Carbon electrodes for double-layer capacitors. I. Relations between ion and pore dimensions

We characterized activated carbon electrodes for electrical double-layer capacitor (EDLC) systems. High-surface-area carbons were prepared by carbonization of cotton cloth at elevated temperatures (up to 1050 °C), followed by activation at 900 °C by oxidation with CO₂ during different time periods. Specific surface areas and characteristic pore sizes obtained from gas adsorption isotherms were correlated with those obtained from ion electroadsorption at the electrical double layer. Electrolytes studied included aqueous LiCl, NaCl, and KCl solutions and nonaqueous propylene carbonate solutions with LiBF₄ and (C₂H₅)₄NBF₄ salts. We found clear evidence that the porous carbons thus formed exhibit ion sieving properties, and that increasing activation time systematically increases the average pore sizes of these carbons. The electric double layer (EDL) capacity of these samples (calculated from voltammetric measurements) depends strongly on the adsorption interaction of the ions in the pores, and hence the relationship between the average pore size and the effective ion size determines the specific EDL capacitance of these samples. The following order of dimension of adsorbed species was found, based on the ion sieving of the various synthesized carbons of different average pore size N₂; Na⁺(aq); Cl^-(≈3.6 angstroms)<BF₄^-<TEA⁺(PC)<Li⁺(PC).