Single-Step Synthesis of Vertically Aligned Carbon Nanotube Forest on Aluminium Foils

Abstract

Vertically aligned carbon nanotube (VACNT) forests are promising for supercapacitor electrodes, but their industrialisation requires a large-scale cost-effective synthesis process suitable to commercial aluminium (Al) foils, namely by operating at a low temperature (<660 °C). We show that Aerosol-Assisted Catalytic Chemical Vapour Deposition (CCVD), a single-step roll-to-roll compatible process, can be optimised to meet this industrial requirement. With ferrocene as a catalyst precursor, acetylene as a carbon source and Ar/H₂ as a carrier gas, clean and dense forests of VACNTs of about 10 nm in diameter are obtained at 615 °C with a growth rate up to 5 µm/min. Such novel potentiality of this one-step CCVD process is at the state-of-the-art of the multi-step assisted CCVD processes. To produce thick samples, long synthesis durations are required, but growth saturation occurs that is not associated with a diffusion phenomenon of iron in aluminium substrate. Sequential syntheses show that the saturation trend fits a model of catalytic nanoparticle deactivation that can be limited by decreasing acetylene flow, thus obtaining sample thickness up to 200 µm. Cyclic voltammetry measurements on binder-free VACNT/Al electrodes show that the CNT surface is fully accessible to the ionic liquid electrolyte, even in these dense VACNT forests.

Keywords: Al foils; aerosol-assisted catalytic chemical vapour deposition; energy dispersive X-ray spectrometry; supercapacitors; vertically aligned carbon nanotubes.

Figure 1

Scanning electron microscope (SEM) images of carbon nanotube (CNT) synthetized at 580 °C on thin aluminium (Al) discs during 20 min with: (a) high Fe/C ratio (>4 wt.%) and (b) medium Fe/C ratio (between 2 and 4 wt.%) and (c) low Fe/C ratio (<2 wt.%).

Figure 2

(a,b) SEM, (c) transmission electron microscopy (TEM) and (d) high-resolution TEM (HRTEM) images of VACNT obtained in optimized synthesis conditions at 580 °C (left) and 615 °C (right), respectively.

Figure 3

(a) Cyclic voltammograms (CV) curves of the different VACNT carpets in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI) 1.3 M in ACN at 5 mV/s. (b) CV of P7 in EMITFSI 1.3 M in ACN at different scan rates. (c) Influence of the electrolyte composition on the capacitance of VACNT electrode. (d) Variation of capacitance with developed surface area.

Figure 4

(a) SEM images of VACNT carpets obtained at 580 and 615 °C for increasing synthesis duration. Influence of synthesis duration on (b) carpet thickness, (c) CNT number density and (d,e) external and internal CNT diameters and their standard deviations at 580 and 615 °C, respectively.

Figure 5

(a) Chemical cartography, (b) STEM images and (c) profiles of the Al/VACNT interface from syntheses at 580 °C of 1, 20 and 40 min duration, respectively (all images are at the same scale).

Figure 6

SEM images of carpets obtained with sequences of (a) 5 min then 10 min at 580 °C, (b) 4 × 10 min at 580 °C and (c) 8 × 10 min at 615 °C. (d) Fits of experimental data with exponential model (red) and diffusion model (blue). (e,f) SEM images of carpets obtained at 615 °C for 80 min without (e) or with (f) reduction of C₂H₂ flow rate after 20 min. (g) Evolution of VACNT thickness with synthesis duration without (red) or with (green) reduction of C₂H₂ flow rate after 20 min.

Figure 7

VACNT grown on a 30 cm-wide Al foil at NAWATechnologies’ roll-to-roll (R2R) production line (rolling speed of 1 m/h): (a) set-up, (b) Al foils at the oven exit, (c) SEM image of the grown VACNT carpet and (d) HRTEM image of a nanotube.