The role of plasma treatment on electrochemical capacitance of undoped and nitrogen doped carbon nanotubes

Carbon nanotubes (undoped CNTs) and nitrogen-doped CNTs (N-CNTs, 2·6 and 5 at.%) were synthesized using a floating catalyst chemical vapor deposition (CVD) method with ferrocene and xylene, pyridine, or acetonitrile. The CNTs were further treated in acid (3:1 HNO₃:H₂SO₄ mixture for 24 h) or in oxygen plasma for 10, 30, or 50 min. The as-produced and acid/plasma treated CNTs were characterized by microscopic and spectroscopic techniques to understand the modification of structures by different treatment procedures. Electrochemical surface areas of the doped and undoped CNTs before and after acid/plasma treatment were studied. It was observed that the electrochemical surface area was significantly low (∼10^-5 to 10^-6 m²) due to limited exposure and wettability of the CNTs on the exposed electrode area. Cyclic voltammetry was also performed to evaluate the effects of these treatments on the capacitance behavior of electrodes. The specific capacitance of CNTs based on the electrochemical surface area was in the order of ∼0·3-8·0 F/g or ∼2000-18000 F/m², which could be attributed to double layer or Faradaic reactions. It was observed that optimized plasma oxidation or surface modification conditions are necessary to achieve maximum specific capacitances. Plasma duration of 10, 30 and 10 min led to maximum specific capacitance for undoped CNTs, 2·6% N-doped CNTs and 5% N-doped CNTs, respectively. Further improvements in specific capacitances necessitate suitable electrode design, selection of electrolyte and charging/discharging conditions. Overall, such low specific capacitances of CNTs in a physiological electrolyte are of critical importance for applications in energy storage in low-powered microreactors or implantable biomedical devices. Supplementary information is available at http://www.icevirtuallibrary.com/upload/10.1680nme.12.00033_SupplementaryInformation.pdf.