Oxygen-rich porous carbons derived from alfalfa flowers for high performance supercapacitors

Abstract

Alfalfa flowers derived oxygen-rich porous carbons are fabricated via carbonization, acidic washing and KOH activation processes. Supercapacitor electrodes designed from these porous carbons are characterized in both three- and two-electrode measurement systems with 6 M KOH as electrolyte at room temperature by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. The experimental results show that the acidic washing for carbonized carbons and activation temperature perform profound effects on the capacitance performance of the porous carbons. Acidic washing treatment removes most of residues, which do not contribute capacitance for porous carbons. High temperature activation leads to large specific surface area and thus increases the capacitance of porous carbons. In three-electrode system, the electrode designed from carbons activated at 800 °C after acidic washing shows the highest specific capacitance of 350.1 F g−1 at current density of 0.5 A g−1 and 297 F g−1 is kept at current density of 50 A g−1. The specific capacitance of the symmetrical supercapacitor is 201.4 F g−1 at 0.1 A g−1 with 82.7% capacitance retention (166.6 F g−1) at 10 A g−1. The energy density of the symmetrical supercapacitor increases from 23.2 Wh kg−1 to 28 Wh kg−1 with decreasing power density from 10.3 kW kg−1 to 100 W kg−1.