fuel cell performance of palladium-platinum core-shell electrocatalysts synthesized … ·...
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BNL-112220-2016-JA
Fuel Cell Performance of Palladium-Platinum
Core-Shell Electrocatalysts Synthesized in
Gram-Scale Batches
Siddique Khateeb, Sandra Guerreo, Dong Su,
Robert M. Darling, Lesia V. Protsailo, Minhua Shao
Submitted to the ACS catalysis
April 2016
Center for Functional Nanomaterials
Brookhaven National Laboratory
U.S. Department of Energy USDOE Office of Science (SC),
Basic Energy Sciences (BES) (SC-22)
Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under
Contract No. DE- SC0012704 with the U.S. Department of Energy. The publisher by accepting the
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Fuel Cell Performance of Palladium-Platinum Core-Shell
Electrocatalysts Synthesized in Gram-Scale Batches
Siddique Khateeb,1 Sandra Guerreo,
1 Dong Su,
2 Robert M. Darling,
1,† Lesia V. Protsailo,
1,†
Minhua Shao3, *
1UTC Power, 195 Governor’s Highway, South Windsor, CT 06074, USA
2Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY, 11794,
USA
3Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science
and Technology, Clear Water Bay, Kowloon, Hong Kong
†Current address: United Technologies Research Center, East Hartford, CT 06108, USA
Email: [email protected]; +852-34692269
Abstract This paper presents the performance of palladium-platinum core-shell catalysts
(Pt/Pd/C) for oxygen reduction synthesized in gram-scale batches in both liquid cells and
polymer-electrolyte membrane fuel cells. Core-shell catalyst synthesis and characterization, ink
fabrication, and cell assembly details are discussed. The Pt mass activity of the Pt/Pd core-shell
catalyst was 0.95 A mg-1
at 0.9 V measured in liquid cells (0.1 M HClO4), which was 4.8 times
higher than a commercial Pt/C catalyst. The performances of Pt/Pd/C and Pt/C in large single
cells (315 cm2) were assessed under various operating conditions. The core-shell catalyst showed
consistently higher performance than commercial Pt/C in fuel cell testing. A 20-60 mV
improvement across the whole current density range was observed on air. Sensitivities to
temperature, humidity, and gas composition were also investigated and the core-shell catalyst
showed a consistent benefit over Pt under all conditions. However, the 4.8 times activity
enhancement predicated by liquid cell measurements was not fully realized in fuel cells.
Keywords: Core-shell, polymer electrolyte membrane fuel cell, platinum monolayer, oxygen
reduction reaction
1. Introduction
Polymer electrolyte membrane (PEM) fuel cells face hurdles to commercialization in automotive
applications in part because of the high cost of Pt-based catalysts used in the anode and
cathode.1-3
Reducing the amount of Pt in PEM fuel cells has long been a focus among
automotive companies and researchers.4, 5
One of strategies to reduce the amount of Pt is to
increase the rate of oxygen reduction reaction per unit mass of Pt. Core-shell catalysts that
consist of an ultrathin Pt shell and a Pd core have been proposed as candidates to replace pure
Pt.6-10
In addition to the high Pt utilization and lower cost of the Pd core, strain and electronic
effects induced by the Pd core can enhance the activity of the Pt shell for the ORR,11
leading to a
greater activity per mass of Pt than expected based on the utilization improvement alone.
The Cu-mediated-Pt-displacement method developed by Adzic et al. can theoretically
produce a Pt monolayer on a Pd core.7 In this method, a Cu monolayer deposited on a Pd core is
displaced by Pt via a surface limited redox replacement (SLRR) reaction: Cu/Pd + PtCl42-
→
Pt/Pd + Cu2+
+ 4Cl-. Most core-shell catalysts made by the Cu-Pt-displacement method were
synthesized on a microgram scale, i.e., with the Cu UPD and Pt displacement reactions on a
rotating disk electrode (RDE) tip. When this process was transferred to the gram scale, the high
activity of core-shell catalysts was not fully realized. So far, only a limited amount of core-shell
materials have been synthesized at a gram scale and tested in real fuel cells.12-19
For instance,
Naohara et al.13
firstly reported the synthesis of Pt/Pd/C on a gram scale with a Pt mass activity
around 0.6 A mg-1
at 0.9 V (measured in RDE), which was lower than that of the same type of
material synthesized at a RDE tip and measured by the same group (0.96 A mg-1
).20
Furthermore,
the Pt mass activity (0.3 A mg-1
) of the Pt/Pd/C catalyst measured in a fuel cell was only half of
that measured in RDE.15
Various issues related to scaling the batch size have resulted in less
active and stable catalysts. Other possible reasons include differences in testing protocols and
test conditions between RDE and fuel cell.17
Thus, an increasing focus on synthesis at large
scales and understanding the fuel-cell performance is necessary.
Ideally, a uniform Pt monolayer is deposited on the core after all Cu atoms are replaced in the
SLRR reaction. However, our in situ XRD results in an H2 environment demonstrated that Pt
clusters rather than a uniform Pt shell was formed on Pd nanoparticles. 12
The mechanism
underlying this observation is that the SLRR process involves electron transfer from the substrate
(Pd in this case) to PtCl42-
ions, rather than direct electron exchange from Cu.21
This means that
electrons generated anywhere on the surface can move freely through the Pd substrate, reducing
PtCl42-
ions wherever their activity and surface energy are greatest. In other words, the Pt atom
may not deposit on the same site left by Cu dissolution, but rather on Pt that was already
deposited on the core leading to the formation of Pt clusters. Due to incomplete Pt coverage and
low Pt usage, clustered catalysts are expected to have lower activity and stability than perfect
core-shell structures. Thus, the key to synthesizing core-shell materials with good quality on a
large scale is to force Pt atoms to deposit on the surface of the core rather than on Pt atoms
already deposited by manipulating the Cu-Pt displacement reaction mechanisms.
This paper reports the synthesis of highly active Pt/Pd/C core-shell catalysts on a gram scale
by employing additives in the Cu-Pt displacement solution. Their characterization and evaluation
in both liquid and fuel cells are also reported.
2. Experimental Section
2.1 Core-Shell Catalyst Synthesis
A custom reactor was built to synthesize gram-scale batches of core-shell catalysts using
commercial Pd nanoparticles supported on Ketjen Black (Pd/C) as the core materials (TKK, 35
wt.%). The reactor comprised a graphite sheet as the working electrode, carbon cloth as the
counter electrode, and an Ag/AgCl leak-free reference electrode (BASi). The procedure of core-
shell catalyst synthesis is illustrated in Figure 1. The Pd/C nanoparticles were dispersed in a 50
mM H2SO4 solution, and then added to the reactor. Pretreatment of the Pd/C was conducted as
needed to wet the powder. After pretreatment, a deaerated aqueous CuSO4 solution in 50 mM
H2SO4 was added to the reactor to obtain a Cu2+
concentration of 50 mM. After addition of the
CuSO4, the potential was held at approximately 0.37 V vs. Ag/AgCl to deposit an atomically thin
layer of Cu on the Pd nanoparticles. Immediately upon completion of the Cu UPD (under
potential deposition), a deaerated aqueous solution of K2PtCl4 was added dropwise to the reactor
to perform galvanic replacement of the Cu by the Pt. In addition to the Pt precursor, citric acid
was also included in the Pt precursor solution to minimize the formation of Pt clusters.12, 22
After
the reaction was complete, typically in 30 minutes, catalysts were filtered and rinsed with
ultrapure water (Milli-Q® UV-plus water). The resulting catalyst powder, denoted as Pt/Pd/C,
was dried in a vacuum at 60°C. Only a trace amount of Cu (less than 0.4%) remained in the final
product based on inductively coupled plasma – mass spectrometry (ICP-MS) measurements.
2.2 Catalyst Characterization
XRD experiments were performed at Beamline X18A at the National Synchrotron Light
Source (NSLS) at Brookhaven National Laboratory (BNL) in Upton, NY. A detailed
experimental procedure can be found elsewhere.12
Briefly, helium (99.999%, Praxair) and
hydrogen (99.999%, Praxair) were metered through a home-made sample cell by two mass flow
controllers (Brooks 5850) at a flow rate of ~20 mL min-1
. The atmosphere was first purged with
He (helium) to prevent burning the catalyst powders via the combustion of H2. The XRD spectra
were recorded with a 2D detector (Perkin Elmer XRD 1621 N ES) at 20 keV (calibrated by the
Mo K absorption (λ = 0.7107 Å)) and subsequently processed by standard software. The dark
current was collected over 40 seconds (10 exposures at 4 s per exposure) and the diffraction
pattern was recorded over 400 seconds (100 exposures at 4 s per exposure). After recording the
XRD pattern under an inert atmosphere, the cell was purged with H2 for 10 minutes and the XRD
spectra were then recorded under this atmosphere. Further purging with H2 did not affect the
structure of the material. Finally, the cell was purged with He for 10 minutes and a post-H2
exposure XRD spectrum was recorded. The sample to detector distance was approximately 400
mm and was calibrated by the use of titanium dioxide (rutile) powder loaded into the sample
holder of the cell in an identical fashion as done for catalyst characterization. Performing a
calculation of error propagation suggests that an error in sample position of +/- 1 mm will result
in an error in the lattice constant of +/- 0.001 nm. Further, there is an internal calibration
standard for each sample corresponding to amorphous carbon peak, which is present in all of the
XRD spectra collected in this work, and which always lies at the same value of 2θ.
High angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)
images and electron energy loss spectroscopy (EELS) data were collected using a Hitachi
aberration-corrected scanning transmission electron microscope (HD-2700C). An electron probe
diameter of 1.3 Å with a convergence angle of 28 mrad was used. At these experimental
conditions, the contrast present in the HAADF images is directly related to the atomic number
and thickness of the materials. The EELS line scans were collected with a pixel time of 0.02 s
and 1 s, respectively, using a Gatan Enfina-ER detector.
2.3 Electrochemical Evaluation
Approximately 5 mg of catalyst was ultrasonically dispersed in a solvent consisting of 5 mL
of water, 2.5 mL of isopropanol, and 30 µL of 5% Nafion (Aldrich) for 10 min. 5 or10 µL of the
suspension was deposited on a pre-cleaned glassy carbon rotating disk electrode (Pine
Instruments) and allowed to dry in air. The electrode was cycled between 0.02 and 1.2 V for 20
cycles in a nitrogen-saturated aqueous solution of 0.1 M HClO4 at 100 mV s-1
. Then a stable
cyclic voltammetry (CV) curve was recorded at 50 mV s-1
. The charges associated with
hydrogen adsorption/desorption were used to calculate the electrochemical areas (ECAs) of all
the samples evaluated in this paper, assuming 210 μC cm-2
for full HUPD coverage. The oxygen
reduction polarization curves were measured in an oxygen-saturated 0.1 M HClO4 (GFS
Chemicals) solution at a scan rate of 10 mV s-1
at 1600 rpm. The kinetic current jk at 0.9 V was
derived from the Koutecky-Levich equation:
where j is the measured current density, B is a constant, and ω is the rotation rate. A reversible
hydrogen electrode (RHE) and Pt gauze were used as the reference and counter electrodes,
respectively. All potentials were corrected to RHE in this paper. Long term durability of Pt/Pd/C
was tested by potential cycling. A square-wave signal with 5 s each at 0.65 and 1.0 V was
applied on the RDE with Pt/Pd/C in an oxygen-saturated 0.1 M HClO4 solution at the room
temperature. CV and ORR polarization curves were recorded after a certain number of cycles.
2.4 Fuel Cell Tests
Pt/Pd/C core-shell catalyst was mixed with ionomer (Ion Power) in a glass vial with a specific
ionomer/carbon (I/C) ratio of 0.8. Ethanol/water solvent was added into the vial and the ink was
dispersed and mixed by placing the vial in an ultrasonic water bath. The temperature of water in
the bath was maintained at the room temperature and subjected to sonication for a few hours
until the desired catalyst agglomerate size was achieved.
Once the catalyst ink achieved the desired agglomerate particle size, the ink was ready to be
sprayed on a membrane to make the membrane electrode assembly (MEA). The membrane was
obtained from a commercial vendor containing anode catalyst on one side with a Pt loading of
0.1 mg cm-2
and a geometric area of 310 cm2. The cathode catalyst ink was sprayed on the bare
2/1
111
Bjj k
membrane side using an automated sprayer manufactured by SonotekTM
. A few layers of catalyst
ink were uniformly sprayed to obtain a Pt loading of 0.1 mg cm-2
. During spraying, the solvent
evaporated inside the spray chamber. Next, the MEA was hot pressed in a pneumatic mechanical
press to ensure sufficient mechanical bonding between the membrane and the catalyst layer.
The MEA was finally assembled with other components as shown in Figure 2. The entire cell
package after assembly was loaded to a pressure of 400 kPa to ensure good electrical and
mechanical contact between various components to minimize Ohmic losses and ensure uniform
pressure distribution.
To understand effects of temperature and air humidity on performance, cells were tested
under various operating conditions as shown in Table 1. Operating a PEM fuel cell using dry air
without an external humidifier is important for automotive applications to reduce system costs
and complexity. Thus, numerous tests were performed with dry air at different temperatures and
humidity to map the responses of cells. Limited test results are reported here and more results
will be published in a separate paper.
3 Results and Discussion
3.1 Pt/Pd/C Core-Shell Catalyst Characterization
Typical Pt and Pd loadings of the Pt/Pd/C catalyst are 18% and 27%, respectively, measured
by ICP-MS. If one monolayer of Pt were deposited on the Pd/C core, the corresponding Pt
loading should be ~12% based on the Cu UPD charge on the Pd core. The higher Pt loading in
the final core-shell product indicates that the thickness of the Pt shell is between 1 and 2 atomic
layers. Figure 3a shows typical core-shell catalyst dispersion in a HAADF-STEM image. The
inset of Figure 3b shows the HAADS-STEM image of a single core-shell particle and its EELS
line profile of Pt across the particle (Figure 3b). The sharp peaks at the two sides of the profile
indicate the Pt shell on the Pd core. The thickness of the Pt shell determined in the line scan is
0.5±0.15 nm, which is equivalent to ~2 atomic layers and is consistent with the ICP data.
The quality of the Pt shell was also evaluated using a qualitative method developed recently,12
which utilizes the ability of Pd to absorb hydrogen to form a Pd hydride structure. Figure 4
compares the XRD pattern of Pt/Pd/C in He and H2 atmospheres measured in a customized cell.
The absence of large Pt clusters in Pt/Pd/C was confirmed by the uniform shift of the XRD peak.
One would not expect the Pt peak (same position in the He atmosphere) to shift since Pt does not
absorb hydrogen resulting in a shoulder in the higher angle side of the main peak. A shoulder at
the position of Pt peak in H2 was clearly observed in a previous study of Pt/Pd/C catalysts
synthesized without citric acid in the Cu-Pt displacement step. 12
3.2 Pt/Pd/C Electrochemical Evaluation
Figure 5a compares CVs (cyclic voltammograms) of Pt/Pd/C and its core Pd/C in a nitrogen-
saturated 0.1 M HClO4 solution. The oxide layer formation and its reduction shifted to a more
positive potential when a Pt shell was deposited on the Pd/C. The delayed oxide layer formation
on the catalyst surface resulted in a significant improvement in ORR activity.
Figure 5b compares the oxygen reduction polarization curves (positive scan) of Pt/Pd/C and
state-of-the-art Pt/C (TEC10E50E, TKK) in an oxygen-saturated 0.1 M HClO4 solution. The
ORR curves were very similar for these two catalysts even though the Pt loading of the Pt/C (24
µg cm-2
) was 6 times higher than that of the Pt/Pd/C (4.0 µg cm-2
). The Pt mass and area specific
activities (kinetic current normalized to electrochemical active area (ECA)) were compared in
Figure 5c . The Pt mass activity of Pt/Pd/C was 0.95 ± 0.10 A mg-1
Pt, which was 4.8 times higher
than that of Pt/C (0.2 ± 0.02 A mg-1
Pt) at 0.9 V. The ECA of Pt/Pd/C was 160 ± 10 m2 g
-1Pt,
while the ECA of Pt/C was 85 ± 2 m2 g
-1Pt. Since the theoretical ECA of a Pt monolayer is 240
m2/g, the measured Pt/Pd ECA is consistent with a 1.5 monolayer thick shell. In addition to a
higher ECA, the core-shell also has a higher specific activity (0.6 mA cm2
Pt) than Pt (0.24 mA
cm2
Pt). The price of platinum is currently ~1.6 times the price of palladium making the core shell
2.6 times more active than pure Pt on a cost basis. It is worth noting that the ratio of platinum to
palladium prices has varied from 0.5 to 5.5 since 1986.
The stability of the core-shell catalyst was tested with a square-wave signal between 0.65 and
1.0 V. After 10000 cycles, the Pt mass activity dropped by 16% from 0.95 ± 0.10 to 0.8 ± 0.10 A
mg-1
. Pt/Pd/C was more stable than Pt/C, which degraded 42% under the same conditions (Figure
5d).
3.3 Cell Test Results
The performances of cells containing Pt/Pd/C core-shell catalyst (18% Pt, 27% Pd) and the
conventional Pt/C catalyst (20% Pt, TKK) on oxygen and air are compared in Figure 6. Both
cathode catalyst layers contain 0.1±0.03 mg cm-2
Pt. The coolant exit temperature was
maintained at 80oC, the coolant flow rate was 300 mL min
-1, gases entered the cell with dew
points of 80oC (yielding approximately 100% relative humidity at the local cell temperature), and
gases exited the cell at 140 kPaabs. Oxygen utilization was 50% in the pure oxygen experiment
and 67% in the air experiment, while hydrogen utilization was 67% in the pure oxygen
experiment and 83% in the air experiment. These are tests 1 and 2 in Table 1. The core-shell
cell is consistently better than the Pt cell on both oxygen and air. The oxygen curves are nearly
vertically offset, which is consistent with improved oxygen-reduction kinetics. The carbon
support, ionomer, and ionomer to carbon mass ratio are identical in the two catalyst layers. The
estimated electrode thicknesses, assuming 50% porosity, are 5.2 and 6.5 m for the core-shell
and Pt catalyst layers, respectively. The visible similarity of the polarization curves measured on
air and oxygen indicates that the two electrodes achieved similar Ohmic and oxygen-transport
losses, consistent with design intent. Switching from air to oxygen should boost performance by
45 mV in the kinetic region for a Tafel slope of 67 mV/decade. The platinum electrode showed
this expected behavior, but an unexpectedly small boost was observed on the core-shell
electrode. This discrepancy could not be reproduced at the RDE level by switching from oxygen
to air and remains unexplained.
Table 2 compares electrochemical areas and mass activities at 0.9 V in RDE and in cell. Mass
activities were estimated for air and oxygen at the temperature of the cell assuming Tafel slopes of
65 mV decade-1
. Mass activities in H2/air cells were corrected for oxygen concentration. For the Pt
cell, mass activities estimated from the cell data on air and oxygen were similar and consistent with
the corresponding RDE measurements. For the core-shell cell, a substantially higher mass activity
was estimated from the air data than from the oxygen data. The mass activity determined with RDE
exceeded both of these values. Hydrogen adsorption measurements on smaller (25 cm2) electrodes
gave electrochemical surface areas of 70 m2
gPt-1
for the core-shell and 50 m2 gPt
-1 for Pt. Both of
these ECAs are significantly lower than those measured in RDE, but the difference is more drastic
for the core shell. Certainly, the low core-shell ECA observed in the cell is partially responsible for
not achieving the expected mass activity. The relatively low ECA of the core shell in the electrode
could be due to an inability of the polymer electrolyte to contact some sites that can be contacted by
liquid electrolyte, or damage occurring during ink fabrication or spraying.
Figure 7 shows voltage differences between the core-shell cell and the Pt cell as a function of
current density on oxygen, air, and helox (21% O2 in He). Application of various gases enables a
rough separation of various losses. Transport in the catalyst layer is complex, involving Knudsen
diffusion of gases in small (< 100 nm) pores, permeation of oxygen through ionomer, proton
movement in ionomer, and multiphase water transport.23
The performance on oxygen is expected to
be dominated by kinetic and Ohmic losses. Switching from pure oxygen to helox activates oxygen
transport losses in ionomer and in nanopores. Normal gas-phase diffusion losses become important
when the balance gas is switched from helium to nitrogen. The differences are always positive as
the core-shell electrode is more active than the Pt electrode. The voltage difference on oxygen is
approximately 10 mV. This is consistent with the core-shell electrode having a mass activity that is
1.4X higher than the Pt electrode. A difference of 44 mV would be consistent with the 4.8X higher
activity observed in RDE. A higher Ohmic loss is expected in the Pt catalyst layer because it is ~1
m thicker. The sheet resistance of the catalyst layer should increase by ~2 m-cm2 for an ionomer
volume fraction of 20% and an ionomer conductivity of 0.17 S cm-1
. The slope of the oxygen
difference curve in Figure 7 is about 2.5 m-cm2.
Oxygen transport losses are minimal when pure oxygen is fed to the cell. Normal gas-phase
transport losses remain unimportant when the oxidant is switched to helox (21% O2 in He), however
oxygen transport losses in the ionomer and in small pores governed by Knudsen diffusion become
important. The Ohmic losses in the cathode can also increase as the current shifts away from the
membrane. The slope of the difference curve on helox is approximately 4X larger than the slope on
oxygen, indicating that additional transport losses become important. The offset also increases from
approximately 10 mV on oxygen to 20 mV on helox. The activity of the core-shell electrode appears
to be higher on helox than oxygen. A 20 mV offset corresponds to an activity improvement of 2X,
which is less than half of the gain observed on RDE. Normal gas diffusion losses become important
when the balance gas is switched from helium to nitrogen. The air and helox curves are similar
below 1.2 A/cm2, but they diverge at higher current densities. The activities on air and helox thus
appear to be similar; however additional transport losses appear on air at high current densities. The
data indicates that transport losses are lower in the core-shell electrode than the Pt electrode. Thus,
the inability of the core shell to achieve the activity observed in RDE does not carry over to other
types of polarization.
Figure 8 shows how the performances of the two cells change with temperature. While the core-
shell cell is consistently better than the Pt/C cell, the difference is relatively insensitive to
temperature and in general agreement with the polarization curves. Much of the difference can be
attributed to the kinetic improvement that attends changing from Pt to core-shell. The relative
humidity in the cell drops as temperature increases because the inlet dew point is constant.
Consequently, Ohmic resistances of the cells increase and the voltages decrease above ~75oC. The
optimum temperature increases with increasing current density because the water concentration
profile polarizes and rises in the MEA. The voltage declines at 60oC and 1.6 A cm
-2 because the
saturation of the various porous media with liquid water increasing.
Figure 9 shows how performances of the two cells change with the relative humidity of the inlet
air and fuel. The kinetic benefit of the core-shell catalyst is apparent as a vertical offset between the
Pt and core-shell cells. There appears to be a slight tendency for the gap between the two cells to
increase when the feed gases are dry at all current densities.
4. Conclusions
Core-shell catalysts consisting of an ultrathin Pt shell and a Pd core were successfully
synthesized using a gram-scale reactor involving the displacement of a Cu UPD layer. The XRD,
ICP, and TEM results showed that Pt/Pd core-shell catalyst had a nominal composition of 18
wt.% Pt and 27 wt.% Pd. Uniform Pt coverage 1-2 atomic monolayers thick (0.5 ± 0.15 nm) on
Pd was demonstrated by HAADF-EELS and in situ XRD. The mass activity of Pt/Pd/C core-
shell was measured to be 0.95 ± 0.10 A mg-1
, which was 4.8 times higher than that of Pt/C at 0.9
V using RDE in a 0.1 M HClO4 solution. The electrochemical area was 160 ± 10 m2 g
-1Pt, which
is double that of commercial Pt/C. Full scale fuel cell tests were performed to compare the
Pt/Pd/C core-shell catalyst with regular Pt/C catalyst under various temperature and humidity
conditions. The core-shell catalyst was approximately 20 and 60 mV higher than Pt/C in the low
and higher current density regions on H2-air, respectively. The cell with core-shell catalyst did
not show the improvement at the low current densities as observed in RDE. The lower than
predicted performance may be due to ink fabrication and spraying steps. Test results with
different ink preparation and MEA fabrication processes will be summarized in a separate
publication. Overall, Pt/Pd/C core-shell catalyst performs better than regular Pt/C catalyst under
both high and low humidity conditions at all temperatures tested (60 – 85ºC).
Acknowledgments
Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported
by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under
Contract No. DE-AC02-98CH10886. The authors would like to thank Marianne Pemberton and
Michael Humbert for helping synthesis and evaluate the catalysts. The work at the Hong Kong
University of Science and Technology was supported by Research Grant Council of the Hong
Kong Special Administrative Region (IGN13EG05 and 26206115) and a startup fund from the
Hong Kong University of Science and Technology. Use of the National Synchrotron Light
Source (NSLS) and Center for Functional Nanomaterials (CFN) at Brookhaven National
Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Contract No. DE-AC02-98CH10886. Beamline X18A at the NSLS is
supported in part by the Synchrotron Catalysis Consortium (DOE BES grant DE-FG02-
03ER15688).
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Figure Captions
Figure 1. Steps involved in core-shell catalyst fabrication: (1) Pd/C dispersion; (2) Pd/C
reduction; (3) Cu UPD; (4) Cu displacement by Pt; (5) washing, filtering, and drying; (6) catalyst
characterization.
Figure 2. A cross-sectional view of a fuel-cell assembly. GDL: gas diffusion layer; MPL:
microporous layer.
Figure 3. A typical HAADF-STEM image of Pt/Pd/C core-shell catalysts (a) and EELS line scan
profiles of Pt for a single Pt/Pd core-shell particle (inset) (b).
Figure 4. The XRD peak positions of (111) reflection of Pt/Pd/C in He and H2 atmospheres. The
Mo Kα (λ = 0.7107 Å) incident radiation was used in XRD measurements.
Figure 5. Comparison of (a) cyclic voltammetry curves and (b) oxygen reduction polarization
curves (positive-going) between Pt/Pd/C and Pt/C in N2- and O2-saturated 0.1 M HClO4
solutions, respectively. Scan rates = 50 and 10 mV s-1
in (a) and (b), respectively. The currents
were normalized to the geometric area of the rotating disk electrode (0.196 cm2). Pd loadings in
Pd/C and Pt/Pd/C were 10 and 7.7 µg cm-2
, respectively. The Pt loadings in Pt/Pd/C and Pt/C
were 4 and 24 µg cm-2
, respectively. (c) Comparison of Pt mass activity and specific activity
between Pt/Pd/C and Pt/C at 0.9 V. (d) Comparison of Pt mass activity at 0.9 V before and after
potential cycling between 0.65 and 1.0 V. The electrochemical areas were calculated from the
charges associated with HUPD assuming 210 µC cm-2
.
Figure 6. Comparisons of fuel-cell performance between Pt/Pd/C core-shell catalyst (18% Pt,
27% Pd) and regular Pt/C catalyst (20% Pt, TKK) under 100% inlet humidity conditions with
pure oxygen (solid lines) and air (dashed lines). Circle: Pt/Pd/C; triangle: Pt/C.
Figure 7. Voltage of core-shell cell minus voltage of Pt cell as a function of current density on
oxygen, air and helox.
Figure 8. Temperature dependence of Pt/Pd/C core-shell and regular Pt/C catalyst cells. Circle:
Pt/Pd/C; Triangle: Pt/C.
Figure 9. Humidity dependence of Pt/Pd/C core-shell and regular Pt/C catalyst cells. Circle:
Pt/Pd/C; triangle: Pt/C.
Figure 1.
Figure 2.
Figure 3.
Figure 4.
0 1 2 3 4 5 6 7 8 9 10 11 12
No
rmal
ized
inte
rsit
y (a
.u.)
Position (nm)
Pta b
10 nm
5 nm
13.0 13.5 14.0 14.5 15.0 15.5 16.0
Inte
nsi
ty /
(a.
u.)
2q /degree
He
H2
Figure 5.
-1.2
-0.8
-0.4
0
0.4
0.8
0 0.2 0.4 0.6 0.8 1 1.2
j(m
A c
m-2
)
E (V vs RHE)
Pt/Pd/CPd/C
-7
-6
-5
-4
-3
-2
-1
0
0 0.2 0.4 0.6 0.8 1
j(m
A c
m-2
)
E (V vs RHE)
Pt/Pd/C
Pt/C
0
0.4
0.8
1.2
1.6
Pt/Pd/C Pt/C
Mas
s ac
tivi
ty (A
mg-1
)Sp
ecif
ic a
ctiv
ity
(mA
cm
-2)
Mass activity
Specific activity
0
0.4
0.8
1.2
1.6
Pt/Pd/C Pt/C
Mas
s ac
tivi
ty (A
mg-1
)
Initial
10000 cycles
a b
c d
Figure 6.
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Cell
Vo
lta
ge
(V
)
Current Density (A cm-2)
Regular Pt: Air
Pt/Pd Core-Shell: Air
Regular Pt: Oxygen
Pt/Pd Core-Shell: Oxygen
Figure 7.
0
10
20
30
40
50
60
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Vo
ltag
e d
iffe
ren
ce, V
cs-V
Pt (
mV
)
Current density (A cm-2)
Air
Oxygen
Helox
Figure 8.
Figure 9.
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
50 60 70 80 90
Ce
ll V
olta
ge
(V
); C
ell
"IR
" V
olta
ge
dro
p (
V)
Temperature (°C)
0.2 A cm-2
1.0 A cm-2
1.6 A cm-2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
-0.2 0 0.2 0.4 0.6 0.8 1 1.2
Cell
Vo
lta
ge
(V
)
Relative Humidity
0.2 A cm-2
1.0 A cm-2
1.6 A cm-2
Table 1 Cell operating conditions
Test
ID
Cell
temperature
(oC)
Gas exit
pressure
(kPa)
Cathode Anode
Reactant Utilization
(%)
Dew-Point
(oC)
Utilization
(%)
Dew-
Point
(oC)
1 80 140 Oxygen 50 80 67 80
2 80 140 Air 67 80 83 80
3 80 140 Helox* 67 80 83 80
4 80 140 Air 67 53 83 53
5 80 140 Air 67 Dry 60 53
6 85 140 Air 67 Dry 60 53
7 75 140 Air 67 Dry 60 53
8 70 140 Air 67 Dry 60 53
9 60 140 Air 67 Dry 60 53
*Helox: 21% oxygen, 79% Helium
Table 2: Comparisons of ECA and mass activities between Pt/C and Pt/Pd/C in liquid cells and MEAs
(Mass activities on MEA-Air were corrected for oxygen concentration).
Catalysts ECA (m2 g
-1) Mass Activity (A mg
-1)
Liquid MEA Liquid MEA-O2 MEA-Air
Pt/Pd/C 160 70 0.95 0.40 0.58
Pt/C 85 50 0.20 0.28 0.26