Supercapacitor Electrodes Fabricated from Carbon Extracted from Biomass and Commercial Organic Wastes
Authors: Ciftyurek Engin , Oginni Oluwatosin , Bragg Dustin , Levelle Ross , Singh Kaushlendra , Sivanandan Litha , Sabolsky Katarzyna , Sabolsky Edward M. 
Published: February 2020

Up-to-date electronic devices require high level of energy storage with the competence to fast charge and discharge [1]. Supercapacitors have shown to be suitable for these criteria. A wide-ranging selection of carbon based materials such as activated carbons (ACs), templated carbon, carbon nanofibers and nanotubes, carbide derived carbons and graphene are currently being investigated for use as an electrode material for EDLC applications [2]. Nevertheless, ACs are the leading choice for supercapacitor applications due to its abundance and price efficiency, in addition to their satisfactory capacitance performance in the general range from 40 to 100 F/g [3, 4]. Hence, extensive research has been dedicated on new approaches for producing activated carbon material with high porosity levels in a cost-effective way. A range of carbon materials from different sources such as sucrose, cellulose, corn grain, banana fiber, potato starch have been prepared and activated by numerous researchers for use as EDLC electrodes. The crucial aspects in using these kinds of sources are aligned with the reduced cost and the lower ecological effect for using the bio-wastes to produce the value added products [5]. In the present work, ACs acquired by the pyrolysis and hydrothermally carbonized of commercially spent osmotic solutions (SOS) were used and tested as EDLC. Two different commercial SOS waste derived AC materials were used in this work to form the high surface areas carbons. AC-CSOS and AC-BSOS are the terms used for the berry- and cherry-derived SOS materials, respectively.

The micro-structure of carbon electrode materials were characterized via scanning electron microscopy (SEM) with an attached energy dispersive x-ray spectroscopy (EDS) apparatus. The chemical state of the carbon was characterized using X-ray photoelectron spectroscopy (XPS). XPS was used for quantification and chemical state analysis of the elements on the surface of the carbon electrodes. Distinctive attention was paid to the quantification of functional groups on the surface. The Brunauer-Emmett-Teller (BET) surface area, in addition to the gas adsorption/desorption isotherms of the carbon materials, were analyzed using nitrogen (N2) adsorption in a Micromeritics ASAP 2020. Electrochemical characterization was completed by testing the carbon material in a two-electrode assembly within a CR-2032 casing architecture [6]. The AC-CSOS and AC-BSOS materials were lightly ball-milled in alcohol and casted on acid washed stainless steel foil to a thickness of ~800 µm using n-methyl-2-pyrrolidone as the binder. The electrodes were assembled using a Nafion® separator, and a 6 M aqueous solution of KOH was used as the electrolyte solution. Due to the aqueous-based electrolyte solution, 1.0 V was chosen to be a maximum potential to be applied [7]. A Cyclic Charge-Discharge method (CCD) was used to assess the electrochemical performance. The tests were completed using 8 Channel Capacitor/Battery Analyzer (MTI Corp., USA). Each sample was tested to 2000 charge-discharge cycles, or until failure from 0.1 V to 1 V using a 2 mA current. In addition, self-discharge measurements were completed for each supercapacitors by charging to 1 V before removing the current and measuring the voltage for 120 min. The tests were generally repeated for 100 cycles or until failure.

Figure 1 shows the constant current charge/discharge (CCD) profile for the supercapacitors with AC-CSOS and AC-BSOS electrodes. The constant current charge/discharge measurements for both materials showed repeatable cyclic behavior, which was near the same level of performance to ACs formed using pitch-based precursors[2,8]. The CCD curves were linear and symmetrical. The specific capacitance (
Cg

) for the electrode system was ~48 and ~20 F/g over the 1500 cycles for AC-CSOS and AC-BSOS, respectively. These results demonstrate that the AC-CSOS and AC-BSOS based carbon electrodes retain the desired electrochemical reversibility and charge/discharge capabilities.

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Figure 1