Nitrogen plasma functionalization of carbon nanotubes for supercapacitor applications

Surface modification of carbon nanotubes with a simple and fast plasma treatment allows for the design of new nanomaterials with enhanced electrochemical properties. Both structural disorder and nitrogen concentration of the nanotubes increase after a nitrogen plasma treatment. The effect of plasma power and nitrogen pressure on the charge storage capability of the nanotubes has been investigated in detail. Depending on the plasma conditions, nitrogen functionalities such as quaternary nitrogen in the basal planes, and pyrrolic groups at the edges are introduced in the nanotubes structure. The potential difference between anodic and cathodic peaks of the Fe³⁺/Fe²⁺ redox couple decreases from 102 mV down to 75.7 mV after the nitrogen plasma treatment, which accounts for an increased reversibility of the electron transfer process between nanotubes and electrolyte. Moreover, the treated nanotubes show a significant increase in their specific capacitance from 22 up to 55 F g⁻¹ at a scan rate of 10 mV s⁻¹ in a 0.1 M Na₂SO₄ solution. Pyridinic and pyrrolic functionalities are found to play an important role in enhancing the reversibility and specific capacitance of the obtained electrodes.