Electrochemical activation to enhance the volumetric performance of carbon nanotube electrodes

Abstract

In recent years, the floating catalyst chemical vapor deposition (FCCVD) method has established itself as one of the most commercially viable methods to produce carbon nanotubes (CNTs). However, at the final stage of the FCCVD process, densification of the web of CNTs to form a sheet causes a significant loss in the active surface area of the CNTs. This loss of surface area reduces the density of active materials that can be infiltrated into the CNT sheet. Reducing the amount of active material in the sheet reduces the charge storage capacity and causes low volumetric performance for CNT sheet in energy storage applications. Here, we are reporting the use of an electrochemical activation technique to chemically functionalize the CNT surface and recover the lost surface area. Chemical functionalization separates the CNTs and thereby increases the density of active materials that can be integrated into the CNT sheet. In order to avoid deterioration of the CNT structure from excessive electrochemical activation, we have optimized the procedure by comparing the performance of four samples of CNT sheets prepared using different numbers of cycles of electrochemical activation (5, 10, 20, 40 cycles). The performance of the sheet was analyzed using specific capacitance measurement, electrochemical impedance spectroscopy (EIS) analysis, and UV–vis absorption spectroscopy. In addition, the Randle-Sevcik plot was used to select the CNTs with the largest activated surface area and electrochemical specific capacitance (150F/g). Further analysis such as x-ray photoelectron spectroscopy (XPS) was also carried out to unveil a 5% increase of the various oxygen-containing functional groups at the surface of the activated CNTs. Polyaniline (PANI) was then deposited on the surface of the electrochemically altered CNTs via an oxidation polymerization process to create a CNT-PANI composite material. A fully fabricated device with CNT-PANI electrodes and Polyvinylidene fluoride − 1-Ethyl-3-methylimidazolium tetrafluoroborate (PVDF-EMIMBF4) electrolyte showed a significant specific capacitance improvement over a device with Poly(vinyl alcohol) -Sulfuric acid (PVA-H2SO4) electrolyte. The PVDF-EMIMBF4 capacitor device had a volumetric energy density of 833.3 µWh/cm3 at a current density of 0.5 mA/cm2. The capacitor has good mechanical flexibility and high capacitance retention (75% after 5000 cycles), which show the advantages of the electrochemical activation method.