Yazar "Zondlo, John W." seçeneğine göre listele

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    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, Jingxin
    Lignocellulosic 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.
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    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.
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    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.