Effect Of Hydrated Halide Clusters On Wettability And Electrochemical Properties Of Flexible Membrane Electrode

Supercapacitors have been widely studied due to their potential applications in emerging fields including portable electronic devices and hybrid vehicles.Electrode materials are important components of supercapacitors.The demand for flexible wearable devices is increasing day by day with the research and development of new materials and nanomaterials,which makes the research to develop new kind of flexible membrane electrode materials more extensive.However,for most nanomaterials,due to the pore structure and capillary effect,some micropores cannot be infiltrated by electrolyte,which cannot form an effective wetting.The available surface area of the materials is also very limited and the active sites are less.Therefore,due to the adverse effects of poor wettability,the transport speed of electrolyte ions in electrode materials and the specific surface area utilization of electrode materials directly affect the further development of supercapacitors.In fact,except for the structure of pore size,the effective utilization of the specific surface area largely depends on the wettability of the materials.Hence the wettability property of electrode materials is an important factor affecting performance of supercapacitors,such as specific capacitance,power density,and impedance.The enhancement of surface wettability of materials is expected to be an effective way to improve specific capacitance and comprehensive electrochemical performance.Based on this,the followings have been done:(?)Activated carbon membrane with double layer capacitance and vanadium nitride membrane with pseudocapacitance are prepared by traditional liquid-liquid phase conversion technology and the wettability is studied.A simple and mild method,halogen acid treatment,is used to introduce different halogen ions(Cl~-/Br~-/I~-)on the surface of membranes to improve the wettability.The effect of different halogen ion hydrated clusters of the membrane and its effect on electrochemical properties are also explored.Symmetrical and asymmetric devices are assembled respectively.With the increase of radius,the distance between halogen ions and water molecules gradually increases,the number of water molecules combined with halide ions decreases,and the improvement degree of wettability weakened accordingly.In the case of grafting Cl~-on activated carbon membrane,the water contact angle decreased from 74.5°to 62.4°and the specific capacitance increased from 173 F/g to 242.5 F/g.Similarly,for vanadium nitride membrane grafting Cl~-,the water contact angle decreased from 86.5°to 66.5°and the specific capacitance increased from 170.4 F/g to 218.6 F/g,indicating that the improvement of the wettability of electrode materials is beneficial to the increase of specific capacitance.(?)Surface-initiated electrochemically mediated atom transfer radical polymerization(SI-e ATRP)is used to introducing different length of polymer brushes to regulate the distance between halogen ions and active substances by control the time of polymerization.The effect of the distance between halogen ions and active substances on the electrochemical properties of membrane electrodes are studied.Using halogen ion bromine as the research object,in addition to the previous treatment of hydrobromic acid,the relationship between steric effect and halogen ion hydrated clusters and the effect on electrochemical properties are investigated by introducing bromine-containing organic small molecule(2-Bromoisobutyryl bromide,BIBB)and different length of polymer brushes on the surface of membranes.As a result,the electrochemical performance of membranes electrode is increased when the Br~-is introduced by hydrobromic acid treatment and graft BIBB.Because the hydrated clusters formed by halogen ions exist on the surface of active substances and pores,which is beneficial to the transport of electrolyte ions and improve the electrochemical performance.However,when grafting polymer brushes,the specific capacitance decreases regardless of the polymerization time,because the polymerization speed is fast and the long molecular chains can be reached in a short time.At this time halogen ions exist at the end of polymer chains,so as the molecular chain grows,halogen ions are pushed far away,which makes the halogen hydrated clusters located far from the membrane.And the effect of the clusters is weakened.In addition,due to the long chain of the polymer brushes,the steric effect is obvious,which will be entangled or even blocked in the membrane surface or pore channel.At this point,the effect of steric effect on the membranes is greater than that of the halogen hydrated clusters,and the electrochemical performance of the electrode material becomes worse.