Nitrogen and phosphorous dual-doped carbon nanotubes (N,P/CNT) have been grown in a single-step direct synthesis process by CVD method using iron-loaded mesoporous SBA-15 support, as an electrode material for the energy storage device. For comparison, pristine nanotubes, nitrogen and phosphorous individually doped nanotubes were also prepared. The basic characterization studies clarify the formation of nanotubes and the elemental mapping tells about the presence of the dopant. Under three-electrode investigations, N,P/CNT produced a maximum specific capacitance of about 358.2 F/g at 0.5 A/g current density. The electrochemical performance of N,P/CNT was further extended by fabricating as a symmetric supercapacitor device, which delivers 108.6 F/g of specific capacitance for 0.5 A/g with 15 Wh/kg energy density and 250 W/kg power density. The observed energy efficiency of the device was 92.3%. The capacitance retention and coulombic efficiency were 96.2% and 90.6%, respectively, calculated over 5000 charge–discharge cycles.