Preparation And Performance Studies Of Carbon Based Electrode Materials For Supercapacitors

Recently,carbon is widely used in energy storage devices as electrode material for electrochemical capacitors and lithium-ion batteries,because of its abundant source,low cost,high electrical conductivity and excellent chemical stability.Graphene and activated carbon are recognized as the leading electrode material in supercapacitor,however,suffering from the limitation of low specific capacity and energy density,which are controlled by their specific surface area,conductivity,surface chemistry property,pore size distribution and porosity.In addition,its poor rate performance resulted from barren porous structure has restricted its further applications.In this study,we focus on improving or optimizing specific surface area and porous structure of graphene and activated carbon to improve their electrochemical performance.The modified graphene hydrogels with three-dimensional porous network structure were constructed by chemical reduction of graphene oxide through a facile hydrothermal process.The activated carbon materials with developed porosity were also prepared by activating and etching.Moreover,the reaction conditions such as reactant ratio,hydrothermal temperature and carbonization temperature were systematically studied to optimize the microstructure of the electrode materials.Thus,the specific capacity,energy density and rate capability were enhanced,and maintain the high power density of supercapacitor.?1?Reduced graphene oxide hydrogel?rGOHG?with 3D porous structures was successfully synthesized by treating GO with various concentrations of NaH₂PO₂ with one-step hydrothermal reaction.In this process,the microstructure properties of rGOHG may be adjusted by changing the concentration of NaH₂PO₂.The measurement reveals that the rGOHG prepared with low concentration shows uniform pore distribution and has a high specific capacitance at low scanning rate.The cell assembled by rGOHG^-1 exhibits an specific capacitance as high as 176.6 F g^-1 at 1 mV s^-1.However,The cell assembled by rGOHG-9 shows a improved rate capability and cycle stability due to its expanded pores formed at high concentration.The CV curve measured at 500 mV s^-1 is still able to keep a good rectangle,and maintains the capacitance of 89 F g^-1.Moreover,it also exhibits power density as high as 54 kW kg^-1 even the current density reaches to 200 A g^-1.?2?Reduced graphene oxide hydrogel was successfully prepared by one-step hydrothermal process using C6H5 OH as the reducing agent,which characterizes a well-defined self-assembled 3D porous network.Moreover,the effects of the amount of reducing agent and the hydrothermal temperature on the synthesis of hydrogels have been investigated systematically.The result demonstrates that the GHPhs prepared at 160oC?mass ratio of GO/C6H5OH=1:1.25?delivers a improved specific capacitance of 260 F g^-1 at 1 mV s^-1 as well as excellent rate capability(53.1% of capacitance retention at 500 mV s^-1).Meanwhile,it shows a improved energy density as high as 8.89 Wh Kg^-1,and the excellent cycling stability?98.3% of capacitance retention after 12,000 cycles?.It is an effective method to prepare high-performance electrode materials based on graphene by introducing unique 3D interconnected networks.?3?A series of reduced graphene oxide hydrogels with unique 3D network structure have been simply achieved with a trace amount of benzenediols via one-pot hydrothermal process.The optimized samples reduced by o-,p-,and m-benzenediols show different electrochemical properties due to the different positions of-OH groups on the aromatic ring.The maximum specific capacitances of the capacitors based on o-rGOHG80,p-rGOHG40 and m-r GOHG40 have been measured to be 253,240 and 238 F g^-1 at 1 mV s^-1,respectively.The energy density is also largely improved to 8.02,7.88 and 7.88 Wh kg^-1.After 10,000 charging/discharging cycles,the capacitance retention reaches up to 98.1%,96.9% and 95.5%,respectively.In addition,the energy density of the cells based on o-rGOHG80 still maintains 5.13 Wh kg^-1(in this case,power density is 21840 W kg^-1)when the discharging current density reaches to100 A g^-1.?4?Cornstalk-core,a renewable agricultural by-product,was effectively used as electrode materials for supercapacitor through activated by KOH.Compared with original materials without activation,a large number of microporous were introduced into as-prepared PCMs,increasing the effective surface areas and pore volumes.The electrochemical measurements of PCM obtained at optimal conditions were performed in 6 M KOH.The specific capacitance is 317 F g^-1 at a scan rate of 1 mV s^-1.Meanwhile,the rate performance was also enhanced.The maximum power density of the assembled supercapacitors is as high as 28.34 kW kg^-1,the energy density remains 4.25 Wh kg^-1,even the current density is increased to 250 A g^-1.?5?The binder-free composite films were prepared by mixing GO and carbon powders derived from cornstalk,and that abundant micro-,mesopores were formed after activation with KOH.During the preparation,the GO sheets are water-soluble to facilitate the dispersed carbon powders,and bind them together via the interaction of oxygen-containing functional groups.Meanwhile,the carbon particles may act as pacers between graphene sheets to decrease the aggregations and stacking from p-p interaction.More importantly,the as-prepared composite films with high surface area and complex pore structure could be directly used for assembling supercapacitor.The optimized sample of rGO/ACCS1/1 has specific capacitance as high as 278 F g^-1 at 1 mV s^-1.In addition,the smallest ?0 value for the cell of rGO/ACCS1/1 indicates its better rate capability.?6?Monolithic porous carbon?MPC?materials were fabricated via reacting polyvinyl alcohol with Sn salt through hydrotherm-carbonization process,in which tin oxide as a porogen and polyvinyl alcohol as carbon precursor.The results of SEM and N2 adsorption-desorption test demonstrate that micropores size in the samples is mainly about 0.75 nm while the mesopores are distributed around 3.2 nm.The electrochemical performance of the MPC-based capacitors was tested using a two-electrode method in 6 M KOH electrolyte.The cell assembled by MPC-P170-C700 exhibits the best electrochemical performance.It delivers a high specific capacitance of 225 F g^-1 at 1 mV s^-1,and exhibites an outstanding cycling stability?95.2% of capacitance retention after 10,000 cycles?.In addition,through increasing the carbonization temperature,supercapatior based on MPC-P170-C900 shows a better rate capability with the smaller constant ?0 of 0.21 s.It also possesses a high power density of 18.35 kW kg^-1 at current density as high as 100 A g^-1.