Influence of Processing Routes on the Electrochemical Performance of Supercapacitors (EDLCs) Fabricated from Biomass-Derived Activated Carbons

Abstract

Various electrode materials have been developed over the years for use in the electrochemical double layer capacitors (EDLCs) or supercapacitors. Among all, activated carbon materials are widely studied and used due to their pore structure, very high specific surface area, low cost and ease of processing. It is known that the electrochemical performance of supercapacitors highly depends upon surface area, pore size/shape, overall pore volume, pore size distribution, surface functionality and electrical conductivity. In addition, microstructure and surface chemistry have significant influence on their electrochemical performance. In order to enhance the performance, stability and cycle life of the supercapacitors fabricated from activated carbon materials, their pore structure, surface area, microstructure and surface chemistry need to be controlled and modified via various processing routes. This study investigated the influence of processing routes on the properties of the activated carbon materials, and also electrochemical performance and stability of the fabricated supercapacitors. Lignocellulosic biomass can be easily, and cost effectively processed to produce activated carbon materials with unique microstructure and surface properties as electrodes. The aim of this study was to utilize different processing routes to understand their effect on the pore structure, microstructure, surface chemistry and final electrochemical performance. Therefore, three major processing techniques, which are air torrefaction (or oxidative thermostabilization), pyrolysis and chemical activation, were used in this study. The air torrefaction pretreatment was performed at low temperature regime (200°-300°C) and in air to stabilize and control the microstructure at the initial stage. Selected samples were also pyrolyzed at 450°C under nitrogen to achieve biochar samples. Chemical activation process was conducted using potassium hydroxide (KOH) to further modify the pore structure and surface chemistry of the biomass-derived carbon. Prior to fabrication of the supercapacitors, properties of the activated biomass and biochar samples were characterized by BET, SEM, TGA, XPS and Raman spectroscopy. Activated carbon materials with ultra-high specific surface area (up to 3265 m2/g), high cumulative pore volume (1.99 cm3/g) and controlled microstructure were successfully obtained. The results also demonstrated significant changes in the relative concentration of oxygen- and carbon-containing surface functional groups depending upon the processing route. To fabricate supercapacitors, activated biomass and biochar samples were mixed with polyvinylidene fluoride (PVDF) binder and carbon black for 24 h to prepare electrode inks. The electrodes were then casted on a stainless steel sheet using a doctor blade. The supercapacitors were then fabricated using a typical CR-2032 parts, KOH as an electrolyte, and nafion as a separator. Constant-current charge-discharge and self-discharge tests were conducted at the range of 0.1-1.0 V and 0.1 A/g, up to 1000 and 50 cycles, respectively. The activated biochar samples showed higher specific capacitance (107-123 F/g) compared to activated biomass (70-92 F/g), presenting the positive influence of the pyrolysis process. Initial air torrefaction process substantially increased the electrochemical performance, since there was a 22-25 F/g increase in the specific capacitance. This may be related to better microstructural control and further change in surface chemistry. All supercapacitors fabricated from the activated biomass and biochar samples showed high electrochemical stability up to 1000 cycles. The average capacitive loss after 1000 cycles was found to be around 2.0%. Acknowledgements: The U.S. Department of Agriculture (USDA), McIntire Stennis Grant Program supported this work under accession number of 1007044 and project number of WVA00118. The authors also would like to acknowledge the West Virginia University Shared Research Facilities for support through materials characterization. Dr. Yumak acknowledges financial support from the Scientific and Technological Research Council of Turkey (TUBITAK) BIDEB-2219 Postdoctoral Research Program.