Research On Application Of Coconut Fibers Activated Carbon In Supercapacitors

Energy storage and utilization is an important part which supports the efficient operation of human society. The depletion of the earth's resources and increasing environmental problems have spawned the rise of renewable energy and become an ideal choice for sustainable and healthy development of human beings. As a new type of green energy storage device, supercapacitor has the characteristics of fast charging and discharging, environmental friendliness and long cycle life. However, its wide application is limited by the low energy density. The major challenges for further development of supercapacitors are maintaining excellent cycle stability also increasing energy density and reducing production costs.The advanced supercapacitors need to improve energy density without sacrificing power density and cycle stability. Based on the energy density formula ?E=1/2CV2?, the key to obtain high energy are increasing the specific capacitance and the working potential window. Thus, an effective method is to develop a porous, nano-sized electrode material for improving the capacitance. Another approach is to build a hybrid/asymmetric supercapacitor that can effectively utilize the potential gap between the two types of electrodes to increase the overall working voltage ?V?. Therefore, the article carried out the following research work:First, high specific surface three-dimensional porous activated carbon electrode materials were prepared by using coconut fibers as raw material and KOH as activator.They showed high specific capacitance and excellent rate performance. In the 6 M KOH electrolyte, the specific capacitance can reach 266 F g^-1 and 202 F g^-1 at 0.1 A g^-1, 100 A g^-1,respectively. And the electrode material also displayed good stability with 84% retention rate after 10000 cycles at 1 A g^-1. Additionally, results prove that 3DHPAC electrode demonstrates ideal capacitive behavior with high capacitance of 155 F g^-1 and 142 F g^-1 at 0.1 A g^-1 and 10 A g^-1 in EMIMBF4 electrolyte, respectively. The cyclic stability test over 10000 cycles at 0.5 A g^-1 also demonstrating excellent stability of HPAC-4 with 81%retention rate. Most notable, the material reported herein can simultaneously achieve high energy density up to 53 Wh kg^-1 and a high power density of 8224 W kg^-1. According to the above tests, 3D HPAC possesses high specific capacitance, excellent rate performance and long cycle life, demonstrating that it is more promising material as advanced electrodes for electrochemical supercapacitors.Next,the nano-slice Ni_0.5Co_0.5?OH?₂/Graphene composite material was prepared by coprecipitation method. The effects on capacitance of sodium dodecyl sulfate ?SDS?surface active agent and graphene loading were investigated. Firstly, the electrochemical properties of Ni_0.5Co_0.5?OH?₂ and Ni_0.5Co_0.5?OH?₂/Graphene composites and coconut fibers based activated carbon were tested by three electrode system. Test results showed that the positive and negative all exhibited good electrochemical performance. At 1 A g^-1, the specific capacitance of NC/G-2 and HPAC-4 could achieve 1253 F g^-1 and 368 F g^-1,respectively. According to the charge balance principle, the new type of positive and negative electrode system was built. The potential window of Ni_0.5Co_0.5?OH?₂/Graphene//coconut fiber activated carbon asymmetric supercapacitor can reach 1.4 V. At 0.1 Ag^-1,5 Ag^-1, the specific capacitance was as high as 110 F g^-1 and 59 F g^-1, respectively, which showed good rate performance. NC/G-2//HPAC-4 asymmetric supercapacitor had a high energy density,up to 29.9 Wh Kg^-1; Even at a high power density of 2267.3 W Kg^-1, the energy density was still 16.1 Wh Kg^-1.