Study On Derivatization And Electrochemical Properties Of Keggin Molybdates With Arsenic As Heteroatoms

The continuous growth of population has led to a significant consumption of non renewable energy,and the existing technological level cannot achieve a high resource utilization rate.At the same time,it may also bring about continuous deterioration of the ecological environment.The development of new energy has become particularly important.Supercapacitors stand out among most new energy storage devices because of their higher power density,better cycling efficiency and lower cost.As an important branch of classical polyacids,Keggin type arsenopolybdates often exhibit mixed valence of Mo atoms,that is,some Mo ^VIis reduced to Mo ^V,exhibiting good electron delocalization and strong reversible redox ability,making them good pseudo capacitive materials.However,due to the instability of precursors and intermediates in the one-step synthesis process of arsenic molybdate,the synthesis of their compounds is difficult,limiting the research on the properties of these compounds.In this paper,based on the different coordination abilities of different metals and the different coordination advantages of rigid ligands,seven Keggin type arsenic molybdate derivatives that have not been reported in the literature were induced and synthesized by adjusting the types of ligands and transition metals as well as p H in the reaction system through a one-step hydrothermal method.The synthesis results show that p H 2.5 is the optimal condition for the precipitation of tetravanadium substitution types,and the substitution activity of vanadium greatly decreases when p H reaches 3.The molecular formulas of compounds 1-7 were determined by elemental analysis,infrared absorption spectroscopy,thermal stability,and single crystal X-ray diffraction as follows:（H₂bpa）₂（H₂bpd）{（VO）₂As Mo₃^ⅤMo₅^ⅥV₄O₄₀}（1）（Hbpa）₃（bpa）{（VO）₂As Mo₈V₄O₄₀}·2H₂O（2）（H₂bpa）₄{（Cu^ⅡO）₂As Mo^ⅤMo₇^ⅥV₄O₄₀}·H₂O（3）{Zn^Ⅱ（diim）₂（pyz）}₂{Zn^Ⅱ（diim）₃}₂{（VO）₂As Mo₅^ⅤMo₃^ⅥV₄O₄₀}·4H₂O（4）[{Cu^Ⅱ（phz）₂}₂{Cu^Ⅱ（bpa）（phz）}₂{（Cu^ⅡO）₂As Mo^ⅤMo₇^ⅥV₄O₄₀}]（5）{Cu^Ⅱ（bimi）₂}₄[{Cu^Ⅰ（bimi）₂}₂{（VO）₂As Mo₇^ⅤMo^ⅥV₄O₄₀}]·2H₂O（6）[{Cu^Ⅰ（bipy）₂}₂{As₂^ⅢAs^ⅤMo₅^ⅤMo₇^ⅥO₄₀}]（7）（bpa=Bis（4-pyridyl）amine,bpd=Bis（3,4-pyridyl）diamine,diim=Diimidazole,pyz=Pyrazole,phz=Phenazine,bimi=Benzimidazole,bipy=2,2’-Bipyridyl）By analyzing the crystal structure,we found that the compounds were all double cap Keggin type arsenopolybdates,and 1-6 were tetravanadium substituted.Compounds1-3 are supramolecular networks modified by organic ligands,and 1 and 2 are double vanadium cap tetravanadium substitution types induced by two and one rigid ligand,respectively;Compound 3 is a single electron reduction type modified with double copper caps;Compound 4 consists of two metal complexes of mixed ligand type{Zn^Ⅱ（diim）₂（pyz）}and{Zn^Ⅱ（diim）₃}as counter ions;Compound 5 is a four supported structure modified by two metal complexes,{Cu^Ⅱ（phz）₂}and mixed ligand{Cu^Ⅱ（bpa）（phz）};Compound 6 is a double supported structure modified by a{Cu（bimi）₂}metal complex;Compound 7 is a two-dimensional pore structure formed by alternating linking{Cu^Ⅰ（bipy）₂}complexes with Keggin clusters.The spatial structures of compounds 4-7 are characterized by abundant hydrogen bonds,π-πstacking,hydrogen bonds and pore structures,and covalent bonding to form pore structures.Using compounds 1-7 as active substances,tests were conducted using glassy carbon electrodes and carbon paper electrodes,respectively.Cyclic voltammetry,constant current charge and discharge,AC impedance impedance,and cyclic stability tests were conducted.The results show that the specific capacitance,conductivity,and cyclic stability of compounds 4-7 modified with metal complexes are superior to those of compounds 1-3 that are organic ligand antagonists,due to the introduction of metal complexes increasing the redox center of the Faraday process and promoting ion electron transfer.The main reason for the higher specific capacitance and cycle stability of compounds 6 and 7 may be that their unique pore structure provides necessary transport channels for electron transport,promoting electron transfer.In addition,the study of electron transfer mechanism shows that both 1-7 are electrochemical storage processes with different proportions of diffusion control and pseudocapacitance control.Compound 6,which has the most excellent capacitive performance,was selected to evaluate its feasibility in practical applications.A water-based asymmetric supercapacitor device（ASC）was assembled using 6-CPE as the electrode negative electrode for cyclic voltammetry,constant current charge and discharge,AC impedance impedance,and cyclic stability tests.The results show that the device has a large specific capacitance(97.5 F·g^-1at a current density of 3 A·g^-1)and good cycle stability,and exhibits excellent energy density(26.54 Wh·kg^-1)and good power density(2123W·kg^-1),further proving that the compound is a good pseudocapacitance material.