Enhanced methanol oxidation performance on platinum with butterfly-scale architectures: Toward structural design of efficient electrocatalysts

Article abstract of DOI:10.1039/c3ra45388j

Enhanced methanol oxidation performance was achieved by combining elaborate butterfly-scale architectures into platinum electrocatalysts using an electroless deposition-detemplating method. The electrochemically active surface area and the forward peak cu

Full text of DOI:10.1039/c3ra45388j

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Enhanced methanol oxidation performance on
platinum with butterfly-scale architectures: toward
structural design of efficient electrocatalysts†
Cite this: RSC Adv., 2014, 4, 3748
Received 26th September 2013
Accepted 18th November 2013
Xingmei Guo, Han Zhou, Di Zhang and Tongxiang Fan*
DOI: 10.1039/c3ra45388j
www.rsc.org/advances
Enhanced methanol oxidation performance was achieved by available to interact with reactants, since mass transport plays
18,19
combining elaborate butterfly-scale architectures into platinum an important role in electrochemical reactions.
There is still
electrocatalysts using an electroless deposition-detemplating much room for improvement in electrode performances,
method. The electrochemically active surface area and the forward considering the complexity and irregularity of the nano-
peak current density for methanol oxidation were dramatically structures of electrocatalysts. In fact, the spatial arrangement of
increased by 5 and 5.2 times, respectively, for lamellar-ridge archi- nanostructured materials strongly impacts their mass transport
tectured Pt compared to its unarchitectured counterpart. Excellent properties, and their architecture design is highly necessary to
20
mass transport properties and efficient catalytic surface accessibility realize their full catalytic potential. For instance, platinum
were further demonstrated by finite element simulation.
nanowire networks have been prepared by chemical and elec-
trochemical deposition using templates with interconnected
21–23
channel structures.
However, to what extent the architecture
Fuel cells are attracting considerable attention due to their
potential applications in the areas of energy storage and
impacts electrochemical behavior and what arrangement can be
considered to be effective for efficient mass transport are still
ambiguous. Considering the current limited patterns of
synthesized architectures, a more delicate design is required,
especially with precise morphology and dimension control, to
explore the architecture-assisted electrochemistry and to
provide a prototype for future architecture design.
1–3
conversion. In addition to offering the advantages of high
energy conversion and low pollutant emissions, direct meth-
anol fuel cells (DMFC) are considered to be some of the most
promising systems because of their low operating temperature
and the high potential density of liquid methanol fuel, which is
3–5
suitable for portable electronic devices. Essential to many
catalytic systems, platinum has been commonly used as an
Compared with modern techniques such as lithography and
nanoprinting, which are oen costly, time-consuming, low-
6
,7
anode catalyst to promote methanol oxidation. However,
achieving widespread commercial use remains a challenge
mainly because of high cost and limited catalytic performance,
throughput, and can only achieve relatively simple surface
24,25
patterns for electrodes,
nature is a free conguration gallery
that provides us with hierarchical architectures scaled from
nano-, micro- to macro-sizes; among them, butteries are quite
extraordinary since their wings exhibit different vivid colors
8,9
due to low platinum utilization ratio. Different methods, such
as the carbon-supporting technique and the platinum-mono-
9
–12
layer approach,
have been investigated in order to maximize
26–30
intricately related to their complex micro/nanoarchitectures.
the platinum utilization ratio and reduce costs. Furthermore,
Nearly 20 000 species of buttery can be identied solely by the
efforts to synthesize various nanostructured platinum, such as
patterning of their wings, which comprise various hierarchical
13
14
15
16
nanoparticles, nanowires, nanotubes, nanodendrites,
31,32
architectures on a micro- and nano-scale.
We believe that
17
hollow nanospheres, etc., have also been carried out to
produce more efficient electrocatalytic behavior. These
enhanced performances mainly derive from their high surface
areas; however, not all catalytic active surfaces are efficiently
these three-dimensional hierarchical arrangements, once intro-
duced into an electrochemical system, may exhibit great mass
transport properties and may offer a vast pool for the structural
design of electrocatalysts. Here, we chose the blue scales from
Morph buttery wings, whose brilliant blue color derives from
33,34
their lamellar-ridge architecture,
as a template and synthe-
State Key Lab of Metal Matrix Composites, Shanghai Jiaotong University, 800
Dongchuan Road, Shanghai, 200240, China. E-mail: txfan@sjtu.edu.cn; Fax: +86- sized hierarchical architectured platinum (lamellar ridge-Pt-1)
2
†
1
1-34202749; Tel: +86-21-54747779
35
using the electroless deposition method. Unarchitectured
Electronic supplementary information (ESI) available. See DOI:
0.1039/c3ra45388j
platinum (at-Pt-1) was also synthesized using the at wing
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Scheme 1 Schematic diagram of the preparation process of archi-
tectured Pt and unarchitectured Pt using lamellar-ridge architectured
blue scales and flat membrane from Morph butterfly wings as sample
templates through surface functionalizing, electroless deposition, and
detemplating procedure.
Fig.
1
Micro/nanoarchitectures of original Morph butterfly scale
template and as-synthesized lamellar ridge-Pt-1. (a) FESEM image of
the scale template. (b) TEM image showing the cross-sectional view of
lamellar-ridge architecture of the original scale template. (c and d)
FESEM images showing the front and cross-sectional views of lamellar
ridge-Pt-1. Scale bar: 500 nm for all.
membrane of the Morph buttery as a template under the same
procedure, as a contrast sample to demonstrate the effect of
nanostructure arrangement on enhancing electrochemical
performance for methanol oxidation. Furthermore, by using the
yellow hind-wing scales and black fore-wing scales from the
Troides aeacus buttery as templates, we also synthesized inverse-
V ridge-Pt-2 and ridge/nano-hole-Pt-2 in order to obtain a more
comprehensive understanding of different architecture effects.
Their unarchitectured counterpart (at-Pt-2) was also synthesized
using the wing membrane of the Troides aeacus buttery as a Fig. 2 FESEM images showing the micro/nanoarchitectures of as-
synthesized inverse-V ridge-Pt-2 (a) and ridge/nano-hole-Pt-2 (b).
Scale bar: 2 mm for both.
template. Additionally, nite element modeling and simulating
were carried out to analyze the mass transport behavior in these
ridge-type architectures and to illustrate the architecture effect
from a mass transport perspective.
The synthesizing process of platinum with buttery-wing and S13†). The crystalline size, calculated by the Williamson–
scale architecture is depicted in Scheme 1. Aer functionalizing Hall method, was 9.6 nm and 9.9 nm for lamella ridge-Pt-1 and
the template surface, electroless deposition, followed by a at-Pt-1, respectively. In contrast, however, the crystalline size
detemplating process, was carried out to obtain architectured was 2.7 nm, 2.9 nm, and 2.7 nm for inverse-V ridge-Pt-2, ridge/
and unarchitectured Pt. A detailed illustration of the synthe- nano-hole-Pt-2, and at-Pt-2, respectively, which were much
sizing procedure is provided in the ESI S1.† As shown in Fig. 1, smaller than that of Pt-1. Furthermore, X-ray photoelectron
the delicate lamellar-ridge architecture of Morph buttery-wing spectroscopy (XPS) was conducted for surface characterization
scales was successfully duplicated by electroless deposition. (Fig. S14 and S15†). The spectra indicate that the architectured
Aer rinsing in phosphoric acid for 72 h, the chitin template and unarchitectured Pt exhibited similar surface conditions.
was successfully removed and the nal architecture of as- Most Pt species existed as metallic Pt with binding energy of
2
+
synthesized Pt was hollow lamellar ridges with a thickness of 71.3 eV (Pt4f7/2) and 74.6 eV (Pt4f5/2), while Pt chemical state in
about 26 nm. The ridge height of lamellar ridge-Pt-1 was either PtO or Pt(OH) with binding energy around 72.3 eV and
2
approximately 1.7 mm, and the distance between ridges was 75.5 eV also appeared on the catalyst surface which may be due
about 650 nm. Each ridge was substructured by 7–8 lamellae to incomplete reduction of Pt from the electroless deposition
staggeringly patterned on either side of the ridge stem, with a solution and the oxygen chemisorption at the step and kink
10,36–38
lamellar distance of about 120 nm. The morphology of its sites presented on the Pt surface.
unarchitectured counterpart at-Pt-1, with a thickness of about the net content of platinum in the synthesized samples, thermal
20 nm, is displayed in Fig. S1.† Also, the morphology of inverse- gravimetric analysis (TGA) was carried out both before and aer
V ridge-Pt-2, ridge/nano-hole-Pt-2, and their unarchitectured removing the templates. The nal weight percentage of plat-
counterpart at-Pt-2 are exhibited in Fig. 2 and S4.† inum aer detemplation was 83% and 81% for lamella ridge-Pt-
In order to determine
1
X-ray diffraction (XRD) patterns reveal that both the as- 1 and at-Pt-1, respectively (Fig. S17 and S18†); the proportion
prepared architectured and unarchitectured Pt-1 and Pt-2 was 83%, 79%, and 82% for inverse-V ridge-Pt-2, ridge/nano-hole-
exhibited a typical face-centered cubic lattice structure (Fig. S12 Pt-2, and at-Pt-2, respectively (Fig. S19–S21†).
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To evaluate the electrochemical activity of as-prepared Pt,
Considering the same composition, crystalline size, and
cyclic voltammetry was conducted in aqueous solution of sul- surface conditions for lamella ridge-Pt-1 and at-Pt-1, the
phuric acid with and without methanol, using a conventional dramatic promotion of electrochemical active surface area and
three electrode system. Pt samples were rst dispersed in N,N- methanol oxidation behavior of the architectured sample
ꢀ
1
dimethylformamide (1 mg ml ) by sonication. Then, a 5 ml should be attributed to its nanostructure arrangement. To
drop of solution was placed on a freshly polished 3 mm diam- investigate the architecture effect from a mass transport
eter glassy carbon electrode, followed by drying under ambient perspective, we constructed lamellar-ridge architectured
conditions to obtain a working electrode. Platinum wire and a models and carried out nite element simulation using a
saturated calomel electrode (SCE) were used as the auxiliary and diffusion domain approach with a one-ridge period as a domain
41
reference electrode, respectively. The cycling was repeated for unit. Both outer and inner catalytic surfaces of lamellar ridges
0 cycles to obtain reproducible voltammetric curve before the were investigated, as well as an unarchitectured counterpart
3
measurement curves were recorded. Fig. 3a shows the cyclic with an ideal at surface. Detailed model dimension parame-
voltammograms of lamella ridge-Pt-1 and at-Pt-1 in 0.5 M ters are presented in Fig. S1 and Table S1.†
ꢀ
1
H
(
2
SO
EASA) was estimated by integrating the voltammogram corre- involves a multistep process that includes the adsorption of
sponding to hydrogen desorption from the electrode surface, reactants, the formation of intermediate products, and further
4
at 50 mV s . The electrochemically active surface area
The oxidation of methanol on anode catalytic surfaces
ꢀ2
42
using a reference value of 210 mC cm for the charge corre- oxidation. Its overall anodic reaction formula is:
sponding to the desorption of one monolayer of H at the surface
2
39
+
+ 6H + 6e
ꢀ
of 1 cm Pt. The EASA of lamella ridge-Pt-1 was calculated to be
CH
3
OH + H
2
O / CO
2
(1)
2
ꢀ1
18.6 m g
, which was 5 times higher than its unarchitec-
Pt
2
ꢀ1
tured counterpart (3.7 m gPt ). Fig. 3b exhibits the cyclic vol-
4
tammograms in the solution of 1 M CH OH in 0.5 M H SO .
To simplify the calculation process, most theoretical analysis
considers the several consecutive steps as one single step
3
2
Both samples displayed two characteristic methanol oxidation depicted by Bulter–Volmer or Tafel kinetic equations, with
42–45
peaks at about 0.63 V (vs. SCE) and 0.44 V (vs. SCE). The former calibrated or assumed parameters.
Corresponding to the
was attributed to the primary oxidation of methanol on catalyst electrochemical experiment in this work, we here focus on
surfaces, and the latter was derived from the removal of illustrating the mass transport behaviors of methanol triggered
incompletely oxidized carbonaceous species formed during the by simplied anodic methanol oxidation under semi-innite
40
forward scan. The peak current density at 0.63 V (vs. SCE) was diffusion conditions, while neglecting other factors. The initial
Ptꢀ
1
3
CH OH
193.3 mA mg
for lamella ridge-Pt-1, which was 5.2 times diffusion ux (N
) for methanol on the electrocatalyst
ꢀ1
higher than the value of 37.0 mA mgPt for its unarchitectured surface area was calculated from the exchange current density
counterpart at-Pt-1. Obviously, the lamellar ridge nano- (I
3
CH OH
0
a
) and overpotential (h ) as the boundary condition on
structure arrangement endowed Pt with signicantly enhanced the electrode–electrolyte interface. The mass transport of
methanol oxidation efficiency.
methanol was described by Fick's second law of diffusion,
neglecting convection and migration. Detailed simulation
processes and parameters are listed in ESI S2.†
As shown in Fig. S27,† the calculated methanol diffusion ux
under constant equilibrium potential (h ¼ 0) for outer and
a
inner catalytic surface models of lamellar-ridge architecture are
ꢀ
3
ꢀ2 ꢀ1
ꢀ3
ꢀ2 ꢀ1
1
.46 ꢁ 10
mol m
s
and 1.26 ꢁ 10
mol m
s ,
respectively, which are approximately 14.0 times and 12.1 times
their unarchitectured counterpart (1.04 ꢁ 10 mol m
ꢀ4
ꢀ2 ꢀ1
s ).
This result implies outstanding accessibility for both outer and
inner catalytic surfaces which exhibit similar surface areas,
both 14.4 times the unarchitectured model. It is also note-
worthy that the outer surface model represents higher diffusion
efficiency than the inner surface one, which indicates the
signicant impact of transport route on diffusion performance,
in addition to high surface areas. To investigate mass transport
behavior, concentration proles at 100 s were portrayed as
shown in Fig. 3c–e. Obviously, compared to the simple one-
direction planar diffusion for the unarchitectured model, the
Fig. 3 Electrochemical performance analysis. (a and b) Cyclic vol-
ꢀ
1
tammograms for flat-Pt-1 and lamella ridge-Pt-1 at 50 mV s in an lamellar-ridge arrangement shows a zigzag route, which is a
2 4 3 2 4
aqueous solution of 0.5 M H SO (a) and 1 M CH OH/0.5 M H SO (b).
brilliant unimpeded approach to access catalytic surfaces effi-
ciently. Additionally, the outer surface is more accessible than
the inner surface through zigzag diffusion because of its much
broader inlet and less chance of trapping and depleting the
electrolyte between adjacent lamellae, the so-called “thin layer
(
c–e) Reactant concentration profiles at t ¼ 100 s in the simulating
process at electrocatalyst architectures and local bulk solution for
unarchitectured model (c), outer surface model of lamellar-
ridge architecture (d), and inner surface model of lamellar-ridge
architecture (e).
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46
effect”. Here, we note that the calculated 14.0 and 12.1 times counterparts, which indicates lower tolerance of carbonaceous
50–52
increase is much higher than the moderate 5 times increase of adsorbates (COad).
Additionally, the onset oxidative potential
EASA for the experimental results. One reason is that the was slightly higher for architectured samples with marginal
experimental counterpart sample (at-Pt-1) was not an ideally driing of peak potential in the positive direction, as well (Table
innite plane as assumed in the simulation process. Another S4†). This may be attributed to the tip effects for E, or in other
important reason for this should be the uneven electric eld words, irregularly high electric eld intensity around architecture
intensity (E) around the lamellar-ridge architecture, due to its tips, which may cause uneven electromigration and aggregation
high structural curvature, which would possibly induce uneven of charged intermediate products and lead to more difficulties
electromigration of charged reactants/products in the complex for intermediates removal/oxidation (Fig. S29 and S30†). Specif-
reaction process; whereas, only uncharged methanol diffusion ically, more COad formed and accumulated on tip surface sites via
was assumed in the above mass transport simulation. In fact, adsorption and oxidation; this accumulation effect resulted in a
according to further study, E in the interior domain of lamella- high coverage of Pt–COad and blocked Pt active sites on tip
ridge architecture was almost zero, indicating a negligible surfaces for adsorption or formation of further oxide intermedi-
inuence of electromigration in the interior zigzag mass ates, such as Pt–OHad, which is critical for the removal of
53–55
transport behavior; there was also no difference for E in bulk poisonous COad
.
So, in future architecture design for articial
solution for all three models. However, irregularly high E fabrications or applications, we should avoid sharp tips with
occurred around the ridge tips for the outer surface model and extremely high curvature to diminish the tip effect for E. None-
around the inlet corners for the inner surface model, due to tip theless, these three-dimensional hierarchical arrangements
effects which may cause accumulation or repulsion of some inspired by buttery-wing scales still show excellent mass trans-
charged intermediates (Fig. S29†). In addition, the incomplete port properties and efficient catalytic surface accessibility, and
removal of the chitin template from the platinum catalyst also may hold great promise for applications on fuel cells and other
lowered the actual catalytic surface, and the architecture of the electrochemical arenas. In addition to ridge-type architecture,
as-prepared working electrodes was not as innitely highly this methodology may also be extended to other buttery-wing
ordered as the simulation assumption, as well.
congurations or other biological species.
Electrocatalytic performance for methanol oxidation was
In summary, we synthesized platinum catalysts with different
also investigated on inverse-V ridge-Pt-2 and ridge/nano-hole-Pt-2. ridge-type architectures inspired by buttery wings using an
Table 1 shows the summary of characterization results for all electroless deposition method, followed by a detemplating
samples. The most signicant enhancement for methanol process. These elaborate architectures, particularly the lamellar-
oxidation was exhibited by lamellar-ridge architecture, as dis- ridge arrangement, showed a signicant enhancing effect for
cussed above. As for other architectures, the electrochemical electrocatalytic methanol oxidation. The electrochemically active
active surface area for inverse-V ridge-Pt-2 and ridge/nano-hole-Pt- surface area and the peak current density for methanol oxidation
2
was 2.4 and 4.3 times higher than that of at-Pt-2, respectively; were dramatically increased by 5 and 5.2 times, respectively, for
the forward peak current density for cyclic voltammograms of lamellar ridge-Pt-1 compared to its unarchitectured counterpart.
methanol oxidation was 2.7 and 3.9 times that of their This signicant enhancement should be attributed to the
unarchitectured counterpart, respectively. Their unimpeded unimpeded zigzag mass transport property and efficient surface
mass transport behaviors were demonstrated by nite element accessibility for the lamellar-ridge architecture, which was
simulation, as well (Fig. S28†).
demonstrated by nite element simulation. This work demon-
Comparing the two unarchitectured samples (at-Pt-1 and strates a great architecture effect on electrochemical performance
at-Pt-2) with different grain sizes, we found a “negative-particle- and may provide a prototype for architecture design of efficient

size effect” for EASA, onset oxidative potential, and current electrochemical congurations. Meanwhile, this work may also
density, except for the I /I value, which coincides with literature present an exemplary strategy for optimizing articial design
Another innegligible fact is that the samples with through inspiration from natural organisms.
f
b
47–49
results.
elaborate buttery-wing scale architectures for both Pt-1 and Pt-2
exhibited lower If/Ib values than their unarchitectured
Acknowledgements
The authors acknowledge the consistent nancial supports of
National Natural Science Foundation of China (No. 51172141 ),
National Basic Research Program of China (No. 2011CB610301)
and the Research Fund for the Doctoral Program of Higher
Education (No. 20100073110065 and 20110073120036).
Table 1 Summary of characterization results for all samples
Particle EASA
size (nm) (m g
Onset potential I
f
) (V vs. SCE)
2
ptꢀ1
Ptꢀ1
(mA mg ) I
Samples
f b
/I
Flat-Pt-1
Lamella
9.9
9.6
3.7
18.6
0.42
0.44
37.0
193.3
1.9
1.4
Notes and references
ridge-Pt-1
Flat-Pt-2
Inverse-V
ridge-Pt-2
Ridge/
2.7
2.9
2.5
6.1
0.41
0.43
30.5
82.1
2.1
1.6
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