Optimization of the characterization of porous carbons for supercapacitors

Abstract

The optimization of carbon-based supercapacitors is of great technological importance for electrical energy storage. This challenge stresses the relevance of current attempts to increase the surface-related capacitance C/S (F m⁻²) of carbons. We show that for microporous carbons this property is practically constant for pore widths between 0.7 and 1.8 nm. The study is based on two independent approaches: (i) the effective surface area accessible to the ions, which may differ considerably from the BET-surface and (ii) the volumetric capacitance in the micropores. One obtains 0.094 F m⁻² in (C₂H₅)₄NBF₄/acetonitrile and approximately 0.100 F m⁻² in aqueous H₂SO₄ after correcting for pseudo-capacitance effects. This pattern is supported by recent modelling which takes into account the solvent. It is also suggested that constant values of the surface based-capacitance may reflect a gradual decrease of the dielectric constant of the electrolytes in smaller micropores due to desolvation.