Journal of The Electrochemical Society, 156 共5兲 B614-B619 共2009兲
B614
0013-4651/2009/156共5兲/B614/6/$23.00 © The Electrochemical Society
Sputter-Deposited Pt PEM Fuel Cell Electrodes: Particles vs
Layers
M. D. Gasda,a R. Teki,b T.-M. Lu,c,* N. Koratkar,d G. A. Eisman,a,* and
D. Galla,z
a
Department of Materials Science and Engineering, bDepartment of Chemical and Biological Engineering, cDepartment
of Physics, Applied Physics and Astronomy, and dDepartment of Mechanical, Aerospace and Nuclear
Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, USA
Platinum catalyst layers with Pt loadings w = 0.05–0.40 mg/cm2 were deposited by magnetron sputtering from a variable deposition angle ␣ onto gas diffusion layer 共GDL兲 substrates and tested as cathode electrodes in proton exchange membrane 共PEM兲
fuel cells using Nafion 1135 membranes and Teflon-bonded Pt-black electrode 共TBPBE兲 anodes. Layers deposited at normal
incidence 共␣ = 0°兲 are continuous and approximately replicate the rough surface morphology of the underlying GDL. In contrast,
glancing angle deposition 共GLAD兲 with ␣ = 87° and continuous substrate rotation yields highly porous layers consisting of
vertically oriented Pt particles, 100–500 nm high and 100–300 nm wide, that are separated by 20–100 nm. The particle electrodes
exhibit a higher 共lower兲 mass-specific performance than the continuous-layer electrodes for a high 共low兲 current density i. This is
attributed to a higher porosity but lower overall electrochemically active surface area for the particles compared to the continuous
layer. Increasing w in particle cells from 0.05 to 0.10 to 0.18 mg/cm2 yields increasing potentials, but w = 0.40 mg/cm2 causes a
voltage drop at i ⬎ 0.4 A/cm2, associated with the reduced pore density at large w. Comparison cells with a TBPBE cathode
exhibit comparatively low Tafel slopes but a lower Pt mass specific performance than the sputtered catalysts. Quantitative analyses
of kinetic and mass-transport losses in the polarization curves suggest a competing microstructural effect, favoring mass-transport
performance and an efficient oxygen reduction reaction for particle and continuous layer electrodes, respectively. The overall
results suggest that in addition to the well-known promise of sputter-deposited Pt catalysts as an approach to increase Pt utilization
at low loading, GLAD provides the unique ability to control Pt porosity and to achieve efficient reactant flow for high-currentdensity operation.
© 2009 The Electrochemical Society. 关DOI: 10.1149/1.3097188兴 All rights reserved.
Manuscript submitted December 12, 2008; revised manuscript received February 2, 2009. Published March 24, 2009.
Polymer electrolyte membrane or proton exchange membrane
共PEM兲 fuel cells are energy conversion devices that show promise
for future use in portable, stationary, and transportation
applications.1 A key hurdle for mass-market adoption of fuel cell
technology is overall system cost, driven in large part by materials
and materials processing costs in the stack. Therefore, a considerable effort in fuel cell research is directed toward minimizing Pt
loadings for a given target efficiency.2 A Pt catalyst loading w of
4.0 mg/cm2 is typical for Teflon-bonded Pt-black electrodes 共TBPBE兲. TBPBEs consist of a hydrophobic, porous structure of
micrometer-sized Pt agglomerates and were used in early space applications due to their high efficiency and long lifetime.3,4 Further
optimization and the addition of ionomer to the electrode surface
resulted in the reduction of TBPBE loading to below w
= 1 mg/cm2 without significant performance loss.5 Current fuel cell
technology mostly employs ionomer-bonded thin-film electrodes
with 2–5 nm wide Pt nanoparticles supported on carbon 共designated
Pt/C兲, with typical w = 0.4 mg/cm2. However, the carbon support
also introduces some challenges, including reduced operating lifetimes due to carbon corrosion,6 peroxide attack of the membrane,7
and sintering and growth of the Pt particles over time.8 Further
reduction of w below 0.4 mg/cm2 typically results in voltage losses
consistent with the fundamental kinetic losses associated with the
oxygen reduction reaction 共ORR兲.9
Sputter deposition of Pt has been explored by various researchers
as a potential path for manufacturing ultralow Pt loading electrodes
in a controlled process. Pt has been sputtered onto the gas diffusion
layer 共GDL兲,10-17 the membrane,17-22 or deposited onto poly共tetrafluoroethylene兲 共PTFE兲 sheets or similar support substrates and
subsequently transferred to the membrane.23,24 Most researchers
found that the sputtered Pt morphology strongly affects fuel cell
performance and have reported increases in catalyst utilization
through roughening the membrane,19 patterning the catalyst,20,21 increasing the Pt porosity,13,14 depositing on polymer nanowhiskers,24
or selecting appropriately rough GDL substrates.10,11 Depositing ad-
* Electrochemical Society Active Member.
z
E-mail: [email protected]
ditional conductive layers such as Cr,12 carbon,22,25 Au,23 or Nafioncarbon ink16-18 also affects performance by enhancing the gas transport or electron conduction within the electrode. Work is already in
progress to characterize the angular distribution of the sputtered Pt
flux for the possible application of such methods to commercial
manufacturing.26 Rabat and Brault22 deposited carbon layers at 45°
with respect to the substrate, coated the resulting columnar morphology with Pt, and found a higher open-circuit voltage of the assembled fuel cell as compared to a commercial electrode. Sputtering
Pt onto carbon nanofibers grown directly on the GDL results in
3.6⫻ higher Pt utilization as compared to a commercial cathode.25
In this paper, we present an alternative approach to engineer the
nanoscale structure of fuel cell electrodes by varying the deposition
angle during sputtering of Pt onto the GDL. Normal-incidence deposition results in a conformal, nearly continuous layer that covers the
porous surface of the GDL. In contrast, deposition from a highly
oblique angle of ␣ = 87° causes Pt to nucleate preferentially on
raised areas of the GDL surface and form a porous coating consisting of vertically oriented Pt particles. This approach is based on the
glancing-angle deposition 共GLAD兲 technique27 which exploits
atomic shadowing effects during line-of-sight physical vapor deposition to engineer nanorods into various shapes such as
nanopillars,28-30 zigzags,31,32 nanospirals,33 nanotubes,34 twocomponent rods,35-37 and branched nanocolumns,38-40 with such potential applications as photonic crystals,33 sensors,32 catalyst
supports,41 magnetic storage media,42 radiation-resistant coatings,43
and field emitters.44 We evaluate the deposited Pt layer and particle
electrodes as cathodes in PEM fuel cells in order to understand the
effect of GLAD on the Pt utilization. At high power, particle cells
have higher Pt utilization than cells with “normally deposited” continuous layers of Pt. The superior mass-transport performance of the
Pt-particle layers is due to the porous, columnar morphology. Highporosity Pt layers formed by GLAD offer a potential path to highperformance sputtered fuel cell electrodes.
Experimental
All electrodes were deposited onto the microporous layer 共MPL兲
of as-received, commercially available GDLs 共SIGRACET 35DC
supplied by SGL Technologies, GmbH兲. The MPL is a Teflon-
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Journal of The Electrochemical Society, 156 共5兲 B614-B619 共2009兲
B615
Figure 1. 共Color online兲 Schematics of Pt
layer deposition onto MPL substrates with
corresponding cross-sectional SEM micrographs from fuel cells after testing, for
the case of 共a兲 normal deposition 共␣
= 0°兲 and 共b兲 GLAD 共␣ = 87°兲, yielding
a continuous Pt layer and separated Pt particles, respectively. The dashed lines on
the left in the micrographs indicate the position of the Pt 共bright contrast兲 which is
deposited on the MPL 共bottom兲 and is partially embedded in the membrane electrolyte 共top兲.
bonded layer of carbon particles that provides mechanical support
for the electrode. The MPL substrates were introduced into a magnetron sputter deposition system with a base pressure of ⬍8
⫻ 10−7 Torr, placed 15 cm from a 7.6 cm diam Pt target 共Plasmaterials, 99.99% pure兲, tilted by ␣ = 0 and 87° for continuous layer
and particle depositions, respectively, and rotated about the surface
normal at 0.5 Hz 共30 rpm兲 in order to achieve particle orientation
normal to the surface. It is important to clearly define our geometry
by noting that ␣ is the angle between the substrate normal and a line
drawn through the centers of the target and the substrate as in Ref.
27. Ar 共99.999% pure兲 was introduced into the system to reach a
constant pressure of 2.3 mTorr. No external heating was applied to
the substrate during the deposition process. A constant dc power of
200 W yielded deposition rates of 2 and 0.3 nm min−1 for the Pt
layer and particles, respectively. This corresponds to Pt loading rates
of 0.05 and 0.007 mg cm−2 min−1, as determined using differential
weighting of 0.2 and 1.5 mg samples in an electronic semimicrobalance 共Sartorius 2024 MP6, standard deviation ⫾0.02 mg兲.
The sputter-deposited electrodes were integrated as cathodes into
fuel cells with an active area of 10 cm2. Nafion 1135 polymer electrolyte membranes 共Ion Power兲, 1100 equivalent weight and 89 ␮m
thick, were activated by boiling in 0.5 M H2SO4 for 30 min, followed by three separate 30 min boiling steps in distilled water, and
stored in distilled water until being assembled into a membraneelectrode assembly 共MEA兲. Anode 共counter兲 electrodes were TBPBE with 1 mg/cm2 Pt and were prepared following the procedure
described in Ref. 5. To facilitate electrode/electrolyte interfacial contact, a 15 wt % dispersion of Nafion in water, isopropanol, and
methanol 共Ion Power Liquion LQ-1115兲 was applied to the surface
of the electrodes before bonding to the membrane. After this coating
dried, the electrodes contained 5–10 wt % Nafion. Finally, electrodes were bonded to the membrane at a pressure and temperature
of 350 psi 共2.4 MPa兲 and 126°C for 5 min.
Fuel cells were operated at 70°C with atmospheric pressure H2
and air reactants that were humidified to a dew point of 70°C 共100%
relative humidity兲. Prior to performance testing, all cells were “incubated” by applying a constant voltage of 0.4 V and allowing the
output current to stabilize for 24 h. H2 and air flow rates were set to
stoichiometric ratios of 1.2 and 2.0, respectively, for 1.0 A/cm2,
corresponding to 84 and 333 sccm. The raw polarization curves
were corrected for shorting, crossover, and internal ohmic resistance
共iR兲 in order to isolate the performance of the cathode electrode
from confounding effects, as discussed in the Results section. Typical values for the current correction due to shorting and crossover
were 5–6 mA/cm2, as determined by applying H2 and N2 to the
anode and cathode, respectively, and measuring the steady-state H2
crossover current at high cathode potential.9 The iR correction for
the voltage, ranging from 0.15 to 0.30 ⍀ cm2, was obtained from
the slope of the linear polarization curve of the cell in hydrogen
pumping mode, as described in Ref. 5. Electrochemical surface area
共ECSA兲 measurements were performed via cyclic voltammetry
共CV兲 with a Parstat 2273 共Princeton Applied Research兲 under humidified N2 at 70°C and a scan rate of 20 mV/s. ECSA was calculated from the raw CV curve by integration under the
H-underpotential deposition region after subtracting the doublelayer charging current as described in Ref. 45. After fuel cell testing,
MEAs were prepared for scanning electron microscopy 共SEM兲
analysis by freeze-fracturing after immersion for 15 min in liquid
N2. SEM micrographs were collected on a Carl Zeiss Supra SEM in
secondary electron mode at 2.5 kV.
Results and Discussion
The schematics in Fig. 1 illustrate the geometry for 共a兲 normal
and 共b兲 glancing-angle deposition of the Pt catalyst onto the MPL. In
the case of normal deposition, where the angle between the substrate
surface normal and the deposition flux ␣ = 0°, the Pt forms a continuous layer that approximately replicates the rough surface morphology of the underlying MPL, as shown on the left side in Fig. 1a.
The corresponding cross-sectional scanning electron micrograph on
the right is from an electrode with w = 0.25 mg/cm2. It shows at the
bottom the 30–50 nm wide carbon nanoparticles from the uppermost
part of the MPL and at the top the membrane electrolyte, which
exhibits little contrast. The catalyst layer at the interface between
MPL and membrane appears bright, because the high-Z Pt exhibits a
higher secondary electron yield during SEM imaging. Its measured
thickness is 50 nm, which is slightly less than the nominal thickness
of 70 nm. We attribute this difference to the roughness of the layer
which conforms to the rough MPL surface, resulting in an effective
Pt layer area that is larger than the nominal 共flat兲 area. This observation shows that the morphology of the catalyst is strongly affected
by the MPL surface, which may explain previous reports indicating
a strong impact of substrate morphology on fuel cell performance.10,11
Figure 1b illustrates the case for an oblique deposition angle ␣
= 87°. The incoming Pt atoms preferentially deposit on substrate
features that protrude out of the surface, as the flux to the lowerlying GDL is blocked as a result of a line-of-sight deposition process. During continued deposition, this atomic shadowing effect
leads to the formation of well-separated particles which grow vertically due to a circular symmetry achieved by continuous substrate
rotation about the surface normal. Statistical fluctuations in the substrate roughness and the deposition process cause differences in the
growth rates of individual particles and lead to a competitive growth
process. The taller particles capture a larger fraction of the incident
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B616
Journal of The Electrochemical Society, 156 共5兲 B614-B619 共2009兲
Figure 2. 共Color online兲 Potential E 共iR-corrected兲 vs mass activity im polarization curves for PEM fuel cells with cathodes comprising Pt particles, a
continuous Pt layer, and a TBPBE.
Figure 3. 共Color online兲 Potential E 共iR-corrected兲 vs current density i polarization curves for cells with cathodes comprising Pt particles with loadings w = 0.05, 0.10, 0.18, and 0.40 mg/cm2 Pt.
particle flux while their smaller neighbors are shadowed and terminate their growth prematurely, as previously reported for GLAD
nanorods.46,47 Consequently, the particles exhibit a relatively large
distribution in both height and width, as also observed in the corresponding SEM micrograph on the right of Fig. 1b from a cell with
0.40 mg/cm2 Pt loading. This cross-sectional micrograph shows Pt
particles, 100–500 nm high and 100–300 nm wide, that have nucleated on the MPL surface and are embedded in the membrane. The
gaps between the particles exhibit widths ranging from 20 to over
100 nm. These pores become partially filled with electrolyte during
the drying of the Nafion dispersion and the membrane bonding process, as described above.
Figure 2 shows polarization curves for three fuel cells that were
assembled using a Pt layer, a Pt particle, and a conventional TBPBE
cathode electrode with loadings of 0.25, 0.18, and 0.88 mg/cm2,
respectively. The plot shows the iR-corrected potential E vs the
mass activity im, which is the current density normalized to the
amount of Pt. Therefore, the graph shows a direct comparison of the
catalyst utilization efficiency. The potential for the continuous Pt
layer decreases with increasing current density for im ⬍ 0.6 A/mg.
Such a decrease is typical for PEM fuel cells48 and is consistent with
activation polarization. However, E exhibits a distinct drop from
0.67 to 0.37 V as im increases from 0.6 to 1.0 A/mg. This drop is
caused by a significant concentration overpotential that dominates
the performance of the continuous layer at high current density,
indicating poor electrode porosity, as discussed below. The fuel cell
with the Pt-particle electrode shows a slightly steeper decrease in E
at low current in comparison to the Pt-layer electrode, with E
= 0.65 and 0.69 V at im = 0.45 A/mg for Pt particles and the Pt
layer, respectively. However, the particle cell shows negligible concentration overpotential, leading to a mass activity of 2.7 A/mg at
E = 0.5 V, which is more than a factor of 3 larger than 0.86 A/mg,
the value for the continuous layer. Figure 2 also contains a plot of
the mass activity from a cell with a TBPBE cathode, which serves in
this study as comparison data. This cell with w = 0.88 mg/cm2 exhibits the highest mass activity out of a set of TBPBE cathodes with
different values of w. Higher w values result in lower mass activity,
because a smaller fraction of Pt/PTFE agglomerates is in ionic contact with the bulk electrolyte, while lower 共w ⬍ 0.88 mg/cm2兲 TBPBE loadings also result in a reduced mass activity, associated with
an increasing fraction of the Pt/PTFE agglomerates that seep into the
MPL during the application and drying process. This effect results in
the smallest possible TBPBE loading within our processing ap-
proach of w ⬇ 0.5 mg/cm2, which is considerably higher than what
is achieved by both types of sputtered Pt. The absolute 共nonnormalized current兲 performance of the TBPBE cell is higher than
for any sputter-deposited electrode cell of this study. However, in a
mass-specific comparison as shown in Fig. 2, the layer electrode
V
V
= 0.068兲 outperforms the TBPBE 共i0.8
= 0.033兲 at a high
共i0.8
m
m
V
= 2.7兲 outperpotential of 0.8 V, while the particle electrode 共i0.5
m
0.5 V
forms the TBPBE 共im = 0.77兲 at a low potential E = 0.5 V. We
attribute the high performance at low im of the layer electrode to a
high Pt fraction that is in direct contact with the electrolyte, facilitated by the relative thinness of this electrode. In contrast, the relatively high voltage of the particle electrode at high current is associated with the excellent electrode porosity, which facilitates oxygen
transport from the bulk GDL pores to catalyst sites throughout the
electrode.
Figure 3 shows plots of the cell potential E vs the current density
i for Pt-particle fuel cells with increasing catalyst loadings w
= 0.05, 0.10, 0.18, and 0.40 mg/cm2, obtained by increasing the
GLAD time from 7.5 to 60 min. All cells show a characteristic decrease in E with increasing i. However, the slope of the E共i兲 curves
is less steep for higher Pt loadings, corresponding to a higher current
density at a given potential. This indicates that increasing the particle size leads to larger catalytically active surface areas. Adding a
given amount of Pt has the greatest effect at low loadings. For example, i at 0.60 V doubles from 0.04 to 0.08 A/cm2 for an increase
in w from 0.05 to 0.10 mg/cm2, but only increases by 23%, from
0.17 to 0.21 A/cm2, for an increase in w from 0.18 to 0.40 mg/cm2.
The trend of increasing i with increasing w is eventually reversed for
the two highest Pt loadings at high current. The voltage of the
0.40 mg/cm2 cell drops below the 0.18 mg/cm2 cell for i
⬎ 0.38 A/cm2. This indicates a mass-transport limitation for w
= 0.40 mg/cm2, which we attribute to limited oxygen transport
through the pores of the catalyst layer. Increasing w leads to longer
and wider particles and, in turn, reduced pore density and longer
diffusion paths, resulting in inefficient oxygen transport from the
GDL to the active Pt-membrane interface. This causes a drop in E
for i ⲏ 0.35 A/cm2 and w = 0.40 mg/cm2. Nevertheless, this drop
is much less pronounced than for the electrode with the continuous
Pt layer shown in Fig. 2.
Figure 4 shows comparative plots of the mass activity vs Pt
loading for both particle and layer cells in the low and high current
density regimes defined by E = 0.80 and 0.50 V, respectively. The
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Journal of The Electrochemical Society, 156 共5兲 B614-B619 共2009兲
Figure 4. 共Color online兲 Mass specific activity vs loading w for cells with
cathodes comprising a continuous Pt layer or Pt particles at 共a兲 0.80 and 共b兲
0.50 V.
V
high-potential mass activity i0.8
for the particle cells is largest for
m
the smallest loading, 0.11 A/mg for w = 0.05 mg/cm2, and decreases monotonically with increasing w to 0.024 A/mg for w
= 0.40 mg/cm2, as shown in Fig. 4a. This decrease is associated
with a decrease in the surface-area-to-volume ratio as the particles
grow both longer and wider. Larger particles have a larger total
surface area, resulting in higher absolute performance, but a smaller
fraction of their Pt is at the surface, yielding a lower mass-specific
performance. The plot in Fig. 4a also shows, for comparison, the
current density for cells with continuous Pt layers. The absolute
open-circuit voltage of the continuous layer cell with the lowest Pt
V
loading 共0.05 mg/cm2兲 was less than 0.80 V, and i0.8
was estim
mated by extrapolation from the crossover- and short-corrected current values. This cell has approximately the same mass activity,
0.12 A/mg for w = 0.05 mg/cm2, as the particle cell with the same
loading. The nearly identical mass activities of the particle and layer
cells with the same w suggests that in the limit of low loading, the
active surface area of the particles approaches that of the continuous
V
layer. The layer with w = 0.25 mg/cm2 exhibits i0.8
m
0.8 V
= 0.068 A/mg. This is 30% higher than im
for the particles at
comparable loadings. That is, the continuous layer has a higher Pt
utilization at 0.80 V than the particles. We attribute this, as noted
above, to a higher fraction of Pt that is in direct contact with the
membrane for the layer than for the particles. The TBPBE compariV
= 0.034 A/mg. This value is higher than what
son cell exhibits i0.8
m
would be expected if the mass activity vs w curve for particle cells
were extrapolated to a high w = 0.88 mg/cm2. However, all particle
B617
V
cells with w 艋 0.18 mg/cm2 exhibit higher i0.8
-values than that of
m
the TBPBE cell, which are loadings that are too low to achieve with
the TBPBE architecture, as discussed above.
The plot of the low-potential 共high current兲 mass activity, shown
in Fig. 4b for the same cells as in 共a兲, reveals different trends. The
V
particle electrodes exhibit a maximum in i0.5
of 2.7 A/mg at an
m
2
intermediate loading of w = 0.18 mg/cm . The mass activity decreases slightly with decreasing w, indicating that flooding by product water of the smaller pores 共at smaller w兲 is more likely. As w is
V
decreases to 1.1 A/mg. This is atincreased to 0.40 mg/cm2, i0.5
m
tributed to the decreased pore density and increased pore length with
increasing w, because longer pores result in decreased oxygen flux,
and lower pore density combined with wider particles increases the
oxygen diffusion path through the electrolyte to access active sites
on the particle tips that are embedded in the membrane. The lowpotential mass activity of the continuous layers, also shown in Fig.
4b, is by more than a factor of 2 lower than that for the particle cells
with comparable w. This dramatic difference is attributed to masstransport-limiting effects of the continuous layers. The low porosity
of the layer inhibits efficient reactant 共oxygen兲 distribution or product 共water兲 rejection. The TBPBE comparison cell has a mass activV
for all particle
ity of 0.76 A/mg at 0.5 V, which is lower than i0.5
m
electrodes but within the range of the continuous layer electrodes.
The polarization curves for all fuel cells shown in Fig. 2-4 were
analyzed using a semiempirical model which provides insight into
kinetic and mass-transport losses as described in greater detail in
Ref. 49. The fuel cell voltage E at the current density i is described
by
关1兴
E = E0 − iR − b log i − m exp共ni兲
where E0 is given by
关2兴
E0 = ER + b log i0
ER is the reversible open-circuit voltage which is 1.17 V at 70°C,50
R is the ohmic resistance which was measured independently and is
controlled by proton transport in the membrane as well as electron
transport within the cell components, i0 is the exchange current density, i.e., the equilibrium rate of reaction at the electrode/electrolyte
interface, and b is the characteristic slope associated with the Tafel
relationship of current and overpotential.51 The last term in Eq. 1 is
an empirical quantity that describes the high current density overpotential which is associated with mass transport. A nonlinear curve fit
routine was used to determine the values of the unknown parameters
E0, b, m, and n in Eq. 1 for each polarization curve, as summarized
in Table I and plotted with the measured data on a logarithmic scale
in Fig. 5. The Tafel slope b is lowest for the cell with a continuous
w = 0.25 mg/cm2 Pt layer and for the TBPBE comparison cell, with
b = 97 and 101 mV/dec, respectively. The particle electrodes have
considerably higher slopes, ranging from 142 to 171 mV/dec, and
the low loading w = 0.05 mg/cm2 continuous layer Pt cell has
201 mV/dec. We attribute the higher Tafel slope for the particles to
losses within the electrode,52 because the slope is expected to double
when the ORR rate is determined by reaction kinetics plus either 共i兲
Table I. Kinetic and mass-transport parameters given by Eq. 1. R is the slope extracted from the hydrogen pumping polarization curve as
discussed in the text, and E0, b, m, and n are obtained from a nonlinear curve fit.
Electrode
Layer
Particles
TBPBE
Loading
共mg/cm2兲
R
共⍀ cm2兲
E0
共mV兲
b
共mV/dec兲
m
共mV兲
0.05
0.25
0.05
0.10
0.18
0.40
0.88
0.294
0.156
0.241
0.204
0.200
0.152
0.310
810
927
869
922
943
934
946
201
97
171
170
153
142
101
1.08 ⫻ 10−1
4.54
6.50 ⫻ 10−2
4.71 ⫻ 10−2
4.65 ⫻ 10−1
1.74 ⫻ 10−1
1.09 ⫻ 10−6
n
共cm2 /mA兲
3.20
1.68
5.28
3.07
8.63
1.31
2.72
⫻
⫻
⫻
⫻
⫻
⫻
⫻
10−2
10−2
10−2
10−2
10−3
10−2
10−2
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B618
Journal of The Electrochemical Society, 156 共5兲 B614-B619 共2009兲
Conclusion
Cathode electrodes consisting of Pt layers and particles were
deposited onto GDL substrates by sputtering from deposition angles
of 0 and 87°, respectively, and integrated into PEM fuel cells. A
comparison of polarization curves from cells with Pt layers and
particles shows that the mass-specific activity is higher for Pt layers
at low current densities but higher for Pt particles at high current
densities. These trends are attributed to a higher catalytically active
surface area for the Pt layer but a more efficient mass transport
共oxygen or water兲 for the particles. These results illustrate the performance tradeoff associated with the Pt surface area and porosity
and demonstrate that the GLAD technique is a promising method to
control the catalyst nanostructure in that regard.
Acknowledgments
Figure 5. 共Color online兲 Tafel plots for cells with cathodes containing Pt
particles, continuous Pt layers, and a TBPBE. The solid lines through the
measured data points are from a fit using Eq. 1.
This work was performed within the Center for Fuel Cell and
Hydrogen Research at Rensselaer Polytechnic Institute and was supported by the National Science Foundation through the Integrated
Graduate Education and Research Traineeship on fuel cells with
award no. DGE 0504361, as well as through award no. ECCS
0506738, ECCS 0403789, and CMMI 0727413.
Rensselaer Polytechnic Institute assisted in meeting the publication costs
of this article.
References
oxygen transport within the electrode or 共ii兲 proton migration within
the electrode.53 We expect the former, because the particles have
been coated with a solution of ionomer and are therefore fully immersed in the membrane electrolyte 共Fig. 1兲, making the oxygen
diffusion within the electrode the probable rate-determining step.
We attribute the relatively high Tafel slope for the continuous layer
cell with w = 0.05 mg/cm2 also to limited oxygen transport, because
the fraction of Pt that is immersed in ionomer increases with decreasing w. The parameters m and n quantify the mass transport
limitation at high i, revealing the enhanced reactant diffusion for
particle vs continuous layer electrodes. The current density at which
the mass transport overpotential m exp共ni兲 = 50 mV is ⬃4⫻ higher
for particle cells 共0.54 A/cm2 at w = 0.18 mg/cm2兲 than for continuous layer cells 共0.14 A/cm2 at w = 0.25 mg/cm2兲. The ohmic resistance R increases with decreasing w, because lower Pt loadings yield
smaller contact areas with the membrane electrolyte, as reported in
Ref. 54. The effect of Pt loading on resistance highlights the importance of correcting for iR in order to isolate the cathode electrode; it
is otherwise possible to misinterpret high ohmic resistance as poor
kinetic or mass transport performance, particularly at high current
density.
ECSA measurements done by CV indicate that layer cells have
more Pt in contact with the electrolyte than particle cells, in perfect
agreement with the above discussion. For example, continuous layer
electrodes with w = 0.05 mg/cm2 have ECSA values between 18.5
and 19.8 mC/cm2, whereas particles at the same low loading exhibit
15.9–18.0 mC/cm2. Increasing w results in higher ECSA, reaching
19.4–20.5 mC/cm2 for layers with w = 0.25 mg/cm2 and
19.0–19.9 mC/cm2 for particle cells with w = 0.40 mg/cm2. That is,
particle electrodes require 60% more platinum than layer electrodes
to achieve a comparable ECSA. The TBPBE comparison electrode
共w = 0.88 mg/cm2兲 exhibits 38.3–38.7 mC/cm2, which is almost a
factor of 2 higher than the value for any sputtered electrode in this
study, however, with 3.5⫻ more Pt. The higher ECSA of the continuous layers confirms that they have more Pt in contact with the
membrane, which explains the superior efficiency of layer cells at
high potential. The ESCA results also indicate that the high current
at low potential of the particle electrodes cannot be explained by the
presence of additional active catalyst area on the particles and therefore must be due to the enhanced reactant transport within these
porous electrodes at high oxygen demand.
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