On the specific double-layer capacitance of activated carbons, in relation to their structural and chemical properties

Twelve well-characterized activated carbons with average micropore widths between 0.7 and 2 nm, total surface areas of 378–1270 m 2 g −1 and specific capacitances C up to 320 F g −1 have been investigated, using H 2SO 4 2 M as electrolyte. Some of the carbons have also been oxidized with (NH 4) 2S 2O 8, which leads to specific oxygen contents between 0.4 and 7.1 μmol m −2 of carbon surface area. It appears that C o, the limiting capacitance at a current density of 1 mA cm −2 of electrode surface, does not depend significantly on the oxygen content. An empirical equation is proposed to describe the decrease of C with increasing current density d (1–70 mA cm −2 of electrode surface), as a function of the oxygen content. As suggested by different authors, C o can be expressed as a sum of contributions from the external surface area S e and the surface of the micropores S mi. A closer investigation shows that C o/ S mi increases with the pore size and reaches values as high as 0.250–0.270 F m −2 for supermicropores. It is suggested that the volume W o * of the electrolyte found between the surface layers in pores wider than 0.7–0.8 nm contributes to C o. However, this property is limited to microporosity, like the enthalpy of immersion of the carbons into benzene. The latter is also correlated to C o, which provides a useful means to identify potential supercapacitors.