Conversion of carbon nanotubes into curved graphene with improved capacitance

Multi-wall carbon nanotubes (MWCNTs) have achieved mass production, but their lengths are in the millimeter range, which is unfavorable for the diffusion of electrolyte ions into their innermost tube. We report an oxidation method to simultaneously cut and unzip MWCNTs along transverse and longitudinal directions, which leads to the formation of curved graphene sheets (CGSs). SEM shows that the curved morphology was retained but the diameters were large after unzipping. This could be caused by the interaction of oxygen-containing functional groups between layers on the edges of the CGSs. Because of the larger number of active sites the specific capacitance is improved. To further increase the capacitive performance, a sample was put into a 0.1 mol L⁻¹ KMnO₄ to incorporate MnO₂. The microstructure of the resulting CGS-MnO₂ hybrid was revealed by electron microscopy, Raman spectroscopy and powder X-ray diffraction. The results indicate that amorphous MnO₂ successfully grew on the surface of the CGSs. The capacitive behavior was measured by cyclic voltammetry in a 1 mol L⁻¹ Na₂SO₄ solution. The CGS-MnO₂ had a specific capacitance of 236 F g⁻¹ at 2 mV s⁻¹ (even 127 F g⁻¹ at 100 mV s⁻¹), which is superior to that of MWCNTs (15 F g⁻¹), CGS (88 F g⁻¹) and MWCNT-MnO₂ (111 F g⁻¹). In addition, excellent cycling performance was achieved for the CGS-MnO₂ hybrid electrode with a 97% capacitance retention over 1000 cycles.