The Regulation Of Redox Behaviors Of Polyoxometalates For Electrochemical Hydrogen Production And Li-S Batteries

Electrochemcial energy storage system plays an important role in the full utilization of intermittent renewable energy,among which electrochemcial hydrogen production and secondary battery storage systems are the most widely studied at present.Polyoxometalates(POMs)have structural diversity and controllability,reversible multi-electron redox activity,electron-coulped proton transfer ability,and photo/electro-catalysis.Therefore,POMs are suitable for hydrogen storage and generation,or as catalysts in battery energy storage systems.In this thesis,we mainly investigate the applications of polyoxometalates in electron-coulped proton-buffer-mediated water electrolysis,photocatalytic hydrogen production from methanol,and lithium sulfur batteries.The main results are shown as follows:1.A strategy to modify the POMs at molecule/atom level to tune the number of reversible redox electrons and the redox potentials in aqueous states is developed.By substituting the heteroatom template from P5+,Si4+to Zn2+in the Keggin-type polyoxometalate cluster,H6ZnW12O40 exhibits double number of electrons and protonation in the redox reactions(from two to four).When applied as electron-coulped proton buffer for on-demand hydrogen storage and generation,H6ZnW12O40 shows a high hydrogen evolution efficiency of 95.5%and an electrochemical energy efficiency of 83.3%.Besides,nearly 100%of hydrogen capacity retention could be achieved in the long-term separated time and space water electrolysis.2.A POMs-mediated system for photocatalytic hydrogen production from methanol is constructed.Under the UV irradiation,excitated POMs could oxidize methanol to methanal,and POMs would be reduced and coulped with protons released from methanol synchronously.Then the reduced POMs can be electrochemically oxidized to the original state.By introducing POMs as photocatalysts and electroncoulped proton buffers,photocatalytic oxidation of methanol is decoulped with hydrogen evolution.On the carbon electrode,the initial oxidation potential of reduced POMs(<0 V vs.NHE)is much lower than that of methanol(>2 V vs.NHE),thus significantly reducing the energy consumption of hydrogen production from methanol.3.The ’indirect’ methanol fuel cell based’ on the photocatalytic effect of POMs is proposed and preliminarily explored.The open circuit potential is close to 0.9 V and an excellent power density of 33 mW cm-2 could be achieved.4.The POM-based catalyst for sulfur conversion reactions is proposed.The[Co4(PW9O34)2]10-cluster catalyst can be uniformly dispersed on the surface of rGO to obtain Co4-POM/rGO by layer-by-layer self-assembly synthetic strategy.When applied for sulfur cathode in lithium sulfur battery,Co4-POM/rGO could exihibit enhanced adsorption of polysulfides compared with pristine rGO,inhibiting the "shuttle effect"of polysulfides and achieving high sulfur utilization and cyclic Coulombic efficiency.Besides,due to the excellent ionic conductivity of POMs and good electronic conductivity of rGO,the synergistic effect can significantly boost the charge transfer in the sulfur conversion reaction.As a result,the Co4-POM/rGO/S composite electrode shows excellent cyclic stability and rate performance.After 1000 cycles at 2C,the specific capacity can still be maintained at about 600 mAh g-1 with a capacity decay of 0.033%per cycle.