Capacitance response of double-walled carbon nanotubes depending on surface modification

The capacitive behaviors of high-purity double-walled carbon nanotubes (DWNTs) were investigated before and after oxidation using nitric acid (HNO3). The electrodes prepared from the HNO3-oxidized DWNTs have exhibited higher capacitances than the pristine nanotube electrode in both aqueous and non-aqueous systems, despite the deterioration of their specific surface area after HNO3 oxidation. The superiority of the HNO3-oxidized DWNTs in capacitance properties is caused by the variations of surface wettability and the interstitial pore structure of nanotube bundles, which result from the introduction of polar oxygen functional groups onto the nanotube surface by HNO3 oxidation.