(Invited) Roll-to-Roll Synthesis of Vertically Aligned Carbon Nanotubes with Embedded Redox Polymers

Electrochemical double layer capacitors (EDLCs), often referred to as supercapacitors, have been the focus of considerable research to develop ultra-high surface area materials with high power and energy density. Activated carbon represents the state-of-the-art EDLC electrode material due to its high porosity, high surface area, and chemical stability. More recently, vertically aligned carbon nanotubes (VACNTs) have emerged as an alternative material due to their high conductivity, stability, facile synthesis and the ability to control the ion-accessible area by varying nanotube density.

VACNT electrodes have been used to achieve high power EDLCs; however, scalable manufacturing methods remain elusive, and the ability to grow VACNTs directly on current collectors will help overcome many hurdles facing commercial application. While the chemical vapor deposition (CVD) method is relatively versatile in terms of controlling CNT characteristics (e.g., tube diameter, number of walls, and dopant ratio), three primary factors limit continuous manufacturing of nanomaterials: i) substrate size determined by reactor geometry, ii) requirements of complex catalytic substrate preparation, and iii) high operating temperatures that are incompatible with traditional current collectors (e.g., Al foil).

In this work, we present a continuous, roll-to-roll process for synthesizing VACNT forests directly on inexpensive Al foil using the ferrocene-xylene liquid injection floating catalyst technique. The Al foil is continuously drawn through a chemical vapor deposition reactor operating at ambient pressure and a relatively low temperature (600 °C). EDLC electrodes comprising VACNT forests synthesized in continuous and stationary CVD processes were directly assembled into supercapacitor cells, which yielded high power densities (1270 W/kg) and energy densities (11.5 Wh/kg). These devices exhibited very low internal series resistance due to their intimate contact with the current collector and excellent cycle stability with no loss in performance over multiple thousands of cycles.

The energy density of VACNT electrodes is increased by integrating high charge capacity redox polymers within the VACNT eelctrodes. In the continuous process, electroactive conducting polymers and redox polymers are embedded into VACNTs using electrochemical methods and physical adsorption processes. Current progress on the performance and limitations of incorporating redox polymers with carbon nanomaterials will be discussed along with the advantages and challenges in future material development.