Using sputter deposition to increase CO tolerance in a proton-exchange membrane fuel cell

Article abstract of DOI:10.1149/1.1479727

Placing a layer of Ru atop a Pt anode increases the carbon monoxide tolerance of proton-exchange membrane fuel cells when oxygen is added to the fuel stream. Sputter-deposited Ru filter anodes composed of a single Ru layer and three Ru layers separated by Nation-carbon ink, respectively, were compared to Pt, Pt:Ru alloy, and an ink-based Ru filter anodes. The amount of Pt in each anode was 0.15 mg/cm² and the amount of Ru in each Ru-containing anode was 0.080 mg/cm². For an anode feed consisting of hydrogen, 200 ppm CO, and 2% O₂ (in the form of an air bleed), all Ru filter anodes outperformed the Pt:Ru alloy. The performance of the Pt + single-layer sputtered Ru filter was double that of the Pt:Ru alloy. (0.205 vs. 0.103 A/cm² at 0.6 V). The performance was also significantly greater than that of the ink-based Ru filter (0.149 A/cm² at 0.6 V). Within the filter region of the anode, it is likely that the decreased hydrogen kinetics of the Ru (compared to Pt) allow for more of the OH_ads formed from oxygen in the fuel stream to oxidize adsorbed CO to CO₂.

Full text of DOI:10.1149/1.1479727

A868
Journal of The Electrochemical Society, 149 ͑7͒ A868-A872 ͑2002͒
0013-4651/2002/149͑7͒/A868/5/$7.00 © The Electrochemical Society, Inc.
Using Sputter Deposition to Increase CO Tolerance
in a Proton-Exchange Membrane Fuel Cell
b
Andrew T. Haug,a, Ralph E. White,
Steven Shi,
John W. Weidner,
Wayne Huang,
*
a,**,z
a,***
c
c
c
b,***
Narender Rana, Stephan Grunow, Timothy C. Stoner,
and Alain E. Kaloyerosc
aCenter for Electrochemical Engineering, Department of Chemical Engineering, University of South
Carolina, Columbia, South Carolina 29208, USA
bPlug Power, Incorporated, Latham, New York 12110, USA
cEnergy and Environment Applications Center, Institute for Materials, and School of Nanosciences and
Engineering, State University of New York at Albany, Albany, New York 12203, USA
Placing a layer of Ru atop a Pt anode increases the carbon monoxide tolerance of proton-exchange membrane fuel cells when
oxygen is added to the fuel stream. Sputter-deposited Ru filter anodes composed of a single Ru layer and three Ru layers separated
by Nafion-carbon ink, respectively, were compared to Pt, Pt:Ru alloy, and an ink-based Ru filter anodes. The amount of Pt in each
anode was 0.15 mg/cm2 and the amount of Ru in each Ru-containing anode was 0.080 mg/cm2. For an anode feed consisting of
hydrogen, 200 ppm CO, and 2% O2 ͑in the form of an air bleed͒, all Ru filter anodes outperformed the Pt:Ru alloy. The
performance of the Pt ϩ single-layer sputtered Ru filter was double that of the Pt:Ru alloy ͑0.205 vs. 0.103 A/cm2 at 0.6 V͒. The
performance was also significantly greater than that of the ink-based Ru filter ͑0.149 A/cm2 at 0.6 V͒. Within the filter region of
the anode, it is likely that the decreased hydrogen kinetics of the Ru ͑compared to Pt͒ allow for more of the OHads formed from
oxygen in the fuel stream to oxidize adsorbed CO to CO2 .
© 2002 The Electrochemical Society. ͓DOI: 10.1149/1.1479727͔ All rights reserved.
Manuscript submitted August 15, 2001; revised manuscript received January 10, 2002. Available electronically May 15, 2002.
Proton-exchange membrane-based fuel cells ͑PEMFCs͒ are gain-
the reversible hydrogen electrode ͑RHE͒ and above.13,15,16 Ruthe-
nium has the ability to form OHads from water at significantly lower
potentials than Pt, 0.35 V for 50 atom % Ru and 0.2 V for 90 atom
% Ru,12,13,16 allowing for a certain amount of CO tolerance. At low
temperatures ͑70-85°C͒, the Pt:Ru ͑1:1 atomic ratio͒ alloy catalyst
provides nearly equivalent performance to pure H2 on Pt for CO
concentrations up to 100 ppm in the feedstream.8,12
The injection of oxygen into the fuel stream increases catalyst
tolerance to CO.14,17 However, even the addition of high levels of
oxygen to the feedstream ͑2-4% by volume of hydrogen͒ provides
approximately 100 ppm CO tolerance. Roughly one out of every
400 O2 molecules oxidizes an adsorbed CO molecule, with the bal-
ance reacting with hydrogen.14 The placement of a layer of Ru cata-
lyst before the Pt electrode to act as a filter has been shown to
increase the effectiveness of oxygen addition over conventional
Pt:Ru alloy catalysts.18 This method also eliminates the process of
alloying the Pt and Ru metals, and by using a filter, the Pt loading in
the anode is free to be varied.
ing popularity due to their high operating efficiency and environ-
mental friendliness. Because of the difficulties inherent to storing
hydrogen, hydrocarbon fuels such as propane, natural gas, and gaso-
line are used to produce reformate gas. Dry reformate is typically
composed of 40-75% hydrogen, 15-25% carbon dioxide, 10-10,000
ppm carbon monoxide, and a balance of nitrogen, depending on the
fuel processing system used.1,2 It has been shown extensively that
CO poisons the platinum catalyst used in PEMFC systems.3-7 Car-
bon monoxide chemically adsorbs onto available Pt catalyst sites as
shown in
CO ϩ Pt → COads
͓1͔
As little as 10 ppm CO in the fuel stream can lower the power
output of the PEMFC by 50%.6,7 For the reforming process to be
effective in the fuel cell system, this problem must be solved. At-
tempts to find catalysts both tolerant to CO and equivalent in per-
formance to Pt have led to the alloying of Pt with Ru, Mo, W, Co,
Os, Ir, Ni, and Sn.7-11 Used by themselves as catalysts, these metals
do not provide the high hydrogen activity necessary to achieve
the current densities that make PEMFCs competitive in the
marketplace.9,12,13 The most commonly used alloy is Pt:Ru. The
Pt:Ru alloy combines the high catalyst activity of pure Pt with the
increased CO tolerance of pure Ru catalyst.12-14
It is believed that the following mechanism is occurring in the
Ru filter for a fully humidified fuel stream containing oxygen, hy-
drogen, and carbon monoxide14,18-33
Ru ϩ H2 ↔ 2Hads
Ru ϩ CO ↔ COads
͓4͔
͓5͔
The oxidation of COads from the Pt:Ru catalyst surface in the
O2 ϩ 2Ru ↔ 2 Oads
ads ϩ Oads → OHads ϩ Ru
͓6͔
anode shown in Eq. 2 follows Langmuir-Hinshelwood kinetics12,13
H
͓7͔
COads ϩ OHads → CO2 ϩ Hϩ ϩ 2M ϩ eϪ
͓2͔
COads ϩ OHads → CO2 ϩ Hϩ ϩ 2Ru ϩ eϪ
͓8͔
where M represents Pt or Ru. The reactions by which OHads is
formed on Pt and Ru are shown in Eq. 315,16
Hads ϩ OHads ↔ H2O ϩ Ru
͓9͔
M ϩ H2O → OHads ϩ Hϩ ϩ eϪ
͓3͔
COads ϩ Oads → CO2 ϩ 2Ru
Hads → Hϩ ϩ eϪϩ Ru
͓10͔
͓11͔
The formation of OHads , shown in Eq. 3 is the rate-determining
step of this reaction and occurs on platinum at potentials of 0.7 V vs.
Reactions 4, 5, and 6 represent the adsorption of species onto the
Ru catalyst. Reaction 7 represents an intermediate reaction on Ru
resulting in the formation of OHads . Reactions 8-11 represent com-
peting desorption reactions on the Ru catalyst. Equation 10 is well
documented as following Langmuir-Hinshelwood kinetics on Pt and
Ru.34,35 Of the desorption reactions, Reactions 8 and 10 are desired
* Electrochemical Society Student Member.
** Electrochemical Society Fellow.
*** Electrochemical Society Active Member.
z E-mail: white@engr.sc.edu
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Journal of The Electrochemical Society, 149 ͑7͒ A868-A872 ͑2002͒
as they result in the oxidation of COads . Earlier work18 showed that
A869
Table I. Types of ink-based MEUs tested.
the reaction shown in Eq. 8 is the primary means by which COads is
oxidized within the filter, resulting in the formation of Hϩ. Thus,
like the Pt region following the filter, a three-phase interface of
catalyst ͑Ru here͒, carbon and Nafion are required for the filter to
operate. The above mechanism is not a comprehensive list of all
reactions occurring within the anode region. The mechanism focuses
on those reactions that lead to CO oxidation. For example, evidence
has been found that H2O2 is formed as an intermediate during oxy-
gen reduction at overpotentials as low as 0.5 V.19 However, H2O2
breaks down to oxygen-containing compounds at or even before
reaching the catalyst surface.4,7,17
The goal of this work is to increase the effectiveness of the Ru
filter by applying it through sputter deposition as opposed to con-
ventional catalyst ink-based application methods. Sputter deposition
is widely used for integrated circuit manufacturing and has been
investigated for us in fuel cells for more than a decade.36-39 Hirano
et al.40 sputter deposited platinum on uncatalyzed gas-diffusion
layers ͑GDLs͒ resulting in cell performances at loadings of
0.10 mg Pt/cm2 equivalent to those of standard methods at loadings
of 0.40 mg Pt/cm2. Witham et al.41 achieved direct methanol fuel
cell ͑DMFC͒ anode catalyst activities one to two orders of magni-
tude higher than those of conventional ink-based catalysts, suggest-
ing that DMFC anodes may be manufactured containing less than
one-tenth the amounts presently used (2.5-4 mg Pt/cm2) without
loss in performance. Holleck et al.42 have sputtered catalyst mix-
tures at the front surface of the anode electrode. These mixtures
included Pt:Ru:X where X represents Ni, Pd, Co, Rh, Ir, Mn, Cr, W,
Nb, and Zr. For low levels of CO ͑10 ppm͒, specific Pt:W and
Pt:Ru:W alloys performed better than Pt:Ru.
In this work, the Ru filter is sputter-deposited as a single layer
and as a series of three layers ͑separated by Nafion-carbon ink͒ and
compared to a conventional ink-based filter. By manufacturing
membrane-electrode assemblies ͑MEAs͒ composed of multiple
sputter-deposited Pt layers, Cha and Lee43 were able to increase the
Pt catalyst activity significantly over conventional ink-based MEAs
by increasing the amount of Pt in contact with Nafion and carbon.
This process was further improved upon by reducing the amount of
Nafion-carbon ink ͑NCI͒ separating the Pt layers, resulting in thin-
ner, more effective electrodes.44 For the same loading of catalyst,
sputter-depositing catalyst between layers of NCI increases the ac-
tive area of the catalyst vs. a single sputter-deposited layer.43,44 For
Pt catalyst, this results in greater performance of the fuel cell
electrodes.43,44 It is predicted that for the Ru filter, this process will
create more sites upon which CO can be oxidized, resulting in a
more CO-tolerant PEMFC MEA.
MEU name
Pt
Description
0.15 mg Pt/cm2 anode
0.15 mg Pt/cm2 cathode
Uncatalyzed Toray GDLs on anode and cathode
0.23 mg Pt:Ru/cm2 anode
Pt:Ru
0.15 mg Pt/cm2 cathode
Uncatalyzed Toray GDLs on anode and cathode
Pt ϩ Ru ͑Ru filter͒ 0.15 mg Pt/cm2 anode
0.15 mg Pt/cm2 cathode
0.080 mg Ru/cm2 coated Toray GDL on the anode
Uncatalyzed Toray GDL on the cathode side
To form a MEA with ink-coated decals, appropriate decals were
placed on either side of the PEM ͑Nafion 117, protonated form͒.
This assembly was hot-pressed to ensure bonding. It was then
cooled to room temperature, before the decals were carefully pealed
from the assembly.
To form a membrane-electrode unit ͑MEU, equivalent to a
GDL ϩ MEA ϩ GDL͒ with ink-coated GDLs, appropriate GDLs
were placed on either side of a PEM ͑Nafion 117, protonated form,
soaked in deionized water for 1 h͒. This assembly was hot-pressed
to ensure a well-bonded MEU. The types of ink-based MEUs made
are shown in Table I.
Development of sputter-deposition-based MEAs.—Plasma modi-
fications and sputter-deposition augmentations/additions were both
completed using an Anatech Hummer 10.2 sputter-coating tool. A
modified sample stage was used to support Nafion 117 substrates up
to 6 ϫ 6 cm while masking 1.5 cm about the membrane’s perimeter.
It has been shown previously that alternating current ͑ac͒ plasma
should be used for all plasma modifications, as Nafion 117 and
MEAs from Nafion 117 suffer no ill effects from this treatment.44
An aluminum target was used for ac plasma modifications, while the
Ru ͑Kurt J. Lesker͒ target and carbon evaporation system ͑Anatech͒
were used for sputter augmentations/additions.
All MEAs subject to sputter deposition were first ac plasma
cleaned for 5 min at 5 mA and 1.2 kV to remove residual buildup
from the target as well as roughen the substrate surface. All treat-
ments were completed at ϳ62 mTorr. A separate vacuum chamber
was used to evacuate each substrate to ϳ45 mTorr, before it was
placed in the sputter-coating tool to minimize contaminant outgas-
sing in the deposition chamber. A potential of 1.8 kV and a current
of 8 mA were maintained to control the deposition rate for Ru. A
SiO2 sample was sputter-deposited in situ with each MEA. The re-
sultant metal/SiO2 stack was subjected to cross-sectional view scan-
ning electron microscopy ͑SEM͒ imaging to verify the thickness of
the sputter-deposited film and top-view SEM imaging to determine
surface characteristics of the film.
Experimental
Development of ink-based MEAs.—The method described in
U.S. Patent 5,211,984 provides an outline for the catalyst ink prepa-
ration and MEA fabrication performed in this project.45 The follow-
ing catalyst inks were prepared: (i) Nafion ϩ carbon only ͑Nafion-
carbon Ink or NCI͒, (ii) Nafion ϩ 20% Pt on carbon, (iii)
Multilayered Ru filters were manufactured by a method similar
to that described by Cha and Lee.43 On the MEA anode, a layer of
NCI was sprayed on the surface of the sputter-deposited Ru layer.
The MEA was then dried at 80°C under vacuum for 10 min. Addi-
tional layers of sputter-deposited Ru and NCI were added until the
desired Ru loading and number of Ru layers were attained.
MEAs containing sputter-deposited catalyst layers were manu-
factured into MEUs through similar methods as the ink-based
MEAs.
Nafion ϩ 20% Pt:Ru on carbon, and (i ) Nafion ϩ 20% Ru on
carbon.
v
The inks were prepared for Pt, Pt:Ru, Ru by adding the E-TEK
catalyst ͑20% catalyst on XC-72 Carbon͒ to a solution of 5 wt %
Nafion ͑DuPont͒. For NCI, XC-72 Carbon was added to a solution
of 5 wt % Nafion.
Decals ͑Teflon, 10 cm2, three-ply͒ were weighed prior to appli-
cation of catalyst ink. The ink was drawn across the surface of the
decals using a Meyer rod. The coated decals were dried in an oven
at 105°C under ambient pressure for 10 min.
Cell assembly and testing.—The MEUs were placed in a 10 cm2
cell assembly and incubated for 4 to 8 h at ambient pressure,
cell temperature of 70°C, stoichiometric ratio ͓͑actual flow͔/
͓stoichiometric flow͔ required for a 1.0 A/cm2 current͒ of 1.5 at the
anode and 2.0 at the cathode. Fuel cell performance curves were
obtained under the conditions given in Table II.
Target loadings were 0.15 mg Pt/cm2 ͑anode͒, 0.230 mg
Pt:Ru/cm2 ͑anode͒, and 0.15 mg Pt/cm2 ͑cathode͒. Target loadings
of 0.230 mg Pt:Ru/cm2 and of 0.080 mg Ru/cm2 for the Ru filter
were chosen so that a 1:1 atomic ratio of Pt:Ru is maintained, while
Pt catalyst is maintained at a loading of 0.15 mg/cm2.
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Journal of The Electrochemical Society, 149 ͑7͒ A868-A872 ͑2002͒
Table II. Fuel cell test conditions.
Pressure
1 atm
Cell temperature
70°C
1.5 hydrogen
Stoichiometric ratio ͑at 1 A/cm2͒
2.0 air
Feedstreams
Anode: hydrogen,
H2 ϩ CO, H2 ϩ CO ϩ air bleed
Cathode: air
Humidification
Air bleed
Complete humidification of anode
and cathode gas streams for all trials
Figure 2. Diagram of the Ru filtered anode. The filter is a layer of Ru
catalyst placed between the Pt catalyst and the gas-diffusion layer for oxi-
dizing CO present within the anode. Filter 1 is the standard Ru filter prepared
from a catalyst ink. Filter 2 is a single sputter-deposited layer of Ru sepa-
rated from the Pt electrode by a layer of NCI. Filter 3 is a three-layer
sputter-deposited Ru filter separated from the Pt electrode by a layer of NCI.
All filters have a loading of 0.08 mg Ru/cm2 and were placed atop a Pt
electrode with a loading of 0.15 mg Pt/cm2.
2.0% O2 ͑in the form of an air bleed
relative to the volumetric flow of
hydrogen in slm͒
CO amounts
50, 200 ppm
Results
Determination of sputter-deposition rates and catalyst load-
ing.—The sputter-deposition rates for Ru were determined with
loadings calculated from cross-sectional and top-view SEM images
of sputter-deposited Ru films on SiO2 substrates. The top-view SEM
images in Fig. 1 were analyzed to determine the surface coverage of
the sputter-deposited film ͑Pt ϳ 65%, Ru ϳ 56%͒. Ru did not form
a continuous film on the SiO2 substrate, but rather agglomerated.
This is consistent with literature.46 The surface coverages were used
in conjunction with the bulk density of Ru ͑12.2 g/mL͒ and the film
thicknesses from the cross-sectional SEM images to calculate the
subsequent Ru loadings. The Ru sputter-deposition rate was constant
with time at 3.3 ␮g Ru/cm2/min.
The cross-sectional view in Fig. 1 shows that the thickness of 30
and 45 min sputter-deposited Ru is roughly two and three times the
thickness of a 15 min deposition of Ru, respectively. Semiquantita-
tive analyses via energy dispersive X-ray spectroscopy ͑EDXS͒ and
Rutherford backscattering spectrometry ͑RBS͒ confirm these Pt and
Ru deposition rates.
tively. As shown in Fig. 2, NCI is used to separate the layers of
sputter-deposited Ru layered between Nafion-carbon ink ͑NCI͒.
Figure 3 compares various Ru filters to the Pt:Ru alloy and the
baseline MEA at 70°C for an anode feed of hydrogen ϩ 50 ppm
CO. All MEAs prepared with Pt, Pt:Ru alloy, and Pt ϩ Ru filters
1-3 exhibited a dramatic loss of performance when 50 ppm CO was
added to the hydrogen feedstream. In all cases, fuel cell performance
dropped to less than 40% of the baseline MEA running on pure
hydrogen fuel. It is clear that the filters do not completely oxidize
the CO in the feedstream to CO2 . The remaining CO passes through
the filter and poisons the Pt portion of the anode, resulting in per-
formance resembling that of a typical Pt anode as evidenced in Fig.
3. The Pt:Ru alloy demonstrates CO tolerance over a Pt anode con-
sistent with literature, more than doubling the current density of any
other anode configuration at 0.6 V.
When 2% O2 ͑in the form of an air bleed͒ is added to the anode
feed, all Ru-containing anodes show significant performance im-
provement over the Pt baseline as shown in Fig. 4. For the 25 min
Ru (0.08 mg Ru/cm2) sputter-deposited filter, there is almost no per-
formance loss except at lower voltages. The Pt:Ru alloy and stan-
dard Ru filter show almost identical performance, roughly 20% less
than the baseline ͑0.227 vs. 0.275 A/cm2 at 0.6 V͒. The performance
of the Ru filter cannot be explained by the formation of a Pt:Ru
alloy as there is no interface between Pt and Ru. A layer of NCI
separates the sputter-deposited Ru from the Pt electrode for the
sputter-deposited Ru filters. The multilayered filter performed the
CO testing.—The three types of Pt ϩ Ru filter anodes prepared
are shown in Fig. 2. They were made to the following specifications:
filter 1: 0.08 mg/cm2 ink-based 20% Ru/C, filter 2: NCI ϩ 25 min
(0.08 mg/cm2) of sputter-deposited Ru, and filter 3: NCI ϩ 3
ϫ
͑8.33 min of sputter-deposited Ru ϩ NCI͒. The total Ru loading
was 0.08 mg/cm2.
Filter 1 is an ink-based Ru filter, while filter 2 and 3 are com-
prised of one and three layers of a sputter-deposited Ru, respec-
Figure 1. Top and cross-sectional SEM
images of sputter-deposited Ru on
Si/SiO2 substrates.
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Journal of The Electrochemical Society, 149 ͑7͒ A868-A872 ͑2002͒
A871
Figure 5. Performance comparison of MEUs with sputter-deposited Ru filter
for an anode feed of hydrogen ϩ 200 ppm CO ϩ 2% O2 . P ϭ 1 atm,
T ϭ 70°C. The filter configurations are defined in Fig. 3.
Figure 3. Performance comparison of MEUs with sputter-deposited Ru filter
under H2 ϩ 50 ppm CO conditions. P ϭ 1 atm, T ϭ 70°C. The filter con-
figurations are defined as follows: filter 1 ͑—᭡—͒ is the standard Ru filter
prepared from an ink; filter 2 ͑—᭺—͒ is a single 25 min sputter-deposited
layer of Ru separated from the Pt electrode by a layer of NCI; filter 3
͑—ᮀ—͒ is a three-layer sputter-deposited Ru filter ͑8.33 min per layer, see
Fig. 2 for design͒ separated from the Pt electrode by a layer of NCI.
of an air bleed. The Ru filter in Ref. 18 showed a 50% loss in
performance when the CO concentration was doubled to 200 ppm.
Of the three Ru-filtered MEAs tested, filter 3 performed the
worst at 200 ppm CO, but suffered only a 12% loss compared to its
performance at 50 ppm CO ͑0.138 vs. 0.157 A/cm2 at 0.6 V͒. That
there is so little drop-off from this three-layer filter configuration
suggests that the lower performance compared to the single-layer
sputter-deposited filter 2 is due to diffusional resistances caused by
the increased thickness of this filter. The layers of NCI are very thick
relative to each Ru deposition ͑ϳ40 nm for each sputter-deposited
Ru layer compared to ϳ12 ␮m for each NCI layer͒,44 accounting
for more than 99.5% of the Ru filter layer thickness. The increased
thickness of this three-layer filter requires hydrogen to travel further
to reach the Pt vs. a single-layer filter, and vs. the baseline MEA
giving rise to greater diffusional resistances. By reducing the
amount of NCI used in each sputter-deposited layer ͑by diluting
the NCI͒, a thinner, more effective multilayered Ru filter may be
developed.
The addition of a layer of NCI between the Pt electrode and the
sputter-deposited Ru filter is evidence that the CO oxidation is not
the result of a layer of Pt:Ru alloy being formed at the interface of
the filter and the Pt electrode. Thus, it can be hypothesized that the
CO oxidation is occurring very fast in relation to the diffusion of the
gases through the filter region. Nearly all the CO is oxidized within
a region ϳ100 to 120 nm thick ͑see Fig. 1͒. Therefore, it is assumed
that a filter of identical loading (0.08 mg Ru/cm2) would perform
similarly in front of a Pt electrode of any loading.
worst of the Ru-containing anodes. This is likely due to the in-
creased diffusional resistances caused by the thickness of the Ru
filter.
As the amount of CO is increased from 50 to 200 ppm, the
benefit of the Ru filters is seen more clearly. Figure 5 shows that all
three Ru filter types outperform the Pt:Ru alloy. For the anode con-
sisting of Pt ϩ Ru filter 2, the performance is double that of the
Pt:Ru alloy ͑0.205 vs. 0.103 A/cm2 at 0.6 V͒. The MEA containing
filter 2 loses only 20% of its performance ͑0.255 vs. 0.205 A/cm2 at
0.6 V͒ as the CO concentration is increased from 50 to 200 ppm CO,
and loses only 25% vs. the hydrogen baseline. There have been other
reports of 200 ppm CO tolerance with an air bleed, but the condi-
tions involved higher temperatures and higher loadings.14 The MEA
containing filter 2 significantly outperformed ink-based filter 1 by
more than 35% ͑0.205 vs. 0.149 A/cm2 at 0.6 V͒. These results also
constitute a significant improvement over the previous work done on
the Ru filter.18 Under conditions identical to those shown in Table II,
the Ru filter of 0.21 mg/cm2 developed in Ref. 18 showed an
equivalent loss in performance to filter 2 of this work when only 100
ppm CO was added to the anode feed containing 2% O2 in the form
It is assumed that all oxygen reacts almost immediately within
the filter region and that the hydroxyl groups produced then react
with CO to produce CO2 . Even with 200 ppm CO tolerance, only
one out 200 O2 molecules is being used to oxidize CO. Further
research into catalysts with lower activity than Ru with respect to
hydrogen may provide even further CO oxidation by further shifting
the selectivity of the oxygen reactions toward the oxidation of CO to
CO2 .
Conclusions
The sputter-deposited Ru filters developed here, containing less
than 40% the amount of Ru ͑0.080 vs. 0.21 mg/cm2͒ of the previous
work,18 achieved twice the CO tolerance ͑200 ppm CO vs. 100 ppm
CO͒ under similar operating conditions ͑H2 ϩ 2% O2 anode feed, a
cathode feed of air, 70°C operating temperature͒.
For an anode feedstream consisting of hydrogen, 200 ppm CO,
and 2% oxygen, all MEAs containing Pt ϩ Ru filter anodes showed
increased CO tolerance compared to a Pt:Ru alloy containing similar
amounts of Pt and Ru ͑0.150 and 0.080 mg/cm2, respectively͒. The
Figure 4. Performance comparison of MEUs with sputter-deposited Ru filter
for an anode feed of hydrogen ϩ 50 ppm CO ϩ 2% O2 . P ϭ 1 atm,
T ϭ 70°C. The filter configurations are defined in Fig. 3.
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Journal of The Electrochemical Society, 149 ͑7͒ A868-A872 ͑2002͒
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MEA containing a single sputter-deposited Ru filter exhibited
greater CO tolerance than that of the MEA containing the ink-based
Ru filter ͑0.205 vs. 0.149 A/cm2 at 0.6 V͒.
Attaining 200 ppm CO tolerance by using 2% oxygen is equiva-
lent to one out 200 O2 molecules being used to oxidize COads mol-
ecule, while the rest likely react with Hϩ to form water. Catalysts
with characteristics similar to Ru but with lower hydrogen activity
would likely allow for a higher percentage of oxygen in the feed-
stream to react with adsorbed CO, resulting in a more effective filter.
While the three-layered sputter-deposited Ru filter ͑filter 3͒ per-
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as the CO in the anode feed was increased from 50 to 200 ppm
͑balance H2 ϩ an air bleed containing 2% O2͒ was the smallest of
all the anodes tested. NCI accounts for 99.7% of the multilayered
Ru filter thickness. Further research can be done to optimize this
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portion of the electrode, resulting in thinner, more effective filters.
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region. This would result in an extremely thin (ϳ1 ␮m) filter that
may provide high Ru activity with minimal diffusional resistances.
And by applying this Ru/C/Nafion filter in a single application to the
PEM, the process is less time-consuming and thus more economical.
Because benefits of the Ru filter occur at high level air bleed ͑2%
O2͒ and the Pt:Ru alloy provides CO tolerance even without air
bleed, it is suggested that the anode configuration that would pro-
vide optimal CO tolerance would consist of a sputter-deposited Ru
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Acknowledgments
The authors acknowledge financial support from the National
Institute of Standards and Technology under cooperative agreement
no. 70NANB8H4039.
The University of South Carolina assisted in meeting the publication
costs of this article.
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