Capacitance behavior of KOH activated mesocarbon microbeads in different aqueous electrolytes

A series of microporous carbons with wide scope of porosity development (surface area SDFT = 1200–2460 m2 g−1, micropore volume VDR = 0.43–1.2 cm3 g−1, average micropore width L0 = 0.9–1.5 nm) was prepared by KOH activation of coal-tar pitch derived mesocarbon microbeads (MCMB). The activated MCMB were tested as an electrode material of capacitor in two-electrode system operating in various aqueous electrolytic solutions using galvanostatic, voltammetry and impedance spectroscopy techniques. The study shows that the capability of charge accumulation in electric double layer is controlled primarily by electrolyte properties. Independent of porosity development the capacitance of activated MCMB decreases in the order: 6 mol L−1 KOH, 1 mol L−1 H2SO4, 4.5 mol L−1 KCl, 2 mol L−1 KNO3 and 0.5 mol L−1 Na2SO4. The sequence corresponds basically to the solution conductivity. Activated MCMB of moderate surface area (SDFT = 1450–1650 m2 g−1) and relatively narrow micropores (L0 = 0.9–1.05 nm), which were produced from MCMB carbonized at 600–700 °C by activation at 700 °C using 2:1 KOH/MCMB ratio, seem to be most promising electrode material. They demonstrate superior both the gravimetric and volumetric capacitances (in 6 mol L−1 KOH 280–300 F g−1 and above 160 F cm−3, respectively) and a good performance at the current density up to 20 A g−1. Apart from the suitable pore size distribution, the specific particle morphology and a high extent of orientation of constituting defective graphene layers seem to contribute to the superior capacitance properties of activated MCMB compared to ordinary activated carbons.