Improving Formate and Methanol Fuels: Catalytic Activity of Single Pd Coated Carbon Nanotubes

Article abstract of DOI:10.1021/acscatal.6b02023

The oxidations of formate and methanol on nitrogen-doped carbon nanotubes decorated with palladium nanoparticles were studied at both the single-nanotube and ensemble levels. Significant voltammetric differences were seen. Pd oxide formation as a competitive reaction with formate or methanol oxidation is significantly inhibited at high overpotentials under the high mass transport conditions associated with single-particle materials in comparison with that seen with ensembles, where slower diffusion prevails. Higher electro-oxidation efficiency for the organic fuels is achieved.

Full text of DOI:10.1021/acscatal.6b02023

Research Article
pubs.acs.org/acscatalysis
Improving Formate and Methanol Fuels: Catalytic Activity of Single
Pd Coated Carbon Nanotubes
Xiuting Li,†,§ Hannah Hodson,†,§ Christopher Batchelor-McAuley,† Lidong Shao,‡
and Richard G. Compton*,†
†Department of Chemistry, Physical & Theoretical Chemistry Laboratory, Oxford University, Oxford OX1 3QZ, United Kingdom
‡Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, Shanghai University of Electric Power,
2103 Pingliang Road, Shanghai 200090, People’s Republic of China
S
* Supporting Information
ABSTRACT: The oxidations of formate and methanol on nitrogen-doped carbon
nanotubes decorated with palladium nanoparticles were studied at both the single-
nanotube and ensemble levels. Significant voltammetric differences were seen. Pd
oxide formation as a competitive reaction with formate or methanol oxidation is
significantly inhibited at high overpotentials under the high mass transport conditions
associated with single-particle materials in comparison with that seen with ensembles,
where slower diffusion prevails. Higher electro-oxidation efficiency for the organic
fuels is achieved.
KEYWORDS: formate, methanol, palladium, carbon nanotube, catalytic activity
electrodes in these studies are macroelectrodes, at which the
mechanism and kinetics can be masked by the limited, slow
mass transport conditions.
1. INTRODUCTION
Increased interest in alternative fuel sources has led to the
separate application of both formate and methanol in fuel cells.
Their use as power sources is appealing due to their high
energy density and ease of transportation and storage, in
contrast to hydrogen fuel cells.1−5 Pd-based materials have
been intensely studied as superior and cost-effective electro-
catalysts in the oxidation of both formate and methanol.6 For
formate oxidation, it has been suggested that Pd-based catalysts
have a better performance in comparison with Pt, as the
catalytic reactions on Pd follow a direct pathway and no
poisoning intermediates are produced.7,8 Pd-based materials are
also considered to be better catalysts in alkaline media for
methanol oxidation than Pt-based materials owing to their
improved kinetics and, again, lower poisoning from adsorbed
intermediates.9 However, the instability of the Pd-based
catalysts can lead to relatively low energy transformation
efficiency.10,11 Improving the electrocatalytic stability remains a
critical obstacle in the development of Pd-based electrocatalysts
in both formate and methanol fuel cells, and a fundamental
understanding of Pd deactivation due to its high surface
sensitivity is still incomplete.6,12
In this work, we first develop an improved electrode
modification method by modifying a carbon microwire
electrode in situ with well-separated individual catalyst entities.
This modification is achieved by inserting the wire electrode in
a very dilute suspension of the nitrogen-doped carbon
nanotubes decorated with palladium nanoparticles (N-CNT-
Pd). Also present in the solution is the redox species of interest
(formate or methanol). Over a period of time individual
catalyst entities (N-CNT-Pd) continuously and randomly
collide with the wire electrode due to their Brownian motion.
Some of the N-CNT-Pd becomes immobilized on the electrode
surface, forming a layer of low coverage. After immobilization
the voltammetric response of the modified electrode is
recorded in the same solution. In addition to these surface
modification studies, a single-particle electrochemical method
based on “nanoimpacts” was applied. This technique relies on
the random collision of individual nanoparticles with a
potentiostated electrode where control of the potential leads
to mediated electron transfer on the surface of the impacting
nanoparticle.20−27 The current amplification from mediated
electron transfer is analyzed and can be applied to the
The electrocatalytic behavior of newly produced materials are
commonly assessed by drop-casting the catalysts onto electrode
surfaces and then performing electrochemical tests to analyze
their performance. However, the interpretation of such a
multilayered, porous drop-cast surface is complex and hard to
analyze.13−19 Moreover, the most commonly used types of
Received: July 19, 2016
Revised: September 13, 2016
© XXXX American Chemical Society
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Figure 1. (A) Voltammograms of a GC electrode (d = 3.0 mm) modified with 0.96 μg of N-CNT-Pd recorded in 5 M NaClO4(aq) in the absence
(dashed line) and presence (solid line) of 5 M formate. Also shown is the voltammetic response of the bare GC electrode in 5 M NaClO4 + 5 M
formate (dotted line). Scan rate: 50 mV s−1. (B) Voltammograms of a GC electrode (d = 3.0 mm) modified with 1.20 μg of N-CNT-Pd in 1 M
NaOH(aq) in the absence (dashed line) and presence (solid line) of 10 M methanol. Also shown is the voltammetric response of the bare GC
electrode in 1 M NaOH + 10 M methanol (dotted line). Scan rate: 50 mV s−1.
fundamental mechanisms and dynamics of electrochemical
processes at the single-entity level.28 In this work, formate
oxidation on individual impacting N-CNT-Pd was studied with
the nanoimpact method. The above methods, including both in
situ modification and the nanoimpact experiments, lead to the
spatial isolation of the catalyst particles on the electrode surface,
creating high mass transport conditions for the oxidation
processes, which provide new insights into the electrocatalytic
mechanisms of formate and methanol oxidation on palladium
and how this is influenced by mass transport.
used as the working electrode, with platinum foil and a
saturated calomel electrode (SCE) as the counter electrode and
the reference electrode, respectively. Before the measurements
a solution containing 5 M formate and 5 M NaClO4 (or 10 M
methanol and 1 M NaOH) was bubbled with N2 for 10 min to
remove dissolved O2, and an atmosphere of N2 was maintained
during the experiment. Two control experiments were carried
out: an unmodified GC electrode in 5 M formate + 5 M
NaClO4 (or 10 M methanol + 1 M NaOH) and a modified GC
electrode in 5 M NaClO4 (or 1 M NaOH).
2.3. CV of Formate and Methanol Oxidation on the
Carbon Microwire Electrode in Situ Modified with N-
CNT-Pd and the Nanoimpact Experiments. In the in situ
modification experiments, the working electrode was carbon
fiber microwire electrodes (ca. 7 μm in diameter and ca. 1 mm
in length), which were fabricated according to the procedure
introduced in our previous work.30 Formate or methanol
solution was degassed with N2 for 10 min, and then a known
concentration of N-CNT-Pd suspension was added and
bubbled with N2 for 30 s to make a well-dispersed and diluted
suspension of N-CNT-Pd. CV was recorded immediately after
a clean carbon microwire electrode was inserted in the
suspension containing formate or methanol and after the
electrode was in situ modified in the same solution for a certain
time. The nanoimpact experiments were conducted by
scanning chronoamperometry immediately after a clean carbon
microwire electrode was inserted in the above solutions. For
counting and analysis of the current spikes, the software
“SignalCounter” was used.31
2. EXPERIMENTAL SECTION
2.1. Materials. Sodium formate (≥98%), methanol
(99.99%), NaClO4, and NaOH were received from Sigma-
Aldrich. Nitrogen gas (N2) was supplied from BOC, Surrey,
U.K. All solutions were prepared with deionized water of
resistivity not less than 18.2 MΩ cm (25 °C, Millipore). The
preparation and characterization of nitrogen-doped carbon
nanotubes decorated with palladium nanoparticles (N-CNT-
Pd) were detailed in previous papers.28,29
2.2. Cyclic Voltammetry (CV) of Formate and
Methanol Oxidation on the GC Electrode Drop-Cast
with N-CNT-Pd. A suspension of N-CNT-Pd (9.2 × 10−12 M)
was prepared by adding 2.4 mg of N-CNT-Pd to 5 mL of
chloroform, followed by sonication in a Fisher Scientific
FB15050 ultrasonic bath for 1 min. A glassy-carbon (GC)
macroelectrode (d = 3.0 mm) was polished with three grades of
diamond spray (3.0, 1.0, and 0.1 μm) and then sonicated in
water and left to dry under an N2 atmosphere. The as-prepared
N-CNT-Pd suspension was then drop-cast on the GC electrode
and dried under an N2 atmosphere.
3. RESULTS AND DISCUSSION
Electrochemical measurements were conducted in a double
Faraday cage with an Autolab potentiostat (Metrohm-Autolab
BV, The Netherlands) using a three-electrode system at 298 K.
For the drop-cast experiments, the modified GC electrode was
We first investigate the CV of formate and methanol oxidation
on glassy-carbon (GC) electrodes drop-cast with N-CNT-Pd to
evidence the catalytic ability of the N-CNT-Pd toward these
oxidation processes. Mechanistic considerations are made.
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Second, we measure and analyze the CV of first formate and
second methanol oxidation on carbon microwire electrodes
which have been modified in situ with individual spatially well
separated N-CNT-Pd tubes via their random collision and
immobilization on the wire electrode. Third, nanoimpact
experiments are conducted by recording chronoamperometry
at a carbon microwire electrode in an aqueous suspension of N-
CNT-Pd in the presence of formate. Current spikes in the
chronoamperograms resulting from formate oxidation on
individual (single) impacting N-CNT-Pd are detected and
analyzed. Similar experiments are attempted for the oxidation
of methanol. Finally, the results from all the experiments are
compared and chemical mechanisms inferred and discussed.
3.1. CV of Drop-Cast N-CNT-Pds. The cyclic voltammetric
responses of formate and methanol oxidation were separately
investigated on N-CNT-Pd modified GC electrodes (of ca. 3.0
mm in diameter). To study the formate oxidation, an
unmodified GC electrode was first voltammetrically scanned
in an aqueous solution containing 5 M NaClO4 and 5 M
formate. No voltammetric signal was observed, as shown in
Figure 1A (dotted line). Subsequently 0.96 μg of N-CNT-Pd
was drop-cast onto the GC electrode and left to dry under an
N2 atmosphere. The amount of N-CNT-Pd drop-cast in the
experiment corresponds to a coverage of ∼1 monolayer,
assuming the CNTs are perfectly tesselated on the electrode
surface (see the Supporting Information). In reality a tangled,
irregular multilayer is likely formed. Figure 1A shows the CV of
the above modified GC electrode in 5 M NaClO4 in the
absence (dashed line) and presence (solid line) of 5 M formate,
respectively. The CV of the modified GC electrode in the
formate solution shows a clear formate oxidation peak at −0.08
V vs SCE and a back anodic peak at around −0.02 V vs SCE
(solid line). No oxidation feature was observed in the absence
of formate (dashed line). The formate oxidation is known to be
highly surface sensitive, and at high overpotentials it is inhibited
by the formation of a palladium oxide layer.6,11 It has been
suggested that on the reverse scan the reduction of the oxide
layer leads to the recommencement of the electrochemical
process.11
In order to characterize the palladium oxidation process, the
voltammetric response of a bulk palladium microelectrode
(radius 5.4 μm) was recorded in the absence of formate. The
CV (black line in Figure S1A in the Supporting Information) in
5 M NaClO4 shows a pair of asymmetric redox peaks for Pd
oxide formation and reduction: a broad Pd oxidation wave on
the forward scan and a sharp peak for Pd oxide reduction on
the backward scan. The reduction of the Pd oxide is found to
occur at ∼0.0 V vs SCE. This potential is comparable to the
peak potential of formate oxidation (−0.08 V vs SCE) as
recorded on the drop-cast N-CNT-Pd (solid line in Figure 1A).
On the reverse scan of the voltammetry of formate oxidation at
a drop-cast N-CNT-Pd electrode, a sharp increase of the
oxidation current is observed (solid line in Figure 1A). This
sharp increase in current is found to occur at the potentials of
Pd oxide reduction where Pd active sites are released.11 This
observation further evidences that the decrease of the oxidation
current on the forward scan is caused, at least in part, by the
formation of a Pd oxide layer.
peak was observed at −0.18 V vs SCE on the modified GC
electrode in 1 M NaOH containing 10 M methanol (solid line
in Figure 1B), and a small oxidation peak was found on the
backward scan at around −0.45 V vs SCE. In the absence of
methanol, the voltammetric responses of both the drop-cast N-
CNT-Pd (dashed line in Figure 1B) and the Pd microelectrode
(black line in Figure S1B) demonstrate that Pd oxide reduction
occurs near −0.4 V vs SCE in 0.1 M NaOH. This value of −0.4
V is more negative than the peak potential for methanol
oxidation (−0.18 V vs SCE) on the drop-cast N-CNT-Pd (solid
line in Figure 1B). Hence, a significant surface coverage of Pd
oxide has probably formed before the methanol oxidation peak
current, and a further deactivation of the Pd active sites leads to
significant inhibition toward methanol oxidation.10 Moreover,
the methanol oxidation is significantly less favorable at the
relatively negative potentials for Pd oxide reduction; hence, a
much smaller back anodic peak (solid line in Figure 1B) is
observed in comparison to that found with the formate
oxidation (solid line in Figure 1A). The measured back-peaks in
the two cases are independent of the potential limit (beyond a
minimum threshold value). It should be noted that the
increased current at high potentials (>0.3 V vs SCE) in Figure
1B (solid line) corresponds to further methanol oxidation on
the background carbon of the nanotube and also the oxidation
of the solvent.
Formate oxidation on palladium is reported to result in the
formation of carbon dioxide via a direct two-electron-transfer
pathway,7,8 while methanol oxidation is reported to undergo a
six-electron-transfer process at low overpotentials.32 For a
microdisk electrode, the equation for the expected diffusion
limited current is33
Ilim = 4nFDCre
where n is the number of electrons transferred, F is the Faraday
constant (96485 C mol−1), D is the diffusion coefficient (6.5 ×
10−11 m2 s−1 for formate34 and 4 × 10−10 m2 s−1 for
methanol35), C is the concentration of formate or methanol,
and re is the electrode radius. Therefore, for 0.5 M formate or
methanol, the theoretical diffusion-limited currents at a Pd
microelectrode with radius of 5.4 μm can be estimated as 1.4 ×
10−7 and 2.5 × 10−6 A, respectively. However, the
experimentally measured oxidative peak currents of 0.5 M
formate (2.2 × 10−9 A) and 0.5 M methanol (2.6 × 10−8 A) at a
Pd microelectrode (r = 5.4 μm) (red lines in Figure S1 in the
Supporting Information) are much smaller than the hypo-
thetical diffusion-limited values, evidencing that both the
formate and methanol oxidation on Pd are surface reaction
rate limited processes, rather than transport controlled.
It is worth mentioning that the modified GC electrode
showed a slightly higher peak current of formate oxidation (8.3
× 10−5 A) in the solution without the supporting electrolyte
NaClO4 (see Figure S2 in the Supporting Information),
indicating that the extremely high concentration of formate
used is able to be self-supporting during the measurement, and
2−
suggests that some Pd active sites may be blocked by ClO4
.
Aqueous 5 M formate solution was used in the following
experiments.
3.2. CV of a Carbon Microwire Electrode in Situ
Modified with a Dilute Coverage of Individual N-CNT-
Pds. The previous section reported the voltammetric response
of a multilayered, porous drop-cast surface. The interpretation
of such a layer is complex and is hard to analyze.13−16 In order
to study the formate oxidation on adsorbed but spatially well
We next consider the oxidation of methanol at a N-CNT-Pd
modified GC electrode and at the Pd microelectrode.
Voltammograms are shown in Figure 1B and Figure S1B in
the Supporting Information, respectively. The results for
methanol oxidation mirror those found for formate. An anodic
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Figure 2. (A) CV recorded immediately after a carbon microwire electrode (l ≈ 1 mm; d ≈ 7 μm) was inserted in a 0.65 pM suspesion of N-CNT-
Pd containing 5 M formate (dashed line) and after the electrode was in situ modified in the same solution for 30 min (solid line). Scan rate: 50 mV
s−1. (B) CV recorded immediately after a carbon microwire electrode was inserted in a 0.65 pM suspesion of N-CNT-Pd containing 1 M NaOH +
10 M methanol (dashed line) and after the electrode was in situ modified in the same solution for 60 min (solid line). Scan rate: 50 mV s−1.
separated individual N-CNT-Pds, a carbon microwire electrode
(l ≈ 1 mm; d ≈ 7 μm) was first treated with an in situ
modification method by dipping the wire electrode in a
suspension of N-CNT-Pd (0.65 pM) containing formate for 0.5
h, followed by a CV sweep. During the modification process,
about 100 individual N-CNT-Pds (see the Supporting
Information) collided with and became immobilized on the
microwire electrode, giving an electrode modified with enough
N-CNT-Pd to give a clear formate oxidation signal in CV. For
the wire electrode used, one monolayer of N-CNT-Pds,
assuming the CNTs are perfectly tessellted on the electrode
surface, would be 3.4 × 104 CNTs (see the Supporting
Information). Therefore, only 0.3% of the surface area is
covered by 100 N-CNT-Pds, which gives large spatial
separation of N-CNT-Pds on the wire electrode surface. No
voltammetric response was detected when CV was recorded
immediately after the wire electrode was dipped in the
suspension of N-CNT-Pd (dashed lines in Figure 2A). After
in situ modification in the formate solution dispersed with N-
CNT-Pd for 0.5 h, the wire electrode showed a Faradaic current
resulting from formate oxidation in the CV (solid lines in
Figure 2A). Similar experiments were conducted to study
methanol oxidation in 1 M NaOH containing 10 M methanol.
It was observed that a methanol oxidation signal was also
shown in the CV at the wire electrode after 1 h of in situ
modification in a suspension of N-CNT-Pd containing
methanol (solid lines in Figure 2B).
electrode surface. Such Faradaic current fluctuations have been
explained in detail in a recent paper.36
3.3. Nanoimpacts of N-CNT-Pd. Nanoimpact experiments
of N-CNT-Pd were performed with the aim of separately
studying the formate and methanol oxidation on individual
impacting single N-CNT-Pd. To achieve this, a clean carbon
microwire electrode was first inserted in a suspension of N-
CNT-Pd (0.65 pM) containing 5 M formate. Chronoampero-
grams were then recorded at a series of applied potentials from
−0.5 to +0.25 V vs SCE. Figure 3A shows a typical
chronoamperogram at +0.2 V vs SCE where oxidative spikes
are observed, evidencing the impact of the individual carbon
nanotubes. No spikes were detected at this potential in the
absence of N-CNT-Pd (Figure S3 in the Supporting
Information). A total number of 135 current spikes at +0.2 V
vs SCE were recorded, giving an average current of (4.1 1.5)
× 10−11 A, and their height distribution is shown in Figure 3B.
The average duration of the spike is short, about 30 ms. This
short duration is possibly caused by the formation of CO2 gas
bubbles on the palladium surface37 or due to adsorbed carbon
intermediates, COad, produced by the further reduction of
CO2,38 leading to the deactivation of Pd active sites. The mean
currents of the individual spikes as a function of potential from
nanoimpacts of N-CNT-Pd are shown in Figure 3C, where it is
observed that the average current at potentials more positive
than −0.2 V vs SCE shows a plateau at around 40 pA. No
spikes were detected at potentials more negative than −0.4 V vs
SCE (Figure S4 in the Supporting Information). The average
current from the spikes is found to have an onset at the
potentials at which formate oxidation is observed in the CV
(Figure 1A), evidencing that the detected current spikes
correspond to Faradaic currents associated with formate
oxidation catalyzed by the individual impacting N-CNT-Pd.
By modeling the N-CNT-Pd as a cylindrical electrode (r =
6.5 × 10−8 m; l = 5 × 10−6 m), for a mass transport controlled
process, the expected peak current on single N-CNT-Pd from
the oxidation of formate (5 M) can be estimated as 3.68 × 10−7
In sharp contrast to the clear oxidation peaks shown in the
CV of the drop-cast experiments, the wire electrode after in situ
modification with N-CNT-Pd exhibits steady state currents at
high overpotentials for both formate and methanol oxidation,
indicating that neither process is inhibited by Pd oxide
formation under these experimental conditions! Note that, for
the CV of the N-CNT-Pd modified wire electrode, current
fluctuations, as shown in Figure 2, are variable due to the
different states of absorbed carbon nanotubes on the wire
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Figure 3. (A) Typical chronoamperogram obtained at 0.2 V vs SCE for a carbon microwire electrode in a 0.65 pM suspesion of N-CNT-Pd
containing 5 M formate. (B) Height distribution of 135 current spikes at 0.2 V vs SCE. (C) Squares denoting the mean current of individual spikes at
different potentials from nanoimapcts of N-CNT-Pd, where n is the number of spikes. (D) Comparison of cyclic voltammetry of formate oxidation at
the wire electrode in situ modified with well-separated N-CNT-Pd (black line) and nanoimpact analysis (red line) of formate oxidation on individual
impacting N-CNT-Pd.
A. The average current of the spikes from formate oxidation is
about 40 pA, which is significantly smaller than the expected
peak current (3.68 × 10−7 A) for a mass transport control
process. This further evidences that formate oxidation on
individual N-CNT-Pds is a surface reaction rate limited process,
rather than being under mass transport control. Moreover, from
the drop-casting experiments, the expected current of formate
oxidation at a single N-CNT-Pd (ca. 5 μm in length, 130 nm in
diameter) can be estimated as 7.9 pA by directly dividing the
peak current (8.3 × 10−5 A) (Figure S2 in the Supporting
Information) by the amount of drop-cast N-CNT-Pds (1.05 ×
107) (see the Supporting Information). This value of 7.9 pA is
much less than the average current (∼40 pA) detected from the
nanoimpact experiment. Therefore, the higher currents likely
result from the physical isolation of the carbon nanotubes in the
impact experiment; in contrast, the drop-cast material will
inevitably form irregular aggregated multilayers on the GC
electrode surface.
oxidation on individual N-CNT-Pd remains stable even at the
high overpotentials. The stable oxidation current is in
agreement with the steady state current observed in the in
situ modification experiments (Figure 3D), further evidencing
that under high mass transport conditions Pd oxide formation
is significantly inhibited, and as a result higher electro-oxidation
efficiency for formate was achieved.
Nanoimpact experiments were also performed to detect the
signals of methanol oxidation on individual N-CNT-Pds, but no
spikes were detected (Figure S5 in the Supporting
Information). On the basis of the drop-casting expeiments,
the expected current from methanol oxidation on single N-
CNT-Pd is calculated to be 1.3 pA. Although the average
current from individual impacting N-CNT-Pds might be several
times larger than 1.3 pA in the nanoimpact experiments
according to the results of formate oxidation, it is still too small
to be detected due to the comparable background noise (∼20
pA) (Figure S5).
In the nanoimpact experiments, the collision of individual N-
CNT-Pd with the wire electrode creates extremely high mass
transport conditions for formate oxidation because of the small
physical size of the individual carbon nanotubes.28,29 Nano-
impact analysis shows that the average current from formate
3.4. Comparison and Inferences. We next compare the
CVs of formate oxidation and methanol oxidation on the GC
electrode drop-cast with N-CNT-Pd and their CVs at the wire
electrode modified with a dilute coverage of well separated N-
CNT-Pd. A striking difference is that a steady state current is
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Figure 4. Comparison of cyclic voltammograms of formate oxidation (A) and methanol oxidation (B) on an N-CNT-Pd drop-cast GC electrode
(black lines) and a carbon microwire electrode in situ modified with well-separated N-CNT-Pd (red lines).
observed in both formate and methanol oxidation at spatially
well separated individual N-CNT-Pd, instead of the current
peaks as shown in the CV response of the drop-cast N-CNT-Pd
(Figure 4). Consequently, it is concluded that, for well-
separated individual N-CNT-Pd, the palladium oxidation (eq 3)
as a competitive reaction is significantly inhibited; hence, the
majority of Pd active sites on palladium nanoparticles remain
active even at highly oxidizing potentials. The Pd active sites
therefore continue to contribute to the catalysis of the formate
or methanol oxidation at high potentials (eqs 1 and 2):
confined volume of the available redox species.17−19,40,41 CVs
similar to those seen with the drop-cast N-CNT-Pd were
observed when the carbon microwire electrode was inserted in
a suspension of N-CNT-Pds of an extremely high concen-
tration (9.2 pM) (10 times higher than that used in the in situ
modification method) for about 10 s, creating a higher
population of large N-CNT-Pd ensembles which stick to the
wire electrode surface and are then transferred into the solution
containing formate or methanol to run a CV. As shown in
Figure S6 in the Supporting Information, the oxidation current
falls off at high overpotentials in the CV of both formate and
methanol oxidation, further evidencing that current inhibition
by Pd oxide formation occurs on the large ensembles of the N-
CNT-Pd. It is also demonstrated that the voltammetric
response of the N-CNT-Pd toward formate or methanol
oxidation can be easily tailored by using different methods to
modify the electrodes.
HCOO− → CO2 + H+ + 2e−
(1)
CH3OH + H2O → CO2 + 6H+ + 6e−
(2)
Pd + H2O → PdO + 2H+ + 2e−
(3)
It was reported previously that palladium oxide formation was
retarded in the presence of formic acid or methanol.10,39 Thus,
a likely factor responsible for the observed steady state current
is that well-separated individual N-CNT-Pds absorbed on the
wire electrode surface experience very high mass transport
conditions for formate or methanol oxidation. This provides a
basis for the preference of formate or methanol oxidation
during the competitive processes with palladium oxidation. In
contrast, an intermingled porous “mat” of carbon nanotubes on
the GC electrode is inevitably formed when using the drop-cast
technique. The resulting relatively limited mass transport
conditions for the oxidation of formate or methanol facilitate
the formation of the Pd oxide layer; probably the mass
transport is a mixture of linear semi-infinite diffusion and thin-
layer transport. For well-separated individual N-CNT-Pds,
there are large numbers of redox species around which create
high mass transport conditions, while an overlapping diffusion
regime is formed on the “mat” of N-CNT-Pds due to the
Overall, Pd oxide formation and formate or methanol
oxidation are two competitive processes. The inferior efficiency
of formate and methanol oxidation on Pd in their application to
fuel cells is due in part to the significant inhibition of Pd oxide
formation toward these oxidation processes. From the above
comparison, it is seen that Pd oxide formation as a competitive
reaction can be retarded under high mass transport conditions
for formate and methanol oxidation, which in this work was
created on the well-separated catalyst N-CNT-Pd. This
highlights how the design of the catalyst in fuel cells and the
local mass transport conditions play an important role in the
energy conversion performance.
4. CONCLUSIONS
In this work, we recorded and compared the voltammograms of
formate/methanol oxidation on drop-cast N-CNT-Pd and on
wire electrodes modified with well-separated individual N-
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N-CNT-Pd, where high mass transport conditions prevail.
Further evidence was seen in the study of formate oxidation on
a single N-CNT-Pd via the nanoimpact method, in which the
oxidation process was also not hindered by Pd oxidation at high
overpotentials. The discovery of the superior electrocatalytic
performance under high mass transport conditions suggests
requirements for the design of fuel cell development and offers
the scope for new catalytic approaches to be developed.
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ASSOCIATED CONTENT
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S
* Supporting Information
(22) Ly, L. S. Y.; Batchelor-McAuley, C.; Tschulik, K.; Katelhon, E.;
̈
̈
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b02023.
Compton, R. G. J. Phys. Chem. C 2014, 118, 17756−17763.
(23) Li, X.; Batchelor-McAuley, C.; Tschulik, K.; Shao, L.; Compton,
R. G. ChemPhysChem 2015, 16, 2322−2325.
Figures S1−S6 as described in the text, estimation of the
number of N-CNT-Pds in 1 monolayer on the GC
electrode surface and the wire electrode surface,
estimation of the number of N-CNT-Pds drop-cast on
the GC electrode, and estimation of the amount of N-
CNT-Pds adsorbed on the wire electrode with in situ
modification method (PDF)
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(30) Ellison, J.; Batchelor-McAuley, C.; Tschulik, K.; Compton, R. G.
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AUTHOR INFORMATION
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Corresponding Author
*E-mail for R.G.C.: richard.compton@chem.ox.ac.uk.
(31) Ellison, J.; Tschulik, K.; Stuart, E. J. E.; Jurkschat, K.; Omanovic,
́
Author Contributions
D.; Uhlemann, M.; Crossley, A.; Compton, R. G. ChemistryOpen 2013,
2, 69−75.
§These authors contributed equally.
(32) Takamura, T.; Mochimaru, F. J. Electrochem. Soc. 1967, 114,
1251−1254.
Notes
The authors declare no competing financial interest.
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1982, 140, 237−245.
ACKNOWLEDGMENTS
(34) Wang, Y.; Wu, B.; Gao, Y.; Tang, Y.; Lu, T.; Xing, W.; Liu, C. J.
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This research was supported by the European Research Council
(ERC) under the European Union’s Seventh Framework
Programme (FP/2007-2013), ERC Grant Agreement no.
320403. The China Scholarship Council is gratefully acknowl-
edged for funding the Ph.D. of X.L. L.S. thanks the National
Natural Science Foundation of China (No. 21403137) for
support.
(35) Danaee, I.; Jafarian, M.; Mirzapoor, A.; Gobal, F.; Mahjani, M.
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