Preparation And Capacitance Performances Of Rice Husk Based Capacitance Carbon With Hierarchical Structure

In recent years, supercapacitors have received considerable attention because oftheir high power density, fast recharge capability and long cycle life. The electrodematerial is an important part of supercapacitor and is the decisive factor for theperformance of supercapacitor. Activated carbons with various porous structures areconsidered to be the most promising electrode materials for supercapacitors owing tolow cost, high surface area, and abundant raw materials.Recently, the preparation of activated carbon (capacitance carbon) using ricehusk as precursor for supercapacitor is a new research hot. However, the ricehusk-based activated carbon (RHC) was found to have the following drawbacks inresearch: the non-ideal capacitive behavior, high production cost with high emissionof contaminates, which greatly restricted the application of RHC in supercapacitors.In order to realize the practical application of RHC in supercapacitors, a series ofresearches have been made. In this paper, the activated carbons with different porestructure were prepared via chemical activation method using rice husk as precursorfor supercapacitors. The effect of activation conditions and the structure andcomposition of rice husk on the pore structure and capacitive performance ofactivated carbon have been discussed in detial. Meanwhile, the cycling stabilities of activated carbon in acid and alkaline electrolytes were also investigated using RHC.The ageing mechanism of activated carbon in alkaline medium was studied. Based onthe aging mechanism, the methods on improving the cycling stability of activatedcarbon in alkaline medium were proposed. Besides, the green technologies to producerice husk-based capacitance carbon were developed and preliminarily studied. Themain contributions of this research are as follows:(1) Using the self-template structure of rice husk, hierarchical activated carbonwith high surface area was prepared by carbonization, NaOH-leaching and KOHactivation. The formation processes of hierarchical porous structure were discussed indetail, where silica in RH plays a crucial role. The rice husk-based hierarchicalactivated carbon (RHHAC), with a3D porous structure, was composed ofmacrochannels, mesopores and micropores both on its surface and inner channels.The RHHAC exhibited a surface area as high as2804m2g-1with total pore volume of1.797cm3g-1. These characteristics endow the RHHAC with a high capacitance (278F g-1at0.5A g-1) and excellent rate capability (77.2%retention at20A g-1) in6molL-1KOH. Furthermore, the symmetric supercapacitor fabricated with RHHACdelivered a high energy density of7.4Wh kg-1even at a power density of6195W kg-1,which reveals the promising application of RHHAC in high-performancesupercapacitors.(2) The effects of silica in rice husk on the formation of pore structure in KOHactivatation process were studied. Experiments results indicate that, when the silica inrice husk was not removed, the obtained activated carbon exhibited highmicroporosity; when the silica in rice husk was removed, the obtained activatedcarbon exhibited high mesopores and macropores. In KOH activation process, KOHpreferentially reacts with silica in upper and lower epidermis of rice husk to producepotassium silicate, which hindered the KOH activation for carbon in upper and lowerepidermis. Meanwhile, the massive KOH led to the destruction of vascular bundle inrice husk via excessive activation. The formation of mesopores and macropores andenlargement of mesopores were hindered under the presence of silica. (3) The cycling stability of activated carbon for surpercapacitor in H2SO4andKOH media were studied, where activated carbon prepared from rice husk by KOHactivation was used. The results show that the prepared RHC exhibited an excellentstability in H2SO4medium but a poor stability in KOH medium. The differentreactions of oxygen-containing functional groups in two media caused the differentchange of positive potential during charge/discharge process of supercapacitors,which led to the different cycling stability of RHC in two media. In KOH medium,the capacitance of positive electrode decreased, which caused the increase ofmaximum positive potential resulting in the oxidation of positive electrode. Thedecreased capacitance of positive electrode originates from the electrostatic repulsionand steric hindrance effect of the hydrolysis products from the carboxyl and lactonesgroups, which hinders OH-into the pores of RHC. However, the reversible redoxreactions of quinone-like groups did not cause the obvious change of maximumpositive potential in H2SO4medium. The methods to improve the cycling stability ofRHC were studied based on the study about the aging mechanism of RHC in KOH.The cycling stability of RHC was improved via removal of oxygen-containingfunctional group on the RHC surface by thermal treatment. The capacitance retentionof RHC increased from28.3%to94.7%via thermal treatment after10000GCDcycling. But, the capacitances of RHC reduced30%. Besides, the increase of positivepotential of RHC was inhibited via increasing mass ratio of positive electrode tonegative electrode, which improved the cycling stability of RHC. When the massratios of positive electrode to negative electrode were increased from1:1to2:1, thecapacitance retention of RHC increased from28.3%to95.3%after10000GCDcycling. At the same time, the capacitance of RHC reduced12%. The improvement ofthe mass ratio of positive electrode to negative electrode is simple and can be easilyrealized compared with thermal treatment.(4) Based on the preparation of rice husk-based capacitance carbon, two greenpreparation techonoly have been proposed and preliminarily researched. In greenpreparation techonoly of rice husk-based activated carbon with high specificcapacitance, the liquid waste coming from the preparation of RHC was acidized and electrolyzed. The nano-silica was prepared. Meanwhile, the recycle of water andKOH were also realized.This green techonoly is low cost, environmental friendly,which is feasible in economy and technology. In the green preparation technology ofRHHAC, the liquid waste produced in the preparation of RHHAC was transformedinto activating agent by lime milk. This green technology is technically feasible. Thegreen technologies has guiding significance for industrial development of ricehusk-based activated carbon.