Synthesis And Supercapacitor Properties Of Polyoxometalate Hybrid Based On {P₂W₁₈}

Supercapacitors play an increasingly important role in the rapid development of modern electronic industry as an energy storage device due to their with large specific capacitance,long cycle life,high power density,fast charge-discharge rate and super long cycle stability.According to different charge storage mechanisms,supercapacitors can be divided into electric double-layer capacitors and pseudocapacitors.The electrode materials are important factors affecting the performance for supercapacitors.Carbon materials are usually used in double-layer capacitors,while metal oxides have been used as electrode materials for pseudocapacitors.Polyoxometalates（POMs）are a large class of metal oxygen clusters formed by different metals through oxygen connections.Due to their superior multi-electron redox properties,structural diversity and low environmental effects,which are well suited to achieve high capacity supercapacitor for application.However,the low conductivity of POMs reduces speed as electrode materials,while the high solubility of POMs limits their application in water supercapacitors.In this work,eight polyoxometalate hybrids with Dawson phosphotungstic polyanions({P₂W₁₈})as building blocks were synthesized repeatedly by hydrothermal method for using the precursor.At the same time,a hybrid based on{P₂W₁₈}zeolite imidazole-like framework materials was synthesized by simple solution method.By elemental composition analysis and TG analysis,the chemical formula is determined to be:[K₂{Cu₂（biim）₂}₂(P₂W₁₈O₆₂)]（1）（biim=biimidazole）,[{Cu（1,4-bimb）₂}{Cu（1,3-bimp）（1,4-bimb）}₂(P₂W₁₈O₆₂)]（2）,(1,4-bimb=1,4-bis（imidazole-l-yl）butane;1,3-bimp=1,3-bis（imidazole-l-yl）propane[{Cu_0.5（Hpyr）}₂{Cu_0.5（pyr）}₄{Cu（pyr）₂}₂{(P₂W₁₈O₆₂)₂}]（3）（pyr=pyrazole）,[Cu（4,4’-Hbipy）]₂[P₂W₁₈O₆₂]（4）（4,4’-bipy=4,4’-bipyridine）,[K₂{Ag₂（biim）₂}₂(P₂W₁₈O₆₂)]（5）,（4,4’-H₂bipy）[{Ag（4,4’-bipy）₂}4(P₂W₁₈O₆₂)]（6）,[Ag（pyr）₂]₄[{Ag（pyr）₂}₂(P₂W₁₈O₆₂)]（7）,（Hpz）[Ag₇（pz）₆（OH）₂][P₂W₁₈O₆₂]·2H₂O（pz=pyrazine）（8）.The structure of compound was characterized.In order to explore electrochemical performance,the synthetic compounds were measured supercapacitor performance as electrode materials to by cyclic voltammetry,galvanostatic charge-discharge,impedance and so on.The results show that the specific capacitances of the compounds are 1070.0、496.6、695.5、953.6、2156.9、1521.0、744.8、1369.1F g^-11 at the current density of 2A g-1,respectively.After cycling,the specific capacitance retention is 89.8,93.4,90.2,92.9,94.5,92.1,91.3 and 92.1%respectively.At the same time,in order to study the performance of symmetrical supercapacitor,carbon cloth was used as collector fluid and carried out in three electrode system.After circulation,the specific capacitance retention rates are 91.2,93.0,91.3,97.3,96,92.6,93.0 and 90.6%,respectively.The energy density of the symmetrical electrode was 0.71、0.105、0.031、0.145、0.585、0.521、0.265、4.49W h kg^-1,and the power density was 116.17、29.1、9.3、6.06、37.6、23.16、13.07、598.6W kg^-1,respectively.Theoretical analysis shows that the synthesized compounds all exhibit pseudocapacitance behavior,and their capacitance is caused by the non-diffusion control charge storage mechanism.Compound 5 has the largest specific capacitance,probably due to the existence of a three-dimensional structure with large pore-channel results.This pore-channel structure was instrumental in the transport of electrolyte solution ions and electrons.Moreover,the unique potassium wrapped in Dawson type polyoxometalates which make the compound more stable as electrode materials.Compound 1 isomorphic to compound 5,but the electrochemical performance of compound 5 is better,due to the fact that compound 5 is silver ion coordination.And it has higher conductivity than copper ion coordination in compound 1.Meanwhile,{P2W18}@ZIF-67 hybrids synthesized by solution method and the mass of specific capacitance is 588.9 F g-1 at current density of 2 A g-1.After 1000cycles,the capacitance retention was 93.4%.The carbon cloth was used as the collecting fluid to test the composition for the water system supercapacitor.The mass specific capacitance was 86.1 F g-1 at 2 A g-1,and power density was 583.496 W kg-1 for energy density was 4.366 Wh kg^-1.