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 Fg(-1) have been investigated, using H,H2SO4 2M as electrolyte. Some of the carbons have also been oxidized with (NH4)(2)S2O8, which leads to specific oxygen contents between 0.4 and 7.1 mu mol m(-2) of carbon surface area. It appears that C-o, the limiting capacitance at a current density of I 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, 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. (c) 2005 Elsevier B.V. All rights reserved.