Correlation between heats of immersion and limiting capacitances in porous carbons

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 C. 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 Delta H-i(C6H6) and Delta H-i(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 Delta H-i(C6H6) and Delta H-i(CCl4). Carbon tetrachloride has a critical dimension (0.65 nm), which is close to the size of the (C2H5)(4)N+ ion (0.68 nm) and therefore Delta H-i(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. (c) 2008 Elsevier Ltd. All rights reserved.