| Capacitive deionization(CDI)technology is a promising electroadsorption desalination technology operated under low applied voltage(generally lower than the electrolysis voltage of water of 1.23V).The salts in the water can be removed through the charge and discharge cycles of the effluents.CDI is considered to be great potential because of its merits such as its low energy consumption,simple electrode regeneration,no secondary pollution and environment-friendliness.The electrode material is the heart of the CDI cell.Usually,the ideal electrode material should have a high specific surface area,good conductivity and cycle stability and so on.The activated carbon,due to its abundant pores,high specific surface area,good cycle stability,and a variety of sources,is considered to the most easy to achieve its industrial production.However,in practical desalination applications,the desalination capacity of activated carbon is pretty low due to the poor conductivity,narrow potential window and low specific capacitance.Polyaniline(PANI)is a typical conducting polymer material with conjugated electronic structure.It has the advantages of simple preparation,high environmental stability and unique doping and doping properties so that it has been widely applied as the electrode material for capacitor and battery application.It has been reported that the electron conductivity of PANI nanotubes is orders of magnitude higher than that of conventional PANI polymers,but so far,there is rare report about the PANI nanotube-activated carbon composite electrode materials for the CDI capacitive desalination research.Nano tubular materials possess three-dimensional structure,large specific surface area and rich active sites.Electrospinning technique possesses the characteristics of low cost,simple equipment and convenient operation.The electrospun fibres with the size of nanometer to micrometer can be industrialized with large-scale productivity.The obtained micro/nano fiber has high specific surface area,controllable diameter and porosity,is very potential for sensing,photocatalysis and battery energy storage application.High yield nanotube materials can be obtained by using electrospun fibers as templates.In this paper,the polyaniline nanotube is fabricated by chemical polymerization of aniline on the electrospun sulfonated polystyrene(PS)nanofiber,followed by the removal of PS.The inner diameter of the nanotube depends on the diameter of PS template.With electroactive Pani T as the additive,the Pani T-AC composite electrode is obtained by simply mixing Pani T with activated carbon(AC)granules in the presence of Teflon emulsion.The inverted hybrid-capacitive desalination(i-HCDI)configuration was constructed with PANI-AC as anode and AC as cathode.The extremely high desalination performance has been achieved on the i-HCDI,owing to the contribution of the electrolyte doping/dedoping of Pani during the charging and discharging process.The effect of PANI nanotubes size on the desalination performance of the capacitor has been investigated.The main results are as follows:The PS nanofibers with different diameters were prepared by adjusting the concentration of the PS spinning precursor.After sulfonation of PS fiber,polyaniline can be easily polymerized and deposited on the surface of the PS nanofibers to form PANI/PS composite fibers in the presence of oxidant ammonium persulfate.And the hollow structured PANI nanotubes was prepared after washing with THF.It was found that the diameter of PANI nanotubes increased with the increase of the concentration of the PS solution.The PANI tube with the inner diameters of 130 nm,346 nm,426 nm and 1370 nm,was fabricated,corresponding to the PS concentrations of 10 wt%,15 wt%,20 wt% and 30 wt%,respectively.When PAN concentration is too low,the wall of PANI nanotubes is too thick with large PANI particles on the tube surface.Too high PS concentration gives the PS fiber with large fiber diameter,and thus large PANI tube with high inner diameter.Too large hollow structure brings to the poor ion diffusion due to long transportation distance.The PANI-AC composite electrode was prepared by mixing PANI nanotubes into the activated carbon and used in electrochemical and capacitive deionization studies.The PANI-AC composite electrode was characterized by SEM,and it was found that PANI nanotubes could be distributed evenly between the activated carbon particles and form auniform composite.The hydrophilic functional groups on the surface of PANI nanotubes effectively improve the hydrophilicity of the electrodes.Cyclic voltammetry tests show that the pseudo-capacitance produced by the redox reaction of PANI nanotubes has been effectively increased with the increase of the inner diameter of the tubes.The PANI-AC composite electrode with PS precursor amount of 20% is found to be 286 F/g,which is much higher than that of AC electrode of 198 F/g.The ion diffusion of PANI-AC composite electrode surface is obviously improved in the light of impedance analysis.The presence of PANI tube in the composite greatly improves the capacitance performance of the asymmetric PANI-AC?AC capacitor.The inverted hybrid-capacitive desalination(i-HCDI)configuration was constructed with PANI-AC as anode and AC as cathode.The extremely high desalination performance has been achieved on the i-HCDI,owing to the contribution of the electrolyte doping/dedoping of Pani during the charging and discharging process.The desalination ability is related to the PANI tubular structure.Under the optimal conditions,the maximum deionization capacity of 30.47 mg/g(cutoff voltage of 1.8 V)was obtained,which is almost 3 times higher than that of the symmetric AC//AC capacitor(13.39 mg/g,cutoff voltage of 1.8 V).The existence of PANI in the composite electrode contributes much to the excellent desalination amount and super charge efficiency.Therefore,PANI-AC has good electrochemical performance and desalination effect,and the PANI-AC based capacitor is potentially prospective for desalination application. |
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