In operando imaging of self-catalyzed formaldehyde burst in methanol oxidation reactions under open circuit conditions

Article abstract of DOI:10.1039/c7sc05347a

We employ a surface plasmon resonance imaging (SPRi) technique to monitor the in operando process of formaldehyde (HCHO) production during methanol oxidation with high spatial and temporal resolutions. While common wisdom suggests HCHO is generated as an intermediate during continuous electron transfer towards CO₂, we find that the majority of HCHO is produced via self-catalyzed chemical and electrochemical reactions under open-circuit conditions, which lead to an unprecedented HCHO burst immediately after withdrawal of external potential. Because open-circuit conditions better represent the operating environments of practical direct methanol fuel cells (DMFCs), this work uncovers a hidden pathway of HCHO accumulation by adopting a quantitative and in operando SPRi technique for the first time. These theoretical and technical advances are anticipated to help the fundamental understanding of the comprehensive mechanism of methanol oxidation with implications for improving the performance of DMFCs.

Full text of DOI:10.1039/c7sc05347a

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ISSN 2041-6539
EDGE ARTICLE
Francesco Ricci et al.
Electronic control of DNA-based nanoswitches and nanodevices
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In operando imaging of self-catalyzed formaldehyde burst in
methanol oxidation reactions under open circuit conditions†
Received 00th January 20xx,
Accepted 00th January 20xx
Liang Yuan, Meng Li, Tinglian Yuan, Yimin Fang, and Wei Wang*
DOI: 10.1039/x0xx00000x
We employ a surface plasmon resonance imaging (SPRi) technique to watch the in operando process of formaldehyde
(
HCHO) production during methanol oxidation with high spatial and temporal resolutions. While common wisdom
www.rsc.org/chemicalscience
2
suggests HCHO is generated as an intermediate during continuous electron transfers towards CO , we find majority of
HCHO is produced via self-catalyzed chemical and electrochemical reactions under open-circuit conditions, which lead to
an unprecedented HCHO burst immediately after withdrawal of external potential. Because open-circuit conditions better
represent the operational environments of practical direct methanol fuel cell (DMFC), this work uncovers a hidden
pathway of HCHO accumulation by adopting a quantitative and in operando SPRi technique for the first time. These
theoretical and technical advances are anticipated to help the fundamental understanding on the comprehensive
mechanism of methanol oxidation with implications for improving the performance of DMFC.
Tao’s group recently found that, when triggering
electrochemical reactions on the gold substrate and
monitoring the time-lapsed SPRi images at the same time, a
Introduction
Despite of the theoretically promising future hold by DMFC in
portable devices and electric vehicles, their commercialization
has been largely hampered by some of the technical
map of electrochemical current could be obtained because
reaction products often exhibited different refractive index
1
6-18
1
, 2
from reactants.
Inspired by this principle, the capability of
challenges. One of the major concerns is the insufficient cell
voltage and current density due to the formation of undesired
SPRi to investigate electrochemical reactions of individual
electroactive nanoparticle has been demonstrated by others
3
-5
byproducts such as HCHO via 2-electron process. However,
the detailed mechanism of HCHO production in DMFC remains
under debate because it has proven difficult to detect HCHO
with existing in situ spectro-electrochemical techniques such
as infrared spectroscopy and mass spectrometry due to the
1
9-22
and us.
However, the utilization of SPRi to study the
methanol oxidation, an important anode reaction in DMFC,
has not been demonstrated yet.
In the present work, we employed SPRi technique to
monitor the production of HCHO during electro-oxidation of
methanol on Pt catalysts in a quantitative and in operando
manner. Because the refractive index of HCHO (product) is
4
, 6-9
interference of other species in solution.
determine the final concentration of HCHO after reaction
And they can only
3
, 10
completed.
It have revealed that HCHO was the major
3
much higher than that of CH OH (reactant), the
byproduct of methanol oxidation on Pt catalysts, especially
3
when operating at lower cell potential.
, 5, 10
electrochemical reactions lead to increased optical signals in
the time-lapsed SPRi images. Temporal and spatial resolutions
of SPRi allow for studying the reaction kinetics and spatial
distribution of this important byproduct HCHO, respectively.
While common wisdom suggests HCHO would be produced as
an intermediate during the continuous electron transfer
So an in operando
capability to detect HCHO as a function of time (or potential) is
highly desired to uncover the long-standing mystery of HCHO
formation in DMFC.
SPRi is a wide-field optical imaging technique that utilizes
planar SPR effect to map the distribution of refractive index
with sub-micron spatial and sub-millisecond temporal
2
towards CO , our results revealed an unexpected HCHO burst
1
1-13
which rapidly diffused from the Pt catalyst to surrounding
medium immediately after the withdrawal of external
potential (i.e., under OCP). Systematical investigations
attributed this phenomenon to the rapid chemical and
electrochemical oxidation of methanol by surface-adsorbed
oxidative species on Pt catalysts. A self-catalyzed production of
HCHO was further detected by analyzing the corresponding
kinetics data, which underscored the quantitative and in
operando features of SPRi technique.
resolutions.
Since its invention in 1990s, SPRi has become a
powerful and popular technique for determining the binding
kinetics between biomolecules and their ligands in protein and
1
4, 15
DNA microarrays.
In addition to biological applications,
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Results and discussion
HCHO burst phenomenon under open circuit conditions
We first prove the electrocatalytic activity of Pt nanoparticles
(PtNPs) for efficient methanol oxidation. PtNPs are deposited
on gold-coated coverslip to form a porous and disk-like Pt-
catalyst (Pt-disk), which can efficiently catalyze the
electrochemical oxidation of methanol (Fig. S1† and S2†). This
Pt-disk chip is subsequently placed on top of a prism for SPRi
experiments. A representative SPRi image is shown in Fig. 1a.
The round-shaped dark-pattern in the center represents the
Pt-disk because the reflectivity from the thick Pt-layer is much
smaller than surrounding solution where total internal
reflection condition remains valid. Any increase (decrease) in
the local refractive index will quantitatively increase (decrease)
the intensity in the corresponding region-of-interest (ROI) in
SPRi images.
A diffusive burst is observed when watching the time-
lapsed SPRi images during the methanol oxidation experiments
whose procedure is described as follows. The SPRi images are
continuously recorded at a speed of 7.6 fps for 30 seconds in
the absence of external potential to acquire a baseline.
Subsequently, a pre-oxidation potential of +500 mV vs NHE is
applied to the Pt-disk in the presence of electrolyte (0.5 M
2 4 3
H SO +1.0 M CH OH, defined as EL-1) and kept for another 60
seconds. During this period of time, SPRi images (Fig. 1a)
remain unchanged after removing the charging background
which is uniform all over the gold substrate. At 90th second,
the pre-oxidation potential is turned off again (i.e., open-
circuit condition). Surprisingly, the solution surrounding Pt-disk
undergoes a wave-like burst in the recorded SPRi images (Fig.
.
1
b-d and Movie S1†), clearly indicating a massive release
production and diffusion) of some chemical species with
(
higher refractive index.
Quantitative analysis confirmed the diffusion nature of
this burst. Fig. 1e shows the SPR intensity curves of nine
adjacent ROIs, whose locations are indicated by colored dots in
Fig. 1c. The kinetic features of these curves are well consistent
with the 3-dimensional semi-infinite diffusion model. When
assuming M molecules are released from a point-source in the
substrate-solution interface at time zero and subsequently
diffuse into the solution, species concentration C(r, t) as a
function of radial distance (r) and time (t) can be resolved from
the following equation:
function of its distance (L) to the center of the Pt-disk
1/2
ꢖꢗ
(
ꢆ
L =
√
6Dtmax , ESI section 2.2). As shown in Figure 1f,
experimental results indeed follow this relationship. The
ꢇꢈꢉ
ꢀ
ꢁꢂ, ꢃꢄ ꢅꢆ
ꢐ
ꢈ ꢑꢒꢓ ꢔꢕ
ꢘ
(1)
ꢊ√ꢋꢌꢍꢎꢏ
ꢋꢍꢎ
-9
2
diffusion coefficient is thus determined to be 4.8×10 m /s,
which is in the reasonable range of small molecules’ diffusion
coefficient in water.
where D is the diffusion coefficient. For each ROI, the
initial concentration before diffusion is zero. Subsequently,
local concentration goes up when species are diffusing over
this area. Since the species amount is limited, the local
concentration would eventually decrease towards zero again.
Because of the mm-scale size of Pt-dot, it is not suitable to
directly apply this simplified equation involving point-source.
Instead, the diffusion distance (L) was calculated by adding the
radial diffusion length from the edge of the Pt-dot to its radius
We further examine the dependence of HCHO burst on
the pre-oxidation potential (Fig. 1g). When the pre-oxidation
potential is lower than 200 mV, no HCHO burst is observed at
all (black curve). When applying a potential of 300 mV, gradual
release of HCHO appears in the potential ON stage, which
gradually decreases after the withdrawal of potential. Most
significant HCHO burst is observed when applying a potential
of 400 mV. Further increasing the potential is also able to
(r=0.4 mm). Diffusion model predicts that, for each ROI, the
time required to reach the maximal concentration (tmax) is a
2
| Chem. Sci., 201x, 00, x-x
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.
intensity of a ring-shape region-of-interest surrounding the Pt-
disk (Fig. S5†). Third, when performing the same experiments
trigger the HCHO burst, but with an obvious delay in tmax for
the same ROI (Fig. S3†), which will be discussed in the last
3
by replacing CH OH with HCHO or HCOOH, no burst is
section. These results demonstrate that HCHO burst is a
consequence of the electrochemical reactions in the pre-
oxidation stage, and it is triggered by the withdrawal of
potential. Note that HCHO is continuously producing in pre-
oxidation stage when applying a higher potential (E>0.4V).
However, if a high potential is hold, the HCHO is released in a
observed at all (Fig. 2c). These results indicate the role of 2-
electron oxidation product HCHO. In addition, optical
calibration curves (Fig. 2d) and the experimentally determined
refractive index values (Fig. S6†) obtained by adding different
solutions into the EL-1 (0.5 M H
the refractive index of HCHO solution is much larger than that
of CH OH solution with the same molar concentration. It
means that when converting CH OH to HCHO, SPR intensity
would increase. The increased refractive index due to added
SO solution is also displayed and the influence of adding
HCHO is about 5 times larger than that of H SO (Fig. S6†).
During the oxidation of CH OH, protons and HCHO are
2 4 3
SO + 1.0 M CH OH) show that
slower rate due to the effective production of CO
blue, magenta and green curves).
2
(Fig. 1g
3
3
Identification of HCHO as the chemical species during burst
H
2
4
We subsequently clarify the chemical identification of the
2
4
3
burst. First, the optical signal is proportional to the CH OH
3
concomitantly generated and both will change the refractive
index. It is difficult to differentiate the conctibution from each
other, and the SPR signal is actually contributed by both HCHO
concentration, and no burst is observed in the absence of
methanol (Fig. 2a). These results together with the
dependence on pre-oxidation potential (Fig. 1g) indicate that
the electro-oxidation products of methanol are responsible for
the burst. Second, a well-established spectrometric assay (see
18
molecules and protons. According to the dependence
between the HCHO burst and pre-oxidation potential (Fig. 1g)
as well as the consistence between final HCHO concentration
and optical signal intensity (Fig. 2b), the generated HCHO
molecules play critical roles in the obtained SPR signal of burst.
Many literature reports using ex-situ techniques have
also shown that HCHO is one of the major byproducts of
Fig. S4† for details) is utilized to determine the end-point
concentration of HCHO under different pre-oxidation
potentials, which show excellent agreement with the optical
2
3, 24
measurements (Fig. 2b).
It should be noted that the
maximal SPR intensity was calculated by averaging the SPR
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methanol electro-oxidation on Pt electrode. Systematic studies HCHO burst. Instead of directly turning off the potential after
by Korzeniewski’s group using fluorometric assays revealed 30-s pre-treatment at 0.5 V, different potentials are applied to
that HCHO yield was firstly increasing with electrode potential, the Pt-disk for another 30 s before turning off (upper panel in
3
reaching the maximum at 500 mV. Higher electrode potential Fig. 3b). The corresponding SPR intensity curves surrounding
would facilitate the 4-electron and 6-electron process to the Pt-disk are shown in the lower panel. When a 0.4 V
produce HCOOH or CO
2
, leading to the decreased HCHO yield. potential is applied, HCHO burst would not appear until the
(magenta curve). It
Such potential dependence is in good agreement with our potential withdrawal at 120th
s
results. Chromatography analysis conducted by Iwasita’s group demonstrates that the OCP needs to be lower than 0.4 V to
5
also confirmed these conclusions. In a recent work by Chen, massively produce HCHO. The application of 0.3 V potential
HCHO was attributed to the major product when mixing results in the HCHO burst in the first 30 seconds. Another small
methanol with a pre-oxidized Pt electrode under open circuit peak shows up after the potential withdrawal (green curve),
25
conditions. Therefore, we conclude that the rapid production suggesting further production of HCHO at OCP lower than 0.3
and diffusion of HCHO on Pt-disk immediately after the V. Similar but smaller secondary peak is also observed for 0.2 V
potential withdrawal is responsible for the burst phenomenon. (blue curve). Electrode potential of 0.1 V (red curve) or 0 V
(black curve) immediately leads to the HCHO burst and no
Reaction mechanisms of HCHO burst under open circuit conditions further peak is detected when turning off the potentiostat at
We next discuss the reaction mechanisms of HCHO burst after 120th second. These results indicate that HCHO burst is a
the withdrawal of potential. It is known that electrochemical consequence of reduced interfacial potential as long as it is
reactions under OCP still take place in a similar way as it is lower than 0.3 V. Lower potential leads to faster HCHO
2
5
driven by external potential. The major difference is that the production rate in the range from 0.3 to 0 V. This potential can
driving potential under OCP is changing over time during the be provided by either OCP or potentiostat. An oxidative
reaction process. Normally in pure H
declines slowly and reverts to the original equilibrium state chronoamperometry current of stage 4 in the presence and
Fig. S7
). Interestingly, it is found that the OCP rapidly absence of reactant methanol, further indicating the electron
decreases from 0.5 V to ca. -0.1 V within 5 seconds in the injection caused by methanol oxidation on Pt-disk under open-
presence of CH
OH (Fig. 3a). OCP drops of pre-oxidized Pt circuit conditions (Fig. S8 ). A total amount of charge injected
2
SO
4
solution, the OCP current
can
be
obtained
by
differencing
the
(
†
†
3
electrode in the presence of methanol have been frequently (Q) is calculated to be 1.5 mC (14 nmol electrons) by
reported as an indicator of electron injection from solution to integrating above differential current over time.
2
5, 26
the electrode due to the methanol oxidation.
In the other hand, the amount of HCHO produced in one
A controlled potential experiment is also conducted in burst is calculated by two independent methods. In the first
order to measure the amount of injected electrons during the approach, we determine the final concentration of HCHO in
the solution by using the spectrometric assay. Results reveal
that 69.3 μM (42 nmol) HCHO is produced in one burst. The
second approach utilized the diffusion model to calculate the
amount of HCHO (M) from the time-dependent concentration
evolution (Fig. 1e). Analysis to Fig. 1e gives us a total HCHO
amount of 48 nmol. The consistency between two methods
not only demonstrates the validity of both approaches, but
also suggests that the observed HCHO burst contributed
majority of the HCHO molecules generated during the entire
experiments. In other words, in operando results in the
present work prove that majority of the HCHO byproduct is
produced in the open-circuit conditions, rather than being
generated as an intermediate species during continuous
2
electron transfer processes towards CO .
Production of HCHO follows both electrochemical and
chemical pathways (Fig. 4). Electrochemically produced HCHO
molecules should be Q/2 (7 nmol) because it is a 2-electron
process, which is six times less than the total HCHO amount
(
M). The difference must be contributed by chemical reactions
between methanol and pre-oxidized species on the Pt-disk.
Previous spectrometric evidences have shown that Pt-C=O is
the major oxidative species in this potential range (0.5 V).
Therefore, we propose the following electrochemical [Eq. (2)]
and chemical [Eqs. (3-5)] pathways:
ꢆꢆꢆꢆꢆꢙꢎꢆꢆꢆꢆꢆ
+
OH ꢚꢛꢛꢛꢛꢜ HCHO + 2H + 2e
ꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆ
-
CH
3
(2)
(3)
+
-
3 2
Pt + CH OH ꢚꢛꢛꢜ Pt-(CH OH)ads + H + e
4
| Chem. Sci., 201x, 00, x-x
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ꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆ
-
+
Pt-COads + Pt-(CH
or,
2
OH)ads + H + e
ꢚꢛꢛꢜ 2HCHO + 2Pt
(4)
(5)
In summary, we have demonstrated the capability of an in
operando SPRi technique to study the dynamic production of
HCHO during methanol oxidation for the first time, leading to
ꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆꢆ
3
Pt-COads + CH OH ꢚꢛꢛꢜ 2HCHO + Pt
Chemical process is enabled by surface-adsorbed oxidative the discovery of an unexpected HCHO burst phenomenon
species Pt-COads. Although certain interfacial potential is still under the open circuit conditions. Self-catalyzed
required, the net interfacial electron transfer is zero [Eq. (5)].
In order to validate the capability of Pt-COads species to oxidative Pt-COads species is responsible for the rapid
oxidize CH OH chemically, another chemical oxidation production and subsequent diffusion of HCHO molecules,
electrochemical and chemical and oxidation of methanol by
3
experiment is also conducted in the absence of potential accompanying with the observed HCHO burst. The present
control. We continuously blow CO gas (bubbles) into the acidic work provides both theoretical and technical advances in
methanol electrolyte, and detect the concentration of HCHO in understanding the detailed reaction pathways in DMFCs. From
the solution with spectrometric assay. We clearly observed the the theoretical point of view, most of the methanol electro-
production of HCHO after bubbling CO for 15 minutes (Fig. 3c). oxidation studies have been conducted under controlled
HCHO concentration gradually increase when extending the potential conditions. However, the anodic potential is not well
reaction time. The abnormal decrease of HCHO production controlled when DMFC is operating. Therefore, the observed
after 60-minutes may be a result of the escape of HCHO HCHO burst is likely to better represent the actual scenario
molecules from the solution due to long-term gas flow. These when DMFC device is switching between operating (with
results demonstrate that Pt-COads species is able to oxidize the potential) and resting (open circuit) conditions. From the
methanol molecules in the solution without external potential, technique point of view, in situ determination of HCHO has
thus supporting the chemical pathway proposed above.
been standing as a technical challenge for a long time. The
present work demonstrates the capability for quantitatively
and label-free visualizing the HCHO production based on its
refractive index. The sufficient temporal and spatial
resolutions of SPRi thus promise the possibility to clarify the
debating mechanism underneath the production of HCHO in
methanol oxidation reactions.
Self-Catalyzed methanol oxidation
The methanol oxidation under open-circuit conditions exhibits
7-29
2
an obvious feature of self-catalysis.
After the potential
withdrawal, it often takes several seconds before the
appearance of HCHO burst as shown in Fig. 1g. Longer delay is
observed at higher pre-oxidation potential (Fig. S3
because both chemical and electrochemical oxidation of
CH OH required fresh metallic Pt atoms to enable the
dissociative adsorption of CH
†). That’s
Conflicts of interest
3
3
0
There are no conflicts to declare.
3
OH molecules [Eq. (2)].
However, most of the surface Pt atoms have already been
oxidized to form Pt-COads under high pre-oxidation potentials.
Therefore, after potential withdrawal, one un-occupied Pt
Acknowledgements
atom (with rather low chance) adsorbs one CH
3
OH molecule
We acknowledge financial support from the National Natural
Science Foundation of China (Grants No. 21522503, 21527807
and 21327902), and the Natural Science Foundation of Jiangsu
Province (BK20150013 and BK20150570).
and reacts with the adjacent Pt-COads to produce two HCHO
3
1
molecules and to release two fresh Pt atoms [Eq. (5)]. This
chain-like process increases the number of un-occupied Pt
atoms, and further accelerates the HCHO process via a self-
catalysis mechanism, until all the Pt-COads species are
exhausted. Higher pre-oxidation potential will cause less un- Notes and references
occupied Pt atoms in the beginning, and thus requires a longer
1
2
3
X. Zhao, M. Yin, L. Ma, L. Liang, C. P. Liu, J. H. Liao, T. H. Lu and
time to initiate the HCHO burst (Fig. 1g). This hypothesis is also
supported by the absence of HCHO burst at a pre-oxidation
potential of 0.3 V (red curve in Fig. 1g). Because this potential
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Chemical Science
View Article Online
DOI: 10.1039/C7SC05347A
A wave-like HCHO burst is in-situ observed during electro-oxidation of methanol on Pt
under open circuit conditions by SPR imaging.