Preparation And Electrochemical Properties Of Polyaniline Decorated Carbon Based Flexible Free-standing Supercapacitor Electrodes

In recent years,wearable electronic devices have been shown to be highly desired in human's life.To this end,it becomes critically important to develop matchable portable and foldable energy storage systems.Flexible freestanding supercapacitors?FFSCs?have attracted a great deal of interests attributing to a number of potential advantages in performance,including high electrochemical performance,easy to assemble with no need of conductive collectors and insulating binders.As the key components of FFSCs,electrodes usually possess high conductivities and excellent flexibilities,and can serve as both the electrodes and the current collectors.To synthesis high performance FFSCs electrode materials,carbon nanotubes?CNTs?and/or graphene with excellent conductivity and mechanical characteristic can act as the flexible“skeleton”,and the PANI with high Faradic activity is the“skin”,resulting the high performance“skeleton/skin”structured FFSCs electrode materials.Firstly,we used the crosslinked multi-walled carbon nanotubes?C-CNTs?films which are prepared by facile filtering the dispersion of the micrometer-scaled 3D framework of crosslinked carbon nanotubes with common water pump as the high-strength conductive skeleton.The preparing conditions were optimized for the best electrochemical property of the flexible C-CNTs/PANI composite films.As free-standing electrodes for FFSCs,the highest specific capacitance of 507 F/g or 314 F/g,energy density of 70.4 or 43.6 Wh/kg and power density of 0.44 or 0.43 kW/kg were achieved with the in-situ chemical oxidative polymerization or solution coating technique,respectively.Then the independently double-crosslinked carbon nanotubes/polyaniline?CCNTs/TC-PANI?composite films were obtained by thermal crosslinking?TC?the PANI coatings in the CCNTs/PANI films.Their specific capacitance,energy density,and power density increased to 531 F/g and 455 F/g,73.8and 63.2 Wh/kg,0.49 and 0.47 kW/kg with the in-situ chemical oxidative polymerization or solution coating technique respectively,and their electrochemical cyclic stability was also enhanced distinctly.Secondly,we prepared a new kind of reduced graphene oxide/carbon nanoparticles?RGO/CNs?composite films with good flexibility and high mechanical strength via a facile hydrothermal process of the?-cyclodextrin modified graphene oxide??-CD/GO?hydrogel in presence of ascorbic acid?VC?.The resulted CNs could enlarge the interlayer distance of graphene sheets not only to prevent their agglomeration,but also to improve the transfer speed of the electrolyte ions throughout the film electrodes during the charge/discharge process.The flexible and robust RGO/CNs composite films were then used as highly conductive supports for the electrodeposition of PANI to further improve the electrochemical performance.As flexible and robust paper-like electrodes,the reduced graphene oxide/carbon nanoparticles/polyaniline?RGO/CNs/PANI?composite films exhibited high electrochemical activity,such as high specific capacitance of 787.3 F/g at the current density of 1 A/g and 564.0 F/g at the current density of 10 A/g,as well as excellent cyclic stability.Thirdly,sandwich-structured S-doped reduced graphene oxide/CNTs/PANI?SRGO/CNTs/PANI?composite membranes were designed as free-standing electrodes for high-performance supercapacitors,by electro-deposition of PANI onto the flexible and robust sandwich structured S-doped reduced graphene oxide/carbon nanotubes?S-RGO/CNTs?composite films as highly conductive supports,which were prepared through a mild hydrothermal reduction and doping reaction of graphene oxide?GO?nanosheets in presence of multi-walled CNTs?MW-CNTs?,with Na₂S as not only a sulfur precursor,but also a high-efficient reducing agent.The S-RGO/CNTs/PANI composite membrane electrode possessed high specific capacitance?812 F/g?in 1.0 M H₂SO₄ aqueous solution,with excellent rate capability and cycling stability for supercapacitors without the need of current collectors and binders,showing significant potential in flexible energy storage devices.Except for the 2D FFSCs materials,we also investigated the graphene and/or CNTs based fiber shaped FFSCs.The 1D RGO/CNTs composite fibers?RC?were prepared firstly via a simple in-situ reduction of GO in presence of CNTs in quartz glass pipes,possessed excellent mechanical performance of>193.4 MPa of tensile strength.Then PANI was coated onto the RC fibers by electrodepositing technique to construct PANI coated RGO/CNTs composite fiber?RCP?.The electrochemical properties of the RCP fiber-shaped electrodes are optimized by adjusting the feeding ratio of carbon nanotubes.The optimized one exhibits good electrochemical characteristic such as highest volumetric specific capacitance of 193.1 F/cm³ at 1 A/cm³,as well as excellent cyclic retention of 92.60%after 2000 cyclic voltammetry cycles.Furthermore,the all-solid-state symmetric supercapacitor,fabricated by using the final composite fiber as both positive and negative electrodes pre-coated with the poly?vinyl alcohol?/H₂SO₄ gel polyelectrolyte,possesses volumetric capacitance of 36.7 F/cm³ at 0.2 A/cm³ and could light up a red light-emitting diode easily.Finally,we used polystyrene sulfonate?PSS?microspheres as hard template to build the one-dimensional microporous graphene oxide fiber?MG?through a simple mass production method.The MG fiber exhibited outstanding flexibility and excellent tensile strength of 183.7 MPa.Then the polyaniline?PANI?coated microporous graphene?MGP?fibers were prepared by in-situ electro-polymerization of aniline.Benefiting from the unique microporous structure,the optimized MGP fiber electrode exhibited remarkable electrochemical characteristics,such as highest specific capacitance of 36.95 mF/cm at current density of 0.25 mA/cm?204.2 F/cm³ at current density of 1.38 A/cm³?,as well as excellent capacitance retention of 96.31%after 2000cyclic voltammetry cycles.More than that,the MGP fiber electrode remained high retention rate of its initial specific capacitance even under tensile stress or bending,indicating its good tolerance to external damage.