Abstract Multi-walled carbon nanotubes (MWCNTs) directly grown on a stainless steel mesh were functionalized using a fast, solvent-free radiofrequency (RF) plasma technique. Two different gas mixtures of Ar/C₂H₆/O₂ and Ar/C₂H₆/N₂ were used to add oxygen- and nitrogen-containing functional groups to the surface of MWCNTs. The binder-free forest-like MWCNTs before and after functionalization were tested as supercapacitor electrodes. The structural and electrochemical characteristics of the electrodes were evaluated. The RF power (10–30 W) and functionalization time (1–20 min) were optimized to achieve the highest capacitance. Both nitrogen- and oxygen-functionalized MWCNTs showed improved capacitive behavior due to the presence of the functional groups on the MWCNT surface. The highest capacitance was obtained for the MWCNT electrode functionalized with the RF plasma employing the Ar/C₂H₆/O₂ gas mixture at 20 W for 15 min, reaching the specific capacitance of 37.3 ± 1.1 mF cm⁻² (196.3 ± 5.8 F g⁻¹).