Surface-related capacitance of microporous carbons in aqueous and organic electrolytes

• The surface-related capacitance is constant in microporous carbons. • The anomalous increase reported for small pores results from the unreliable BET area. • Constant capacitances imply a decrease in relative permittivity in narrow pores. This paper examines the surface-related capacitance C/ S of carbons with average micropore widths between 0.73 and 1.80 nm, using 1–2 M H 2SO 4, 6 M KOH and 1 M (C 2H 5) 4NBF 4 in acetonitrile as electrolytes. Following corrections for pseudocapacitance effects for the aqueous electrolytes and the use of an average surface area suggested by independent techniques, different from the BET area, it appears that C/ S is practically independent of the micropore width. The analysis of the data with the help of recent models suggests that the dielectric constants ɛ r of the different electrolytes may decrease with the pore size. It is surprising that the coexistence of two sets of values for the surface area of microporous carbons and its consequence on C/ S have not received more attention in the past.