Controllable Fabrication Of Polyaniline/Graphene Based Composites And Its Electrochemical Capacitive Performance

Supercapacitor is a novel energy storage device which has both large energy density of battery and high power density of traditional capacitor. According to the charge storage mechanisms of the electrode materials, supercapacitors are categorized as electrochemical double layer capacitors (EDLCs) and pseudo-capacitance based on fast Faradaic reactions. Carbon matrials, mental oxides, conducting polymers and their composites are used as supercapacitor electrode materials.As a conducting polymer, polyaniline (PANI) is applied in supercapacitor electrode materials due to its chemical stability, low cost and unique doping/dedoping properties. PANI has multiple oxidative states and high theoretical capacitance (as high as2000F g-1in H2SO4.) which provide high pseudocapacitance when used as supercapacitor material. The electrochemical cycling stability and conductivity of PANI is poor. It is reported that these properties would be improved when combined with carbon materials. Graphene is a new emerging carbon material with huge specific surface area, high conductivity and excellent mechanical property, which can be used as supporting materials for pseudocapacitive materials. The power density and cycling stability of graphene are outstanding while the specific capacitance and energy density are low, however, a superior materials with large energy density and high power density would obtained when combined graphene with pseudocapacitive materials. The chemical properties of graphene and the structure of composites have a great influence on the electrochemical capacitive properties. Our research focused on the aforementioned issues:first, the uniform PANI/graphene oxide (GO) composites were produced by adopting the method of controllable polymerization of PANI; second, the effect of reduction degree of graphene on the capacitive performance of PANI/graphene composites was investigated; the PANI/graphene/CNTs with three-dimensional porous structure were built in order to reduce internal-resistance and maintain high capacitance at rapid charge/discharge process. Homogeneous PANI/graphene oxide (GO) composites were prepared using GO and H2O2as oxidants. GO accepted the lone-pair electrons from adsorbed aniline monomers resulting in increasing of electron density and mobility. The electron enriched GO would activated H2O2to form OH which polymerized the aniline only on the surface of GO. Thus the polymerization of aniline in the bulk solution was suppressed as well as the formation of PANI nanofibers. SEM showed that PANI uniformly coated on the surface of GO in the form of nanoparticles with small size. The weight ratio of aniline and GO affected the coating amount of PANI and the specific capacitance of the composites. The largest specific capacitance was obtained when the weight ratio of aniline and GO was30(GP30). As for the PANI/GO composite (GPC) prepared with traditional method, there were nanoparticles coated on the surface of GO and nanofibers with large size formed in the bulk solution. The capacitance of GP30was797F g-1, and the internal-resistance of which was smaller than that of PANI and GPC. After being cycled for500times, the capacitance of GP30increased18%, which was decreasing for PANI and GPC with the increasing cycling number.GO was reduced by adopting glucose and ammonia as reducing agents. The reduction degree was controlled by varying the reducing time. And finally,0min,30min,60min and120min were selected as the reduction time. A series of reduced graphene oxide (RGOs) were characterized by XPS, FTIR, Raman and EIS. XPS and FTIR showed that the oxygen groups on the surface of GO were removed with the increasing reducing time. Raman results showed that with the increasing reducing time, the ID/IG increased which mean the defects increased. The conductivity increased with the increasing reducing time according to the EIS results. And over reduction lead to the aggregation of RGO and made the hydrophilicity worse. The PANI/RGOs composites were prepared through the in situ polymerization. The results showed that the capacitance was increased first and then decreased with the increasing reducing time. The capacitance reached to1045F g-1when the reduction time was60min. The influence of aniline concentration on capacitance performance was investigated. The loading amount of PANI on RGO increased with the increasing of aniline concentration according to Elements Analysis results. At low scanning rate, the capacitance of composites increased with the increasing aniline concentration. However, at high scanning rate, the capacitance decay rate was faster when the loading amount was larger.PANI/RGO/CNTs with three-dimensional porous structure were prepared by in situ polymerization. SEM and TEM showed that PANI uniformly coated on the surface on RGO and CNTs, PANI/CNTs embedded between PANI/RGO sheets and/or bridged the isolated ones. The diameter of PANI/CNTs was about40nm and the thickness of PANI was about10nm. BET results exhibited that the specific surface area was increased after composing with RGO and CNTs. The pore volume was very small for PANI/RGO due to the sheets restacking. PANI/RGO/CNTs showed the largest pore volume (0.160cm3g-1) with meso-and large pores due to the addition of CNTs. PANI/RGO/CNTs exhibited high capacitance and low internal-resistance at rapid charge/discharge process due to the porous structure and increased conductivity. With the increasing of the current density, the PANI/RGO/CNTs showed better rate performance than that of PANI/RGO and PANI. Because of the addition of high conductive carbon materials, the internal-resistance of composites was smaller than that of PANI.