Correlation between heats of immersion and limiting capacitances in porous carbons

Abstract

Based on more than 80 carbons, the paper shows that immersion calorimetry into benzene, water and carbon tetrachloride can be used to assess with a good accuracy the limiting capacitance Co at low current densities in both acidic (2 M H2SO4) and aprotic (1 M tetraethyl ammonium tetrafluoroborate in acetonitrile) electrolytic solutions. The enthalpies of immersion ΔiH(C6H6) and ΔiH(H2O) provide information on Co-acidic, where both the surface area and the oxygen content play a role. On the other hand, in the case of the organic electrolyte the oxygen content has only a small influence and Co-aprotic is directly related to ΔiH(C6H6) and ΔiH(CCl4). Carbon tetrachloride has a critical dimension (0.65 nm), which is close to the size of the (C2H5)4N+ ion (0.68 nm) and therefore ΔiH(CCl4) provides better information in the case of carbons with small micropores. The advantage of this approach lies in the fact that immersion calorimetry, in itself a useful tool for the structural and the chemical characterization of carbons, can also be used to evaluate directly the gravimetric capacitances of these solids at low current densities.