Rapid screening of bimetallic electrocatalysts for oxygen reduction in acidic media by scanning electrochemical microscopy

Article abstract of DOI:10.1149/1.2186208

Additional bimetallic electrocatalysts for the oxygen reduction reaction (ORR) in acidic media were designed using a previously reported thermodynamic selection guide. The electrocatalyst mixtures were prepared in large arrays on glassy carbon substrates and the electrocatalytic activity was screened using scanning electrochemical microscopy (SECM). Activities were measured for a range of bimetallic combinations that showed a synergetic electrocatalytic effect during screening, including Au-V, Ag-V, Pd-Mn, and Pd-V. Upon initial screening, a highly active electrocatalytic combination consisting of 60:40 Pd-V was identified. Using rotating disk electrode (RDE) experiments, the high activity of this combination for the ORR in acidic media was confirmed when the electrocatalysts were supported on Vulcan carbon. The electrocatalytic activity of Pd-V was close to that exhibited by Pt, the electrocatalyst of choice for the ORR in acidic media, and thus is another example of a nonplatinum catalyst with high activity that follows the previous strategy for catalyst design.

Full text of DOI:10.1149/1.2186208

Journal of The Electrochemical Society, 153 ͑6͒ E99-E103 ͑2006͒
E99
0013-4651/2006/153͑6͒/E99/5/$20.00 © The Electrochemical Society
Rapid Screening of Bimetallic Electrocatalysts for Oxygen
Reduction in Acidic Media by Scanning Electrochemical
Microscopy
*
**,z
Darren A. Walsh, José L. Fernández, and Allen J. Bard
Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712, USA
Additional bimetallic electrocatalysts for the oxygen reduction reaction ͑ORR͒ in acidic media were designed using a previously
reported thermodynamic selection guide. The electrocatalyst mixtures were prepared in large arrays on glassy carbon substrates
and the electrocatalytic activity was screened using scanning electrochemical microscopy ͑SECM͒. Activities were measured for
a range of bimetallic combinations that showed a synergetic electrocatalytic effect during screening, including Au–V, Ag–V,
Pd–Mn, and Pd–V. Upon initial screening, a highly active electrocatalytic combination consisting of 60:40 Pd–V was identified.
Using rotating disk electrode ͑RDE͒ experiments, the high activity of this combination for the ORR in acidic media was confirmed
when the electrocatalysts were supported on Vulcan carbon. The electrocatalytic activity of Pd–V was close to that exhibited by
Pt, the electrocatalyst of choice for the ORR in acidic media, and thus is another example of a nonplatinum catalyst with high
activity that follows the previous strategy for catalyst design.
© 2006 The Electrochemical Society. ͓DOI: 10.1149/1.2186208͔ All rights reserved.
Manuscript submitted November 7, 2005; revised manuscript received January 25, 2006. Available electronically April 6, 2006.
Polymer electrolyte membrane fuel cells ͑PEMFCs͒ have been
widely proposed as potentially inexpensive, efficient, and clean en-
ergy sources.1-5 Research in this area is directed at a number of
approaches, including the development of improved polymeric
membrane materials,6,7 nanocarbon electrode supports,8-11 and the
search for replacement cathode materials for the oxygen reduction
reaction ͑ORR͒.12,13 Of these, the latter is particularly important if
PEMFCs are to achieve widespread application as the current Pt
electrocatalysts are very expensive and even with Pt the nonideal
kinetics for the ORR leads to significant overpotentials ͑ϳ0.4 V͒
and power losses.14,15 In an effort to reduce the quantity of Pt re-
quired for fuel cell applications, a range of Pt alloys has been re-
ported that exhibits good activity for the ORR.16-20 Despite this,
replacement of the Pt in PEMFCs with a significantly less expensive
non-Pt electrocatalyst in PEMFCs, and catalyst improvement for the
ORR remain major goals in this field.
reduction͒ as shown in Fig. 1, which extends the range of metals
given previously.24 The applicability of the selection guide and
screening method was examined using combinations of Pd, Au, and
Ag ͑as metal 2͒ with Co ͑as metal 1͒, and a reduction in the ORR
overpotential was observed in each case. Therefore, the selection
guide was a very useful starting point when deciding which systems
to screen. Upon detection of promising electrocatalytic combina-
tions, they can be studied in greater detail by preparing mixtures
with carbon and examining by rotating disk electrode ͑RDE͒ volta-
mmetry and then scaled up for testing under actual fuel cell condi-
tions.
The work reported here extends the previous studies using
the reported selection guide24 by selecting metals that form metal
oxides ͑i.e., break the O–O of O2͒ more easily than Co, so that
it might be possible to design bimetallic electrocatalysts that
show better enhancement of the synergetic effect. In particular,
this is illustrated using combinations that contain Mn and V. Each
of these metals has large negative free energies for oxide formation
͑Ͻ−350 kJ mol−1͒ and so readily form metal oxides. These metals
have been combined with metals whose oxides are easily reducible
͑Au, Ag, and Pd͒ to test further the guidelines and search for en-
hanced ORR electrocatalysis at bimetallic spots containing these
metals compared to the single metal. Significantly, an additional
Pd-based combination that exhibits high activity for the ORR has
been identified. RDE measurements confirm the high activity of this
electrocatalyst and a comparison with commercial Pt electrocatalyst
is described.
The search for novel electrocatalysts has benefited in recent
years from the introduction of several rapid screening methods.21-23
In particular, a method of automatically preparing test arrays of
bimetallic ORR electrocatalyst spots on an inactive ͑glassy carbon͒
substrate followed by rapid scanning electrochemical microscopy
͑SECM͒ screening was recently described.24 SECM screening in-
volved the evolution of O2 at a metal tip, which diffused to the array
component immediately beneath the tip, where it was electrore-
duced. By holding the substrate at different potentials, it was pos-
sible to identify those mixtures with high activities for the ORR and
to construct current-potential curves for the ORR reaction at that
composition. The SECM screening method used very little material
and was rapid, so it was possible to screen very large arrays of
different bimetallic combinations rather quickly. The mixture com-
ponents were selected using simple guidelines based on thermody-
namic principles involving the so-called “direct route” of oxygen
reduction, where one of the metals was selected for good dissocia-
tive chemisorption of O2 onto the surface ͑metal 1, M͒, forming
adsorbed oxygen atoms ͑O•͒ ͑e.g., 2M + O2 → 2MO͒. Electrore-
Experimental
Chemicals
.— Glassy carbon ͑GC͒ plates ͑1 mm thick, 50
ϫ 50 mm͒ were purchased from Alfa Aesar ͑Ward Hill, MA͒. Prior
to use, the GC plates were cut into smaller squares to produce the
substrates ͑15 ϫ 15 mm͒. ͑NH4͒2PdCl4 ͑Aldrich͒, HAuCl4 ͑Alfa͒,
AgNO3 ͑Aldrich͒, VCl2 ͑Aldrich͒, and Mn͑NO3͒2 ͑Aldrich͒ were
used as obtained and Milli-Q water was used to prepare all solu-
tions. Vulcan carbon XC-72R was from Cabot Co. ͑Alpharetta, GA͒.
Commercial carbon-supported Pt ͑20 wt % platinum͒ was from
Johnson Matthey ͑Ward Hill, MA͒.
duction of O• at the surface by metal 2, M , with the addition of 4
Ј
protons and 4 electrons, then results in the overall reduction of O2 to
H O ͑2M O + 4H+ + 4e → 2M + 2H O͒.24 Bimetallic combina-
Ј
Ј
2
2
tions were chosen that combined a metal that easily forms adsorbed
O• by breaking the O–O bond of O2 ͑i.e., a highly negative free
energy for metal oxide formation͒ with a second metal whose oxide
is easy to reduce to water ͑i.e., a positive potential for metal oxide
Preparation of catalyst spots.— Catalyst mixtures were pre-
pared on GC supports as described previously.24 Briefly, a piezoelec-
tric pico-dispenser ͑Microjet AB-01-60, MicroFab, Plano, TX͒ was
installed onto a digital plotter head ͑Houston Instruments DMP-5,
Houston, TX͒ to dispense picoliter amounts of metal-salt solutions
at programed locations on a glassy carbon substrate by application
of 50–60 V pulses ͑pulse time 25 ␮s͒ to the dispenser. Metal-salt
solutions typically contained 0.3 M salt in a 3:1 solution of water/
*
Electrochemical Society Student Member.
Electrochemical Society Fellow.
**
z E-mail: ajbard@mail.utexas.edu
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Journal of The Electrochemical Society, 153 ͑6͒ E99-E103 ͑2006͒
Figure 2. SECM TG-SC images of oxygen reduction activity measured on
Au–V arrays in 0.5 M H2SO4. Tip-substrate distance = 30 ␮m, tip current
= −160 nA, scan rate = 50 ␮m each 0.2 s. ͑A͒ ES = 0.1 V vs NHE. ͑B͒
ES = 0.2 V vs NHE. ͑C͒ ES = 0.3 V vs NHE. WM is the atomic ratio of metal
M in the electrocatalyst spot.
Figure 1. Thermodynamic guidelines for bimetallic catalyst design for ORR
͑see the text for details͒ ͑Ref. 24͒. Reaction 1. 2M + O2 → 2MO. Reaction
2. 2M O + 4H+ + 4e → 2M + 2H O.
Ј
Ј
2
glycerol. Voltage pulses were applied using a DAQ board ͑Keithley
Series 500, Cleveland, OH͒ coupled to a high-voltage dc OP Amp
͑Burleigh PZ-70, Fisher, NY͒. After agitation to thoroughly mix the
components, reduction of the metal-salt solutions to metal was car-
ried out as follows. HAuCl4 and VCl2 were dispensed to form the
Au–V array and the resulting array was treated with H2 ͑1 atm͒ at
550°C for 1 h. Ag–V arrays were prepared using the same proce-
dure by dispensing AgNO3 and VCl2 solutions. Pd–Mn and Pd–V
arrays were deposited using solutions of ͑NH4͒2PdCl4, Mn͑NO3͒2,
and VCl2. The precursor spots were reduced by dispensing 10 drops
of 10 mM NaBH4 directly onto each spot. The complete array was
then rinsed with water and a subsequent thermal treatment under H2
at 550°C for 1 h was applied.
ments a Hg/Hg2SO4 reference electrode was used and the potential
values reported were adjusted to the normal hydrogen electrode
͑NHE͒ scale, where Hg/Hg2SO4, H2SO4͑0.5 M͒ = 0.65 V vs NHE.
Results and Discussion
SECM ORR activity screening.— Figure 2 shows SECM images
obtained upon screening an array containing mixtures of Au and V
over a range of substrate potentials, ES. At ES = 0.1 V pure Au
is active for the ORR and so are electrocatalyst combinations
that contain low amounts of V, indicated by essentially the
same substrate current ͑iS͒ in the color scale plot ͑Fig. 2a͒. Figure 2b
and c shows the SECM images recorded for the Au–V array with
ES at more positive potentials. At ES = 0.2 V ͑Fig. 2b͒ the electro-
catalytic activity of pure Au is almost zero, while mixtures con-
taining 10–30% V clearly exhibit some activity for the ORR. At
ES = 0.3 V the activity of pure Au is zero and it is possible to
distinguish among the activities of those mixtures containing low
amounts of V and the mixture containing Au:V at a ratio of 80:20,
which exhibits the highest activity for the ORR at this potential.
Figure 3 shows the SECM images recorded for electrocatalyst
mixtures containing Ag–V over a range of substrate potentials. Upon
screening, the behavior was similar to that observed for the Au–V
array. At the most negative potential, ES = 0.05 V vs NHE ͑Fig. 3a͒,
pure Ag and those mixtures containing relatively low amounts of V
exhibit similar activities. As the substrate potential was made more
positive ͑Fig. 3a and b͒, the electrocatalytic activity of pure Ag
decreased to zero while those combinations containing low amounts
of V showed some activity, even at the most positive potential mea-
sured, ES = 0.25 V. Pure Au and Ag are poor electrocatalysts for the
ORR and both show significant improvements for the ORR upon
mixing with V, although the absolute activities are still inferior to a
good electrocatalyst such as Pt. Nevertheless, both of these cases
follow the general guidelines proposed for bimetallic electrocatalyst
design, i.e., improving the behavior of metal 2 ͑Au, Ag͒ by the
addition of a metal 1 ͑V͒.
Rotating disk electrode preparation.— Supported Pd and Pd–V
electrocatalysts ͑20 wt % metal͒ were prepared on carbon by reduc-
tion of metal-salt precursors with NaBH4, and dispersed in Nafion
solutions. Initially, 160 mg of Vulcan XC-72R was suspended in
50 mL of water containing sufficient amounts of the salts to produce
40 mg of metal in total. The suspensions were then sonicated for
30 min and 10 mL of 0.1 M NaBH4 solution was added dropwise
under continuous stirring. The suspensions were stored overnight to
ensure complete reduction of the metal salts. The supported electro-
catalysts were then centrifuged at least four times to separate the
liquid phase and to wash the solid. The resulting powder was dried
at 87°C and then heated under H2 ͑1 atm͒ at 550°C for 1 h in a tube
furnace ͑Barnstead International, Dubuque, IA͒. Inks of the
electrocatalyst-containing powders were prepared by dispersing
8 mg of electrocatalyst in 200 ␮L of a 1 wt % Nafion solution ͑Al-
drich, Milwaukee, WI͒ in isopropanol and agitating ultrasonically
for 15 min. A drop ͑0.1 ␮L͒ of this ink was deposited on a 1 mm
diameter GC RDE and dried for 30 s at 40°C. RDE measurements
were performed using a standard three-electrode cell with a carbon
rod counter electrode and a Hg/Hg2SO4 reference electrode. RDE
polarization curves were recorded in O2-saturated 0.5 M H2SO4 at a
sweep rate of 1 mV s−1
.
As mentioned previously, relatively high electrocatalytic activi-
ties for the ORR have been observed for Pd-based mixtures contain-
ing small amounts of Co.24,25 However, the free energy for the for-
mation for CoO is relatively low ͑−220 kJ mol−1͒. In this study, a
number of metals with much more negative free energies for oxide
formation were chosen to combine with Pd. Mn and V have oxide
SECM screening of electrocatalyst arrays.— Tip generation-
substrate collection SECM screening was performed using a CH
Instruments model 900B SECM ͑Austin, TX͒ as described
previously.15,24 A constant current was applied to the working elec-
trode using a 9 V battery between the working ͑25 ␮m Au disk
sealed in glass͒ and auxiliary ͑Au wire͒ electrodes. In all experi-
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Journal of The Electrochemical Society, 153 ͑6͒ E99-E103 ͑2006͒
E101
Figure 3. SECM TG-SC images of oxygen reduction activity measured on
Ag–V arrays in 0.5 M H2SO4. Tip-substrate distance = 30 ␮m, tip current
= −160 nA, scan rate = 50 ␮m each 0.2 s. ͑A͒ ES = 0.05 V vs NHE. ͑B͒
ES = 0.15 V vs NHE. ͑C͒ ES = 0.25 V vs NHE. WM is the atomic ratio of
metal M in the electrocatalyst spot.
formation free energies well below −350 kJ mol−1 ͑Fig. 1͒ and
readily promote dissociative chemisorption of O2. Figure 4 shows
SECM images recorded for a series of Pd–Mn combinations at dif-
ferent potentials. As ES was adjusted in a positive potential direction
from 0.4 V vs NHE ͑Fig. 4a͒, the activity of pure Pd decreased and
was almost zero at ES = 0.6 V ͑Fig. 3c͒. The mixture containing
30% Mn showed the highest activity at the most positive potential
measured, and the enhancement is comparable to the improvements
observed in Ag upon addition of relatively small amounts of V.
Figure 5 shows SECM images recorded for combinations of
Pd–V over a relatively wide range of ES values. Again, at the most
negative potential ͑ES = 0.3 V, Fig. 5a͒, almost all of the Pd–V
combinations ͑from 0–60% V͒ showed activity for the ORR. How-
ever, at more positive potentials the activity of pure Pd and those
Figure 5. SECM TG-SC images of oxygen reduction activity measured on
Pd–V arrays in 0.5 M H2SO4. Tip-substrate distance = 30 ␮m, tip current
= −160 nA, scan rate = 50 ␮m each 0.2 s. ͑A͒ ES = 0.3 V vs NHE. ͑B͒
ES = 0.5 V vs NHE. ͑C͒ ES = 0.6 V vs NHE. ͑D͒ ES = 0.7 V vs NHE. ͑E͒
ES = 0.8 V vs NHE. WM is the atomic ratio of metal M in the electrocatalyst
spot.
mixtures containing low amounts of V clearly decreased to zero.
However, those Pd–V combinations containing 30–60% V still
showed ORR activity at ES = 0.8 V vs NHE, a decrease in the ORR
overpotential of approximately 400 mV when compared with pure
Pd.
By combining metals such as Pd, Au, and Ag with Co it was
possible to enhance the electrocatalytic activity of the metals ͑Pd,
Au, Ag͒ by 50–100 mV. As shown here, by combining these metals
with metals that have more negative free energies for oxide forma-
tion, one can enhance the electrocatalytic activity of each metal
significantly, up to an almost 400 mV decrease in the overpotential
in the case of Pd–V. This effect is illustrated in Fig. 6, in which the
ORR overpotential at each pure metal is shown relative to that ob-
served for the optimum bimetallic combination. For clarity, only the
overpotential at the best electrocatalytic metal of the combination is
shown, i.e., the ORR overpotentials at Ag and Ag–V are shown, but
pure V is not shown as it is the poorest electrocatalyst of the mix-
ture. In addition, the overpotentials described in Fig. 6 are the ES
values at iS = 20 nA ͑after background current subtraction͒ at each
electrocatalyst spot, subtracted from the thermodynamic cell poten-
tial for the ORR, 1.229 V vs NHE.26
Using the screening method described here, we also found a
number of bimetallic combinations that did not exhibit any syner-
getic effect upon mixing. These combinations include Ag–Mn and
Pd–Cd. Cd has a relatively positive free energy for oxide formation
͑−228.4 kJ mol−1͒ and any synergetic effect may be so small that it
cannot be resolved using the screening technique. Moreover, the
present selection guide relies on alloy formation or nanometer-scale
Figure 4. SECM TG-SC images of oxygen reduction activity measured on
Pd–Mn arrays in 0.5 M H2SO4. Tip-substrate distance = 30 ␮m, tip
current = −160 nA, scan rate = 50 ␮m each 0.2 s. ͑A͒ ES = 0.4 V vs NHE.
͑B͒ ES = 0.5 V vs NHE. ͑C͒ ES = 0.6 V vs NHE. WM is the atomic ratio of
metal M in the electrocatalyst spot.
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Journal of The Electrochemical Society, 153 ͑6͒ E99-E103 ͑2006͒
Figure 7. RDE polarization curves measured by slow potentiodynamic scans
͑1 mV s−1͒ of Vulcan carbon-supported commercial Pt ͑᭿͒, Pd–V 60:40
͑b͒, and Pd ͑᭺͒ in O2-saturated ͑1 atm͒ 0.5 M H2SO4. Rotation rate:
2000 rpm.
Figure 6. Comparison of the electrocatalytic activities of each pure metal
with the corresponding bimetallic electrocatalyst. Colorless bars represent
the ORR overpotential at the pure metal and the gray bars represent those at
the bimetallic electrocatalysts. The pure metals and combinations are ͑1͒ Ag
and Ag:V, 50:50, ͑2͒ Au and Au:V, 80:20, ͑3͒ Pd and Pd:Mn, 70:30, ͑4͒ Pd
and Pd:V, 60:40.
combining Pd with V, the overpotential for the ORR at Pd decreases
substantially. At relatively high currents, a decrease in the ORR
overpotential of up to 400 mV when compared to Pd is observed, in
excellent agreement with the SECM screening observations. Even
more significantly, the electrocatalytic activity of Pd has been im-
proved by addition of V to such an extent that, at moderate currents,
the difference in overpotentials between pure Pt and Pd–V is less
than 100 mV. The observed performance was stable and reproduc-
ible for at least 6 days. This aspect may be linked to the fact that V
and V alloys easily form surface oxides that passivate the metal and
protect it from dissolution. The performance of this material in a
membrane electrode assembly ͑MEA͒ in a small-scale PEMFC will
be evaluated to test the catalyst at high current densities where
ohmic effects may be important. Moreover, modifications of the
preparation technique may yield electrocatalytic materials with de-
creased particle sizes with well-defined single phases. Such devel-
opments may help to improve even further the activity of these
materials.
intimate contact between each metal of the bimetallic mixture.
Therefore, Pd–Cd and Ag–Mn ͑which, based on the guide, might be
expected to produce a large synergetic effect͒ are not in intimate
contact in the electrocatalytic array spots. This may be due to non-
alloy formation in the reduction process. Based on an analysis of the
phase diagrams of these combinations,27 one expects Ag and Mn to
form an alloy, however. In the case of Pd–Cd, the phase diagram is
quite ambiguous and it is possible that a Pd–Cd phase was not
formed using our method. Clearly, the method used to synthesize the
electrocatalytic material can have a very large effect on the observed
enhanced electrocatalytic activity. For example, Pd–V and Pd–Mn
combinations that were prepared only by reduction with H2 at
350°C did not exhibit any synergetic effect. However, as discussed
here, when reduced with NaBH4 and then thermally treated under
H2, Pd–V and Pd–Mn showed much higher activity than the respec-
tive pure metals. These observations suggest that BH−4 reduction of
the salts, which can yield nanoparticles, improves the overall contact
between the two metals and at the nanometer scale rapid interdiffu-
sion of the metallic components is possible. Indeed, ready alloy
formation from nanometer-size particles is an important advantage
of preparation of arrays using these techniques. Issues of particle
size and phase formation in ORR electrocatalysis are currently being
addressed, and a recent contribution describes these issues in the
case of Ti-based electrocatalysts.28
Conclusions
Based on a previously reported selection guide for the design of
improved electrocatalysts for the ORR in acidic media, several ad-
ditional bimetallic systems have been developed and their electro-
catalytic activity probed. Bimetallic arrays were deposited on GC
substrate by reduction of metal-salt precursors and electrocatalytic
activities were determined using a rapid screening SECM technique.
A series of bimetallic combinations that exhibited enhanced electro-
catalytic activity when compared with the pure, constituent metals
was identified including Au–V, Ag–V, Pd–Mn, and Pd–V. In most
cases, the enhancement of electrocatalytic activity was greater than
that previously described for Co-based combinations and a greater
thermodynamic ability of the V and Mn to break the O–O bond of
oxygen to produce the metal oxide may be an important factor. In
particular, Pd–V ͑30 to 60 atom % V͒ exhibited remarkably high
activity for the ORR upon screening, and RDE measurements were
used to verify the high activity when the electrocatalyst was sup-
ported on Vulcan carbon, i.e., a configuration closer to a fuel cell.
Finally, the bimetallic electrocatalyst was compared with Pt and
found to exhibit activity remarkably close to that of the commercial
electrocatalyst.
RDE activity measurements.— The enhancement of the electro-
catalytic activity of Pd upon combining with V, as determined using
the SECM screening method, was remarkable and as can be seen in
Fig. 5, the mixture containing 40% V exhibited electrocatalytic ac-
tivity for the ORR at 0.8 V, close to that typically expected only by
Pt, some Pt alloys, and some of the previously described bimetallic
and trimetallic catalysts. However, the SECM screening method is
used as an initial rapid screening technique to identify those mate-
rials that appear to show good activity for the ORR. The applicabil-
ity of rapid automated synthesis and high-throughput screening tech-
niques for novel electrocatalysts has been previously questioned as
the structure of deposited metallic arrays may not be comparable to
the structure when employed in true fuel cell devices.29 Therefore,
after identification of candidate electrocatalysts that exhibit high ac-
tivity using the screening method, RDE experiments were used to
determine the electrocatalytic activity in configurations that ap-
proach more closely those encountered in true fuel cells. To do this,
i-E curves for 60:40 Pd–V, supported on Vulcan carbon were ob-
tained ͑Fig. 7͒. This figure also shows the response obtained for pure
Pd as well as Pt, the reference ORR electrocatalyst. Clearly, by
Acknowledgments
This work has been supported by the National Science Founda-
tion ͑grant CHE 0451494͒. J.L.F. thanks the Fundación Antorchas
͑Argentina͒ for a postdoctoral fellowship.
University of Texas at Austin assisted in meeting the publication costs of
this article.
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Journal of The Electrochemical Society, 153 ͑6͒ E99-E103 ͑2006͒
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