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Öğe Activated carbons prepared by indirect and direct CO2 activation of lignocellulosic biomass for supercapacitor electrodes(Pergamon-Elsevier Science Ltd, 2020) Jiang, Changle; Yakaboylu, Gunes A.; Yumak, Tugrul; Zondlo, John W.; Sabolsky, Edward M.; Wang, JingxinLignocellulosic biomass was converted into hierarchical porous carbon by using a physical activation technique under a carbon dioxide environment. Both direct and indirect CO(2 )activation routes were utilized to investigate the effect of processing parameters and the kinetics of the activation. The porosity, surface chemistry, and morphology of the activated carbons were characterized in addition to their proximate and ultimate analyses. This was followed by the preparation of the activated carbon electrodes and the fabrication and electrochemical testing of these electrodes within a symmetrical supercapacitor cell. The results showed a dominant microporous structure along with the limited content of larger pores for the activated carbons prepared via both direct and indirect activation. Along with the preserved natural pore structure of the biomass, an engineered pore structure was achieved which is highly beneficial for the supercapacitors with respect to the transport and storage of ions. The morphological analysis also revealed their tortuous porous structure. The maximum specific capacitances of 80.9 and 92.7 F/g at the current density of 100 mA/g were achieved after direct and indirect activation routes, respectively. The surface functional groups were also found to play a significant role in the resultant electrochemical performance of the supercapacitors. (C) 2020 Elsevier Ltd. All rights reserved.Öğe Comparison of the electrochemical properties of engineered switchgrass biomass-derived activated carbon-based EDLCs(Elsevier, 2020) Yumak, Tugrul; Yakaboylu, Gunes A.; Oginni, Oluwatosin; Singh, Kaushlendra; Ciftyurek, Engin; Sabolsky, Edward M.Activated carbon-based electrodes with high surface area were synthesized by direct and indirect chemical activation of switchgrass biomass and its biochar, and decorated with MnO2 in order to enhance and compare the capacitive performance of the fabricated supercapacitors. N-2 physisorption, Raman, XPS and SEM techniques were used to examine the differences in the microstructure and surface characteristics of biomass-derived activated carbon samples and MnO2 loaded samples. All fabricated supercapacitors were highly stable and showed high-rate capacitive performance. Direct KOH and H3PO4 activation increased the specific surface area up to 1272 and 1373 m(2)/g, respectively. As a result, the capacitive performance increased from 50 to 140 F/g for directly KOH activated biomass, and from 49 to 95 F/g for directly H3PO4 activated biomass. MnO2 loading led to an increase in specific capacitance (110 F/g) for the KOH activated biochar, even though no significant effect was observed for the H3PO4 activated biochar. The effectiveness of direct chemical activation of the biomass to enhance the electrochemical performance of the fabricated supercapacitors was demonstrated by comparing it with pseudocapacitive material loading (MnO2) and indirect chemical activation of biochars.Öğe Effect of high temperature preoxidation treatment on the oxidation behavior of MoSi2 - and WSi2 -Al2O3 composites(Elsevier Science Sa, 2020) Yakaboylu, Gunes A.; Yumak, Tugrul; Sabolsky, Katarzyna; Sabolsky, Edward M.Composites of 30 vol% MoSi2 -70 vol% Al2O3 and 30 vol% WSi2-70 vol% Al2O3 were prepared via sintering at 1600 degrees C in argon. The high-temperature preoxidation treatments were applied at 1000 degrees-1200 degrees C for 10 - 120 min in air. The non-isothermal oxidation tests were conducted in air at temperatures ranging between 50 degrees and 870 degrees C to study and understand the influence of the alumina phase and preoxidation process on the low-temperature oxidation behavior. The optical and scanning electron microscopy, X-ray diffraction, X-ray photoelectron and Raman spectroscopy were used to characterize the surface layers formed. The oxidation-induced mass gains were substantially reduced by 82.1-99.8%, implying their enhanced oxidation resistance. This was due to the formation of highly dense, protective surface layers with a sufficient thickness (3.1-14.2 mu m). The structural and surface analyses revealed their complex chemistries, since the ternary (Mo-Si-Al, W-Si-Al) intermetallic, mullite, alumina and silica phases with high oxidation resistances were identified within these surface features. In addition, the high-temperature electrical properties of the composites were highly preserved after the preoxidation treatment, and their electrical conductivities were measured as 45.1-78.6 S/cm at 800 degrees C and 40.1 - 69.1 S/cm at 900 degrees C. (C) 2019 Elsevier B.V. All rights reserved.Öğe Effect of synthesis methods on the surface and electrochemical characteristics of metal oxide/activated carbon composites for supercapacitor applications(Elsevier, 2019) Yumak, Tugrul; Bragg, Dustin; Sabolsky, Edward M.MnO2/Activated Carbon (cAC) and NiO/cAC composites were synthesized by both hydrothermal and precipitation methods in order to investigate the effect of preparation methods on the surface chemistry and porous structure of composite-based electrodes and electrochemical properties. X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), N-2 physisorption, Raman spectroscopy and scanning electron microscopy (SEM) techniques were used to investigate the surface chemistry, chemical composition, pore characteristics and surface morphology of the synthesized composites. Synthesized composites were used as electrode materials within button cell supercapacitors. It was found that hydrothermal treatment to deposit the nano-oxides led to an increase in specific surface area, also this treatment resulted in oxygen-containing surface functionalities, which led to an improvement in electrochemical properties. Although the metal oxide loading caused a decrease in the specific surface area, the pseudocapacitive effect of MnO2 and NiO, and oxygen-containing surface functionalities increased the specific capacitance. MnO2 and NiO loading led to a 50% and 150% increase in specific capacitance, respectively. NiO/cAC samples obtained by precipitation method showed a higher specific capacitance compared to hydrothermally synthesized NiO/cAC. The metal oxide loading method has great influence on the surface chemistry, surface area and the resulting electrochemical performance of activated carbon-based fabricated supercapacitors.Öğe Engineered hierarchical porous carbons for supercapacitor applications through chemical pretreatment and activation of biomass precursors(Pergamon-Elsevier Science Ltd, 2021) Yakaboylu, Gunes A.; Jiang, Changle; Yumak, Tugrul; Zondlo, John W.; Wang, Jingxin; Sabolsky, Edward M.For a better process and property control, the effect of chemical pretreatment time on the chemistry and electrochemical performance of activated carbons derived from Miscanthus grass biomass was exam-ined. The microstructure, chemistry and active functional groups were controlled by tuning the pretreatment duration, which provided the removal of certain concentrations of hemicellulose and lignin, as well as, pore development at the initial stage. The optimal KOH pretreatment (12-18 h) resulted in interconnected pore structure, rich oxygen content (18-21 at.%), significant changes in their chemistry and functional groups, and a sheet-like morphology. A high specific capacitance up to 188 F/g and a high cycling stability of 85-91% retention (after 1000-2500 cycles) at 0.1 A/g were achieved. The optimization of the pretreatment time also resulted in high specific energy (8.0 W h/kg) and specific power (377 W/ kg) at 0.5 A/g. The micro/mesopore volume, cellulose content, C/O ratio, and surface chemistry were identified to be major contributors to the electrochemical performance as a result of enhanced electroadsorption, double layer formation, and rapid ion transport. This understanding creates a simple and cost-effective route for controlling the pore network and chemistry, as well as, the resultant performance of the porous activated carbon supercapacitor electrodes. (C) 2020 Elsevier Ltd. All rights reserved.Öğe Preparation of Highly Porous Carbon through Slow Oxidative Torrefaction, Pyrolysis, and Chemical Activation of Lignocellulosic Biomass for High-Performance Supercapacitors(Amer Chemical Soc, 2019) Yakaboylu, Gunes A.; Yumak, Tugrul; Jiang, Changle; Zondlo, John W.; Wang, Jingxin; Sabolsky, Edward M.Seven kinds of highly porous activated carbon were prepared from two different lignocellulosic biomass feedstocks (hybrid willow and miscanthus grass) by utilizing four different processing routes, which generally include variations of the pyrolysis, slow oxidative torrefaction, and KOH chemical activation. The activated carbons were evaluated for potential application within the electrodes of double layer supercapacitors. The synthesized activated carbons showed high specific surface area (up to 3265 m(2)/g), hierarchical pore structure composed of micro-/meso-/macropores with large pore volume (up to 1.535 cm(3)/g), and rich oxygen content (10.9-19.2 at. %). Their surface area, pore structure/volume, microstructure, and surface functional groups were highly influenced by processing routes, which in turn determined their electrochemical performance and stability. In particular, pretreating the biomass samples via slow oxidative torrefaction substantially increased their surface area, total pore volume, and meso-/micropore volume, and the surface chemistry of these materials showed a higher concentration of carboxyl groups. The performance of two-electrode symmetrical supercapacitors was evaluated in a 6 M KOH aqueous electrolyte. They exhibited relatively high specific capacitance of 70.2-162.3 F/g under constant current density of 100 mA/g, with a high cycling stability based on the capacitance retention of 95.1-99.9% after 1000 cycles. In addition, an increase of 25.0-62.2 F/g was achieved in specific capacitance by including the pyrolysis and/or slow oxidative torrefaction in the synthesis protocol. The sample (HW-D) that exhibited the best performance also maintained 94.1% of its specific capacitance after 5000 charge/discharge cycles at 100 mA/g. The synthesis strategies including the slow oxidative torrefaction pretreatment showed great promise for preparing low-cost, porous carbon materials from renewable biomass sources that are highly suitable for incorporation in supercapacitors and other electrochemical applications.
