Drastic change of electric double layer capacitance by surface functionalization of carbon nanotubes

In this letter we show that it is possible to drastically change electric double layer capacitance, from a large increase to complete disappearance, by introducing various surface functional groups on carbon nanotubes. The introduction of surface carboxyl groups created a 3.2 times larger capacitance due to the increased hydrophilicity of the multiwalled carbon nanotubes in an aqueous electrolyte. In contrast, the introduction of alkyl groups resulted in a marked decrease of capacitance. Notably, we unexpectedly observed the complete disappearance of capacitance for samples functionalized with longer alkyl groups than octyl, indicating the perfect block of proton access to the carbon nanotubes’ surfaces by extreme hydrophobicity.