(468b) Development of Biochar As a Promising Carbon-Based Candidate in Electric Double Layer Applications

Biochar, a residual by-product of fast pyrolysis of woody biomass, is studied as a low-cost, novel, and renewable carbon-based electrode material for electric double layer (EDL) based applications. The main EDL-based applications include water and wastewater purification (e.g., electrosorption of industrial effluents, capacitive deionization of brackish water) and energy storage capacitor fabrication (e.g., supercapacitor fabrication). The activation of biochar for increasing the surface area, tuning the pore structure, and modifying the surface chemical groups is investigated for developing biochar as a promising carbon-based candidate for EDL-based applications. Based on the combination of chemical (i.e., impregnation in 7 M KOH) and thermal activation (i.e., carbonization at 675 or 1000⁰C under N₂ atmosphere), the surface area and the total pore volume of the untreated biochar increased from 1.66  m² g^-1 and virtually negligible up to 990 m² g^-1 and  0.90 cm³ g^-1, respectively. Different activation conditions resulted in activated biochar samples with various properties such as structure, surface area, porosity, and functional groups. These properties have been studied through FT-IR, XRD, and Elemental Analyses. To test the EDL performance of activated biochar samples, electrodes were fabricated by spraying the activated biochar on a flexible thin Nickel mesh (150 x 150 mesh size) as current collector. To increase the adhesiveness of activated biochar on Nickel mesh, different concentrations of Nafion^® (a cation exchange binder) from 0 to 30 wt.% have been applied and the subsequent effect on the EDL performance was investigated. This electrode fabrication technique can potentially overcome major operational difficulties in commercial EDL processes units such as significant potential and pressure drop of the electrolyte flow through electrode. Furthermore, This electrode fabrication technique could be advantageous for the design and assembly of capacitors with various configurations, e.g., stacked or rolled screens.

The electrochemical properties of the activated biochar electrodes were investigated through Cyclic Voltammetry (CV) and Galvanostatic charge/discharge investigations. For comparison, electrodes have also been made from untreated biochar (as-received biochar) and a commonly used carbon black candidate in EDL applications, i.e., Vulcan XC-72. Electrodes prepared from untreated biochar showed negligible EDL behaviour as studied by CV analyses. However, the electrodes prepared from the activated biochar samples showed typical EDL behaviour as in form of a rectangular-based CV diagram. These results confirm the importance of chemical activation on the EDL performance of biochar electrodes. CV analysis was conducted at sweep rates in the range of 1-50 mV/s to quantify the total capacitance of each electrode. The electrolyte was a mixture of 0.1 mol L^-1 NaCl in 0.1 mol L^-1 NaOH. The maximum capacitance obtained was 167 F.g^-1 for the electrode prepared from activated biochar at 675⁰C and 2 hour residence time under 258 mL.min^-1 N₂ flow. Results have further suggested that increasing the Nafion^® content during electrode preparation yields lower total capacitances, i.e., 43 F.g^-1 capacitance reduction while increasing the Nafion^® content from 0 to 30 wt.%. The activated biochar electrodes have also showed promising charge/discharge behaviour through Galvanostatic analyses. The maximum discharge capacitance of activated biochar electrodes was 55.4 F.g^-1 which is considerably higher than that of Vulcan XC-72 electrode prepared at identical conditions. Furthermore, the total capacitances of the activated biochar electrodes are competitive with much more expensive systems such as carbon nanotubes CNTs and graphene-based electrodes in the literature [1-3].

The activated biochar electrodes are novel, green, and low-cost candidates for EDL-based applications with promising characteristics such as high surface area, high content of carbon oxygen groups, specific type of structure, and comparable electrical conductivity with conventional carbon-based electrodes. Developing this new generation of electrodes can further improve the economical feasibility of commercial units of EDL-based processes, e.g., electrosorption units and supercapacitor fabrication, due to much less costly preparation methods compared to those of promising carbon electrodes (e.g., carbon aerogels and carbon nano-tubes) in the literature. Moreover, the value-added utilization of biochar as carbon-based material in EDL applications would increase the overall economic outlook of woody biomass pyrolysis.

References

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[3]        Y.-K. Hsu, Y.-C. Chen, Y.-G. Lin, L.-C. Chen, and K.-H. Chen, “High-cell-voltage supercapacitor of carbon nanotube/carbon cloth operating in neutral aqueous solution,” J. Mater. Chem., vol. 22, no. 8, p. 3383, 2012.