High Performance Electrochemical Supercapacitors Based On Activated Carbon Derived From Rice Husk

Electrochemical supercapacitors have attracted significant attention, due to theirhigh power density, long cyclelife, high current efficiency and competitive in electricvehicles and energy storage field. But their limited energy density has hindered theirwider application. How to improve the energy density and reduce its costs havebecome research hotspot of electrochemical supercapacitors. To achieve these goals,rice husk carbon (RHC) produced from rice husk was studied as electrode material ofelectrochemical supercapacitors, the properties of RHC in KOH, Na2SO4and LiPF6were also discussed. Symmetric electrochemical supercapacitors operating between0to1.6V and0to1.8V in aqueous electrolyte with RHC as electrode materials, andLi-ion capacitor with RHC as positive electrode and LTO as negative electrode weredesigned, and both with significant improved energy density. The results showed thatRHC is a promising electrode material of electrochemical supercapacitors.RHC has good power performance in6M KOH, and show typical electric doublelayer capacitance at different scan rate from10mV/s to2V/s. The energy density andpower density of symmetric capacitor with RHD electrode is4.88Wh/kg and522.12W/kg at2.27A/g, and the specific capacitance of RHC only decrease5.8%after10000galvanostatic charge/discharge (GCD) cycles. The frequency behavior of RHCin6M KOH is also studied through electrochemical impedance spectroscopy (EIS)with equivalent circuit, complex capacitance and complex power. The ESR and time constant of RHC is low and slightly increased during cycles. The pore size affects thecapacitance of RHC significantly. Microporous contribute to electric double layercapacitance at low current density, while Mesoporous lead to higher capacitance athigh current density.The potential window of RHC in Na2SO4is about twice larger than in KOH, whichis beneficial to improving energy density. The energy densities of0-1.6V and0-1.8Vsymmetric capacitors with1M Na2SO4as electrolyte were1.85and2.38times ofsymmetric capacitors in KOH, respectively. Tens of thousands cycles of GCDindicated the excellent cycle life of1.6V and1.8V symmetric capacitors. Thespecific capacitance of1.6V symmetric capacitor keep stable in100000cycles, whilethe ESR remain stable after slightly increase and inner resistance Rs keep constantafter small decreased. Through detecting the potential change of positive and negativeof symmetric capacitor, we found that the difference between two-electrode test andthree-electrode test derived from different scope of electrode potential changes.In1mol/L LiPF6, RHC also has a good capacitance performance between3.0V-4.5V (vs. Li+/Li), and shows specific capacitance of138F/g and specific capacity of57.5mAh/g. The tests indicate that RHC has good cycle and power performance. Thespecific capacitance of Li-ion capacitor with LTO as negative electrode and RHC aspositive electrode is60.7F/g. The energy density of Li-ion capacitor is53.3Wh/kg,which is four times of symmetric capacitor assembled RHC in organic electrolyte.