Stable ionic-liquid-based symmetric supercapacitors from Capsicum seed-porous carbons

Abstract

In this study, a symmetric ionic-liquid based supercapacitor was assembled with porous carbon derived from Capsicum (bell pepper) seeds. The “peppered”-activated carbon (ppAC) was synthesized using varying amounts of KHCO3 activating agent (AA) at 850°C carbonization temperature. The best device performance reported was recorded with optimum amounts of AA to raw material. The need for less amount of AA is crucial if the entire activation/carbonization process is to be scaled-up with the cost and final product yield also being important for a viable synthesis. A mechanism of saturation of pores with unreacted AA which leads to lower porosity metrics in the samples with increasing the amount of AA during carbonization/activation was also proposed. Using an ionic liquid electrolyte, 1-ethyl-3-methylimidazolium bistrifluorosulfonylimide (EMIM-TFSI), the ppAC-based supercapacitor operated up to a maximum cell voltage of 3.20V.A specific energy of 37Whkg−1 was obtainable with a corresponding practical power density of 0.6kWkg−1 at 0.5 A g−1. A specific energy of ∼26Whkg−1 was still achievable when the applied current was doubled to 1.0 A g−1 and a high cyclic stability (approx. 99% coulombic efficiency) was proven over 25,000cycles. Further ageing test performed on the device revealed a remarkable improvement in the electrochemical performance after a 180h (ca. 1week) floating time. The obtained results also confirmed a uniquely distributed porous carbon in which the complete utilization of the entire less-corrosive KHCO3 AA for optimal pore activation at elevated carbonization temperatures. Thus, the efficient design combinations for stable, high-energy and power ionic liquid-based supercapacitors with cheaper biomass-based materials are demonstrated.