Construction And Electrochemical Properties Of Transition Metal Modified Keggin Type Arsenic Molybdate

Keggin-type arsenomolybdates,an important component of classical polyacids,are 1:12-type poly acid structures formed with As as the heteroatom and Mo as the coordination atom.Keggin-type polyacids are often used as building blocks for organicinorganic hybrids because of their low spatial resistance,high coordination ability and abundance of oxygen atoms on the surface.As atoms in arsenomolybdates can exhibit either+3 or+5 valence,and Mo atoms often exhibit+5 and+6 valence,showing good mixed-valence delocalisation,making them ideal electrochemical energy storage materials.However,due to the toxicity of arsenic and the difficulty of precursors precipitating from solution and the poor stability of arsenomolybdenum series compounds,the synthesis of arsenomolybdates is difficult,thus limiting the development and properties of this class of compounds.In this paper,a series of vanadium-substituted cap arsenomolybdate hybrids were designed and synthesised to investigate their electrochemical catalytic and energy storage properties by varying the material ratio and pH of the mixed solution in a one-step hydrothermal method,taking advantage of the high coordination ability of the rigid ligands and the coordination diversity of the flexible ligands,and introducing suitable transition metal units.The following findings have been summarised by crystallographic testing,analysis and testing of their electrochemical properties.The molecular formulae of compounds 1-8 were determined by means of elemental analysis,infrared absorption spectroscopy,thermal stability tests and structural characterisation analysis such as single crystal X-ray diffraction,as follows:(Hbipy)2{As?2AsMo?Mo?7V4O40}·5H2O(1)[{Co(phen)2(H2O)}2{Co2AsMo?Mo?7V4O40}]·2H2O(2)(bipy)0.5[{Ag2(bipy)4} {(VO)As?AsMo8V4O40}]H2O(3){Cu?(imi)2}5{Cu?(VO)AsMo?Mo?7V4O40}(4)(Hbpa)4{(VO)2AsMo?Mo?7V4O40}·8H2O(5)(Hbpp)2{As?2As?Mo?Mo?7V4O40} 2H2O(6)(Hbib)2(biyb)3{As?As?Mo8V?4O40}2·2H2O(7){Cu?3btp2} {Cu?2AsMo10V2O40}·4H2O(8)(bipy=4,4'-Bipyridyl,phen=1,10-Phenanthroline,imi=Imidazole,bpa=Bis(4pyridyl)amine,bpp=1,3-Di(4-pyridyl)propane,bipy=2,2-bipy,bib=1,4-Bis(Imidazol-1yl)butane,biyb=1,4-Bis(imidazol-l-ylmethyl)benzene,btp=1,3-bis(1,2,4-triazol-4yl)propane)The crystal structure analysis revealed that all eight new compounds are new arsenomolybdates formed by the substitution of some molybdenum atoms by vanadium,and all have a double-capped structure.Compound 1 is a supramolecular network consisting of a 4,4'-bipyridine ligand attached to a polyacid anion by supramolecular interaction;Compound 2 is a supramolecular network consisting of a {Co(phen)2(H2O)}complex unit modified on the surface of the poly acid anion while being constructed with adjacent structural units by supramolecular interaction;compounds 3 and 4 are new compounds with one-dimensional structures formed by the linkage of {Ag2(bipy)4} and{CuI(imi)2} complex units,respectively.Compounds 3 and 4 are novel compounds with one-dimensional structures formed by the action of {Ag2(bipy)4} and {Cu?(imi)2}complex units respectively.Compounds 5 and 6 are two compounds with 3D network structures formed by protonation of two flexible organic ligands,Hbpa and Hbpp,respectively,as supramolecular linker units;compound 7 is a 3D highly connected supramolecular network expanded by the combined action of two different flexible organic ligands,Hbib and biyb;Compound 8 is a novel three-dimensional mesh compound formed by linking spatially adjacent structural units through {Cu?3btp2}complex units.Eight new vanadium-substituted arsenomolybdate compounds were mixed with acetylene black and ground and sonicated with ethanol to produce a glassy carbon electrode.The electrochemical properties of the eight compounds were analysed by an electrochemical workstation,including cyclic voltammetry,constant current charge and discharge,impedance,cyclic stability and electrocatalytic redox tests.The results show that the covalently linked three-dimensional compound 8 has higher bifunctional catalytic effect,specific capacitance and cyclic stability than the rest of the compounds;among the supramolecular compounds,those with a high proportion of organic ligands are higher than the simple compounds,while the performance of the compounds modified by transition metals is higher than that of the same type of unmodified arsenomolybdate compounds.The main reason for this is that the transition metal modification with the formation of the cap structure leads to the formation of more redox centres and the large number of H-bonding interactions in the structure facilitates the promotion of proton/electron transfer.In addition,all eight examples of compounds have a higher specific capacitance than the normal Keggin compounds,which may be due to the synergistic interaction between the substituted V and the ligand atom Mo in the backbone as well as the cap.The electrocatalytic test results show that compounds 1-8 can all catalyse the oxidation of AA and the reduction of H2O2,making them potential bifunctional catalytic materials.