| As a new-type of green energy storage system,the supercapacitor has attracted great attention on account of their high power density,long cycle life,wide temperature range,and low cost.Although supercapacitor has many outstanding features,it is necessary to improve the energy density and cycle stability of the supercapacitor.Meanwhile,the low energy density of the supercapacitor has impeded its practical application.According to the formula of the energy density,the energy density is proportional to the product of the specific capacitance and the operation voltage.So,the energy density of the supercapacitor could be increased through improving the specific capacitance by choose pseudocapacitance material with high capacitance or extending the operation voltage by fabricating the asymmetric supercapacitor?ASC?.At present,the negative electrode materials are rarely studied,and the negative electrode materials commonly used are carbon materials.However,the capacitance of traditional carbon materials are generally low,and therefore,researching new negative electrode materials is inevitable.Polyoxometalate-based coordination polymers?POM-based CPs?with negative potential window and high specific capacitance are suitable for the negative electrode materials of supercapacitor.In this work,eleven kinds of novel POM-based CPs have been synthesized by a hydrothermal method.And they have been characterized by a single crystal X-ray diffraction,SEM,etc.The supercapacitor performances of the compounds have been studied.Then,the influences of test system,electrolyte and current collector on the performance of electrode supercapacitors have been explored.Further,a new kind of asymmetric supercapacitor device was fabricated.The main research contents are as follows:1.Compounds 1-5 constructed by distinct Keggin-type structures were designed and prepared via a simple one-pot hydrothermal synthesis method.And they have been characterized by a single crystal X-ray diffraction,powder X-ray diffraction,SEM,etc.Compounds 1-5 are heterogeneous isomorphic,while they exhibit a interdigitated architecture.Further more,the POM-based CPs?compounds 1-5?were explored as supercapacitor electrodes,and their electrochemical properties were investigated in the three-electrode system.The results indicate that compounds 1-5 showed high capacitance and good cycling ability.Among them,compound 4 showed the highest capacitance of 399.2 F g-1 at 100 A g-1.2.Two new POM-based CPs constructed by a high-connected and excellent redox-active Wells-Dawson POM??-K6P2W18O62·15H2O?,namely compounds 6 and 7,were obtained.Subsequently,they have been characterized by a single crystal X-ray diffraction,powder X-ray diffraction,SEM,etc.The two compounds were obtained and further explored them as electrode materials of supercapacitors.The supercapacitor performance of the compound 7 with 3D structure exhibited higher specific capacitance(339.8 F g-1 at 5 A g-1)than compound 6.Also,the cycling stability of compound 7?91.3% capacitance retention after 5 000 circles?is better than the compound 6 with 1D chain structure containing dinuclear copper?II?fragments.Compared with parent ?-K6P2W18O62·15H2O,POM-based CPs of compounds 6 and 7 can effectively overcome the weak cycle stability and low electrical conductivity.These results show that the Dawson-type POM-based CPs are more feasible and superior for supercapacitors electrode materials.3.To research supercapacitor properties of the POM-based CPs with three dimensional structures,via a simple one-pot hydrothermal synthesis method,two new-type POM-based CPs with high connection three-dimensional structures,namely compounds 8 and 9 were obtained and further explored them as electrode materials in supercapacitors.Each of POM clusters in the compounds acts as an octadentate connector,connecting with eight silver-brtmb subunits to generate a three-dimensional structure.When employed compounds 8 and 9 as the electrode materials in supercapacitors,compound 9 exhibited a higher specific capacitance(396.8 F g-1 at 110 A g-1)than many other POM-based composite electrodes.In a three-electrode configuration,with 1 M H2SO4 as the electrolyte,the supercapacitor performance of the compound 9 formed by parent H3PMo12O40 exhibited a higher cycling stability?96.7% capacitance retention after 1 000 circles?than the isomorphic compound 8.What is more,a battery of electrochemical measurements of POM-based CPs electrode materials was carried out in a two-electrode configuration for the first time.These results further confirm that the POM-based CPs materials are promising candidates for applications in electrochemical supercapacitors.4.Two new-type POM-based CPs with large range of negative potential windows,namely compounds 10 and 11 were obtained and further explored them as an electrode materials in supercapacitors.Compound 11 contains a lot of lattice water molecules in channel structure,and therefore it exhibits the highest specific capacitance(at 15 A g-1,890.2 F g-1)among the compounds 1-11 as well as many other typical supercapacitor electrode materials.Using compound 11,an asymmetric supercapacitor device was fabricated.Namely,the compound 11 was acted as a negative electrode and selected an active carbon served as a positive electrode.Notable,the prepared supercapacitor device possesses an ability to light the light emitting diode?LED?,which demonstrates promising prospects in the applications of supercapacitors.The 11-based electrode exhibits higher energy density and power density(23.4 W h kg-1,3 864.4 W kg-1)than many other POM-based composite electrodes,even metal oxide materials.The above results reveal that POM-based CPs may be a new-generation of alternative negative electrode materials for high-performance supercapacitors. |
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