Design And Syntheses Of Polyoxometalate-based Composites For Electrochemical Energy Storage System

Polyoxometalates?POMs?,which are considered as electron reservoirs or sponges,are promising electrode materials for energy storage and conversion applications due to their complex multi-electron transfer properties.However,POMs suffer from low electrical conductivity,partial degradation in electrolyte and poor machinability,which obstruct their application as electrode materials.POM-based inorganic–organic composite materials effectively combine the synergistic effect between POMs and organic substrates and achieve ideal electrochemical performances.In this paper,We design and synthesize a series of POM-based composites for high-performance supercapacitors and Li/Na ion batteries: Keggin-type Mo/W and Mo/V mixed-addenda POMs and isopolyacid are utilized as POM moieties;cationized graphene?RGO?,polydopamine?PDA?and acetylene black?AB?are used as conductive substrates.1.We design a series of new composites through an in-situ hydrothermal method,with dual addendas of POMs,H₃PMo_12-xW_xO₄₀?x=1-6??denoted as PMoW?,and H₄PMo₁₁VO₄₀,H₅PMo₁₀V₂O₄₀,H₆PMo₉V₃O₄₀?denoted as PMoV?as electroactive materials and poly?dimethyl-diallyl?ammonium?PDDA?functionalized RGO as a conductive substrate.The resulting POM-PDDA-RGO composites exhibit a homogeneous honeycomb-like porous structure leading to fast ion transport and short ion diffusion pathways.The positively charged PDDA can change the surface of RGO from negatively charged to positively charged to effectively connect PMoW molecules.Electrochemical analyses on a two-electrode symmetric system reveal that the PMoW-PDDA-RGO composite achieves remarkable rate capability(140 F g^-1 even at a high current density of 10 A g^-1)and good long-term stability(remaining 94.6% of its initial capacitance after 1700 circles at a high discharge speed of 5 A g^-1).And the supercapacitor exhibits an improved energy density of 6.15 W h kg^-1 at a power density of 250 W kg^-1.2.We select bio mimetically synthetic polymers,polydopamine?PDA?as a conductive substrate material,and first report the fabrication of a PMoV/PDA composite(herein,PMoV=H₄PMo₁₁VO₄₀,H₅PMo₁₀V₂O₄₀ and H₆PMo₉V₃O₄₀)via in situ polymerization of dopamine?DA?without additional buffer solution.The POM-assisted one-pot hydrothermal synthesis strategy may be widely exploited for preparation of other POM-based PDA composites.The PMoV/PDA electrode can be an advanced binder-free anode materials for lithium ion batteries,owing to the adhesive ability and unique electron transfer properties of PDA microspheres and the multi-step redox performances of PMoV.The loading of the active material in a work electrode is 80 wt%,which is much higher than most of POM-based carbon materials.For instance,the PMo₁₀V₂/PDA achieves a high discharge specific capacity of 1654 mA h g^-1 at a current density of 20 mA g^-1 with the initial CE of 66.5%,good stable cyclability(no capacity decay after 65 cycles at a current density of 100 mA g^-1 and ⁹3% capacity remained even after 300 cycles at a high current density of 1000 mA g^-1)and remarkable rate capability(559.6 mA h g^-1 at a high current density of 2000 mA g^-1).Our work offers a general approach to prepare other POM-based PDA composite materials,which can be widely used in other application fields such as catalysis,sensors,biomedicine,solar cells and hydrogen generation.3.The isopolyacid Na₆Mo₇O₂₄ is hybridized with acetylene black?AB?by a one-step ball-milling method.The obtained Na₆Mo₇O₂₄/AB nanocomposite is used as anode materials in sodium ion batteries and shows improved performance compared with pristine Na₆Mo₇O₂₄.Thus,the isopolyacid are expanded for applications in energy storage and conversion.