Journal of Membrane Science 256 (2005) 122–133
Polymer blends based on sulfonated poly(ether ketone ketone)
and poly(ether sulfone) as proton exchange
membranes for fuel cells
Steven Swier a,∗ , V. Ramani b , J.M. Fenton b , H.R. Kunz b ,
M.T. Shaw a,b , R.A. Weiss a,b
a
b
Institute of Materials Science, University of Connecticut, Storrs, CT 06269, USA
Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269, USA
Received 17 December 2004; received in revised form 30 January 2005; accepted 6 February 2005
Available online 13 April 2005
Abstract
The importance of the blend microstructure and its effect on conductivity and structural integrity of proton exchange membranes (PEM)
were investigated. Sulfonated poly(ether ketone ketone) (SPEKK) was selected as the proton-conducting component in a blend with either
poly(ether sulfone) (PES) or SPEKK with a different sulfonation level. The second component was added to improve the mechanical stability
in the fuel cell environment. Membranes were cast from solution using N-methyl-2-pyrrolidone (NMP) and dimethylacetamide (DMAc).
Special attention was paid to the ternary solution behavior.
Solution cast SPEKK/PES membranes are homogeneous for all studied compositions, 8/2 through 5/5 (w/w), and sulfonation levels,
1.7–3.5 mequiv./g. Although this polymer pair does not show evidence for intrinsic compatibility, the excellent solvent quality results in a
frozen-in structure during solution casting. The morphology of SPEKK/SPEKK blends can be tailor-made by finding the right balance between
composition, casting solvent and temperature. Co-continuous morphologies can be devised for an SPEKK blend with sulfonation levels of
1.2 and 2 mequiv./g.
Both blends show lower swelling than the parent SPEKK. This results in better stability of PEMs during fuel cell testing.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Poly(ether ketone ketone); Poly(ether sulfone); Polymer blend; Proton-exchange membrane
1. Introduction
A proton exchange membrane (PEM) serves as a solid
electrolyte in a fuel cell, separating the anode (hydrogen or
methanol) from the cathode (oxygen/air) compartments. Proton conduction occurs by a combination of both ion-hopping
and convection mechanisms, and for sulfonate ionomer membranes the presence of water is required [1,2]. Polymers used
for PEMs must exhibit high proton conductivities and a good
resistance to the high-temperature, humid and oxidative environment of a fuel cell.
∗
Corresponding author.
E-mail address: [email protected] (S. Swier).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.memsci.2005.02.013
Perfluorinated polymer electrolyte membranes such as
NafionTM have been the principal choice of PEM in the past.
Such ionomers consist of a hydrophobic backbone with sulfonic acid groups placed at the ends of short perfluoro-ether
side chains [3]. The acid groups aggregate to form nano-phase
separated hydrophilic domains. When swollen with water,
a percolation pathway is formed with water-filled channels
connecting the ionic domains, which results in the high proton conductivity [3,4]. The hydrophobic phase formed from
the perfluorinated backbone provides mechanical integrity
and limits the swelling of the PEM. However, perfluorinated
polymers are expensive, have relatively poor resistance to
methanol transport, which is important in direct methanol fuel
cells (DMFC) and have relatively poor mechanical properties
when highly swollen by water.
S. Swier et al. / Journal of Membrane Science 256 (2005) 122–133
Contemporary PEM research involves the development
of new polymer electrolytes based on hydrocarbon polymers
[5]. Typically, aromatic polymers are functionalized with proton conducting groups, e.g. by sulfonating the phenylene
groups in the backbone. One polymer that has been considered a candidate PEM is sulfonated poly(ether ether ketone)
(SPEEK), which has a reasonably good proton conductivity
[6–8] and has shown promising performance data for both
hydrogen/oxygen [9] and direct methanol fuel cells [10]. An
advantage of the sulfonated hydrocarbon polymers versus
the perfluorinated ionomers is the ability to achieve higher
ion-exchange capacities, IEC (i.e. concentration of sulfonate
groups in equivalents per mass), but since the perfluorosulfonic acid is a stronger acid, for comparable IEC and hydration [4,11], the perfluorinated ionomers have higher proton
conductivities [12].
Polymer blends wherein a sulfonated polymer with high
proton conductivity is combined with a non-conductive engineering thermoplastic chosen to maintain mechanical integrity has become a popular contemporary approach to the
design of improved PEM materials. A variety of different
polymer pairs have been considered, in particular and of relevance to the current paper, SPEEK in combination with
poly(ether imide) (PEI) [10,13–18], nylon 6 (PA6) [16],
poly(ether sulfone) (PES) [4,19,20] and poly(2,6-dimethyl
phenylene oxide) (PPO) [10]. The improvement in mechanical stability can be attributed to the entanglement of these
polymers and to possible (partial) mixing due to specific interactions, e.g., ion–dipole, dipole–dipole and proton transfer [10,13,16,21–24]. Such interactions also are reported to
reduce significantly methanol cross-over in DMFCs [21].
Intermolecular interactions may also weaken the sulfonateion pair, thus reducing the amount of water needed to promote proton transport. Other poly(aryl ether ketone)s like
poly(ether ketone) (PEK) and poly(ether ether ketone ketone) (PEEKK) and poly(ether ketone ketone) (PEKK) have
also been investigated in combination with PBI, PEI, [22]
poly(phenylene oxide) (PPO) and PES [4,10].
Although key fuel cell performance properties such
as proton conductivity, swelling, reactant crossover and
voltage–current performance have been studied for most
polymer-blend PEMs, the role and control of the blend microstructure has been mostly overlooked. When PEI was
used as the minority component in SPEEK/PEI blends, TEM
showed a dispersed droplet morphology with PEI particles
dispersed in the SPEEK matrix [14]. Miscible blends were
found for polymers exhibiting strong acid–base interactions,
while weaker bases showed microphase separation [23,24].
The added engineering thermoplastic was usually present as
the minority component.
The overriding hypothesis of the work described in this
paper is that deliberate control of the membrane microstructure is required to optimize proton conductivity and membrane integrity. The extra degree of freedom afforded by using a polymer blend allows for the separation of the transport and mechanical properties, which is not possible in ho-
123
mopolymer ionomer membranes. This paper discusses the
control of the blend microstructure by using different parent
polymers and mixing ratios and varying the solution casting procedure. Knowledge of the solution behavior of the
constituent polymers is essential. In case of phase separation in a metastable region of the phase diagram, nucleation
and growth is most probable, resulting in dispersed droplet
morphologies, while a co-continuous microstructure is expected for a quench within the unstable, spinodal region. The
droplet morphology might be advantageous when the protonconducting phase is the matrix, assuring percolation, and reinforcement of the ionomer by the engineering thermoplastic
can improve the mechanical properties of the membrane. A
co-continuous, interconnected microstructure is another potentially interesting morphology, where both the conductive
polymer and the engineering thermoplastic constitute the matrix. In this way, percolation of the former polymer assures
high proton conductivities, while the continuous phase of the
engineering thermoplastic should also provide more efficient
mechanical reinforcement. Strong interpolymer interactions
and/or vitrification from the miscible region of the phase diagram may produce homogeneous microstructures where the
ionic microphases are dispersed randomly and entanglements
provide membrane integrity. In this case, percolation of the
acidic groups has to be ensured by carefully selecting the
content of the non-conductive polymer.
The work described herein considers blends of sulfonated poly(ether ketone ketone) (SPEKK) ionomers with
poly(ether sulfone) (PES). This blend can be compared to
the SPEEK/PES system [4,19,20,25]. The thermal properties,
water uptake and performance of SPEKK as a PEM material,
have been described previously [26]. One of the reasons to
select poly(aryl ether ketone)s with higher K/E ratios is the
concomitant increase in oxidative stability [27] and the lower
degree of swelling upon hydration of the sulfonated polymer
[28]. Finally, a promising, novel approach to the design of a
blend PEM will be discussed, in which two different SPEKKs
are blended, one with high and one with low sulfonation.
2. Experimental
2.1. Materials
PEKK with a T/I ratio of 6/4 (OXPEKK SPb, Tg = 154 ◦ C,
Tm = 300 ◦ C) was obtained from Oxford Performance Materials, Enfield, CT. PES was obtained from BASF (Ultrason®
E6020P, Tg = 225 ◦ C, Mw = 51 kg mol−1 , Mw /Mn = 3.5) and
was used as received. Sulfonation of PEKK was performed by
using a 5% (w/v) mixture of 53/47 (v/v) concentrated sulfuric
acid to fuming sulfuric acid as detailed previously [29]. The
sulfonated polymer, SPEKK, was precipitated by drop-wise
addition of the solution into six volumes of rapidly stirred
de-ionized ice water. The SPEKK was filtered, washed repeatedly with de-ionized water to remove excess acid and
dried at 60 ◦ C overnight and then under vacuum at 120 ◦ C
for 3 days. The nomenclature used for the ionomer in this pa-
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S. Swier et al. / Journal of Membrane Science 256 (2005) 122–133
per is SPEKKx, where x is the sulfonation level expressed as
ion-exchange capacity (IEC), defined as the concentration of
sulfonate groups in equivalents per total mass as determined
by titration [26]. The IEC and the degree of sulfonation XS ,
defined as the average number of sulfonate groups per repeat
unit, are related as follows:
XS
IEC (meq/g) = 1000 ×
300 + 81XS
For the sulfonation of PEKK with fuming sulfuric acid, sulfonation can occur on either of the aromatic rings that have
one ether and one ketone substituent; the aromatic ring attached to two ketone groups is not susceptible to an electrophilic substitution reaction [28]. Therefore, the maximum
degree of sulfonation of PEKK is two sulfonate groups per
(EKK)-repeat unit. That corresponds to a maximum ionexchange capacity (IEC) of 4.4 mequiv./g.
Membranes based on blends of SPEKK/PES and
SPEKK/SPEKK were prepared by casting 5% (w/v) solutions in N-methyl-2-pyrrolidone (NMP, b.p. = 202 ◦ C)
or dimethylacetamide (DMAc, b.p. = 165 ◦ C) onto glass
plates. The solution volume and the area of the glass plate
were fixed to reproduce the evaporation rate and the final
membrane thickness. Typically, the solvent was allowed to
evaporate for about 1 day at 60 ◦ C, after which the films were
washed at 25 ◦ C with de-ionized water to leach out residual
solvent. The residual solvent present after this procedure as
obtained from thermogravimetric analysis was below 3 wt.%
for both casting solvents. The effect of casting temperature
was also investigated by using evaporation temperatures
ranging from 40 to 120 ◦ C.
Membrane-electrode assemblies (MEA) were prepared
for selected systems. The catalysts used were 46.5 wt.% Pt/C
on the cathode and 30.1% Pt–23.4% Ru/C on the anode. Both
anode and cathode catalysts were provided by Tanaka Kikinzoku Kogyo K. K., Japan. The catalyst powders were dispersed by adding methanol and Nafion® 1100 (5% solution)
equivalent to a 25 wt.% ionomer loading. The catalyst was
applied to a 5 cm2 area of membrane using an airbrush to
yield the MEA. Successive layers of ink were applied to the
membrane, with a 5–10 min interval between each application to ensure solvent evaporation, which was facilitated by
an infrared heating lamp placed directly behind the membrane. The electrode noble metal loading was in the range
0.3–0.45 mg/cm2 .
2.2. Materials characterization
Blend morphology was studied with a Philips 300 transmission electron microscope (TEM) operating at 80kV and a
Topometrix’ Explorer atomic force microscope (AFM). The
sample preparation for both techniques consisted of embedding membranes in an EponTM -Araldite® embedding mixture followed by ultra-microtomy with a diamond knife to
obtain thin sections. Sections were placed on copper grids
for TEM analysis or pre-cleaned glass cover slips for AFM.
The TEM specimens were stained with a 1 M Pb(NO3 )2 aque-
ous solution, which preferentially darkens the SPEKK phase
in the TEM images. For SPEKK/SPEKK blends, the higher
IEC phase was more noticeably darkened, which provided
the sample contrast. For AFM, drying of the sections on glass
resulted in shrinkage of the swollen ionomer phase (microtomed sections were initially floated onto water), and the
higher IEC SPEKK phase, which shrank the most, corresponded to areas that were lower in topography.
The thermal properties of membranes were measured using a TA Instruments Q100 differential scanning calorimeter
(DSC). The glass transition of SPEKK as a function of water content was obtained by exposing DSC pans containing
the SPEKK membranes to controlled relative humidity environments. Equilibrium was attained after 12 h as confirmed
from transient water uptake experiments. Sample pans were
crimped quickly after about 48 h exposure. To measure the
glass transition temperatures in the presence of water beyond
100 ◦ C, high pressure stainless steel pans were used.
Proton conductivity measurements of hydrated membranes were made at room temperature along the membrane
direction over a frequency range of 10 mHz to 106 Hz and an
ac voltage amplitude of 50 mV using a frequency response
analyzer (Solartron SI 1260, impedance/gain-phase analyzer)
combined with a potentiostat (Solartron SI 1287, electrochemical interface). Membranes were equilibrated in a 98%
relative humidity, R.H., environment using a conductivity cell
based on previous work [5].
Turbidity measurements on ternary solutions were obtained from visual observations during evaporation of the
solvent at 60 ◦ C. Solutions with increasing polymer concentration were allowed to equilibrate at 25 ◦ C for about 1 h.
When a cloudy mixture was obtained, a small amount of
solvent was added to clear the solution and obtain a better
estimate of the cloud point.
Subsequent to anode and cathode catalyst application,
MEAs were placed between two thin Teflon® sheets, which
were then placed between two rubber sheets. The entire assembly was introduced between two stainless steel plates and
compression molded in a Carver hot press at 120 ◦ C and
207 kPa. The MEAs were assembled in a 5 cm2 hardware
with single serpentine flowfields (Electrochem Inc.; model
no. FC05–01SP). Commercial gas-diffusion layers (GDLs)
(SGL Carbon; model no. 10BB) were used. The resistance
of the membrane was obtained using the current-interrupt
technique built into the cell testing system. The conductivity
was estimated from the measured value of resistance using
recorded values of the active area (5 cm2 ) and thickness of
the membrane. The performance of the cell was evaluated by
obtaining polarization curves at 80, 100 and 120 ◦ C. For the
experiments at 80 ◦ C both anode and cathode gases were saturated at 80 ◦ C and 73 ◦ C for 100% and 75% R.H. operation,
respectively. Fuel cell tests at 100 and 120 ◦ C were performed
at 70% R.H. and 35% R.H., respectively. H2 was used at the
anode and air or O2 at the cathode. All data were obtained at a
pressure of 101 kPa (1 atm). Further details of the system used
and testing conditions adopted are provided elsewhere [30].
S. Swier et al. / Journal of Membrane Science 256 (2005) 122–133
Fig. 1. Water content for SPEKK as a function of sulfonation level: hydration
number λ (䊉), wt.% absorption () at 25 ◦ C and 98% R.H. The filled square
() is the hydration number measured for NafionTM 112.
3. Results and discussion
125
Fig. 2. Glass transition temperature of SPEKK1.9 as a function of hydration
number λ. The membranes were equilibrated in different humidity environments at 25 ◦ C (䊉); after immersing in water at 50 ◦ C (). The dashed line
denotes λ from which melting of water crystals were detected, and the solid
line is the Tg prediction from the Fox mixing rule with Tg,H2 O = −139 ◦ C
[33].
3.1. The role of water in SPEKK
Promising proton conductivities were previously reported
for SPEKK with relatively high sulfonation levels; e.g.,
0.06 S/cm for SPEKK2.0 at 25 ◦ C and 98% R.H. [26]. That
value is comparable to that reported for NafionTM 112, which
has an IEC of only 0.9 mequiv./g. The higher intrinsic acidity
of the perfluorosulfonic acid sites, combined with a larger hydrophobic/hydrophilic separation, has been stated to result in
the higher intrinsic conductivity of perfluorinated ionomers
[4,11].
As indicated earlier, water is needed to assure high proton
mobilities in ionomers. Fig. 1 shows that although the water
uptake of SPEKK increases with sulfonation level, the number of water molecules per sulfonic acid site or the hydration
number λ is almost constant at 10 for 25 ◦ C and 98% R.H.,
which is also comparable to the value for NafionTM [1]. Of
the 10 water molecules associated with each sulfonic acid
group, three are strongly bound to the acid sites and the other
seven are loosely bound or free water within the microphaseseparated hydrophilic domains [5,31].
The glass transition temperature (Tg ) of the hydrated membranes was very sensitive to the amount of plasticizing water
that was retained, Fig. 2. A dramatic drop in Tg of SPEKK1.9
from 200 to 85 ◦ C occurred when about three water molecules
per sulfonic acid group were introduced into the ionomer.
That amount corresponds to the solvation of the proton and
the sulfonate ion, and λ = 3 also corresponds to the percolation threshold and the insulator-to-conduction transition [3].
Additional water is not strongly bound to the sulfonic acid
group, as is evidenced by the presence of a melting peak
around 0 ◦ C in DSC (not shown). Melting peaks occur for
values of λ to the right of the dashed line in Fig. 2. In that
region, water swells the polymer, but does not significantly
affect the Tg . This can be attributed to phase separation of a
water-rich phase, which results in a deviation of the polymer
Tg from the Fox mixing rule [32].
The strong effect of water on the Tg as the membranes
were swollen is further revealed by the effect of sulfonation
level. Fig. 3 shows that increasing the sulfonation level of dry
SPEKK membranes led to a small increase in Tg , probably
due to hydrogen bonding interactions. That increase, however, is completely offset by the presence of water. For example, for the water-swollen membranes, Tg decreased by 90 ◦ C
per mequiv./g, which corresponded to ∼7 ◦ C/wt.% H2 O.
The drop in Tg of SPEKK below 100 ◦ C in the presence
of water is important in view of the temperature window for
fuel cell operation, ∼80–120 ◦ C. During operation, the dry
SPEKK membrane undergoes a change from a glassy to an
elastic, gel-like material. Fig. 4 shows the equilibrated water
content for SPEKK membranes with varying IEC as a function of temperature. For SPEKK0.8 and SPEKK1.2, the water
content did not increase significantly with temperature, resulting in a behavior similar to that of NafionTM 112. The low
Fig. 3. Glass transition temperature of SPEKK as a function of IEC:
membranes vacuum dried at 120 ◦ C (, slope: 14 ± 2 ◦ C/(mequiv./g),
P < 0.0001); membranes immersed in water at 25 ◦ C for 48 h (䊉, slope:
−90 ± 14 ◦ C/(mequiv./g), P = 0.003); P is the probability of being wrong by
rejecting the hypothesis of slope = 0, i.e., no trend.
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S. Swier et al. / Journal of Membrane Science 256 (2005) 122–133
Fig. 4. Hydration number, λ, as a function of the water temperature for
SPEKK (T/I 6/4); IEC = 2.1 (), 1.7 (), 1.4 (), 1.2 (), 0.8 (䊉),
NafionTM 112 (). The equilibration time at each temperature was 48 h.
amount of hydrophilic, water-containing sites produced only
a moderate Tg depression. Moreover, the sulfonation level
of SPEKK0.8 was sufficiently low that the polymer retained
some residual crystallinity [26]. However, excessive swelling
of the membranes occurred for SPEKK1.4 above 60 ◦ C and at
even lower temperatures for the higher IEC membranes. Similar water absorption trends were also reported for sulfonated
poly(ether ether ketone ketone) (SPEEKK) membranes [4].
Unfortunately, the high sulfonation levels of SPEKK
needed to attain competitive proton conductivities resulted
in poor membrane integrity and premature failure in actual fuel cell operation. SPEKK2.0, for example, attained
a cell voltage of 0.5 V at a current loading of 1 A/cm2 for
about 4 h, after which fracture occurred around the gasket
edge. These results provided the incentive to alternative membrane designs, namely composite membranes of SPEKK with
a non-conductive engineering thermoplastics, in this case,
poly(ether sulfone), PES.
3.2. SPEKK/PES composite membranes with a
homogeneous morphology
Homogeneous SPEKK/PES blends were obtained by solution casting from DMAc, as indicated by the TEM micrograph in Fig. 5a for an 85/15 (w/w) blend. All blend compositions used, 80/20 through 50/50, and SPEKK sulfonation
levels, 1.7–3.5 mequiv./g, resulted in similar homogeneous
morphologies for solution-cast membranes. The single-phase
morphology could arise from thermodynamic miscibility of
the two polymers or from the solution casting procedure,
which could produce a metastable morphology for the blend
by freezing-in a single-phase state of the ternary solution during solvent evaporation.
Turbidity measurements were performed on SPEKK/
PES/DMAc solutions, and sulfonation levels from 1.7 to
2.5 mequiv./g, with increasing total polymer concentration.
All mixtures studied in this way were transparent, including
the fully dried SPEKK/PES binary film. Similarly, no cloud
Fig. 5. TEM micrograph (cross-sectional view) of (a) 85/15 (w/w)
SPEKK1.7/PES blend and (b) 85/15 (w/w) SPEKK1.7/PEI blend cast at
60 ◦ C from a 5% (w/v) DMAc solution.
point was detected for SPEKK/DMAc and PES/DMAc binary solutions. This is in contrast to the SPEKK/PEI system,
where the limited solubility of PEI in DMAc typically produced phase separation of the polymers at a total polymer
concentration of 15% (w/v) and resulted in heterogeneous
morphologies, see Fig. 5b [22].
Infrared spectroscopy was performed on cast SPEKK/PES
membranes in an attempt to identify a chemical interaction
responsible for the miscibility. The focus was on the specific S–O absorption of the aromatic sulfonic acid group at
926 cm−1 . No shift in that absorption from that in the neat
SPEKK was observed, which is consistent with the absence
of specific hydrogen bonding interactions between the two
polymers. However, an electron donor-acceptor complex or
a similarity in structure cannot be excluded as a possible
explanation for the miscibility. The electron donor-acceptor
complex has been proposed as the origin of miscibility in a
similar blend, SPEEK/PES [17].
A different approach to determine SPEKK/PES miscibility is to bring thin SPEKK and PES films in contact, fol-
S. Swier et al. / Journal of Membrane Science 256 (2005) 122–133
127
Fig. 7. Polarization curves at 80 ◦ C and 75% R.H. for SPEKK1.7 (, failure occurred after 10 h; conductivity σ from current interrupt = 0.02 S/cm,
thickness: 24 ␮m) and a 85/15 (w/w) SPEKK1.7/PES blend (♦, no failure
before 50 h; σ = 0.01 S/cm, thickness: 38 ␮m).
Fig. 6. Proton conductivity (σ at 25 ◦ C, 98% R.H., 䊉) and hydration number
(λ at 90 ◦ C, immersed in water, ♦) for SPEKK1.7/PES membranes as a
function of (A) wt.% PES and (B) for 5/5 (w/w) blends as a function of
SPEKK IEC.
lowed by annealing in a temperature range where miscibility
is expected. The annealing temperature has to be selected
above the glass transition of at least one of the components
(>200 ◦ C) and lower than the desulfonation temperature of
SPEKK (<240 ◦ C). TEM micrographs showed sharp interfaces on a scale of 500 nm in cross-sectional views, indicating
poor interfacial mixing.
Since SPEKK and PES do not seem to be a miscible pair,
the excellent solvent quality of DMAc for SPEKK and PES
seems to be responsible for the homogeneous morphologies
found for solution cast blends. The temperature/solvent history results in a quench from solution, interfering with the
formation of an equilibrium, phase-separated blend. This
is further confirmed by substituting PES for the similar
polysulfone (PSU), resulting in macroscopically phase separated SPEKK/PSU blends. Ternary SPEKK/PSU/DMAc solutions were indeed cloudy at relatively low polymer loadings
(<20%).
Homogeneous SPEKK/PES blends offer significant advantages in regard to membrane swelling, as illustrated for a
range of sulfonation levels and blend compositions in Fig. 6.
The addition of 15 wt.% PES to SPEKK1.7 decreased the
water concentration at 90 ◦ C to below 20 H2 O/SO3 H, even
though the neat SPEKK1.7 exhibited excessive swelling
(>80 H2 O/SO3 H). A similar amount of PEI added to
SPEKK1.7 still resulted in a very high water content,
∼60 H2 O/SO3 H, which was a consequence of poor interaction between the SPEKK and PEI [22] Water-soluble
SPEKKs (IEC > 2.4 mequiv./g) were rendered insoluble by
the addition of 50% PES. While the addition of PES to the
SPEKK lowers the inherent conductivity of the blend, that
can be compensated by increasing the IEC of the SPEKK,
especially if it is no longer water extractable from the membrane. Thus, the design of the SPEKK/PES membranes offers
several degrees of freedom, including the SPEKK IEC and
blend composition that could provide a membrane with high
conductivity, yet reasonably low water swelling and good
mechanical properties.
The fuel cell performance of SPEKK1.7 is compared to
an 85/15 SPEKK1.7/PES blend in Fig. 7. Although a lower
performance was found for the latter PEM, the blend was
stable in the fuel cell environment over 50 h in comparison
to 10 h for the parent SPEKK. Note that our current MEA
set-up does not allow for long-term stability tests.
An alternative to a single-phase structure for the blend is to
develop a well-defined heterogeneous, phase-separated morphology. This may have advantages in light of the increased
interfacial region between the two phases that might concentrate the ionic groups due to the specific interaction between
the two polymers. As indicated before, SPEKK and PES have
limited compatibility. Thermally induced phase separation by
annealing the cast blend PEM’s above their glass transition
temperature (200–240 ◦ C) was attempted, but this was not
successful largely due to the restriction of the temperature to
below the desulfonation temperature of SPEKK, ∼240 ◦ C.
Preliminary results, however, indicated that phase separated
morphologies could be obtained by neutralizing the SPEKK
to its Zn2+ or Mn2+ -salt prior to solution casting. The additional parameters of a metal counter-ion and the extent of
neutralization provide even more flexibility with regard to
the design and control of morphology of these blend PEMs.
Once the desired morphology is achieved using the metal salt
ionomers, one can re-acidify the ionomer by exposure to a
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S. Swier et al. / Journal of Membrane Science 256 (2005) 122–133
strong acid and retain the morphology in the sulfonic acid
form of the blend.
3.3. Co-continuous morphology of SPEKK/SPEKK
blend PEMs
As noted in the discussion of Fig. 4, low IEC SPEKK produces fairly robust membranes, due primarily to relatively
low swelling by water. That ionomer may, therefore be a viable blend component for improving the performance of a
high IEC SPEKK. Although the proton conductivity of the
low IEC ionomer is limited [26], it still has an advantage
over a non-conductive component in that it will contribute to
the conductivity of the system. Several intriguing morphologies were obtained by solution casting high IEC/low IEC,
SPEKK/SPEKK membranes, as illustrated by the micrographs in Fig. 8. For both TEM and AFM the darker regions
in Fig. 8 represent the higher IEC phase (see Section 2). The
morphologies found with both techniques corresponded very
well. When the sulfonation levels of both SPEKK’s were similar, e.g., the SPEKK2.0/SPEKK1.5 blend, a homogeneous
morphology was formed by solution casting from NMP.
When the low IEC component was reduced to SPEKK1.2,
a dispersed-particle morphology was achieved, and for an
SPEKK2.0/SPEKK0.8 blend, the dispersed phase became
larger.
DSC thermograms of the blends and the pure components
are compared in Fig. 9. Following solution casting, all membranes were converted to the sodium salt to increase the Tg
difference between the constituents. This is based on the finding that the Tg of the acid derivative of SPEKK increased
about 14 ◦ C/(mequiv./g), while the stronger intermolecular
ionic interactions present in the salt form result in an increase
of 90 ◦ C/(mequiv./g) [26]. The neutralization procedure did
Fig. 8. TEM and AFM micrographs of 5/5 (w/w) blends of (a) SPEKK2.0/SPEKK1.5, (b) SPEKK2.0/SPEKK1.2 and (c) SPEKK2.0/SPEKK0.8 cast from
NMP at 60 ◦ C; plotting the size of the minority phase particles as a function of IEC ratio results in a positive slope of 5 ␮m with P = 0.07.
S. Swier et al. / Journal of Membrane Science 256 (2005) 122–133
129
Fig. 9. DSC thermograms (scan rate = 10 ◦ C/min) of SPEKKs and
50/50 (w/w) SPEKKx/SPEKKy (x/y) blends cast from NMP at 60 ◦ C.
not affect the blend morphology as confirmed from TEM (not
shown). The thermal behavior of the blends corroborates the
morphologies shown in Fig. 8. The high IEC combination,
SPEKK2.0/SPEKK1.5, exhibited a single Tg , indicating a
single-phase morphology, while the other two blends showed
two distinct Tg ’s, consistent with the phase-separation of the
two.
A combination of factors determines the occurrence of
phase separation in SPEKK/SPEKK blends. It is well known
that differences in monomer content [34] and sequence
distribution [35] can induce phase separation in copolymer/copolymer blends (copolymer effect). In addition, the
effect of the casting solvent has to be understood. One factor that might be crucial in this respect is that the solubility
of SPEKK decreased with decreasing sulfonation level, so
that mixing the two components became increasingly more
difficult as the difference in IEC increased. For example,
SPEKK0.8 was only partially soluble in NMP and DMAc,
while SPEKK with IEC > 1.4 mequiv./g was soluble in those
solvents over the entire concentration range [26]. For SPEKK
with 0.8 < IEC < 1.4 mequiv./g, SPEKK was initially soluble,
but exhibited solvent-induced crystallization after about 20 h,
as evidenced from the resulting opaque solutions and from
X-ray analysis on the dried membranes. This can also account for the SPEKK/SPEKK phase separation when the difference in sulfonation level is increased. Note that extensive
crystallization from solution embrittles the resulting PEMs.
The SPEKK/SPEKK membranes studied here were therefore
quickly mixed and cast from transparent solutions.
Further morphology control can be obtained by changing
the casting conditions, e.g., temperature and solvent. The effect of changing to a lower boiling casting solvent (DMAc)
while keeping the casting temperature at 60 ◦ C is shown by
the AFM micrographs in Fig. 10 for SPEKK2.0/SPEKK1.2
blends with compositions from 4/6 to 6/4 (w/w). Note that
the darker phase corresponds to the high-IEC component. The
corresponding TEM images were similar. The difference in
morphology between the 5/5 (w/w) SPEKK2.0/SPEKK1.2
Fig. 10. AFM micrographs of SPEKK2.0/SPEKK1.2 blends: (a) 6/4, (b) 5/5
and (c) 4/6 (w/w) blends cast from DMAc at 60 ◦ C.
blend cast from NMP (Fig. 8) and DMAc (Fig. 10) is
quite striking; the latter membrane appears to possess a
co-continuous morphology. The faster evaporation rate of
DMAc probably results in a fast quench in the unstable
130
S. Swier et al. / Journal of Membrane Science 256 (2005) 122–133
Fig. 11. Proton conductivity (σ at 25 ◦ C, 98% R.H.) and hydration number
(λ at 90 ◦ C, immersed) for SPEKK2.0/SPEKK1.2 blends as a function of
blend composition. The membranes were cast from DMAc at 60 ◦ C.
region of the phase diagram associated with spinodal-type
behavior. In addition, SPEKK1.2/DMAc solutions show a
stronger tendency to solvent-induced crystallization than
SPEKK1.2/NMP solutions. Phase inversion occurred close to
the 4/6 SPEKK2.0/SPEKK1.2 composition, and disperseddroplet morphologies were observed for larger composition
deviations from 5/5 blends.
Fig. 11 shows the proton conductivities (25 ◦ C for
98% R.H.) and water absorption at 90 ◦ C for SPEKK2.0/
SPEKK1.2 blends cast from DMAc as a function of
blend composition. The conductivities exceeded 0.01 S/cm
for blends containing up to 60 wt.% SPEKK1.2. The
co-continuous morphology found at a 4/6 SPEKK2.0/
SPEKK1.2 composition (Fig. 10) assured a continuous path
for the conductive SPEKK2.0 component in the blend. On the
other hand, having a continuous SPEKK1.2 phase explains
the striking decrease in water content accompanying the transition to a co-continuous morphology around a 5/5 composition. For higher SPEKK1.2 concentrations, the low IEC
component became the only continuous phase and the conductivity dropped accordingly. A PEM with optimum properties will, therefore, most likely be found around the 4/6
SPEKK2.0/SPEKK1.2 composition.
Morphological changes can also be achieved by changing the casting temperature, as illustrated for the 4/6 (w/w)
SPEKK2.1/SPEKK1.15 blend in Fig. 12. Co-continuous
morphologies were attained for the lower casting temperatures. Note that a slightly larger difference in sulfonation
level was used in comparison to the blends in Fig. 10. The
proton conductivity did not change appreciably with casting temperatures from 40 to 80 ◦ C, see Table 1. The highest
casting temperature used, 120 ◦ C, produced a grossly phaseseparated blend and a membrane with a rough surface. The
incompatibility of the two polymers became more distinct
as the casting temperature increased, due presumably to a
higher rate of interdiffusion in the initial stages of solvent
evaporation. Ultimately, a decreasing solvent content in the
membrane will increase its Tg above the casting temperature,
resulting in a frozen-in microstructure.
Fig. 12. TEM micrographs of 4/6 (w/w) SPEKK2.1/SPEKK1.15 blends cast
from DMAc using different casting temperatures.
A measure of the merit of the different membranes can
be obtained by comparing the proton conductivities and
water contents of pure SPEKK with those of SPEKKbased blends as a function of the average IEC (Fig. 13).
The proton conductivities of the blend membranes are
slightly higher than those for pure SPEKK. The most significant improvement was found in the membrane integrity
S. Swier et al. / Journal of Membrane Science 256 (2005) 122–133
131
Table 1
Proton conductivity (␴ at 25 ◦ C, 98% R.H.) and hydration number (␭ at
90 ◦ C, immersed) for 4/6 (w/w) SPEKK2.1/SPEKK1.15 blends cast from
DMAc using different casting temperatures (x indicates excessively swollen
membranes)
Tcasting (◦ C)
σ (S/cm)
λ at 90 ◦ C
40
50
60
80
120
0.019
0.020
0.023
0.020
0.012
69
80
x
x
x
as evidenced by much lower water contents at 90 ◦ C for
SPEKK-based blends. An SPEKK/PES blend membrane
with an average IEC of 1.75 mequiv./g, for example, takes
up less than 20 H2 O/SO3 H, while pure SPEKK membranes
fail at this IEC. Plotting proton conductivity (25 ◦ C) as a
function of water content (90 ◦ C) in Fig. 14 further confirms the advantage of using blend membranes. Although
these properties were not obtained at the same temperature, the proton conductivity follows an Arrhenius-type dependency with similar activation energies for different sulfonation levels [36]. This was confirmed by measurement
of E ∼ 20 kJ/mol for SPEKK1.0 and SPEKK2.0. Fig. 14
again shows that SPEKK/SPEKK and SPEKK/PES blends
exhibit higher proton conductivities with less water content
than do neat SPEKKs, and the two blend PEMs also exhib-
Fig. 13. Proton conductivity (σ at 25 ◦ C, 98% R.H.) and water content (λ
at 90 ◦ C, immersed) as a function of the average IEC for different SPEKKbased membranes; neat SPEKK (䊉), SPEKK/SPEKK (), SPEKK/PES
().
Fig. 14. Proton conductivity (σ at 25 ◦ C, 98% R.H.) as a function of water
content (λ at 90 ◦ C, immersed) for different SPEKK-based membranes; neat
SPEKK (IEC range: 0.8–1.4 mequiv./g) (䊉), SPEKK/SPEKK (IEC range
0.8–2.1 mequiv./g) (), SPEKK/PES (IEC range 1.7–3.5 mequiv./g) ().
ited better mechanical stability and durability in a fuel cell
environment.
The MEA performance data at 80 ◦ C of some selected
SPEKK/SPEKK membranes with different conditions of relative humidity and reactant gases are compared with that of a
neat SPEKK1.4 membrane in Fig. 15. SPEKK1.4 constitutes
the highest sulfonation level for which a PEM was stable for
up to 2 days in a fuel cell at 80 ◦ C and 75% R.H. All blendbased PEM’s performed better than pure SPEKK, due in large
part to the higher proton conductivity that was obtained for
a similar water content (Figs. 13 and 14) and the higher average IEC that could be used for the blend membranes (1.45
and 1.53 mequiv./g for the 32/68 and 4/6 membranes, respectively).
Figs. 15 and 16 show the MEA performance curves
for SPEKK2.1/SPEKK1.15 composite membranes at
Fig. 15. Polarization curves of selected membranes measured at 80 ◦ C:
(䊉) SPEKK1.4 (failure occurred after 36 h, σ = 0.003 S/cm, thickness: 30 ␮m, 75%R.H. H2 /O2 ); () SPEKK2.1/SPEKK1.15 (32/68 w/w)
cast at 60 ◦ C (σ = 0.013 S/cm, thickness: 50 ␮m, 75%R.H. H2 /O2 );
SPEKK2.1/SPEKK1.15 (4/6 w/w) cast at 50 ◦ C (thickness: 51 ␮m): ()
75%R.H. H2 /Air (σ = 0.015 S/cm), () 100%R.H. H2 /Air (σ = 0.046 S/cm),
() 100%R.H. H2 /O2 (σ = 0.038 S/cm).
132
S. Swier et al. / Journal of Membrane Science 256 (2005) 122–133
Fig. 16. Polarization curves (H2 /Air) for an SPEKK2.1/SPEKK1.15
(4/6 w/w) cast at 40 ◦ C from DMAc: (䊉) 80 ◦ C and 100% R.H.
(σ = 0.036 S/cm, thickness: 38 ␮m); () 100 ◦ C and 70% R.H.
(σ = 0.005 S/cm, thickness: 38 ␮m); () 120 ◦ C and 35% R.H. (thickness:
38 ␮m).
80–120 ◦ C. The performance suffers significantly at the
higher temperatures, as a result of the low humidity conditions. As indicated before, this can be attributed to the crucial
role of water for promoting high proton mobility [4,11]. The
literature suggests several ways to rectify this problem, including using acid–base intermolecular interactions between
polymer pairs to induce ionic crosslinking, improve membrane integrity, and to weaken the proton-sulfonate interaction, making proton conductivity less dependent on water
content [10,13,16,21,23–25]. The best improvements in high
temperature, low humidity performance involved complexation of basic polymers using oxo-acids like phosphoric acid
[4,37–40]. Introducing a basic polymer as the second component in SPEKK-based blends, possibly doped with phosphoric acid, is a logical next step in the research described in
this paper.
SPEKK and PES are intrinsically incompatible as indicated by the absence of specific hydrogen bonding interactions and the lack of intermixing between stacked
SPEKK/PES films. On the other hand, obtaining heterogeneous morphologies by high-temperature annealing of cast
films is hampered by the onset of desulfonation of SPEKK.
Preliminary experiments show that neutralizing SPEKK prior
to casting does allow heterogeneous morphology development. In this way, the drop in proton conductivity corresponding to adding the non-conductive PES might be
limited.
A novel strategy was devised based on blends of SPEKK
with different sulfonation levels. Controlled morphologies
can be attained by blending a high-IEC SPEKK, having
high proton conductivity, with a low-IEC SPEKK, exhibiting good resistance to swelling. The best compromise of
these two properties can be obtained in case of a cocontinuous microstructure, characterized by interconnection
between the two SPEKKs. A relatively high IEC for both
components results in homogeneous solution cast membranes. Heterogeneous structures are formed for a larger
difference in IEC. This can probably be attributed mostly
to the decrease in solubility of the low IEC SPEKK in
the casting solvent. Morphology control can be obtained
not only by the IEC difference and composition of the
ionomers, but also by changing the casting temperature and
solvent.
Polarization data on SPEKK/SPEKK and SPEKK/PES
blends at 80 ◦ C and high relative humidities indicate an important improvement in performance and structural stability
over the pure SPEKK membranes. However, since SPEKK
relies strongly on the presence of water to ensure high proton
mobilities, high temperature/low humidity fuel cell operation is difficult to achieve. Employing a basic polymer as the
second component in combination with acid doping will be
attempted to reach this technical goal. Having a tool to control the morphology of these blends will be very useful in this
respect.
4. Conclusions
Although competitive proton conductivities are found for
SPEKK with a high sulfonation level, the presence of water
causes a drop in Tg concomitant with excessive membrane
swelling. This jeopardizes the long-term stability of SPEKKbased PEMs in the fuel cell environment. The added flexibility of polymer blends can resolve this issue to some extent.
As a first strategy, a miscible blend of SPEKK with the
non-conductive engineering thermoplastic PES can be obtained by solution casting. Both components are soluble in
DMAc over the entire concentration range, eventually leading to a frozen-in homogeneous microstructure after solvent
evaporation. Intimate entanglement of the polymer pair results in excellent PEM integrity in the fuel cell environment.
Highly sulfonated, water soluble SPEKK can be turned into
a robust membrane by adding PES, further evidence for the
strong interpenetration.
Acknowledgement
The authors thank the Department of Energy (Grant
69797-001-033D) and Connecticut Innovations, Inc (Yankee
Ingenuity Grant 01Y09) for support of this research, Oxford
Performance Materials for supplying the PEKK samples and
BASF for supplying PES.
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