Research On Interfacial Ion Adsorption And Electron Transport Mechanism Of Polypyrrole-based Electrochemically Switched Ion Exchange Materials

A large number of toxic heavy metals and corrosive ions discharged from industrial wastewater and domestic sewage give rise to a serious threat to human health and ecological environment.Polypyrrole（PPy）with conjugated structure has attracted extensive attention in the field of water treatment because of its excellent charge transport performance.However,the low adsorption capacity results in the low removal rate of target ions.Therefore,the modification of PPy is the key to its development.Among them,PPy-based two-dimensional electrochemically switched ion exchange materials are widely used in the removal of lead ions(Pb²⁺)and halogen ions（X^-）from wastewater due to stable and high-efficiency adsorption and selective separation performance.Nevertheless,the unclear relationship between microstructure and properties at molecular scale results in low development efficiency and difficult design,which limits its large-scale application.Therefore,revealing the structure-activity relationship of different PPy-based materials from a microscopic point of view will guide the development of new functional materials,which also has a certain application value for the separation and removal of toxic heavy metals or corrosive ions in wastewater.In this dissertation,PPy was modified by inorganic substrate materials.Based on theoretical calculation and experiment,different PPy-based two-dimensional electrochemically switched ion exchange materials were constructed.The mechanism of electrically switched separation of target anions and cations in wastewater was systematically studied.Furthermore,the materials were innovatively applied to the field of energy storage of lithium-sulfur（Li-S）batteries.The interface ion adsorption behavior of four representative materials was deeply analyzed,the microscopic electron transport mechanism was revealed,and a set of design and screening criteria for electrochemically switched ion exchange materials were proposed.It will provide theoretical basis for rapid screening and exploring electrochemically switched ion exchange materials with potential application value.The main results are as follows:（1）Aiming at the mechanism of electrically switched separation of Pb²⁺from wastewater,the montmorillonite（MMT）/PPy composite model has been constructed,and the adsorption selectivity of MMT/PPy for Pb²⁺is explored.The results show that the hydrogen bond interaction between MMT and PPy interface can improve the stability of the structure,and the generated internal electric field results in the electronegative for the surface Si—O,and inducing the adsorption of Pb²⁺.Meanwhile,the quasi-one-dimensional ion migration“highway”along the surface of MMT Si—O drives the adsorption and desorption of Pb²⁺.In addition,the calculation results of work function and migration energy barrier show that MMT/PPy exhibits excellent selectivity and electrochemical sensitivity to Pb²⁺,even in the presence of competitive ions because of its high electrical activity,more electron exchange and kinetic diffusion rate.（2）The electrochemical separation and recovery mechanism of halogen anions in waste water is facing severe challenges owing to the same charge and similar ion radius.Bismuth oxychloride（BiOCl）/PPy composite has been synthesized by chemical oxidation method,and its adsorption selectivity for chloride ion（Cl^-）in water has also been studied.BiOCl/PPy localized Cl^-can realize the rapid capture and release of Cl^-through the memory effect of lattice site,and a diffusion channel for ions by the lattice sites,which is different from the selective separation mechanism of MMT/PPy for Pb²⁺.Secondly,MMT/PPy can always maintain a metallic state,showing excellent electrochemical and kinetic behavior due to the built-in electric field generated by the“information transfer”of Cl^-during the adsorption process.（3）Aiming at the selective separation of halogen bromide ion（Br^-）,bismuth oxybromide（BiOBr）has been combined with PPy to form BiOBr/PPy composite.It is found that the built-in electric field generated by the close contact between BiOBr and PPy improves the structural stability and electrical activity.The reversible redox of Br^-is realized due to the directional charge transfer and end-face dependence between Br and Bi.However,the adsorption capacity of BiOBr/PPy to Br^-decreases with the increase of adsorption contents of Br^-.At the same time,BiOBr/PPy shows outstanding adsorption selectivity to Br^-even in the presence of other competing ions due to the shielding effect of PPy,appropriate adsorption energy and low diffusion barrier.In addition,the adsorption retention efficiency of BiOBr/PPy fluctuates almost around 100%during 10 cycles,and the release rate of Br^-remains above 80%.（4）The application of two-dimensional polypyrrole-based material in Li-S battery has been explored based on the excellent performance of electrochemically switched ion exchange material in ion deintercalation.Theα-zirconium phosphate（α-ZrP）/PPy has been applied as the cathode material of Li-S battery to explore the adsorption of polysulfides（Li PSs）and the migration and diffusion capacity of Li⁺.The results show thatα-ZrP and PPy can provide unique ion and electron transfer channels,respectively.In addition,the synergistic effect of them can inhibit the shuttle effect and volume expansion of Li PSs,and promote the kinetics of redox reaction,therefore realize the rapid capture-dissociation-transformation of Li PSs,and improve the rate performance of the Li-S battery.Moreover,Li PSs can act as a switch regulating electron transfer inα-ZrP and PPy redox reactions,which accelerates the heterogeneous conversion,improves the utilization rate of sulfur,and avoids excessive oxidation of polysulfide.（5）The electron-ion transport relationship has been established,and a set of design and screening criteria for electrochemically switched ion exchange materials is proposed by analyzing and comparing the adsorption selection characteristics and electron transport of four PPy-based materials.It is shown that moderate interlayer spacing（～3?）can provide suitable diffusion channels for electrons and ions.Moderate adsorption strength（-3～-5 e V）,low diffusion energy barrier（0.1～0.8 e V）,high electronic conductivity,electrochemical activity and thermodynamic stability can result in rapid electron/ion diffusion and surface electrochemical reaction.In addition,the criteria has been proofed by investigating the performance of removing target ions in wastewater and anchoring Li PSs in Li-S battery for metal phosphate（MP）/PPy.The results show that MP/PPy exhibits excellent performance for adsorbing Pb²⁺and anchoring sulfur.