalysts Based On Polyoxometalates: Design,Preparation And Hydrogen Evolution Properties

As a new type of energy,hydrogen energy has the characteristics of high energy density,high combustion calorie value,and non-polluting products,which is considered to be one of the most potential future energy.Electrocatalytic hydrogen evolution is one of the ideal schemes for hydrogen production.Platinum-based catalyst is currently the best electrocatalytic hydrogen evolution catalyst,but its extremely low content and high price limit its large-scale industrial application.It is one of the important strategies for the design and preparation of electrocatalysts to reduce the amount of platinum and improve its catalytic activity.At the same time of developing single atom platinum electrocatalyst,it is a hot research topic to improve the catalytic activity and utilization of platinum-based catalyst by composition and interface engineering.And it is very important to reveal the electrocatalytic mechanism of single atom platinum by using the catalyst model with clear structure.In addition,the design and development of non-noble metal nanocatalysts to completely replace platinum-based catalysts is another development route.Element doping can regulate the electronic structure of active sites and improve the activity of non-noble metal nanocatalysts.It is an important catalyst design strategy in this field.However,the research on the effect of element doping on the performance of catalysts usually requires the consistency of material composition,morphology and crystalline phase,which poses a challenge to the design and synthesis of materials.Based on the above scientific problems,we have carried out the following research work by using the unique composition,structure and properties of polyoxometalates（POM）:1.A series of transition metal-doped molybdenum carbide nano electrocatalysts were designed and prepared by using Anderson-type polyoxometalates containing transition metals as the molecular pre-assembly platform（TM-Mo2C@C,TM=Ni,Co,Fe and Cr）.The series of nano electrocatalysts are prepared by in-situ pyrolysis method.The Anderson-type polyoxometalates（NH₄）_n[TMMo₆O₂₄H₆]·5H₂O(TM=Ni²⁺,Co²⁺,n=4;TM=Fe³⁺,Cr³⁺,n=3)were mixed with dicyandiamide by solid grinding uniformly and carbonized at high temperature Afterwards,a series of TM-Mo2C@C（TM=Ni,Co,Fe and Cr）composite nanoparticles coated with several layers of graphite carbon shells were obtained.All these composite nanoparticles possess similar size,morphology and TM/Mo composition ratio.Electrocatalytic experiments show that TM doping has an important influence on the HER catalytic activity of Mo2C.The order of electrocatalytic activity is as follows:Ni-Mo2C@C>Co-Mo2C@C>Fe-Mo2C@C>Cr-Mo2C@C.Among them,Ni-Mo2C@C has the highest activity,and the overpotential is 72mV at a current density of 10mAcm^-2.The Tafel slope is 65.6mV dec^-1.DFT calculations show that different metal doping can effectively adjust the surface hydrogen binding energy(ΔG_H*)of Mo₂C,showing the following order:Ni-Mo₂C（-0.3eV）>Co-Mo2C（-0.4eV）>Fe-Mo2C（-0.42eV）>Cr-Mo2C（-0.53eV）.These results are consistent with the results of electrocatalytic experiments,and provide a systematic reference for the effect of transition metal doping on the performance of Mo2C.2.Based on the fact that polyoxometalates have clear structure and can be used to simulate the important characteristics of metal oxide surface,we choose two well-defined Pt-containing polyoxometalates,Na₅[H₃Pt^（IV）W₆O₂₄](abbr.Pt W₆O₂₄)and Na₃K₅[Pt₂^（II）(W₅O₁₈)₂]（abbr.Pt2（W5O18）2）are uniformly dispersed in Ketjen black carbon and electrochemical hydrogen evolution experiment was carried out on.The overpotentials of t W6O24/C and Pt2（W5O18）2/C with Pt content of 1 wt%are 22mV and 26mV at 10mAcm^-2,respectively,and their mass activities are 20.175 A mg^-1 and 10.976 A mg^-1 with the overpotential of 77mV at 20°C,respectively,which is better than those of commercial 20%Pt/C.A series of control experiments,in-situ XAS experiments and DFT calculations show that the Pt-O bond in POMs should be the active site of HER.Specifically,Pt is mainly the electron acquisition center,and its coordinated O atom is the proton capture center.During the HER process,when more electrons and protons are injected,the elongated Pt-O bond accelerates the coupling of protons and electrons,resulting in the rapid release of H2 from the Pt-O bond.3.Based on the clear crystal structure and adjustable composition of polyoxometalates,the electrocatalytic hydrogen evolution performance of polyoxometalates was studied as a single atom model of noble metal on the surface of metal oxides.Na5[H3Pt W6O24],K4[H4Pt Mo6O24],Na5[H2Pt V9O28]and[（C3H9）4N]4[Si Pt W11O40H2]as structure models of single atom Pt electrocatalysts were uniformly dispersed on Ketjen black carbon and electrochemical hydrogen evolution experiment was carried out on.The experimental results show that the single atom Pt on different oxide supports exhibit different hydrogen evolution activities:{Pt W6}/C>{Pt Mo6}/C>.The overpotentials of{Pt W6}/C,{Pt Mo6}/C and{Pt V9}/C are 22、139和310mV at the current density of 10mAcm^-2,respectively.The B-V equation,in-situ XAS and DFT results show that the support will affect the ability of single-atom Pt to obtain electrons,thereby affecting the effective potential applied to the active center and catalytic activity.In addition,although{Pt W6}and{Pt Si W11}have the same number of Pt-O bonds and WO6 substrate loading environment,due to the change of single atom Pt coordination environment,the catalytic activity of{Pt W6}/C（22mV）is higher than that of{Pt Si W11}/C（242mV）due to the coordination environment of single-atom Pt.It is because the coordination environment of Pt during the HER reaction is different,which leads to different reaction mechanisms.