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 MWCNTs are typically entangled tubes with 10 similar to 20 nm outer diameter, around 5 nm inner diameter, and around 220 m(2) g(-1) of BET specific surface area. The galvanostatic measurement (40 mAg(-1), 2 similar to 4 V vs. Li/Li+) in a three-electrode system showed 15 Fg(-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 degrees 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 degrees C or 2300 degrees C with a B-contained graphite crucible. The B-doping can improve the specific capacitance per surface area for the MWCNTs while maintaining the tube morphology. This effect of B-doping on the capacitance can be explained by modification of the space charge layer in carbon.