Contrast structure and EDLC performances of activated spherical carbons with medium and large surface areas

Two spherical carbons of 500 and 3000 m2/g, respectively, activated with NaOH (M500) and KOH (M3000), were examined in relation to their carbon structure and electrochemical behavior to explain their contrast capacitances as EDLC electrode.M500 and M3000 showed capacitances per weight (F/g) and volume (F/ml) of 35 and 35 (M500), 40 and 25 (M3000), respectively. The charge profile of M500 by galvanostat charge indicated that the charge took place rapidly below 1.5 V and then very gradually increased up to the final voltage of 2.7 V in the first charge. Such electrochemical behavior suggests electric field activation of this particular activated carbon at the charge. The charge profile of M3000 was conventional. The structure of M500 suffered a certain marked expansion at the charge, however the 0 0 2 diffractions profile shifted very slightly to a lower angle at the change. Such charge appears reversible while the structure of expansion was more or less irreversible. No expansion was observed with M3000 at the charge to the same voltage.Such a structure allows high efficiency of EDLC due to small pores and electric field activation to induce small pores among the graphitic units. In contrast, M3000 with its larger surface areas of relatively large pores in the graphitic structure showed a larger capacitance per weight. However many useless pores cause lower the capacitance per volume. In addition, large pores lose the efficiency for the formation of an electric double layer per unit surface area of the pore, while the non-graphitic wall of M3000 fails to introduce small pores with a higher capacitance.