Understanding the rate performance of microporous carbons in aqueous electrolytes

Variation of specific capacitance versus current density is studied for microporous carbons. Although literature states that capacitance retention is higher for macro/mesoporous than for microporous carbons, the results reported here show that high capacitance retention can be reached for microporous carbons in combination with aqueous electrolytes (2M H2SO4, 1M KOH and 6M KOH). Six carbon monoliths are studied; three pristine ones and those three heat-treated, so as to reduce their content of surface oxygen groups and develops porosity. The capacitance retention is analyzed based on five parameters: electronic conductivity, surface chemistry and porosity of the monoliths, ionic conductivity and type of electrolyte. The capacitance retention is higher for the monoliths working as negative (H3O+ and K+) electrodes than as positive (HSO4− and OH−) ones, being these results of interest for the use of carbon monoliths in asymmetric and hybrid supercapacitors. The highest capacitance retention is obtained by combining (i) monolith electronic conductivity of 11–14 Scm−1 and micropore size of 0.6–0.8 nm for H3O+, K+ and HSO4−, and of 0.85–0.95 nm for OH−; (ii) electrolyte ionic conductivity above 600 mScm−1 and 6M KOH electrolyte, since this electrolyte performs better than 2M H2SO4 and 1M KOH.