World Journal of Engineering
FACILE PREPARATION AND H2 SEPARATION OF MULTIWALLED CARBON NANOTUBE BUCKYPAPER
MEMBRANES WITH TUNABLE PORE SIZES
Xiaoshuang Yang, Lixiang Yuan, Andrew I. Minett, and Andrew T. Harris
School of Chemical and Biomolecular Engineering, University of Sydney, Sydney,
Australia
(Millipore), washed with deionized water
and dried at 110 ºC.
Three different sonication durations (5, 10,
15 min) were undertaken to prepare
buckypapers with 50 mg CNTs dispersed in
300 ml ethanol. The suspensions were then
filter through 0.45 μm PTFE membranes.
Buckypapers were peeled off from the PTFE
membranes and dried at 110 ºC.
Introduction
Carbon nanotube (CNT) based membranes
have attracted more and more attention and
exhibited great potential to a wide range of
applications, such as water and gas
separation, field emission, and fuel cells [1].
Buckypapers, as the simplest type of CNT
membranes, are self-supporting mats of
tangled carbon nanotubes held together by
van der Waals interactions, and present
inherent flexibility within a mechanically
stable structure. Buckypapers are usually
prepared by filtration of carbon nanotube
dispersions in polymer or surfactant
solutions. Chemical functionalization, crosslinking and surfactant addition are usually
used to assist the CNT dispersion and
buckypaper fabrication [2].
In this study, a facile, sufactant-free
assembling process was used to prepare
multi-walled CNT (MWNT) buckypaper
membranes without the aid of cross-linking
or functionization. The pore structures and
gas permeance of the as-prepared
buckypaper membranes were examined.
Characterization
The as-synthesized and purified CNTs
(20.5-97.8%)
were
examined
using
thermogravimetric analysis (TGA, TA SDT
Q600) and fourier transform infrared
spectroscopy (Bruker IFS66V FTIR
Spectrometer). The thickness and the fine
surface structures of buckypapers were
observed by field emission scanning
electron microscopy (SEM, Zeiss Ultra plus).
300 pores were selected for each
buckypaper sample from the SEM images to
calculate the apparent pore sizes using
ImageJ software. The pore structures of the
as-prepared buckypapers were analyzed by
N2 adsorption/desorption measurements
obtained at 77K (Quantachrome Autosorb).
Experimental
Results and Discussion
Preparation of multi-walled carbon
nanotube buckypapers
MWNTs were produced by chemical vapour
deposition in a fluidized-bed, under the
conditions of 600 ºC, 25% ethylene
concentration and 60 min deposition time
[3]. The as-produced CNTs were purified
using a microwave technique described
previously[4]. In a typical purification
process, 200 mg of as-synthesized CNTs
were added to 20 ml of 5 mol/L sulfuric acid
in a sealed, PTFE-lined, microwave
transparent pressure vessel. The sample was
heated for 20 minutes to 220 ºC (at a rate of
10 ºC/min) in a microwave workstation
(MDS-10, Sineo Microwave Technology).
The purified products were then filtered
through 0.45 μm PTFE membranes
A highly efficient purification process was
employed to purify the as-produced CNTs,
achieving a high purity of 97.8%. FTIR
results (not shown here) show that no
functional groups were introduced to the
CNTs
after
the
microwave-assisted
purification process. In order to examine the
properties of pure CNT buckypapers and
simplify the fabrication process, we avoided
introducing any other functional groups or
agents to the CNTs. In the fabrication
process, sonication time is one of the factors
that influence the structure of buckypapers.
In current study, Figure 1 shows the CNT
dispersion during 5 min sonication time.
CNTs did not form buckypaper when they
were weakly dispersed in ethanol below 5
min. When the sonication duration was
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World Journal of Engineering
increased to 10 and 15 min, the CNTs were
more dispersed in the solvent and the pore
size of the resulting buckypapers increased
from 35.3 ± 9.7, to 40.5 ± 9.1, 43.3 ± 11.9
nm, respectively. The porosity also
increased from 59.4% to 70.2% and 71.8%.
Fig 2. Gas (H2, Ar, C2H4, CO2) fluxes
through the as-prepared buckypaper
membrane at different pressure difference.
Fig 1. CNT dispersions in ethanol after (a)
1min, (b) 3min, (c) 5 min sonication; (d) the
as-prepared CNT buckypapers; side (e) and
top (f) view and of the as-prepared CNT
buckypaper
Conclusion
Tunable pore-size MWNT Buckypapers
were prepared by a facile process of
purification, sonication and filtration,
without the assistance of surfactants, crosslinking or functionalization. This method
enables us to examine the properties of the
resultant buckypapers with pure CNTs. The
pore structures of the as-prepared
buckypaper varied with the sonication
duration to disperse CNTs. H2 selectivities
over other gases show that the buckypaper
membranes can be used as H2 purification
membranes.
The H2 permeance of the buckypaper
membranes prepared by 10 min sonication
was obtained by measuring the gas flux
through them under different pressure
difference at 30 °C. Three other common
laboratory gases (Ar, C2H4, CO2) were also
tested. For all the gases (H2, Ar, C2H4, CO2),
the as-prepared buckypaper exhibited a
linear relationship between the flux and the
pressure difference, indicating a pressureindependent diffusion in the buckypaper
membrane. Based on the pore size of the
buckypaper, the gas permeance behavior can
be predicted by Knudsen diffusion.
However, the gas permeance of the asprepared buckypaper membrane was 2.63 –
3.34 times of that predicted by Knudsen
diffusion, suggesting an enhancing effect
due to the existence of the CNTs. Several
studies have reported that the gas fluxes
through the CNT inner channels were 1 – 2
orders of magnitude higher than those
predicted by Knudsen diffusion due to the
smoothness of the CNT surface. It is likely
to suggest that the outside wall of the CNT
is capable in providing a smooth surface for
fast gas transport similar to the inner wall
does. The ideal selectivities of H2 over other
gases (SH2/Ar = 4.19, SH2/C2H4 = 3.34, SH2/CO2
= 4.27) were calculated based on the single
gas permeance and were found to be similar
to those predicted by Knudsen diffusion.
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