Electric double layer capacitance of multi-walled carbon nanotubes and B-doping effect

The double layer capacitance properties of multi-walled carbon nanotubes (MWCNTs) prepared by CVD were investigated using propylene carbonate electrolytes. The original MWCNTsare typically entangled tubes with 10∼20 nm outer diameter, around 5 nm inner diameter, and around 220 m2 g-1 of BET specific surface area. The galvanostatic measurement (40 mA g-1, 2∼4 V vs. Li/Li+) in a three-electrode system showed 15 F g-1 of gravimetric capacitance and a good rate of this property. Due to the tips opening by thermal oxidation, the specific surface area and the capacitance increased. In contrast, heat-treatment at 3000 °C decreased the surface area and the capacitance of the MWCNT due to defect recovery. A small amount (<1 at. %) of boron was doped to the MWCNT by heat-treatment at 2200 °C or 2300 °C with a B-contained graphite crucible. The B-doping can improve the specific capacitance per surface area for the MWCNTswhile maintaining the tube morphology. This effect of B-doping on the capacitance can be explained by modification of the space charge layer in carbon.