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 m²g⁻¹ and specific capacitances <i>C</i> up to 320 F g⁻¹ have been investigated, using H₂SO₄ 2 M as electrolyte. Some of the carbons have also been oxidized with (NH₄)2S₂O₈, which leads to specific oxygen contents between 0.4 and 7.1 μmol m⁻² of carbon surface area. It appears that <i>C</i>_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 <i>d</i> (1–70 mA cm−2 of electrode surface), as a function of the oxygen content. <br><br> As suggested by different authors, <i>C</i>_o can be expressed as a sum of contributions from the external surface area <i>S</i>_e and the surface of the micropores <i>S</i>_mi. A closer investigation shows that <i>C</i>_o/<i>S</i>_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 <i>W</i>_o^* of the electrolyte found between the surface layers in pores wider than 0.7–0.8 nm contributes to <i>C</i>_o. However, this property is limited to microporosity, like the enthalpy of immersion of the carbons into benzene. The latter is also correlated to <i>C</i>_o, which provides a useful means to identify potential supercapacitors.