Nanoscale electrocatalysis: Visualizing oxygen reduction at pristine, kinked, and oxidized sites on individual carbon nanotubes

Article abstract of DOI:10.1021/ja505708y

There is a prevailing and widely adopted view that carbon nanotubes, which are finding considerable application in energy, healthcare, and electronics applications, are highly (electro)catalytically inert unless modified, doped, or defected. By visualizing the electrochemical reduction of oxygen (hydrogen peroxide generation) at high resolution along pristine (defect-free) regions of individual single-walled carbon nanotubes, we show that there is, in fact, significant activity comparable to that of standard gold electrocatalysts. Moreover, the activity is greatly enhanced at strained (kinked) sites and regions modified by oxidation. Single-walled carbon nanotubes are thus effective electrocatalysts in their own right and not just supports for other materials.

Full text of DOI:10.1021/ja505708y

Communication
pubs.acs.org/JACS
Open Access on 07/25/2015
Nanoscale Electrocatalysis: Visualizing Oxygen Reduction at Pristine,
Kinked, and Oxidized Sites on Individual Carbon Nanotubes
Joshua C. Byers, Aleix G. Guell, and Patrick R. Unwin*
̈
Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, U.K.
S
* Supporting Information
O2 + 2H2O + 2e− ⇌ H2O2 + 2OH−
ABSTRACT: There is a prevailing and widely adopted
view that carbon nanotubes, which are finding considerable
application in energy, healthcare, and electronics applica-
tions, are highly (electro)catalytically inert unless modified,
doped, or defected. By visualizing the electrochemical
reduction of oxygen (hydrogen peroxide generation) at
high resolution along pristine (defect-free) regions of
individual single-walled carbon nanotubes, we show that
there is, in fact, significant activity comparable to that of
standard gold electrocatalysts. Moreover, the activity is
greatly enhanced at strained (kinked) sites and regions
modified by oxidation. Single-walled carbon nanotubes are
thus effective electrocatalysts in their own right and not
just supports for other materials.
(2)
(3)
E° = −0.133 V vs NHE
H2O2 + 2e− ⇌ 2OH−
E° = +0.935 V vs NHE
For most carbon electrode materials, the ORR occurs via the
indirect peroxide pathway.13,14 It is well established that there is
a very large overpotential on graphite, whereas kinetic
measurements on SWNTs have proved difficult because of
issues in defining electrode areas, materials properties, and mass
transport in conventional electrode setups.3
Herein, we study the ORR at well-defined individual SWNTs
using our recently developed multi-probe platform based on
scanning electrochemical cell microscopy (SECCM),15 that
permits high-resolution reaction mapping of substrates to be
correlated with local structure (Supporting Information (SI),
sections 1−3).16−20 The technique provides a means of
positioning a tiny meniscus electrochemical cell on a substrate
of interest that serves as a working electrode, whose area is
defined by the meniscus diameter (Figure 1a).19,20 By coupling
this approach with complementary techniques (SEM, Raman)
applied to the same area, detailed structure−activity correla-
tions are revealed (SI, sections 1 and 3).20 This approach is
particularly powerful as it allows us to investigate structure−
activity relationships of individual nanoscale features. Thus, we
quantitatively capture and characterize the ORR at segments of
straight and kinked pristine SWNTs and investigate the impact
of deliberately oxidizing SWNTs.
A more detailed description of the SECCM operation can be
found in the SI and elsewhere.16,17 Briefly, a silanized theta
pipet, that was pulled to a sub-micrometer diameter, was filled
with an aerated phosphate buffer solution (pH 7.2, Sigma-
Aldrich) and 25 mM KCl. Nanoscale electrochemical imaging
was realized by translating the resulting meniscus confined
electrochemical cell, formed at the theta pipet, over the
substrate in the xyz directions with nanometer control. The
substrate functioned as a working electrode whose potential
was controlled using Ag/AgCl quasi-reference counter electro-
des (QRCEs). An electrochemical current was measured when
the meniscus made contact with the SWNT (Figure 1).
Millimeter long flow aligned SWNTs were grown on an
insulating Si/SiO2 substrate using catalyzed chemical vapor
deposition (SI, section 1), which allowed us to probe the
arbon nanotubes (CNTs) are being actively explored for a
Cwide range of applications due to their exceptional
electronic and surface properties.1−3 The high surface area to
mass ratio makes CNTs particularly promising candidates for
applications in catalysis and energy conversion, but CNTs have,
hitherto, largely been used as support materials for catalytic
nanoparticles or molecules.4−7 Indeed, there is a popular
perception that pristine CNTs function only as support
materials to wire up more active electrocatalytic materials,
unless modified or doped.8,9 Here, we demonstrate that high
quality pristine single-walled carbon nanotubes (SWNTs)
actually exhibit exceptional electrocatalytic properties for
hydrogen peroxide (H2O2) generation via the oxygen reduction
reaction (ORR). This is an important reaction because
electrocatalysis is emerging as a potential route for H2O2
synthesis (300 million tons/annum industrial), and new
electrocatalysts are actively being explored.10 On the other
hand, H2O2 is an unwanted byproduct of the ORR in polymer
electrolyte membrane fuel cells, where CNTs are being widely
considered as supports,11 reducing the efficiency and being
detrimental to cathode stability. These disparate applications
highlight the need to better understand the ORR and H2O2
generation at SWNTs.
The ORR can occur via a direct 4-electron process (pathway
1) or an indirect pathway, which involves two sequential 2-
electron-transfer steps (pathways 2 and 3) where peroxide is
produced as an intermediate.12
O2 + 2H2O + 4e− ⇌ 4OH−
Received: June 6, 2014
Published: July 25, 2014
(1)
E° = +0.401 V vs NHE
© 2014 American Chemical Society
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Figure 1. (a) SECCM setup showing a theta pipet positioned directly above an individual SWNT creating a well-defined tiny meniscus
electrochemical cell for local electrochemical measurements and substrate mapping. A voltage, V1, permits control of the working electrode potential,
while an ion current (IIC) between the two barrels of the pipet, generated by V2, is used for precise pipet positioning. An electrochemical current
(IEC) is measured when the meniscus makes contact with the SWNT. Illustration (not to scale) of the enhanced mass transport and well-defined
geometry of the SECCM setup, where dmeniscus and dSWNT correspond to meniscus and SWNT diameter, respectively. (b) Cyclic voltammograms of
the ORR at a pristine individual SWNT. Scan rate: 100 mV/s. (c) Electrochemical map recorded at a substrate potential of −1.0 V vs Ag/AgCl
QRCE for a pristine individual SWNT. Inset contains a line scan profile illustrating the evolution of the measured electrochemical current as the
meniscus advanced across the SWNT. All measurements carried out in aerated PBS (pH 7.2) with 25 mM KCl.
electrochemical activity of individual SWNTs. Pd metal was
evaporated onto the SWNTs at one end using a shadow mask
to form an ohmic contact for electrical connection. The
working electrode dimensions were determined by the diameter
of the SWNT, dSWNT, and the diameter of the meniscus,
dmeniscus, as shown in Figure 1a.
expected to be revealed. In fact, the electrochemical response is
quite uniform over this long section and represents the intrinsic
activity of an unmodified SWNT. Since the average defect
spacing on SWNTs of this type is ∼4 μm,22 which is much
larger than the spatial resolution in the image (∼0.5 μm, Figure
1c) and in the line profiles (data acquired every 0.5 nm in the
scan direction, Figure 1c), it can be readily deduced that there
is little influence of defects in this electrochemical regime. In
particular, the line profile morphology, in which the current
increases monotonically, reaching a maximum, and then falls as
the SECCM meniscus passes over the SWNT, is consistent
with uniform SWNT electroactivity at the high spatial
resolution (∼nm) of this measurement.20,23,24
Figure 1b shows a cyclic voltammogram (CV) of the ORR at
a fixed position on an individual metallic SWNT. The metallic
nature and diameter of the SWNT were determined by Raman
spectroscopy (SI, Figure S2). Two separate waves were
observed for both the forward (cathodic-going) and reverse
(anodic-going) scans of the CV, but these were more
pronounced in the anodic scan direction. The corresponding
current plateaus ca. −1.25 and −1.70 V vs Ag/AgCl QRCE
during the reverse scan indicate a 2-electron reduction to H2O2
(pathway 2) followed by further reduction to water (pathway
3) at SWNTs, as found in studies of CNT ensembles.14
Measurements under similar conditions for highly oriented
pyrolytic graphite (SI, Figure S3) revealed a much more
negative onset potential (by >0.5 V), highlighting that for these
two sp2 carbon materials, the ORR is greatly facilitated on
SWNTs compared to graphite. The hysteresis between the
forward and reverse current−voltage response for SWNTs was
most pronounced for the first wave, for the reduction of oxygen
to H2O2 (pathway 2). This effect was fairly reversible, with
similar behavior seen in the cathodic and anodic scan directions
during the second voltammetric cycle, with a small increase in
the limiting current. Chemical oxidation of the SWNT sidewall
by the electrochemical generation of H2O2 is unlikely to occur
to a significant extent due to its rapid flux away from the
electrode as a result of the high mass transport of SECCM with
a SWNT.20 Permanent surface modification of the SWNT
surface during the ORR also cannot account for the reversible
nature of the hysteresis, and more likely reflects changes in the
electrochemical double layer and interfacial structure over the
wide potential range investigated.14,21
Electrochemical8 and chemical oxidation25 has recently been
used to increase the number of defect sites along the sidewalls
of multiwalled CNTs to enhance their electrocatalytic activity
toward the ORR. Increased defect densities at kinked regions of
pristine SWNTs have also been confirmed through selective
electrochemical deposition,22 and our technique allowed us to
probe the ORR directly in such regions. Figure 2a shows a
large-scale electrochemical map of a section of a pristine SWNT
that contained a kinked region, and Figure 2b shows a
separately recorded zoom map of the kinked region.
Considerably enhanced electrochemical activity is observed,
as confirmed by the line profiles over this area (Figure 2b),
highlighting the significant extent to which the activity is
enhanced, revealed here using SECCM, as a result of the
increased number of defects at these regions. Complementary
maps obtained simultaneously during all SECCM measure-
ments20 were used to confirm that the meniscus footprint and
effective volume were very consistent over the whole image and
so the enhanced activity in the kink region can be assigned to
the SWNT architecture and not to changes in the meniscus
size. See SI, section S5, for further discussion and an example of
the complementary maps obtained.
Enhancement of the ORR by defect sites was further con-
firmed by their intentional introduction along the SWNT
sidewall by electrochemical oxidation (inset Figure 3a), which is
known to introduce defects and oxygen functionalized moieties
into the sp2-hybridized carbon lattice.26 Using SECCM, it was
The electrochemical map in Figure 1c shows the oxygen
reduction activity over a 10 μm length of a pristine metallic
SWNT just at the onset potential (−1.0 V vs Ag/AgCl QRCE)
where any differences in activity along the SWNT would be
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Communication
no disconnection or cutting of the SWNT in this one terminal
device.
It was possible to construct mass transport corrected Tafel
plots to determine the exchange current density, j0 (at zero
overpotential), by extrapolation. The area of the cylindrical
SWNT working electrode was defined by the meniscus
diameter, dmeniscus, representing the length of the cylinder,
while the effective width of exposed SWNT, WSWNT, was given
24
by (π/2)dSWNT
.
The meniscus width was determined
accurately from a line scan of the SWNT, and was comparable
with the diameter of the pipet itself, measured from an FE-SEM
micrograph of the complementary theta pipet to that used for
imaging (SI, Figure S5). The kinetic current density was then
determined by correcting for mass transport using the following
expression:
Figure 2. (a) Electrochemical map of a pristine SWNT that contained
a kinked region. (b) Separate zoom-in electrochemical map of (a).
Line scans of kinked region (red dashed line in (b)) and pristine site
(black dashed line in (b)) highlighted in map (b). Maps obtained at
−1.0 V vs Ag/AgCl QRCE.
1
j
1
jk
1
=
+
j
(4)
d
where j is the current density, jk is the kinetic current density,
and jd is the diffusion-limited current density. Further details of
the Tafel analysis, in which currents were further normalized by
oxygen concentration, to permit comparison with published
studies, are provided in the SI, section S6.
Tafel analysis of ORR (H2O2 generation) voltammograms
(Figure 4) allowed comparison of the performance of pristine
Figure 3. Electrochemical maps and line scans of a pristine SWNT
before (a) and after (b) electrochemical oxidation; start and end
potential 0 V, return potential 1.30 V at a scan rate of 50 mV/s (inset
of (a)). Maps obtained at −0.75 V vs Ag/AgCl QRCE.
Figure 4. Mass transport corrected Tafel plots. The kinetic current
density ordinates have been normalized by the oxygen concentration
for each system to permit direct comparison. Kinetic current density,
electrode potential, and experimental conditions for Au−C and Au−
polycrystalline were obtained from refs 10, 27, and 28.
possible to directly compare variations in the electrochemical
response in the same area of the SWNT before and after
electrochemical oxidation of the SWNT sidewall (Figure 3a,b).
Prior to electrochemical oxidation, a straight section of SWNT
showed essentially uniform electrochemical activity along its
length (Figure 3a). Activity measurements were made at a low
driving potential of −0.75 V vs Ag/AgCl QRCE, to highlight
any changes in electrochemical activity due to local electro-
chemical oxidation. With the SECCM meniscus fixed over the
SWNT, the electrode potential was then scanned from 0 to
1.30 V and back to 0 V at a scan rate of 50 mV/s, resulting in an
anodic current above ∼1.1 V. Following electrochemical
oxidation, the ORR in the modified and surrounding region
was re-imaged, revealing a 5-fold increase in electrochemical
current in the modified region, and an asymmetric profile with
discrete features, consistent with a heterogeneously active
surface. It is further evident that following activation, the
electrochemical response along the sidewall remained uniform
on either side of the modified region confirming that there was
(black dots) and modified (red dots) SWNT portions to Au-
polycrystalline (green dots) and Au−C (blue dots) electrodes,
as well-known standards for electrochemical H2O2 electro-
generation.27,28 Note that the overpotential is much higher for
the SWNT system as a consequence of the ultrahigh mass
transport rates generated in the SWNT-SECCM format,20
leading to very high current density values. It is evident that the
activity (current density) of the sidewall of pristine SWNTs is
very similar to Au-based electrodes. Moreover, the oxidation of
a SWNT provides an electrocatalyst for oxygen reduction
where the reaction kinetics is approximately 1 order of
magnitude larger than for Au-based electrodes. Thus, pristine
SWNTs stand out as effective electrocatalysts and this activity
can be further enhanced by electrochemical modification.
There is increasing interest in developing catalysts for the
ORR that selectively promote H2O2 generation and permit
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decentralized production.29 From this perspective, SWNTs
represent a powerful nonmetal electrocatalyst, where the
activity can be tailored (enhanced) by building in particular
architectures (kinks) or through surface modification (sidewall
oxidation). On the other hand, the relative ease of H2O2
production at SWNTs may limit their use as supports for the
development of stable fuel cell electrodes and in some
healthcare applications where reactive oxygen species need to
be avoided. In general, pristine SWNTs are not inert: they
exhibit substantial intrinsic electrochemical activity, which
needs to be taken into account in directing their use in the
future.
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(22) Fan, Y.; Goldsmith, B. R.; Collins, P. G. Nat. Mater. 2005, 4,
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ASSOCIATED CONTENT
■
(23) Dudin, P. V.; Snowden, M. E.; Macpherson, J. V.; Unwin, P. R.
ACS Nano 2011, 5, 10017.
S
* Supporting Information
(24) Guell, A. G.; Ebejer, N.; Snowden, M. E.; McKelvey, K.;
̈
SWNT synthesis, experimental setup, Raman spectroscopy, and
additional details of the Tafel analysis. This material is available
free of charge via the Internet at http://pubs.acs.org.
Macpherson, J. V.; Unwin, P. R. Proc. Natl. Acad. Sci. U.S.A. 2012, 109,
11487.
(25) Cheng, Y.; Zhang, H.; Varanasi, C. V.; Liu, J. Sci. Rep. 2013, 3,
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AUTHOR INFORMATION
(26) Wei, D.; Liu, Y.; Cao, L.; Zhang, H.; Huang, L.; Yu, G.; Kajiura,
H.; Li, Y. Adv. Funct. Mater. 2009, 19, 3618.
(27) Jirkovsky, J. S.; Halasa, M.; Schiffrin, D. J. Phys. Chem. Chem.
Phys. 2010, 12, 8042.
■
Corresponding Author
p.r.unwin@warwick.ac.uk
(28) Jirkovsky, J. S.; Panas, I.; Ahlberg, E.; Halasa, M.; Romani, S.;
Schiffrin, D. J. J. Am. Chem. Soc. 2011, 133, 19432.
(29) Edwards, J. K.; Solsona, B.; N, E. N.; Carley, A. F.; Herzing, A.
́
Notes
The authors declare no competing financial interest.
A.; Kiely, C. J.; Hutchings, G. J. Science 2009, 323, 1037.
ACKNOWLEDGMENTS
■
We thank the European Research Council (ERC-2009-
AdG247143-QUANTIF) for support. The research leading to
these results has received funding from the European Union
Seventh Framework Programme (FP7/2007-2013) under grant
agreement no. 329953.
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