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Nano Res
1
Ultra-Thin Layer Structured Anodes for Highly Durable
Low-Pt Direct Formic Acid Fuel Cells
Rongyue Wang1,§, Jianguo Liu2,§, Pan Liu3, Xuanxuan Bi1, Xiuling Yan1,4, Wenxin Wang1, Yifei Meng2,
Xingbo Ge1, Mingwei Chen3, and Yi Ding1 ()
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-014-0517-9
http://www.thenanoresearch.com on June 16, 2014
© Tsinghua University Press 2014
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Nano Research
DOI 10.1007/s12274-014-0517-9
Ultra-Thin Layer Structured Anodes for Highly
Durable Low-Pt Direct Formic Acid Fuel Cells
Rongyue Wang1, § , Jianguo Liu2, § , Pan Liu3,
Xuanxuan Bi1, Xiuling Yan1,4, Wenxin Wang1, Yifei
Meng2, Xingbo Ge1, Mingwei Chen3, and Yi
Ding1()
1 Center for Advanced Energy Materials & Technology
Research (AEMT), and School of Chemistry and
Chemical Engineering, Shandong University, Jinan
250100, China.
2 Eco-Materials and Renewable Energy Research Center,
Department of Materials Science and Engineering,
National Laboratory of Solid State Microstructures,
Nanjing University, Nanjing 210093, China.
3 WPI Advanced Institute for Materials Research, Tohoku
University, Sendai 980-8577, Japan.
4 Resources and Ecologic Research Institute, School of
Chemistry and Bioscience, Yili Normal University,
Xinjiang 835000, China.
§These authors contribute equally to this work.
By confining highly active nanoengineered catalysts into an
ultra-thin catalyst layer, a dramatic decrease in Pt usage down
to 3 microgram per cm-2 is achieved in direct formic acid fuel
cell anode while maintaining impressive electrode activity,
durability and power performance in both single cells and
multi-cell stacks.
Ultra-Thin Layer Structured Anodes for Highly
Durable Low-Pt Direct Formic Acid Fuel Cells
Rongyue Wang1,§, Jianguo Liu2,§, Pan Liu3, Xuanxuan Bi1, Xiuling Yan1,4, Wenxin Wang1, Yifei Meng2,
Xingbo Ge1, Mingwei Chen3, and Yi Ding1 ()
§
Received: day month year
Revised: day month year
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Direct formic acid fuel
cells, Low-Pt loading,
Core/shell structures,
Nanoporous gold,
Dealloying
ABSTRACT
Direct formic acid fuel cells (DFAFCs) allow highly efficient low temperature
conversion of chemical energy into electricity and are expected to play a vital
role in our future sustainable society. However, the massive precious metal
usage in current membrane electrode assembly (MEA) technology greatly
inhibits their actual applications. Here we demonstrate a new type anodes
constructed by confining highly active nanoengineered catalysts into an
ultra-thin catalyst layer of order 100 nm. Specifically, an atomic layer of
platinum was firstly deposited onto nanoporous gold (NPG) leaf to achieve
high utilization of Pt and easy accessibility of both reactants and electrons to
active sites. Those NPG-Pt core/shell nanostructures were further decorated by
sub-monolayer of Bi to create highly active reaction sites for formic acid
electro-oxidation. Thus obtained layer-structured NPG-Pt-Bi thin films allow a
dramatic decrease in Pt usage down to 3 micrograms per cm-2, while
maintaining very high electrode activity and power performance at sufficiently
low overall precious metal loading. Moreover, this kind of electrode materials
show superior durability during half-year test in actual DFAFCs, with
remarkable resistance to common impurities in formic acid, which together
imply their great potential in actual device instrumentation.
1. Introduction
When powered by liquid fuels, polymer electrolyte
membrane fuel cells [1, 2] show great potential to
support portable electronic devices and off-grid
facilities [3, 4]. Compared with methanol, formic acid
has received increasing attention recently due to its
advantages of being non-flammable, less toxic,
higher electromotive force, and lower fuel crossover
[5-7]. Although some prototype examples have been
demonstrated, the commercialization of direct formic
acid fuel cells (DFAFCs) has been hampered by the
massive usage of precious metals such as Pt [8] and
Pd, which is typically on the order of several
milligrams per square centimeter area at anode (see
Table S1).
To decrease precious metal usage while
Nano Research
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Research Article
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2 Nano Res.
maintaining high enough performance, the specific
parameters contributed to the fuel cell voltage decay
should be understood clearly and avoided in the
membrane electrode assembly (MEA) fabrication [9].
For DFAFC anode, three main factors lead to the fuel
cell voltage decline (over-potential): 1) poisoning
intermediate formation on catalyst surface; 2)
insufficient electron conduction; and 3) limits in
proton transportation or reactants diffusion. It is
widely known that pure Pt catalyst is prone to be
poisoned by CO intermediate in formic acid
electro-oxidation [10] which results in large
over-potential. To solve this problem, a variety of
approaches have been developed to inhibit the
formation of poisoning intermediates and improve
the catalytic activity of Pt [11-13]. Although orders of
magnitude catalytic activity enhancements have been
demonstrated in electrochemical measurements, it is
rarely seen under real fuel cell testing conditions
[14-25]. This is related to the other two factors which
are associated with undesired electrode structure
involved in the current MEA construction
methodology, which is based on carbon supported Pt
or Pt-based alloy nanoparticle catalysts. As shown in
Scheme 1a-c, within a typical MEA, carbon
supported Pt nanoparticles are mixed with Nafion
ionomers and pasted onto diffusion layer to make a
catalyst layer with thickness of order 10 µm [26](up
to 40 µm for most reported DFAFCs). The
contribution of catalysts to fuel cell performance
would be restricted by either electron/reactant
transportation (for catalysts near the membrane side)
or proton transportation (for catalysts near the
diffusion layer side) through the ~40 µm thick
catalyst layer (see Scheme. 1a). The Pt efficiency
could be increased by simply decreasing the
thickness of catalyst layer, but cell performance will
then be sacrificed [19, 20], not to mention that only
20-30% Pt was reported to actually contribute to the
apparent performance in a real fuel cell [27]. These
problems could only be solved by concentrating high
performance nanocatalysts in an ultra-thin electrode
layer. The recently reported 3M’s (Minnesota Mining
and Manufacturing Company) nanostructured thin
film catalysts have shown the advantage of this
concept when functioning as a H2-PEMFC cathode,
although their electrochemical surface areas are
much lower than Pt/C catalysts [28, 29]. Direct
growth of Pt or Pd nanoparticles onto carbon paper
would also contribute to performance improvement.
However, with much larger particle size around
15-25 nm, the precious metal utilization was low [18,
30].
An ideal anode electrode in DFAFCs should
simultaneously fulfill the following desired
properties: high Pt utilization, high catalytic activity
toward formic acid electro-oxidation and high active
site concentration in an ultra-thin catalyst region. In
this work, we present such an electrode based on
nanoporous gold (NPG) leaf supported Pt
nanoarchitectures (see Scheme. 1d-f). With three
dimensional bicontinuous porous structure and ~100
nm order thickness, NPG leaf made by dealloying
could be an ideal catalyst support within which its
inter-connected ligament facilitates the collection and
transportation of electrons from the catalyst surface
to current collectors and the open porous structure
allows the surfaces easily accessible by the reactants
[31-34]. Using electrochemical method and/or
chemical method, one can deposit catalytically active
materials such as Pt on the ligament surface of NPG
uniformly at atomic layer precision [32, 35, 36]. In
principle, the Pt utilization of NPG supported Pt
nanocatalysts could achieve a perfect value of 100%
for a monolayer structure (see Scheme. 1e). Different
from carbon materials on which the deposited Pt
atoms tend to aggregate into nanoparticles to
minimize the surface energy, NPG would stabilize
the Pt adlayers by strong metallic bonds (alloying).
Further decorating the Pt surface with
sub-monolayer Bi, one can greatly improve the
catalytic activity of Pt by changing the reaction paths
or facilitating the removal of CO poisoning
intermediate (see Scheme. 1f). By confining NPG leaf
supported Pt/Bi catalysts into an ultra-thin layer, we
demonstrate a high performance DFAFC anode with
extremely low Pt loading down to 3 micrograms per
cm-2. Both single cell and stack tests demonstrated
that these rationally designed anodes possessed very
high activity and durability.
Scheme 1. Schematic illustration of membrane electrode assembly (MEA) structures. a-c) MEA structure of Pt/C based anode. d-f)
MEA structure of NPG-Pt-Bi based anode. For simplicity, only anode side was highlighted.
2. Experimental section
2.1 Sample preparation
NPG was prepared by dealloying 12 carat AuAg
alloy leaves in concentrated nitric acid for 30 minutes
at room temperature. For comparison purposes,
AuAg alloy foils with larger thickness were also
selected. To control the Pt loading, NPG-Pt samples
could be prepared by depositing Pt onto the
ligament surface of NPG using either chemical
deposition or Cu-UPD (under potential deposition)
mediated deposition process. The detailed
preparation procedures of NPG and NPG-Pt samples
could be found in previous literatures [14, 31, 35].
The NPG-Pt catalysts were then immersed into 5 mM
Bi(III) containing 0.1 M HClO4 solution for 5 min to
allow Bi decoration. After cleaning with ultra pure
water, NPG-Pt-Bi samples were reduced by
sweeping the potential to 0 V from open circuit
potential. The Bi(III) containing solution was
prepared by dissolving proper amount of Bi2O3 (5M)
in 0.1 M HClO4. The electrochemical and
electrocatalytic properties of Pt/C, NPG-Pt, and
NPG-Pt-Bi were characterized by cyclic voltammetry
(CV) in 0.5 M H2SO4 and 0.5 M H2SO4+1 M HCOOH
solution. The catalytic activities of different catalysts
were normalized to Pt loadings by dividing the data
in CV curves by Pt mass on the electrodes,
respectively.
2.2 Fuel cell testing
NPG-Pt-Bi catalysts were brushed onto carbon paper
for use as anodes. Pt/C catalysts were also brushed
onto carbon paper to prepare cathode electrodes.
Both anodes and cathodes were hot pressed onto
each side of Nafion® 115 to make an MEA. The MEA
was fixed into a fuel cell to test the performance. The
fuel cell was activated by CV and tested at 40 oC by
supplying 3 M formic acid to the anode (about 2 ml
min-1) and dry air (about 120 ml min-1) to the cathode.
The outlet formic acid temperature was used as
indicator of the stack temperature. Cell discharging
and polarization tests were conducted on a fuel cell
station equipped with a Kikusui PLZ30 (Japan)
electronic load.
2.3 Characterization
The morphology of NPG-Pt-Bi anode and cross
section of MEA were examined by scanning electron
microscopy (FEI NOVA NanoSEM 230). The
composition and actual precious metal loading of
these nanoelectrodes were determined by ICP-AES
(IRIS Advantage). The chemical states of Au, Pt and
Bi in NPG-Pt and NPG-Pt-Bi samples were analyzed
with a VGESCALAB X-ray photoelectron
spectrometer (XPS), using monochromatized Mg Ka
X-ray as the excitation source, and choosing C 1s
(284.60 eV) as the reference line. The microstructure
characterization was performed with a 200 kV
JEM-2100F electron microscope (JEOL) equipped
with two aberration correctors (CEOS GmbH) for the
image- and probe-forming lens systems and X-ray
energy-dispersive spectroscopy (JED-2300T, JEOL)
for chemical composition analysis.
3. Results and discussion
3.1 Half-cell demonstration
To demonstrate the structural and catalytic
uniqueness of these rationally designed catalysts, we
fabricated an NPG supported monolayer Pt catalyst
using copper under potential deposition method [35]
and further decorated it with sub-monolayer Bi
atoms. The cyclic voltammetric (CV) curve of NPG
supported Pt (NPG-Pt) was shown in Figure 1a.
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4 Nano Res.
Although only one monolayer Pt was deposited,
nearly the entire surface of NPG was covered by Pt
as demonstrated by the absence of Au reduction
peak at 1.2 V [35]. This also means almost all Pt
atoms are exposed on the surface thus are accessible
to reactants and current collector simultaneously.
The catalytic activity of NPG-Pt was tested by CV in
0.5 M H2SO4 + 1 M HCOOH mixed solution at room
temperature and the results were presented in Figure
1b, where the data acquired from the commercial
Pt/C catalyst were also listed for comparison. As
shown in Figure 1b, the NPG-Pt catalyst exhibits
similar CV curves as Pt/C catalyst toward formic acid
electro-oxidation. In the forward scan, the NPG-Pt
catalyst shows small oxidation current as the surface
was passivated by CO [37]. However, the Pt mass
specific activity was greatly improved due to much
improved Pt utilization. To further improve the
catalytic activity, NPG-Pt catalyst was decorated
with Bi. And this process was realized by an
irreversible spontaneous adsorption of Bi by simply
immersing the NPG-Pt electrodes into 5 mM Bi(III)
containing 0.1 M HClO4 solution for 5 min. The Bi
coverage could be easily controlled by Bi(III)
concentration and the immersion time [38]. The
successful deposition of Bi was confirmed by CV
curves shown in Figure 1a where the hydrogen
adsorption and desorption charges become evidently
smaller. Different from NPG-Pt and Pt/C catalysts,
NPG-Pt-Bi exhibits a dramatic negative-shift in onset
potential to below 0.2 V and huge anodic peaks at
around 0.6 V as showed in Figure 1b, indicative of
successful inhibition for the formation of poisoning
CO intermediate due to the selection of the direct
reaction path induced by the “third body” effect [14,
15, 38, 39]. That also indicates the continuous Pt
surface has been divided into small Pt ensembles
which are too small for the formation of poisoning
CO intermediate but large enough for the direct
reaction path to occur. By successfully constructing
onto a single platform all three key functions, i.e. (i)
high Pt utilization, (ii) great tolerance to poisoning,
(iii) high conductivity and accessibility, these novel
nanostructures demonstrated very high
electrochemical performance of NPG-Pt-Bi toward
formic acid electro-oxidation, and at 0.6 V the
NPG-Pt-Bi catalyst could exhibit over 570-fold
enhancement in Pt mass specific activity as
compared with the Pt/C catalyst (103.4 vs. 0.18 A
mg-1). The Pt mass specific activity on NPG-Pt-Bi
catalyst is to our knowledge the highest value
reported to date.
Figure 1. Electrochemistry and electrocatalytic activities of three electrodes. (a) Cyclic voltammetric (CV) curves of Pt/C, NPG-Pt, and
NPG-Pt-Bi electrodes in 0.5 M H2SO4. (b) Pt mass specific catalytic activities of Pt/C (enlarged by a factor of ten for better comparison),
NPG-Pt, and NPG-Pt-Bi in 0.5 M H2SO4+1 M HCOOH. The positive potential was restricted to 0.8 V for NPG-Pt-Bi sample to avoid Bi
oxidation. Sweep rate: 50 mV s-1.
3.2 Structural characterization
While half-cell testing demonstrated the tremendous
catalytic activity enhancement for NPG-Pt-Bi
electrode as compared with the commercial Pt/C
catalyst, we are interested in their actual
performance in real fuel cells. To make larger
amount of catalysts, we first mass-produced NPG-Pt
catalysts using the previously developed electroless
plating method [31], on which Bi was further
deposited. The prepared sample was examined by
transmission electron microscopy (TEM), high
resolution TEM (HRTEM), scanning TEM (STEM)
under high-angle annular dark-field (HAADF) and
X-ray photoelectron spectroscopy (XPS). TEM
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5 Nano Res.
images (Figure 2a&b) demonstrate interconnected
ligaments of the NPG-Pt-Bi nanocomposites with
pore size around 20 nm. As shown in Figure 2c, the
entire ligament surface is uniformly covered by a
layer of nanoparticles with size around 1-2 nm. As
the pristine NPG ligament is characterized by a
smooth surface, these small nanoparticles could be
ascribed to Pt. The formation of Bi nanoparticles is
quite unlikely because the loading of Bi is
significantly lower than that of Pt which will be
discussed later. As shown in Figure 2d-f, these Pt
nanoparticles are grown on the surface of NPG in an
epitaxial mode which is consistent with our previous
results [31]. It should be noted that there might exist
an ultra-thin Pt layer between Pt nanoparticles and
NPG surface because electrochemical potential
cycling did not reveal evident Au signals.
Figure 2. Electron micrographs of NPG-Pt-Bi catalysts. (a) TEM image. (b, c) HAADF TEM images. (d-f) HAADF HRTEM images.
To determine the distribution of each element, we
performed EDS mapping on one ligament under
HAADF mode. As shown in Figure 3b, the intensity
of gold is higher in the middle of ligament than the
edge areas which is reasonable as the cross section of
NPG ligament is nearly a circle. However, the
distribution of Pt follows the opposite way where
there are higher intensities along the edge areas. This
is obviously caused by a core/shell type NPG-Pt
structure which could be seen more clearly in the
EDS mapping overlay for Au, Pt and Bi shown in
Figure 3e. The atomic percentage of Au, Pt and Bi is
93.12±0.68, 6.01±0.30 and 0.87±0.18%, respectively.
From the low percentage of Bi, we can conclude that
the nanoparticles on the ligament surface could be
ascribed to Pt. Moreover, the fine EDS mapping
results on one ligament surface also indicate the
correspondence of particle like structures to Pt
signals which are shown in Figure S1. In addition to
the very low percentage of Bi, electrochemical result
shows hydrogen adsorption/desorption on Pt after Bi
deposition which together demonstrate the
sub-monolayer structure of Bi in NPG-Pt-Bi catalyst.
As demonstrated by XPS results (Figures S2&3),
there is no noticeable change for Au and Pt binding
energies after Bi decoration. In contrast to the
metallic state of Au and Pt, Bi species in NPG-Pt-Bi
catalyst was found to be in the oxidation state and
could be ascribed to Bi2O3 [40].
Figure 3. HAADF TEM image of NPG-Pt-Bi catalyst and the corresponding EDS mappings. (a) HAADF TEM image. (b) EDS
mapping for Au. (c) EDS mapping for Pt. (d) EDS mapping for Bi. (e) EDS mapping overlay for Au, Pt and Bi.
3.3 Fuel cell performance
Nanoporous gold based electrodes are robust
enough to be readily incorporated into a fuel cell
MEA by either attaching onto a Nafion® 115
membrane directly or being first dispersed into a
solution and then brushing the mixed slurry with
Nafion® onto carbon paper just like the process to
make an MEA using the traditional Pt/C catalyst.
Both methods resulted in an ultra-thin catalyst layer
in MEA. Figure 4a shows one such MEA using the
second fabrication procedure which is composed of
NPG-Pt-Bi small sheets attached on top of carbon
paper. The porous structure of NPG was effectively
reserved during processing (Figure 4b&c). This
structure ensures the easy accessibility of
catalytically active sites by reactants. And the direct
connection between catalyst sheets and carbon paper
also facilitates the electron transportation. As shown
in Figure 4d, the thickness of the NPG-Pt-Bi based
catalyst layer was estimated to be ~300 nm
depending on the catalyst loading. A closer look into
the catalyst region resolves single, double or triple
NPG-Pt-Bi layers in the cross-sectional SEM images
(Figure S4), indicating the catalyst layer thickness in
the range of 100-500 nm. The respective energy
dispersive X-ray spectroscopy (EDS) results shown
in Figure 4d demonstrate that the anode catalyst was
composed of Au and Pt (the Bi content is too low to
be detectable). In comparison, the catalyst layer with
2.2 mg cm-2 Pt loading is about 40 μm in thickness
for the Pt/C based MEA (Figure 4e), almost two
orders of magnitude thicker.
Unlike the insufficient electron transportation paths
in the traditional catalyst layer, electron
transportation from reaction sites to external circuit
is instantaneous for the ultra-thin NPG-Pt-Bi based
anode, considering that the electron conductivity of
metals is at least 3-4 orders of magnitude higher than
traditional carbon materials. In addition, the open
nanoporosity of the catalyst layer also favors the fast
diffusion of formic acid molecules. This structural
feature was also proved by the fact that galvanic
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7 Nano Res.
replacement reaction between Pt ions in solution and
copper monolayer on ligament surface of NPG leaf
could result in very uniform and nearly complete
coverage of entire porous surfaces with Pt monolayer
as aforementioned. It should be noted that
construction of an ultra-thin catalyst layer would not
necessarily lead to high fuel cell performance if the
concentration of active sites is not high enough to
guarantee a low over-potential for formic acid
electro-oxidation. The electrochemical surface areas
(ECSA) of Pt in anodes were evaluated using
hydrogen adsorption/desorption method. With 40
μm thick catalyst layer, the 2.2 mg Pt/C anode
exhibits an ECSA of 1298 cm2 which equals ~3.2 cm2
Pt surface in every 100 nm thick catalyst layer. In
contrast, a 100 nm thick NPG-Pt-Bi anode shows an
ECSA of 11 cm2. In another word, the reaction site
concentration in NPG-Pt-Bi anode is almost 3.4 times
higher than that in Pt/C based anode. Considering
each individual Pt atom in NPG-Pt-Bi is far more
active than that in Pt/C, the fuel cell performance of
NPG-Pt-Bi anode is thus expected to be very high
even at a much lower Pt loading.
Figure 4. (a-c) Plan-view SEM image of catalyst layer prepared with NPG-Pt-Bi. Cross-sectional SEM images and EDS results of
MEAs with (d) NPG-Pt-Bi and (e) Pt/C anode.
Figure 5a shows the current-voltage (C-V) and
current-power (C-P) polarization curves of
NPG-Pt-Bi catalysts with different Pt loadings. An
MEA made with the commercial Pt/C catalyst (2.2
mg Pt cm-2) was also presented for comparison.
Typically, the NPG-Pt-Bi based anodes showed both
higher open circuit voltage and maximum power
output, although the Pt loading is orders of
magnitude lower. For example, the sample with Pt
loading as low as 3 micrograms (44 mW cm-2)
already exhibited larger maximum power than the
Pt/C catalyst (40 mW cm-2). The maximum power
output for NPG-Pt-Bi 0.02 mg catalyst is ~80 mW
cm-2, which is twice of that for Pt/C catalyst even the
catalyst layer is only 100 nm thick for this specific
sample. The apparent fuel cell power performance
also compares favorably to most literature results,
although here the Pt loading is typically two orders
of magnitude lower (Table S1). Further increasing the
Pt loading didn’t result in apparent increase in fuel
cell performance which may be caused by the
saturation of active sites in the catalyst layer (Figure
S5). The apparent maximum powers were
normalized to the amount of Pt used on anodes and
the results were shown in Figure 5b. The Pt loading
specific maximum power densities of NPG-Pt-Bi
samples are in the range of 4-14.7 W mg-1 with the
highest value for the catalyst with 3 micrograms Pt.
In contrast, the data in literatures are typically in the
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8 Nano Res.
range of 10-280 mW mg-1 (the fuel cell performance
varies under different conditions, see Figure S6 for
example), indicating two orders of magnitude
improvement in specific power efficiency for these
new electrodes. Considering the relatively limited Pt
reserve on our planet, the results of NPG-Pt-Bi
catalysts are of considerable significance since their
anode efficiency even approaches those of hydrogen
fuel cells which are typically below 20 W mg-1 [41].
Even when the mass of NPG substrate was
considered, the anode specific power density
normalized to the total precious metal loading
reaches 667 mW mg-1 for the NPG-Pt-Bi (0.02 mg Pt +
0.1 mg Au) catalyst which is still much higher than
the reported ones. The fuel cell performance can also
be boosted to over 160 mW cm-2 by feeding dry
oxygen as the oxidant (Figure S6). We are now
further optimizing its performance under various
conditions and also trying to replace the gold
substrate by using less precious nanoporous metals.
Figure 5. Single-cell performances of NPG-Pt-Bi and Pt/C based MEAs. (a) C-V and C-P polarization curves for Pt/C catalyst (red)
with 2.2 mg Pt loading and NPG-Pt-Bi samples with Pt loading of 0.003, 0.013, and 0.02 mg per cm2 in the anode. The actual Pt loading
in NPG-Pt-Bi catalyst was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). (b) Metal loadings (Pt or
Pd) and mass specific maximum power densities of literature results listed in Table S1 and that of NPG-Pt-Bi catalysts. (c) Durability
test of NPG-Pt-Bi 0.09 mg catalysts. The fuel cell was first discharged at 100 mA cm-2 for 10 h, after which the fuel cell was stored in
air and further tested in the following 6 months, with each test for more than 4 h. (d) C-V and C-P polarization curves of NPG-Pt-Bi
0.09 mg anode using 3M formic acid without (red squares) or with 100 ppm CH3OH (green circles), HCOOCH3 (magenta triangles),
and CH3COOH (blue triangles) impurities. Fuel cells were operated at 40 (a, b, c) or 50 oC (d), using the same Pt/C (2.2 mg cm-2 Pt)
cathode, with formic acid (3 M) as the fuel and dry air as the oxidant.
The long-term stability of NPG-Pt-Bi catalysts was
tested in a fuel cell with Pt loading of 0.09 mg cm-2.
Operating the fuel cell at 100 mA cm-2 for 10 h did
not see obvious voltage decline. After discharging
test, the cell was stored at room temperature in air.
Repeatedly operating this cell in the following six
months (each for more than 4 h) always generated a
power density around 70 mW cm-2, indicating the
excellent durability of this catalyst (Figure 5c). The
influence of possible impurities in formic acid was
also tested. The maximum power decreased very
slightly after adding 100 ppm contaminants (CH3OH,
HCOOCH3 or CH3COOH) into formic acid solution
(Figure 5d). When fuel cell operated at constant
current density of 100 mA cm-2, the negative effects
of CH3OH and HCOOCH3 are almost negligible
(Figure S7a). While it seems the influence of
CH3COOH is a little larger than other two
contaminants, the potential drop is within 10% of its
initial potential (Figure S7a). More importantly, even
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9 Nano Res.
the impurity concentrations increased to 500 ppm,
the fuel cells still showed stable voltage-time curves
at constant current density of 100 mA cm-2 and the
potential drop was within 12% of the initial potential
(Figure S7c). These results demonstrated the
NPG-Pt-Bi catalyst is robust enough to be fueled by
common formic acid (contaminant concentrations
are often in a range of 15-180 ppm) [42]. The stability
of NPG-Pt-Bi catalyst could be highlighted by
comparing with Pd catalyst. Although Pd/C catalyst
with a Pd loading of 3 mg cm-2 has similar initial
catalytic activity, but the performance declined
quickly even fueled with pure formic acid without
any contaminants (see Figures S8&S9).
3.4 Stack performance
Featured by high activity, superior stability and ultra
low Pt loading, NPG-Pt-Bi catalysts are highly
promising for real fuel cells. To demonstrate the
possibility, we prepared a stack with ten cells (each
cell has an area of 68 cm2, see insert in Figure 6b)
and investigated the performance. Figure 6a shows
the C-V and C-P polarization curves of this stack. The
maximum power density achieved 40.9 W at a
current of 12.9 A, indicating an area specific power
density of 85 mW cm-2 which is a little higher than
that in a single cell. This may be caused by the slight
increase in temperature during stack operation.
When the fuel cell stack was operated at 4.8 A, there
was almost no voltage decline (around 4.5 V) in 10 h
which again proved its high stability. Moreover, the
voltage of each individual cell varied very little
around 0.5 V (Figure 6b&c), demonstrating the
potential of making even larger fuel cells in a
repeatable manner.
Figure 6. Stack performance of NPG-Pt-Bi based MEA. (a) C-V and C-P polarization curves for a 10-cell stack (each cell: 6 cm 8 cm,
with Pt loading of 0.09 mg cm-2) using NPG-Pt-Bi catalysts as anodes. (b) Voltage-time curves at a constant current density of 100 mA
cm-2. Insert is a digital photo of this stack. (c) The voltages of single cells during the constant current discharging at 100 mA cm-2. The
fuel cell stack was operated at room temperature, with formic acid (3 M) as the fuel and dry air as the oxidant, the flow rate of formic
acid and air was 10 ml min-1 and 2 L min-1, respectively. (d) Current–time curves of DFAFC stack under constant voltage discharging at
5 V with 300 ml formic acid solution at different concentration.
Figure 6d shows the variation of discharging current
of DFAFC stack at a constant voltage (5 V) using 300
ml formic acid solution of different concentrations
from 1.0 to 5.0 M. The discharging current was found
to increase rapidly in the early stages for all solutions,
reach a peak value and decrease gradually toward
zero as the formic acid in the fuel tank was
consumed. It was found that the transient stack
operating temperature varied with a trend similar to
the transient discharging current. This behavior
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10 Nano Res.
indicates that the current differences among the
different concentrations of formic acid can be
attributed to the differences in the temperature
variation and mixed potential caused by the different
rates of formic acid crossover.
To investigate the fuel utilization, we define the fuel
efficiency as:
η= Discharging capacity (Ah)/Theoretical
discharging capacity (Ah)
In addition, the energy efficiency of DFAFC can be
defined as:
ξ= Discharging energy(Wh)/Theoretical discharging
energy(Wh)
The fuel efficiencies of the DFAFC stack at 1.0, 2.0,
3.0, 4.0, and 5.0 M are then calculated to be 78, 65, 64,
48, and 39%, respectively, which are much higher
than that of DMFC [43] due to its lower formic acid
crossover. The corresponding energy efficiencies are
26, 22, 21, 16, and 13%, respectively, comparable with
reported values [4]. The above results clearly
demonstrated that these rationally designed low Pt
loading NPG-Pt-Bi catalysts could be used in real
fuel cells.
4. Conclusions
In conclusion, using nanoporous gold leaves as
substrates, we designed and fabricated high
performance anode electrocatalysts for formic acid
fuel cells with ultra low Pt loading and excellent
stability. By concentrating highly active catalysts in
an ultra-thin catalyst layer, the electron
transportation, and reactant diffusion in the
electrode were optimized, which resulted in
unparalleled performance in DFAFCs. With the
development of advanced membrane technology and
poisoning-resistant cathode materials, the
commercialization of various PEMFCs seems on a
steady way to becoming reality, within which
DFAFCs may become an attractive portable power
source. Guiding by the efficiency loss contribution
rules in various clean-energy technologies, the
electrode design strategy demonstrated in this work
may shine light on the development of new
generation, high performance electrodes used for
example in metal-air batteries.
Acknowledgements
This work was sponsored by the National 973
(2012CB932800) Program Project of China, and the
National Science Foundation of China (51171092,
20906045). Y. D. is a Tai-Shan Scholar supported by
the Fundamental Research Funds of Shandong
University. The assistance of Shangling Tian,
Wenliang Zhang, and Zezhong Li in fuel cell testing
is gratefully acknowledged.
Electronic Supplementary Material:
Supplementary material (DFAFC performances in
literatures; EDS mappings of NPG-Pt-Bi catalyst;
XPS results of NPG-Pt and NPG-Pt-Bi samples; SEM
images of NPG-Pt-Bi based catalyst layer; DFAFC
performances of NPG-Pt-Bi and Pd based anodes) is
available in the online version of this article at
http://dx.doi.org/10.1007/s12274-***-****-*
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Electronic Supplementary Material
Ultra-Thin Layer Structured Anodes for Highly Durable
Low-Pt Direct Formic Acid Fuel Cells
Rongyue Wang1,§, Jianguo Liu2,§, Pan Liu3, Xuanxuan Bi1, Xiuling Yan1,4, Wenxin Wang1, Yifei Meng2,
Xingbo Ge1, Mingwei Chen3, and Yi Ding1 ()
§
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
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Table S1. DFAFC performances and detailed parameters in literatures. *It is not of our intention to show all
literature results in this table and only most related data were listed. N/A: data not available. RT: room
temperature.
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Nano Res.
Figure S1. HAADF TEM image of NPG-Pt-Bi catalyst and the corresponding EDS mappings. (a) HAADF TEM
image. (b) EDS mapping for Au. (c) EDS mapping for Pt. (d) EDS mapping for Bi. (e) EDS mapping overlay for
Au, Pt and Bi.
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Figure S2. XPS results of NPG-Pt and NPG-Pt-Bi samples.
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Figure S3. XPS result of NPG-Pt-Bi sample.
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Figure S4. Cross sectional SEM images of NPG-Pt-Bi based catalyst layer.
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Nano Res.
Figure S5. Current–voltage and current-power polarization curves for NPG-Pt-Bi 0.09 mg catalyst. The fuel cell
was operated at 40 oC, using the same Pt/C (2.2 mg cm-1 Pt) cathode, with formic acid (3 M) as the fuel and dry
air as the oxidant.
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Figure S6. Current–voltage and current-power polarization curves for NPG-Pt-Bi 0.09 mg catalyst. The fuel cell
was operated at 60 oC, using the same Pt/C (2.2 mg cm-1 Pt) cathode, with formic acid (3 M) as the fuel and dry
oxygen as the oxidant.
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Figure S7. a and c, Voltage-time curves at a constant current density of 100 mA cm-2 with impurities of CH3OH,
HCOOCH3, and CH3COOH of 100 and 500 ppm, respectively. b, Current–voltage and current-power
polarization curves with impurities of CH3OH, HCOOCH3, and CH3COOH of 500 ppm. Fuel cells were
operated at 50 oC, using the same Pt/C (2.2 mg cm-2 Pt) cathode, with formic acid (3 M) as the fuel and dry air
as the oxidant.
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Figure S8. Current–voltage and current-power polarization curves for Pd 3 mg anode catalyst. The fuel cell was
operated at 40 oC, using the same Pt/C (2.2 mg cm-2 Pt) cathode, with formic acid (3 M) as the fuel and dry air
as the oxidant.
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Figure S9. Voltage-time curves for Pd 3 mg catalyst at constant current density of 100 mA cm-2. The fuel cell
was operated at 40 oC, using the same Pt/C (2.2 mg cm-2 Pt) cathode, with formic acid (3 M) as the fuel and dry
air as the oxidant.
.
Address correspondence to Yi Ding, email: [email protected]