(70c) Roll-to-Roll Synthesis of Vertically Aligned Carbon Nanotube Electrodes for Electrical Double Layer Capacitors

Electrochemical double layer capacitors (EDLCs), also known as supercapacitors, have received notable attention in recent years for applications requiring durable and reliable devices that can deliver high power output with high energy density. EDLCs provide energy densities 2 to 3 orders of magnitude greater than that of electrolytic capacitors, while offering comparable power densities, thus, providing a suitable candidate power systems requiring a combination of power and energy. EDLCs have been commercialized using activated carbon electrodes, primarily due to its high surface area, high porosity and electrochemical stability.

Vertically aligned carbon nanotubes (VACNTs), however, have emerged as an alternative material for EDLCs due to their high conductivity, electrochemical stability, facile synthesis and the ease of controlling the ion-accessible area by varying CNT density on substrates. Although CNTs can be synthesized in large quantities, present processes are not amenable to continuous synthesis of VACNTs directly on current collectors for scalable manufacturing of EDLC electrodes, which is necessary to maintain low costs for commercialization. Several methods have been investigated for CNT synthesis, including electric arc discharge, laser ablation and chemical vapor deposition (CVD). Of these, CVD has emerged as a practical and reliable method for synthesizing VACNT forests with tunable properties (e.g., tube diameter, number of walls, and dopant ratio). During material synthesis, 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 scalable, roll-to-roll process for synthesizing forests of VACNTs on inexpensive Al foil substrates using the ferrocene-xylene liquid injection floating catalyst technique for the continuous production of CNT‑based EDLC electrodes. The Al foil is continuously drawn through a chemical vapor deposition reactor operating at ambient pressure and a relatively low growth 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.