22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma-Modified Carbon Nanotubes for Alternative Energy Storage
M.A. McArthur1,2, M.D.G. Evans1, F.P. Sainct1, S. Omanovic2 and S. Coulombe1
1
Plasma Processing Laboratory & 2Electrochemistry/Corrosion Laboratory, Department of Chemical Engineering,
McGill University, Montréal, Québec, Canada
Abstract: Plasma modification of traditional materials plays a large role in producing high
quality electrodes for electrochemical energy storage (ees) applications. Electrocatalysts
made up of multi-walled carbon nanotubes-supported Ni nanoparticles showed a ~600
times increase in activity towards hydrogen generation relative to a 2D Ni plate. Oxygenfunctionalized multi-walled carbon nanotube electrodes displayed a ~7 times increase in
electrochemical capacitance relative to non-functionalized carbon nanotubes.
Keywords: carbon nanotubes, plasma functionalization, pulsed laser ablation, optical
emission spectroscopy, hydrogen generators, supercapacitors, electrochemistry
1. Introduction
Alternative energy storage sources are required to
reduce and ultimately eliminate our dependence to CO 2 producing fuels [1].
Using nanostructured carbon
electrodes, such as multi-walled carbon nanotubes
(MWCNTs), for electrochemical energy storage (ees) is
one such alternative to the traditional fuels [2]. Although
MWCNTs are perceived as excellent nanomaterials for
energy storage due to their high aspect ratios and good
thermal, electrical, and structural properties, there are no
commercial energy storage devices containing them.
Presently, the robustness and performance of these
materials cannot compete with proven carbons such as
graphite and activated carbon.
In our laboratory, we have devised a thermal CVD
process for the direct growth of MWCNT onto stainless
substrates, thus directly producing MWCNT electrodes,
and several simple plasma processing steps to modify
these electrodes to enhance their electrochemical
performance as both electrocatalyst supports for hydrogen
generators and electrochemical energy storage
(supercapacitors).
With recent pushes in hydrogen powered fuel cell
vehicles, the generation and transportation of this
abundant element is required [3]. Water electrolysis,
when coupled with other green energy technologies, is
one of the cleanest methods of producing high-purity H 2
[4]. However, the search for an inexpensive, non-noble
electrocatalyst is a daunting task. Ni has been suggested
as an alternative due to its relative abundance and its
modest cost [5], [6]. In order to maximize performance
(electrocatalytic activity) and minimize costs, large
surface area electrodes are needed. These conditions can
be achieved by using Ni nanoparticle (NP)
electrocatalysts supported on an open, high area matrix of
robust conductive material.
Of the NP producing
techniques available, pulsed laser ablation (PLA) has been
identified as a promising investigative method at the
bench scale [7]–[9]. Using a high-power ns-pulsed laser,
a target is irradiated at power fluxes on the order of 1012
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W cm-2, which level is well above ablation thresholds.
The resultant metallic plume created in vacuum rapidly
quenches and forms NPs in flight which can deposit onto
a surface. The plasma formed, though small, has a large
impact on the materials produced [10]. The size and
deposited coverage of the NPs produced by PLA can be
tuned to the user-defined specifications and makes for a
versatile bench-scale plasma processing approach.
Supercapacitors (SCs) are well-known ees devices with
a high power density relative to batteries. Charge is
stored in these devices traditionally via the double-layer
or Faradaic (charge transfer) reactions. For MWCNTs,
this value ranges between 7 – 70 F g-1 and is relatively
low compared to other carbons. Oxygen-containing
surface groups (carboxylics, carbonyls, hydroxyls) grafted
onto carbons lead to an increase in specific capacitance
[11].
However, these surface groups, prepared
traditionally by acid exfoliation, are unstable for stressful
and long-term cycling. Plasma functionalization has been
shown to stably graft oxygen groups to surfaces, creating
a hydrophilic surface [12].
In this presentation, we show how simple plasma
processing steps enable drastic enhancements of ees
electrode performance.
2. Experimental Methods
2.1. MWCNT Growth by Thermal-Chemical Vapour
Deposition
MWCNTs were grown in-house using a thermalchemical vapour deposition (t-CVD) technique. Details
of t-CVD can be found elsewhere [12]–[14]. Briefly,
growth occurs on an inexpensive, commercially available
316 stainless steel (SS) mesh (400 series, 25µm grid dia.;
McMaster-Carr, USA) in a tube furnace (Lindberg Blue,
Thermo Scientific, USA) at 700 ºC under an Ar flow
(592 cm3 min-1). Acetylene is used as the carbon source
and is injected into the furnace for 4 min at 68 cm3 min-1.
Once the furnace has cooled, the SS/MWCNT electrodes
are removed.
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2.2. Nanoparticle Decoration of MWCNT Electrodes
by Pulsed Laser Ablation
PLA was performed to stably fix Ni NPs onto the
MWCNT walls following the procedure outlined in ref.
[14]. PLA took place in an evacuated vacuum chamber
(P base ≈ 2.5 mTorr). The MWCNT electrode was placed 7
cm away from a Ni target (99.9%, 2.54 cm dia.; Kurt J.
Lesker Co., USA) mounted on a linear displacement stage
to even out target erosion. The chamber pressure was
regulated with He at the desired pressure (P = P base or
4 Torr).
A computer-controlled frequency-tripped nanosecond
pulsed Nd:YAG (355 nm, 10 Hz, 5 ns pulse duration;
Brilliant B10, Quantel, FR) laser beam was focused to
~500 µm onto the target surface at a 45º angle. Ablation
time (t PLA ) was adjusted for the desired Ni NP loading on
the MWCNTs.
Ni NP-decorated MWCNTs are
henceforth referred to as Ni NP/MWCNTs.
2.3. Plasma Functionalization of MWCNT Electrodes
Electrodes suitable for supercapacitors were prepared
by grafting stable oxygen functionalities to the MWCNT
surfaces by exposure to a low-pressure capacitivelycoupled RF glow discharge (1.25 Torr) [12]. The
atmosphere under which plasma functionalization
occurred was a mixture of Ar/O 2 /C 2 H 6 :250/5/1 cm3 min-1
metered by mass-flow controllers (Brooks 5050E, Brooks,
USA), sustained at 20 W for 5 min using a continuous
wave 13.56 MHz power supply (Advanced Energy Cesar;
Advanced Energy, USA).
Plasma functionalized
MWCNTs are referred henceforth as f-MWCNTs.
MWCNTs are featureless and indicate little-to-no
destruction of the MWCNTs maintaining the open
morphology and high aspect ratio (surface area). From
the SEM image (Fig. 1d), NPs appear to uniformly cover
the MWCNT surfaces. In the inset TEM image, one
notices globular and uniformly dispersed Ni NPs along
the displayed MWCNT (for PLA at P = P base ).
Fig. 1. High-resolution SEMs of SS mesh (a),
bare MWCNTs (b), f-MWCNTs (c), and Ni
NP/MWCNTs (d). TEMs of (c) and (d) are
shown in the respective insets.
2.4. Characterization of Plasma Sources by Optical
Emission Spectroscopy
Optical emission spectroscopy (OES) was performed to
characterize the plasma produced by both sources
described in Sections 2.2 and 2.3. Light emitted from the
RF glow or laser plasma plume passed through a vacuumcompatible quartz window and into an optical fibre to a
spectrometer (Ocean Optics USB2000, Ocean Optics Inc.,
USA) calibrated and corrected in wavelength and
intensity. The exposure time of each acquired spectra was
100 ms, and an average over 10 acquisitions was used to
produce the observed spectra.
3. Results and Discussion
3.1. Morphology of Electrodes
Fig. 1a-b shows high resolution scanning electron
micrographs (SEMs) of the SS mesh electrodes before (a)
and after (b) MWCNT growth by t-CVD. MWCNTs
grown by t-CVD are ~60-100 nm in dia. and extend
~10 µm from the SS grid and form an open, 3dimensional matrix which can be readily functionalized.
Fig. 1c-d show higher magnified views of the f-MWCNTs
(c) and Ni NP/MWCNT for t PLA = 40 min at P base (d).
The insets to Figs. 1c-d show transmission electron
micrographs (TEMs) of the respective conditions. The f-
2
Fig. 2. Particle size distribution for Ni NPs
deposited onto MWCNTs by PLA at base
pressure (a) and 4 Torr of He (b). N = 100.
Fig. 2 displays the synthesized Ni NP size distribution
histograms at P base (a) and P = 4 Torr (b) along with their
respective log-normal fits.
The insets show highresolution TEM images of the NPs on the MWCNTs.
The morphology of the Ni NPs is strongly dependent on
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the pressure of the background He gas within the PLA
chamber. At low pressure, the NPs take on a globular
form. Further, NP coverage of the MWCNTs is relatively
sparse. At high pressure, Ni forms elongated “flake-like”
NPs which completely cover the MWCNT surfaces. At
both pressures, the mean particle size is quite small (3.8
and 2.03 nm for P base and P = 4 Torr, respectively) with a
narrow size distribution (log-normal variance (σ2) of 2.4
and 0.67 for P base and P = 4 Torr, respectively).
Fig. 2 clearly demonstrates the tunability of NPs
formed using the PLA technique. The size distribution
and morphology is affected by the pressure at which
ablation occurs. The customization of the PLA technique
does not end at pressure; NPs formed from various target
materials, such as Au or Ti, can be substituted for Ni in
this case.
Furthermore, PLA of ceramics can be
performed to form NPs [15]. The NP loading can be
adjusted by varying t PLA .
3.2 Optical Emission Spectroscopy of Plasma Sources
Fig. 3 shows the emission spectrum from the RF
plasma functionalization process. The inset to Fig. 3
shows the ultra-violet section of the spectrum. The
OH(A-X) and CH(A-X) systems can be seen in the inset.
These active species contribute to the formation of the
active carboxylic and carbonyl groups grafted to the
MWCNTs. Ar-I lines appear at higher wavelengths (λ >
700 nm), which is characteristic emission of the process
during
plasma
gases
present
(Ar/O 2 /C 2 H 6 )
functionalization of the MWCNT electrodes.
At
intermediate wavelengths, a mixture of Ar and carbon
lines can be observed. At low wavelengths (λ < 350 nm),
the breakdown of –OH is observed. This part of the
plasma plays a strong role in the addition of the stably
bound oxygen species to the MWCNT surfaces.
Fig. 4 shows the emission spectrum recorded for the
PLA system at both P base and P = 4 Torr. The inset to
Fig. 4 shows an expanded view between 420 and 560 nm.
Strong emission lines observed at 355 and 532 nm are
highlighted by vertical dashed lines. These lines are due
to the laser’s primary (355 nm) and secondary (532 nm)
beams from frequency tripling.
In the inset to Fig. 4, at an operating pressure of 4 Torr,
numerous lines are observed relative to operation at P base .
These lines are prescribed to Ni emission in the plasma
plume formed during ablation. Interestingly, these lines
are largely absent (or at least much lower in intensity) at
P base , despite observed Ni NP deposition on the
MWCNTs (Figs. 1d, 2a). This discrepancy in the
emission spectra may be due to the plasma ablation plume
expansion into the background gas (4 Torr He vs
vacuum). The plasma produced by PLA systems is very
similar to those produced by cathodic arc erosion [10]. In
He, the plasma/gas interaction zone is more conducive to
NP formation than in the vacuum condition. This is
logical considering the large NP number and coverage
observed on the MWCNTs at 4 Torr for the sample t PLA
(insets in Fig. 2).
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Fig. 3. OES spectrum for hydrophilic functionalization of MWCNTs.
Fig. 4. OES spectrum for PLA of Ni NPs at
reactor base pressure and 4 Torr He.
3.3 X-ray Photoelectron Spectroscopy of MWCNTs
Fig. 5 displays the X-ray photoelectron spectrograph
(XPS) of the C1s peak after plasma functionalization for
the non-functionalized (“nf-” designation) and f-MWCNT
electrodes. It is clear that after plasma functionalization,
contributions to the XPS spectrum from the oxygencontaining carbonyls (C=O; ~286.8 eV), carboxyls (O–
C=O; ~288.9 eV), and hydroxyls (C–OH; ~285.8 eV) are
present on the MWCNT surfaces.
Fig. 5. High-resolution C1s XPS spectra
contrasting nf- and f-MWCNT electrodes.
Indicators for functional groups present are
given.
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3.4
Electrochemical Applications of Plasma-Modified
MWCNT electrodes
The first application of our plasma-modified MWCNT
electrodes was in the field of electrocatalysts for water
splitting (H 2 generation) in alkaline electrolyte [14].
Electrochemical performance (electrocatalytic activity) of
Ni NP/MWCNT electrocatalysts for various Ni NP
loadings (controlled by t PLA ) was compared to a 2D Ni
plate electrode. Fig. 6a shows the relative electrocatalytic
activities (relative performances) of the electrocatalysts.
The activity of the Ni NP/MWCNT electrocatalysts
increases substantially with t PLA up to 40 min. This
loading yielded the maximum electrocatalyst activity
towards hydrogen generation which is ~600 times larger
than the 2D Ni plate. However, above t PLA = 40 min, a
“crowding” effect occurs on the MWCNTs whereby too
many NPs cover the MWCNT surfaces, resulting in a net
decrease of the electrocatalyst activity. Regardless, for a
relatively low amount of Ni used, the electrocatalytic
activity of the electrocatalysts produced can substantially
increase the amount of hydrogen generated using an
inexpensive material.
Electrodes exposed to a RF glow discharge to graft
stable oxygen functionalities were used as supercapacitor
electrodes [12]. Fig. 6b displays the results of potential
cycling experiments between f-MWCNT and nf-MWCNT
electrodes.
Those electrodes containing oxygen
functionalities substantially increased the specific
capacitance, C sp [F g-1], of the MWCNTs by ~570% (43
to 288 F g-1 at current densities of 0.2 mA cm-2). The
increase in C sp of the f-MWCNTs is thought to be a result
of two effects: (i) the oxygen containing groups undergo
charge-transfer reactions and (ii) the oxygen groups
increase the hydrophilicity of the material [16], increasing
the surface area of f-MWCNTs in contact with the
electrolyte, thus increasing capacitance.
Fig. 6. Electrocatalytic activity of Ni
NP/MWCNT electrocatalysts (a) and the
capacitance of nf- and f-MWCNT supercapacitor
electrodes (b).
4. Conclusions
Plasma-modified
MWCNT
electrodes
for
electrochemical energy storage applications were
produced. Pulsed laser ablation is a useful method of NP
production at the bench-top scale and produces a high
4
degree of reproducibility, tunability, and robustness of the
produced NPs. Plasma functionalization is a useful
method to stably graft oxygen functional groups onto
MWCNT surfaces which increase electrochemical
performance and increase electrode hydrophilicity.
Ni NPs were deposited by PLA onto an open, 3D
matrix of MWCNTs and used as an electrocatalyst for
hydrogen generation. The electrodes showed an increase
in activity, producing more hydrogen relative to a 2D Ni
plate by a factor of ~600.
Oxygen functional groups grafted onto MWCNTs by
exposure to an RF glow discharge were used as
supercapacitor electrodes.
f-MWCNT electrodes
achieved a specific capacitance of 288 F g-1 relative to
MWCNT electrodes (43 F g-1).
5. Acknowledgements
M.A.M. acknowledges the funding support from
McGill University through the McGill Engineering
Doctoral Award and NSERC through the CGSD award.
This research project is funded by NSERC, FRQNT and
McGill University.
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