Optimization of hierarchical porous carbon derived from a biomass pollen-cone as high-performance electrodes for supercapacitors

Abstract

Activated carbons (ACs) with large surface area and hierarchical porous character are the materials of larger interest to use as electrodes in energy storage devices like supercapacitors/ electrical double layer capacitors (EDLCs). Here, we present comparative studies on ACs, derived from a bio-waste pollen cones activated using two activating agents ZnCl2 and KOH, as electrode materials. Optimization of ACs derived from different ratios of raw pollen cone powder and activating agents has been performed by porosity analysis. The AC-powders have been characterized using X-ray diffraction, scanning electron microscopy, Raman and Fourier-transform infra-red spectroscopy. The electrochemical performance of AC-electrodes have been tested by fabricating symmetric configuration of EDLCs with aqueous liquid electrolyte (1 M Na2SO4) and quasi-solid-state, ionic liquid incorporated gel polymer electrolyte (ILGPE). The free-standing film of ILGPE, comprising IL 1-ethyl-3-methyl imidazolium bis(trifluoromethyl sulfonyl)imide (EMITFSI) immobilized in a copolymer poly(vinylidine fluoride-co-hexafluoropropylene) (PVdF-HFP), is found suitable as EDLC electrolyte due to its flexibility, high ionic conductivity (σ ∼2.4 mS cm−1 at room temperature) and wide electrochemical stability window (∼4.6 V versus Ag). Comparative performance of EDLCs fabricated with AC-electrodes (activated using ZnCl2 and KOH), and aqueous liquid electrolyte and ILGPE have been tested using cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge-discharge studies. Performance of pollen-cone derived AC-electrodes has also been compared with commercial AC and un-activated pollen-cone carbon-electrodes. The KOH-activated AC-powder shows better capacitive performance over the ZnCl2-activated and commercial ACs with both the electrolytes. The ILGPE-based quasi-solid-state EDLC, although, offers comparable specific capacitance (∼126–146 F g−1), the specific energy (∼18–21 Wh kg−1 at effective power of ∼110–190 W kg−1) is higher compared to aqueous electrolyte-based EDLC. The quasi-solid-state EDLC exhibits 98–100% Coulombic efficiency and stable cyclic performance for ∼10,000 charge-discharge cycles with ∼12% initial fading in capacitance. Thermal stability of ILGPE-incorporated cells is found for wider temperature range, particularly at lower temperatures.