Carbon-based Electrode Materials For Supercapacitors

Supercapacitors, which are also named as electrochemical capacitors, combine the advantages of high energy density for cells and high power density for conventional capacitors. There are three kinds of supercapacitors depending on the working mechanism: electrochemical double layer capacitors（EDC Ls）, Faradaic pseudocapacitors, and electrochemical hybrid capacitors. EDC Ls are carbon materials with large specific surface area and high conductivity. Faradaic pseudocapacitors include transition metal oxides and conducting polymers with large capacitance and high energy density. Electrochemical hybrid capacitors combine EDLCs and Faradaic pseudocapacitors.The introduction of nano technology and mesopores for carbon materials are main methods for EDLCs. The Faradaic pseudocapacitor electrode materials involve carbon-based composites（carbon-based transition metal oxide or conducting polymers composites） and the materials obtained by element doping. In the paper, a flexible graphite sheet（FGS） was an important experimental material, which was processed by treating natural graphite with anions that intercalate into the graphene layer, heating at high temperature for vermiform expanded graphite and rolling. The attractive force between graphene layers becomes so weak that intercalation ions would readily exfoliate graphene sheets from the FGS. The increased distance between layers could lead to the expansion of FGS and the formation of graphene.The research contents are as follows:1) FGS was treated by electrochemical intercalation in H₂SO₄ solution. Self-supporting graphene（SSG） grew on the surface of FGS by tuning the technological parameters, which decreased the aggregations and stacking of graphene, and the unexpanded FGS substrate acted as a current collector. Then the electrodeposition of a homogeneous and conformal polyaniline（PANI） film was performed on the surface of SSG. The electrochemical measures of the obtained graphene /PANI composite electrode materials were conducted. The areal specific capacitance was 1.36 F ·cm^-2, and the gravimetric specific capacitance was up to 491.3 F ·g^-1 at a scan rate of 10 mV·s^-1. The capacity retention was 86% after 5000 cycles at a scan rate of 50 mV·s^-1. The maximum energy density of the assembled symmetrical supercapacitor was 46 Wh·kg^-1.2) FGS was treated by electrochemical intercalation in（NH₄）₂S₂O₈ solution. SSG was formed on the surface of the expanded FGS. A uniformed and conformed MnO₂ film was deposited by a micro-wave assisted deposition technique. The electrochemical measures of the obtained graphene/MnO₂ composite electrode materials were performed. The areal specific capacitance was 2.8 F ·cm^-2, and the gravimetric specific capacitance was up to 465 F ·g^-1 at a scan tare of 2 mV·s^-1. The maximum energy density of the assembled asymmetrical supercapacitor was 25.6 Wh·kg^-1. The capacity retention was above 87% after 5000 cycles at a current density of 1 A·g^-1.3) Carbon nanomaterials with different morphologies were obtained by thermal treatment of FGS in KNO₃ molten salt. The formation mechanisms of the different carbon nanostructures were discussed. Porous graphenes, carbon nanocages, and carbon nanospheres were obtained in turn at 350, 500, and 600 oC, respectively. The maximum gravimetric specific capacitance of porous graphenes, carbon nanocages, and carbon nanospheres were 309, 285, and 231 F·g^-1 at 2 mV·s^-1, respectively. After 5000 cycles, the capacity retentions of the three nanomaterials were above 96%.4) N-doped mesoporous carbons（N-MCs） were synthesized by the oxypolymerization of aniline monomers with amphiphilic triblock copolymer Pluronic F127 as a template, calcination at 800 oC on N₂ atmosphere and activation in a KOH solution. The gravimetric specific capacitance of N-MCs was 318 F ·g^-1 at a current density of 0.2 A·g^-1. The capacity retention was above 96% after 5000 cycles at different current densities, indicating excellent cycle stability of N-MCs. The maximum energy density of the assembled symmetrical supercapacitor was 11.25 Wh·kg^-1. The capacity retention was 94.3% after 5000 cycles at a current de nsity of 1 A·g^-1.