Systematic Approach to Design Higher Temperature
Composite PEMs
Tony M. Thampan, Nikhil H. Jalani,* Pyoungho Choi, and Ravindra Datta**
,z
Fuel Cell Center, Department of Chemical Engineering, Worcester Polytechnic Institute,
Worcester, Massachusetts 01609, USA
The design of higher temperature composite proton-exchange membranes PEMs with adequate performance under low relative
humidity RH is discussed here based on experimental and theoretical considerations. The approach is based on enhancing the
acidity and water sorption of a conventional polymer electrolyte membrane by incorporating in it a solid acidic inorganic material.
A systematic investigation of the composite Nafion/inorganic additive PEMs based on characterization of water uptake, ion-
exchange capacity IEC, conductivity, and fuel cell polarization is presented. The effects of particle size, chemical treatment,
additive loading, and alternate processing methodologies are investigated. The most promising candidate investigated thus far is
the nanostructured ZrO
2
/Nafion PEM exhibiting an increase of 10% in IEC, 40% increase in water sorbed, and 5%
enhancement in conductivity vs. unmodified Nafion 112 at 120°C and 40% RH. This appears to be an attractive candidate for
incorporation into a membrane-electrode assembly for improved performance under these hot and dry conditions.
© 2004 The Electrochemical Society. DOI: 10.1149/1.1843771 All rights reserved.
Manuscript submitted February 23, 2004; revised manuscript received July 1, 2004. Available electronically December 23, 2004.
It is fair to say that the commercialization and large-scale de-
ployment of polymer electrolyte membrane PEM fuel cells is cur-
rently hamstrung by the limitations imposed by the available poly-
mer electrolyte membranes. For instance, Nafion, one of the oldest
but still one of the best available PEMs, limits the operating tem-
perature of PEM fuel cells to 80°C on the one hand, thus requiring
pure hydrogen as the fuel and consequently imposing severe con-
straints on reformers, while on the other hand it is still far too
expensive, making fuel cells economically unattractive. Unfortu-
nately, the available alternative PEMs compromise performance and
longetivity. Thus, there is world-wide effort currently underway to
find suitable alternatives to Nafion that might allow higher tempera-
ture operation and cost benefit.
This is, however, a particularly challenging task because of the
desired performance characteristics. Thus, a good polymer electro-
lyte membrane must be thin for low resistance, compliant to make a
good contact with electrodes but rigid enough to provide support to
the membrane electrode assembly MEA, thermally and dimension-
ally stable, impervious to gaseous or liquid fuels as well as elec-
trons, must be durable, and should be able to provide excellent pro-
ton conductivity rivaling liquid electrolytes 0.1 S/cm under hot
and dry conditions.
A good proton conductor evidently requires mobile protons.
Thus, inorganic proton conductors
1-3
without a liquid phase, while
conceptually very attractive, require temperatures in excess of
800°C to provide adequate conductivities via a proton hopping
mechanism owing to the high activation energy. At lower tempera-
tures, a liquid-phase for proton conduction is essential, either as a
molten or a solvated acid. When a solvent other than water is used,
the challenge of complete immobilization of the liquid must be first
addressed to ensure stable performance over extended periods.
When water is the solvent, the challenge is to retain water within the
membrane under hot and dry conditions owing to its high volatility.
An alternate approach, first proposed by Malhotra and Datta,
4
is
to incorporate inorganic acidic materials within the conventional
polymer electrolytes such as Nafion, in order to improve water re-
tention while simultaneously increasing the number of available acid
sites. This approach shows promise for developing PEMs that func-
tion adequately at temperatures above 120°C under low relative hu-
midity RH conditions,
4
and has consequently become a very active
area of research. This paper is concerned with a systematic investi-
gation of the issues related to the design and development of such
composite membranes.
Literature Review
A brief literature review of the available ingredients polymer
electrolyte and inorganic additives for designing composite PEMs
is provided below. The available polymer electrolyte membranes
may be subdivided into two categories: i proton-exchange mem-
branes PEMs, e.g., Nafion, in which the acid anion is covalently
attached to the polymer backbone so that only the proton is mobile,
requiring a solvent such as water, and ( ii ) polymer-acid complexes
PACs, e.g., PBI/H
3
PO
4
, in which the acid is simply complexed
with a basic membrane so that both the proton and the anion are
mobile, i.e., the transference number of protons is less than unity.
While a solvent such as water is not essential for conduction in
PACs, it aids by further ionizing the acid, but unfortunately can also
cause leaching of the acid from the membrane, a serious limitation
for long-term stability.
Proton-exchange membranes (PEMs).—Figure 1a shows a sche-
matic of the major components of a proton-exchange membrane,
namely the polymer backbone, chemical cross-links, side chains,
and the pendant acid group. The right combination of these elements
confers the desirable properties listed above. The backbone poly-
mers are: i fluorinated and ( ii ) hydrocarbon polymers. The com-
mon acid groups covalently bound are either: i sulfonic acid
(-SO
3
H), ( ii ) carboxylic acid -COOH,( iii ) phosphonic acid
(-PO
3
H
2
), and ( i v ) sulfonyl imide (-SO
3
NHSO
2
CF
3
). The back-
bone along with any cross-links confers appropriate thermomechani-
cal properties, inertness, and extent of swelling, while the number
equivalent weight, EW) and strength (pK ) of acid groups confers
the electrolyte properties.
The perfluorinated PEMs are the most commercially advanced
membranes owing primarily to their chemical inertness.
5-8
Thus,
Nafion has demonstrated fuel cell lifetimes of over 60,000 h at
80°C,
9
although higher temperature lifetime studies have not yet
been reported. The polytetrafluoroethylene PTFE backbone en-
hances the chemical and mechanical properties of the PEM albeit at
the cost of limited water sorption due to its hydrophobicity. Other
perfluorinated membranes include the Dow membrane which has a
shorter side chain than Nafion but otherwise has similar structural
and morphological properties.
10
Both Aciplex-S and Flemion, avail-
able from Asahi Chemical and Asahi Glass Company, respectively,
have long side chain perfluorosulfonated membranes with perfor-
mance similar to Nafion. Perfluorinated PEMs have been developed
by modification of the acid group.
11-13
Thus, DesMarteau
12,14
re-
placed the sulfonic acid group (-SO
3
H) in Nafion with a sulfonyl
imide group (-SO
2
NHSO
2
CF
3
), which results in an increase in the
water uptake while Kotov et al.
13
developed membranes with a
phosphonic acid group that has the potential for higher thermal sta-
* Electrochemical Society Student Member.
** Electrochemical Society Active Member.
z
E-mail: rdatta@wpi.edu
Journal of The Electrochemical Society, 152 2 A316-A325 2005
0013-4651/2004/1522/A316/10/$7.00 © The Electrochemical Society, Inc.
A316