A Sol-Gel-Based Approach to Synthesize High-Surface-Area
Pt-Ru Catalysts as Anodes for DMFCs
Jin Yong Kim,
a,c
Z. G. Yang,
a
C.-C. Chang,
a,
* T. I. Valdez,
b,
* S. R. Narayanan,
b,
*
and P. N. Kumta
a,
*
,z
a
Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania
15213, USA
b
Jet Propulsion Laboratory, Pasadena, California 91109-8099, USA
A novel sol-gel based chemical process was developed to synthesize Pt-Ru catalysts possessing high specific surface area and good
catalytic activity, for direct methanal fuel cells DMFCs. In this process, tetramethylammonium hydroxide was used to hydrolyze
platinum II acetylacetonate and ruthenium III acetylacetonate to form a homogeneous gel. Phase-pure powders possessing a
high specific surface area as high as 136.9 m
2
/g were produced using controlled oxidizing atmospheres such as 1% O
2
balanced
with N
2
. The resultant powder consisted of nanocrystalline Pt-Ru particles less than 10 nm and also revealed an excellent
catalytic activity in standard three-electrode half-cell tests cell potential of 0.295 V at currents of lnmA/g of 8, demonstrating
the potential of the sol-gel-based processes for synthesizing high-performance catalysts for DMFCs. The key factor in obtaining
phase-pure powders with high specific surface area is the controlled removal of carbon species in the as-prepared precursors. The
rapid kinetics of carbon removal in air caused the growth of particles as well as the formation of Ru oxide, thus leading to a low
specific surface area powder, combined with the segregation of Ru oxide from the homogeneous Pt-Ru precursors. A limited
oxidizing ability of the heat-treatment atmospheres used such as 0.1% O
2
also yielded a low specific surface area powder due to
the insufficient removal of carbon.
© 2003 The Electrochemical Society. DOI: 10.1149/1.1612503 All rights reserved.
Manuscript submitted December 9, 2002; revised manuscript received May 9, 2003. Available electronically September 15, 2003.
Direct methanol fuel cells DMFCs have received considerable
attention as power sources for several defense and automotive ap-
plications due to their lower weight and volume compared with
indirect FCs.
1-5
These advantages in weight and volume are due to
the elimination of additional fuel reformation processes, resulting in
simple design and operation, higher reliability, and reduced costs of
operation and maintenance. Therefore, the capacity of providing
continuous power with the use of a low cost fuel such as methanol
makes it an ideal candidate as a low cost alternative power source
for use in automotive applications in comparison to rechargeable
batteries. One of the key issues arising in the development of high-
performance DMFCs is the improved catalytic activity of the anode.
There have been significant efforts to synthesize anode catalysts
possessing excellent catalytic activity for oxidizing methanol. To
this extent, Pt-based alloys have been developed and mostly inves-
tigated as the anode catalysts.
6-9
It is well known that the catalytic
activity of platinum for methanol electro-oxidation can be increased
by the addition of second elements such as Sn, Ru, Re, and Os.
9
This is because the addition of these second elements enhances
methanol oxidation due to the formation of metal-oxygen bonds,
which is one of the important parameters and is the main rate-
determining step in the oxidation of methanol. Among Pt-based bi-
nary bifunctional catalysts, Pt-Ru is known for its most promising
catalytic activity.
3
There are several approaches reported for synthesizing of Pt-Ru
catalysts. Most of these approaches involve the use of halide salts of
Pt and Ru, which require a follow-up washing treatment to remove
the undesired halide residues.
10-15
As a result, these processes are
cumbersome and can cause significant loss of the starting noble
metal precursors. Identification of an alternative process utilizing a
nonhalogen-containing precursor could therefore be beneficial. This
was the primary motivation for the present study.
In the present work, we report a new sol-gel-based chemical
process to synthesize Pt-Ru catalysts possessing a high specific sur-
face area. This is one of the important factors known to govern the
catalytic activity of this alloy. Precursors derived using the sol-gel
based approach were heat-treated under various conditions. Heat-
treated powders as well as the as-prepared precursors were charac-
terized using X-ray diffraction XRD. The Brunauer-Emmett-Teller
BET technique was used for measuring the specific surface area,
while thermogravimetric and differential thermal analyses TG and
DTA were utilized for analyzing the thermal characteristics of the
precursors and powders. Scanning electron microscopy SEM and
transmission electron microscopy TEM were also used to deter-
mine the microstructure and particle size of the resultant catalyst
powders. Finally, electrochemical tests were also conducted to as-
certain the half-cell and full-cell response of the synthesized Pt-Ru
catalysts.
Experimental
Synthesis and heat-treatment.—Phase-pure and high surface area
Pt-Ru catalyst powders for DMFCs were synthesized using a sol-
gel-based process. Specifically, platinumII acetylacetonate Pt-
acac, PtC
5
H
7
O
2
)
2
, 97%, Aldrich and rutheniumIII acetylaceto-
nate Ru-acac, RuC
5
H
7
O
2
)
3
, 97%, Aldrich were used as the
sources for Pt and Ru. Figure 1 shows a schematic flow chart of the
procedure followed for this process. First, 0.00338 moles of Pt-acac
and the corresponding amount of Ru-acac (Pt:Ru = 1:1, 1:1.25,
and 1:1.5 were dissolved at 50°C in 100 mL of acetone. Tetram-
ethylammonium hydroxide TMAH, (CH
3
)
4
NOH, 25% in metha-
nol, Alfa was then added to the solution to act as a high molecular
weight hydrolyzing agent. Because Pt-acac and Ru-acac phase-
separated during drying and evaporation of the solvent, the addition
of a complex organic agent such as tetramethylammonium hydrox-
ide was found to be beneficial to yield homogeneous amorphous
gels. After stirring for 10 min, the solvent was evaporated until the
solution became viscous, transforming into a gel. The viscous gel
was then transferred to a petri dish and dried in air at 170°C for 10
h. After the gel was completely dried, it was crushed using a mortar
and pestle to yield the as-prepared powders. The as-prepared precur-
sors were then heat-treated under various conditions based on the
results of thermogravimetric and differential thermal analysis TG/
DTA. Table I lists the process parameters and the nomenclature
followed, along with the heat-treatment schedule used for the differ-
ent samples. Accordingly, I–IV indicate the precursors prepared us-
ing various ratios of Pt to Ru and the addition of different amounts
of tetramethylammonium hydroxide. The numbers to the right of
these letters correspond to the atmosphere used for the heat-
treatment. The number 0.1 and 1 correspond to the heat-treatment
* Electrochemical Society Active Member.
c
Present address: AMTEK Research International, LLC, Lebanon, OR.
z
E-mail: kumta@cmu.edu
Journal of The Electrochemical Society, 150 11 A1421-A1431 2003
0013-4651/2003/15011/A1421/11/$7.00 © The Electrochemical Society, Inc.
A1421