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Journal of Membrane Science
j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / m e m s c iComposite polymer electrolytes of sulfonated poly-ether-ether-ketone (SPEEK)
with organically functionalized TiO
2
M.L. Di Vona
a,∗, E. Sgreccia
a,e, A. Donnadio
b, M. Casciola
b, J.F. Chailan
c, G. Auer
d, P. Knauth
e,∗∗ aUniversità di Roma Tor Vergata, Dip. Scienze Tecnologie Chimiche, Roma, ItalybUniversità di Perugia, Dip. Chimica, Via Elce di Sotto 8, Perugia, Italy cMAPIEM, Université du Sud Toulon-Var, La Garde, France dCrenox GmbH, Uerdingen, Germany
eUniversité de Provence-CNRS: UMR 6264 Laboratoire Chimie Provence, Centre St. Jérôme, Marseille, France
a r t i c l e i n f o
Article history: Received 4 August 2010 Received in revised form 11 December 2010 Accepted 21 December 2010 Available online 31 December 2010 Keywords:
Sulfonated aromatic polymers Proton conductors
Hybrid materials Fuel cells
a b s t r a c t
Synthesis and properties of proton-conducting composites of SPEEK with organically functionalized TiO2 are described. Composites with hydrophilic titania particles present an inhomogeneous microstructure with agglomeration of TiO2 particles, high strength and low ductility, high water uptake and proton conductivity. Composites with hydrophobic titania particles have a very homogeneous microstructure, very reproducible mechanical properties, lower water uptake and proton conductivity.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Proton exchange membrane fuel cells (PEMFCs) are devices that employ as electrolyte a solid polymer. This energy conversion tech-nology is one of the most promising in the field of electric vehicles and portable applications.
In order to achieve high PEMFC efficiency, the polymeric membrane should satisfy several requirements: high proton con-ductivity (typically 0.1 S/cm under operation conditions), good chemical, thermal and mechanical stability, and low permeability to reactants. Low cost and ready availability are important econom-ical requirements. Furthermore, the membrane should work at an operative temperature around 120◦C for long time[1–3]. Although there is much interest in the development of an “ideal” mem-brane, there is nowadays no material that can completely satisfy the required performances[4].
The current materials research explores different fields: blend of polymers, cross-linked membranes, ionic liquids, block-copolymers, and organic/inorganic hybrids [5–9]. Organic/inorganic hybrids can offer opportunities in many
∗ Corresponding author. Tel.: +39 0672594385; fax: +39 0672594328. ∗∗ Corresponding author.
E-mail addresses:divona@uniroma2.it(M.L. Di Vona),
Philippe.knauth@univ-provence.fr(P. Knauth).
areas and not last in the field of fuel cell materials[10]. Their main characteristic is the capability to combine the properties of the components. Choosing suitable materials, it is thus possible to reach the right features for different applications[11,12].
Several techniques can be used to obtain organic/inorganic hybrids[13–16]. In this work, we have dispersed an inorganic com-ponent in an organic polymer, obtaining a composite that belongs to Class I hybrids, according to the classification by Judeinstein and Sanchez[17]. The organic matrix is sulfonated poly-ether-ether-ketone (SPEEK). PEEK is an inexpensive, fully aromatic polymer characterized by high thermal resistance, mechanical strength and oxidation stability[18]that needs to be sulfonated for achieving the proper proton conductivity[19]. Increasing the degree of sul-fonation, one enhances the conductivity of the polymer, but, the mechanical properties deteriorate due to a large hydrophilicity of the membrane[20].
The inorganic components added to the organic matrix are organically functionalized TiO2 nano-particles. Incorporation of
nanosized binary oxide materials (SiO2, TiO2, ZrO2) in SPEEK
membranes has several attributes of interest, including decreased membrane swelling, reduced permeability towards methanol and improved morphological stability without compromising proton conductivity at high degree of sulfonation[21–23]. Nanostructured TiO2, with a typical dimension less than 100 nm, is used in many
applications ranging from UV shielding to solar cells and photo-catalysts, owing to its peculiar properties. Its chemical stability,
0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.12.044
M.L. Di Vona et al. / Journal of Membrane Science 369 (2011) 536–544 537
even under strongly acidic or basic conditions, and its capabil-ity to modify the hydrophilic/hydrophobic balance in the hybrid systems make the material suitable to be used as filler in poly-meric electrolyte membranes[24,25]. The presence of the inorganic filler is expected to accentuate the phase separation between the hydrophobic and hydrophilic domains, which is a factor controlling the water channeling and proton conductivity in PEMFCs[26,27].
To produce high-performance composites, the realization of a well-stabilized dispersion in the organic matrix is paramount. When ceramic powders are dispersed, it is difficult to avoid spon-taneous agglomeration and undesired clustering, because of the presence of attractive van der Waals forces between oxide parti-cles, which are the primary cause of the inhomogeneous dispersion of nanosized particles[28].
Different procedures can be used to generate Class I hybrid poly-mers; among them the formation of inorganic components in situ in a polymer matrix seems to be very promising. Recently, we reported the formation of SPEEK/TiO2hybrid membranes using a
non hydrolytic sol–gel route. The results were very encouraging with respect to the homogeneity and properties of the composites, but the synthetic procedure was too complicated for applying at the industrial level[29].
It was also very difficult to achieve homogeneous dispersion within the SPEEK matrix when SiO2particles[30], generated in situ
by a hydrolytic sol–gel procedure, were used as filler. The level of adhesion between the inorganic domains and the polymer matrix in the obtained composite membranes was generally very low, worsening the mechanical properties of the composites. Proper control at an atomic level and dispersion of the inorganic com-ponent is instead feasible using organically functionalized oxides [31]. The functionalization can improve the homogeneity of the system, enhancing the compatibility between the components, avoiding agglomeration of oxide nanoparticles and modifying the hydrophilic/hydrophobic character of TiO2surfaces.
The objective of this work is to determine if simple, inexpensive molecules, readily used for industrial purpose can be applied for the functionalization of inorganic fillers in PEM membranes. This would have a significant cost-saving impact. We have chosen two model cases of functionalization: a typical hydrophilic surface modifier is tri(hydroxymethyl)-propane and a typical hydrophobic surface modifier is silicone oil. In that way, we want to verify the influence on the homogeneity of filler particles distribution and on relevant properties sensitive to the presence of water. The used chemicals, especially silicone oil, are well known to be extremely stable (at least kinetically).
These two different TiO2 particles with organically modified
surfaces are used as inorganic filler in SPEEK [32]. The hybrid membranes are characterized and compared with pure SPEEK membranes. The swelling and thermal behaviour, mechanical strength, and electrical properties are discussed.
2. Experimental
2.1. Membrane synthesis
Poly-ether-ether-ketone (PEEK, Victrex, 450P, MW = 38,300), functionalized titanium dioxide (Hydrous titanium dioxide, anatase, 350 m2/g, Crenox GmbH, Germany) and all other chemicals
(Aldrich) were reagent grade and were used as received. Sulfonated PEEK (SPEEK) was prepared by reaction of PEEK with concentrated sulphuric acid at 50◦C for 2 h[33]. The solution was poured, under continuous stirring, into a large excess of ice-cold water. After standing overnight, the white precipitate was filtered and washed several times with cold water to neutral pH. The sulfonated polymer (SPEEK) (Scheme 1) was dried over night at 80–85◦C. The degree
Scheme 1. Formula of SPEEK.
of sulfonation (DS) was evaluated both by1H NMR and by
titra-tion, according to published procedures[34]. Both methods gave according results, indicating a DS = 0.75 (Ion exchange capacity, IEC = 2.2 meq/g).
The TiO2was obtained from hydrolysis of a TiOSO4solution in
a conventional industrial process (sulphate process). The result-ing nano-sized TiO2(“titanium hydrate”; average diameter 5 nm)
with anatase structure was neutralized using ammonia, thoroughly washed with deionized water, and dried subsequently. Residual sulphuric acid in the TiO2was 0.3% (SO4/TiO2).
Two types of functionalized TiO2 (F-TiO2) were used as
filler. One was functionalized with hydrophilic molecules: 10% tri(hydroxymethyl)-propane was added to the powders (called in the following THP–TiO2sample); the other was functionalized with
hydrophobic molecules: 10% silicone oil (Soil–TiO2 sample). The
functionalization of the TiO2surface was achieved by spraying the
organic compounds (50% tri(hydroxymethyl)-propane in water and 50% polymethylhydrosiloxane MH 15, Momentive Performance Materials, Leverkusen, CAS-Number 63148-57-2) onto the titanium hydrate under vigorous mixing and subsequent drying at 110◦C. Final carbon analysis revealed 8.9% of tri(hydroxymethyl)-propane and 9.5% of polymethylhydrosiloxane on TiO2.
SPEEK-based composite membranes containing 5 wt.% of F-TiO2
were prepared by dissolving 250 mg SPEEK in 20 ml of dimethyl sulfoxide (DMSO) and adding 12.5 mg F-TiO2to the solution. The
resulting mixture was stirred for 4 h, evaporated to 5 ml, cast onto a Petri dish and heated to dryness. After cooling to room temperature, the resulting membranes were peeled off and dried under vacuum at 80◦C for 24 h and then further dried in the oven at 140◦C for 64 h to remove the solvent. Pure SPEEK membranes were prepared following the same procedure for sake of reference. The thickness of the resulting membranes was in the range∼60–90 m. 2.2. Membrane characterisation
2.2.1. Structure and microstructure
X-ray diffraction (XRD) patterns were recorded at room temper-ature using a Siemens D5000 diffractometer with CuK␣ radiation ( = 0.1540 nm). Scanning electron microscopy (SEM) images were made with a Philips environmental SEM and Atomic Force Microscopy (AFM) with an Autoprobe apparatus.
Fourier-transform infrared (FTIR) spectra of membrane samples were collected in transmission mode in the range 4000–400 cm−1 (32 scans, 2 cm−1resolution) with a Bruker Equinox 55. The mem-brane thickness was ca. 60m in all cases. A background spectrum was run and sample spectra were normalized against the back-ground spectrum.
2.2.2. Thermal and water uptake properties
High resolution thermogravimetric analysis (TGA Q500, TA Instruments) was performed under air flux in the temperature range between 25◦C and 550◦C with a maximum heating rate of 5 K/min.
Water uptake was measured by full immersion in deionized water. Excess water was removed with absorbing paper and then the mass change of the samples was measured. The experiments
Fig. 1. X-ray diffraction patterns of: (a) SPEEK; (b) composite of SPEEK and tri(hydroxymethyl)-propane (THP)–TiO2(Blue: TiO2anatase, JCPDS file 021-1272); (c) composite
of SPEEK and silicone oil-functionalized (Soil)–TiO2(Blue: TiO2anatase, JCPDS file 021-1272, green: SiO2JCPDS file 045-0112). (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of the article.)
were performed at different water temperatures in the range 25–145◦C. After immersion in water at 145◦C one sample was dried over P2O5and weighted. The final weight was compared with the
initial one: no mass loss was observed, indicating that TiO2did not
leach-out.
Water sorption isotherms were determined after equilibration with water vapor at different temperatures under 0–95% RH. The water sorption isotherms were recorded using a TA5000 thermo-gravimetric analyzer. Prior to all experiments, the membranes were first dried in situ 3 h at 80◦C under 0% RH. RH was then modified in 5 or 10% steps and the water uptake recorded at each step during 2 or 3 h. The reversibility of water uptake was checked by systematic desorption experiments, reducing RH with same steps from 95 to 0%.
2.2.3. Mechanical properties
Stress–strain tests were performed using an ADAMEL Lhomargy DY30 test machine at room temperature at a constant crosshead speed of 5 mm/min with aluminium sample holders as described in Ref.[35].
Dynamic mechanical analysis (DMA) was performed with a DMA 2980 apparatus from TA instruments in tension mode with samples of approx. 15× 7 mm2 size and 90m thickness. DMA
was operated in air at a fixed frequency of 1 Hz with oscillation amplitude of 10m. This last value was chosen to keep the linear viscoelastic response of samples during experiments. The stor-age (E) and loss modulus (E) spectra versus temperature were obtained at 3 K/min between 50 and 250◦C.
2.2.4. Electrical properties
Dielectric analysis (DEA) measurements were performed using a DEA 2970 dielectric analyzer from TA instruments with ceramic parallel plate (CPP) configuration of electrodes. This apparatus allows a frequency scan ranging from 10 Hz to 100 kHz. All experi-ments were performed on 25× 25 mm2membrane samples under
dry argon atmosphere and with heating rate of 2 K/min. Pressure clamping between electrodes was about 50 N cm−2. The measured current was separated into its capacitive and conductive com-ponents. An equivalent capacitance and conductance were then calculated and used to determine the dielectric permittivity εand the dielectric loss factor ε, which is proportional to conductance.
Ionic conductivity was calculated as follows:
= ε0εω (1)
where ε0 is the absolute permittivity of the free space
(ε0= 8.85× 10−12F/m) and ω is the angular frequency of the applied
sinusoidal voltage.
Through-plane conductivity measurements were carried out on membranes, 8 mm in diameter and 90m thick, sandwiched between gas diffusion electrodes (ELAT containing 1 mg cm−2 Pt loading), which were pressed on the membrane faces by means of porous stainless steel discs. The pressure clamping the membrane between the electrodes (60 kg cm−2) was applied before starting the measurements and not controlled during the experiment. The membrane conductivity was determined as a function of temper-ature and relative humidity by impedance spectroscopy with a Solartron Sl 1260 Impedance Analyser in the frequency range 10 Hz to 1 MHz at a signal amplitude≤100 mV. Relative humidity was controlled as described in Ref.[19]. The conductivity of the sam-ples in the transverse direction was calculated from the impedance data, using the relation:
= d
RS (2)
Here d and S are the thickness of the sample and the electrode area, determined before the measurements. The resistance R was derived from the high frequency intercept with the real axis on a complex plane impedance plot.
3. Results and discussion
3.1. Structure and microstructure
X-ray diffraction patterns of SPEEK and composite membranes are reported inFig. 1. The SPEEK reference membrane is fully amor-phous: the broad signal around the reflections of crystalline SPEEK is indicative of the lack of crystallinity. Composite membranes show a clear amount of crystalline anatase TiO2phase within the
majority amorphous polymer, as can be immediately concluded from the reflections in the diffraction pattern. Furthermore, one can observe a small amount of crystalline silicon dioxide when using hydrophobic TiO2, due to partial oxidation of silicone oil
M.L. Di Vona et al. / Journal of Membrane Science 369 (2011) 536–544 539
Fig. 2. Typical SEM micrographs of SPEEK/TiO2composites and corresponding EDX analysis: (a) composite with hydrophilic THP–TiO2; (b) composite with hydrophobic
Soil–TiO2. EDX analysis: (a1) composite with THP–TiO2; (a2) focus on a TiO2agglomerate; (b1) composite with Soil–TiO2. to influence the mechanical and electrical properties of the hybrid
membranes.
SEM images (Fig. 2) show typical microstructures of SPEEK/TiO2
composites. Fig. 2a shows a membrane with hydrophilic TiO2:
the significant agglomeration of titania particles leads to an inho-mogeneous membrane. The corresponding EDX analysis (a2) near agglomerated TiO2particles shows a large amount of S, due to
sul-fonic acid groups, and Ti from the inorganic oxide. One can assume segregation of sulfonic acid head groups near the hydrophilic TiO2
particles, which enhances agglomeration.Fig. 2b shows a mem-brane with hydrophobic TiO2, which is more homogeneous, due
to smaller interaction between TiO2particles. Here, the presence
of Si is evidenced by EDX analysis (b1) near the TiO2
parti-cles, due to the silicone oil coating. With both filler types, the chemical nature of the functionalized surface is clearly revealed and has a strong influence on the microstructure of the mem-branes.
The following AFM images (Fig. 3) show the characteristics of SPEEK/TiO2composites annealed at 140◦C for 64 h and untreated.
All the surfaces are without pores, but the presence of the sec-ond phase makes the surfaces inhomogeneous with a higher mean roughness for the annealed membranes (Rms= 49 nm for annealed
and Rms= 15 nm for non-annealed hydrophilic TiO2 composites).
With hydrophobic TiO2, the surface is less rough, especially after
annealing, due to lower agglomeration (Rms= 20 nm for annealed
Fig. 3. Typical AFM images of SPEEK/THP–TiO2(a) annealed at 140◦C for 64 h and (b) untreated and SPEEK/Soil–TiO2(c) annealed at 140◦C for 64 h and (d) untreated. Fig. 4shows a comparison between typical FTIR spectra of SPEEK
and of SPEEK/THP–TiO2 and SPEEK/Soil–TiO2 composites. In all
spectra, aromatic groups from PEEK backbone and sulfonic acid groups are observed. The hydrophobic composite exhibits some dif-ferences around 2350 cm−1. There is also a clear difference between these spectra around 3500 cm−1, the region of OH absorption. Fur-thermore, the transmittance between 800 and 400 cm−1is lower in composites. The broad peak in the region of 400–600 cm−1is due to the transverse vibration of the Ti–O bonds; it is associated with the longitudinal vibrational mode in the region of 700–950 cm−1.
3.2. Thermal stability
The thermogravimetric analysis (Fig. 5A) shows that SPEEK and SPEEK/F-TiO2composite membranes have a similar decomposition
profile. InFig. 5B is reported a magnification of the three mass loss curves in the range 25–300◦C. At low temperature, two mass losses can be observed for all samples corresponding to about 7% of their initial mass. The first loss (below 100◦C) can be attributed to water molecules sorbed by hydrophilic groups and lost until the dry state of the sample is reached[36,37].
Fig. 4. Comparison of FTIR spectra: SPEEK/hydrophilic THP–TiO2composite (black line) SPEEK/hydrophobic Soil–TiO2composite (red line) and single-phase SPEEK (green
M.L. Di Vona et al. / Journal of Membrane Science 369 (2011) 536–544 541
Fig. 5. (A) TGA (solid line) and DTG (dashed line) curves for (a) SPEEK (black), (b)
SPEEK/THP–TiO2(blue) and (c) SPEEK/Soil–TiO2(red) membranes. (B) Zoom in the
temperature range of 25–300◦C. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of the article.)
A small loss around 190◦C is due to loss of residual solvent DMSO. A major mass loss starts approximately at 210◦C and is attributed to the decomposition of the sulfonic acid groups of SPEEK. The whole weight loss can be evaluated knowing the degree of sulfonation of SPEEK. It is expected to be around 16% andFig. 5A confirms a weight loss close to this value for all membranes. The presence in the membranes of titanium dioxide slightly modifies the temperature range of decomposition of sulfonic groups, which occurs between 205 and 360◦C for the SPEEK membrane, between 215◦C and 325◦C for SPEEK/THP–TiO2 and between 210◦C and
330◦C for SPEEK/Soil–TiO2. If we relate the width of the
tem-perature range where sulfonic acid loss occurs to a distribution of sulfonic acid groups in various environments, it would mean that the incorporation of TiO2 reduces this distribution, possibly
by segregation of sulfonic acid head groups on the oxide surface as evidenced earlier. It is well-known that the affinity of titanium hydrate to sulphate groups is high.
The last mass loss is attributed to PEEK main chain decomposi-tion. For SPEEK membranes, it is observed between 360 and 500◦C, while for SPEEK/THP–TiO2 and SPEEK/Soil–TiO2 it is recorded in
the temperature range 325–465◦C and 330–460◦C, respectively. Although the decomposition profiles are not modified by the addition of TiO2, its interactions with SPEEK modify the
temper-ature range where the main decomposition reaction occurs. The main chain decomposes at lower temperature in composites. The presence of second phase particles destabilizes the polymer back-bone versus decomposition, possibly by reduction of interactions between macromolecules.
Fig. 6. Water uptake coefficients () obtained for SPEEK (1 h of immersion),
SPEEK/hydrophilic (hphi) THP–TiO2and SPEEK/hydrophobic (hpho) Soil–TiO2
com-posites (steady-state).
3.3. Water uptake behaviour
The solubility properties and the water uptake behaviour of the membranes are important parameters to take into consideration for the performance in PEMFCs. In sulfonated aromatic polymers, such as SPEEK, excessive water uptake may lead to swelling and mechanical degradation, but in contrast a too low water uptake does not permit good conductivity. The formation of the composite drastically modified the water absorption characteristics of SPEEK. The values of water uptake coefficient (the number of mol of water absorbed per mol of acid groups) were obtained using the equation: = (mwet− mdry) mdry · 1000 IEC· M(H2O) (3) mwetand mdryare the weight of the samples after and before the
immersion in water and M(H2O) is the molar mass of water.
Fig. 6shows values obtained by full immersion in water at different temperatures for the three membranes (Table 1). We can observe that at 25◦C the values of the water uptake coefficient are not influenced by the presence of F-TiO2in the matrix. The
pres-ence of functionalized titanium dioxide is instead of fundamental importance at higher temperature; it enhances the stability of the membranes reducing their tendency to absorb water. While SPEEK is soluble in water after more than 1 h at a temperature greater then 75◦C, the two composites reach a stable value even at 145◦C. Obviously, the nature of the chemical modification influences the behaviour of the membranes: the water uptake values are higher if the surface additive is hydrophilic tri(hydroxymethyl)-propane rather than hydrophobic silicone oil.
The results of water vapour sorption experiments at 25◦C for composites with hydrophilic TiO2are shown inFig. 7. The water
uptake obtained is consistent with that obtained by immersion in liquid water. A moderate hysteresis is observed between water sorption and desorption isotherms.
Table 1
Water uptake coefficients in deionized water at different temperatures.
T [◦C] SPEEK SPEEK/THP–TiO2 SPEEK/Soil–TiO2
25 4 4 4
105 103a 77 46
125 114a 90 62
145 219a 100 69
Fig. 7. Water sorption isotherm (䊉, sorption; , desorption) at 25◦C for a
SPEEK/THP–TiO2composite annealed at 140◦C for 64 h.
Fig. 8. Stress strain curves of SPEEK/hydrophobic Soil–TiO2 (red line) and
SPEEK/hydrophilic THP–TiO2(black line) and SPEEK annealed at 120◦C for 168 h.
(For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
3.4. Mechanical properties
Typical stress–strain tests for single-phase and composite mem-branes are presented inFig. 8. The static mechanical properties of SPEEK[38]and SPEEK with hydrophobic TiO2are very reproducible,
which is in accordance with a very homogeneous membrane. In contrast, the mechanical properties determined for the membrane with hydrophilic TiO2show a large scatter, probably related to the
inhomogeneous nature of the membrane, as shown in the SEM image. The data reported inTable 2have therefore considerably dif-ferent standard deviations. Provided this limitation, it seems that the composite with hydrophilic TiO2 shows the highest strength
and lowest ductility. Both composite are considerably less duc-tile than pure SPEEK, which is in accordance with the considerable increase of the glass transition temperature.
Fig. 9presents DMA experiments made on annealed and non annealed SPEEK/F-TiO2composites. As expected, the thermal
treat-ment increases both storage modulus in the glassy state and glass transition temperature. This can be explained by two effects:
Table 2
Young’s modulus (E), ultimate strength (), elongation at rupture (ε) and glass tran-sition temperatures (Tg) of SPEEK/TiO2composites and pure SPEEK annealed at
140◦C for 64 h.
Membrane E [MPa] [MPa] ε [%] Tg[◦C]
SPEEK/THP–TiO2 1400± 500 41± 13 7± 4 200± 5
SPEEK[38] 1240± 120 43± 4 29± 13 190± 10
SPEEK/Soil–TiO2 880± 20 27± 2 8± 2 185± 5
Fig. 9. (a) Storage modulus (E) and (b) tan ı of SPEEK/TiO2composites as function
of temperature from DMA experiments.
removal of residual solvent which has a plasticising effect and mor-phological stabilization as reported for PEEK[39].
However, one can note that modulus increase is more pro-nounced for SPEEK/THP–TiO2composite than for SPEEK/Soil–TiO2
composite. These results in agreement with stress–strain tests can be explained by the functionalization type of TiO2. As mentioned
for SEM results, THP–TiO2leads to important agglomeration, but
with strong interactions between titania particle agglomerates and SPEEK, which leads to an important mechanical reinforcement. On the opposite, silicone oil coating of Soil–TiO2 allows good
parti-cle dispersion, but decreases SPEEK/TiO2interactions, which leads
to a poor mechanical reinforcement. The composites made using hydrophobic TiO2have a slightly higher glass transition than the
ones made using hydrophilic TiO2, probably due to the better
dis-persion of TiO2particles.
3.5. Electrical measurements
Fig. 10shows the ionic conductivity of SPEEK/TiO2
compos-ites annealed at 140◦C for 64 h obtained from dielectric analysis. During heating, an increase of proton conductivity was observed in both samples until reaching a maximum around 95◦C. Prob-ably the hydrophilic nature of the second phase allows reaching higher proton conductivity. Above 110◦C, the proton conductiv-ity decreases until a minimum around 170◦C for Soil–TiO2 and
160◦C for THP–TiO2(at 10 kHz). The lower temperature of the
M.L. Di Vona et al. / Journal of Membrane Science 369 (2011) 536–544 543
Fig. 10. Ionic conductivity of (a) SPEEK/Soil–TiO2and (b) SPEEK/THP–TiO2
compos-ites annealed at 140◦C for 64 h.
Tgrevealed by DMA. Above this temperature an increase of ionic
conductivity is observed due to chain-assisted motion.
The proton conductivity of SPEEK/Soil–TiO2 determined at
10 kHz fixed frequency (Fig. 11) is lower than that of pure SPEEK because of the hydrophobic nature of the composite. The relatively low ionic conductivity is due to the low degree of sulfonation of SPEEK used (DS = 0.75) and to the hydrophobic functionalization of TiO2. In contrast, composites with hydrophilic TiO2show a higher
ionic conductivity; the decrease of conductivity is observed also at higher temperature. Here, the positive influence of the hydrophilic functionalization can be clearly seen.
Proton conductivity data at 100◦C obtained by impedance anal-ysis for pure SPEEK and for a composite with hydrophobic TiO2
can be observed inFig. 12as function of relative humidity. These measurements were carried out at increasing RH from 50 to 90%. All conductivity data of the composite, as well as those of pure SPEEK at
Fig. 11. Comparison of ionic conductivity at 10 kHz vs. temperature: SPEEK (—),
SPEEK/THP–TiO2(), SPEEK/Soil–TiO2().
Fig. 12. Proton conductivity at 100◦C as function of relative humidity for a
SPEEK/Soil–TiO2composite () and pure SPEEK () annealed at 140◦C for 64 h. The
arrow indicates the conductivity change occurred within 12 h after the conductivity had reached a maximum.
Fig. 13. Proton conductivity at 90% RH as function of temperature for a
SPEEK/Soil–TiO2composite () and pure SPEEK () annealed at 140◦C for 64 h in
Arrhenius representation. Arrows indicate the conductivity change occurred within 12 h after the conductivity had reached a maximum.
RH≤ 80%, were collected after the conductivity had attained steady state for at least 2 h. In contrast, the conductivity of SPEEK at 90% RH did not reach a steady state: after increasing RH from 80 to 90%, the conductivity passed through a maximum of 0.03 S cm−1 and then decreased slowly reaching about 5× 10−3S cm−1after 12 h. A
similar behaviour was observed for Nafion 117 and attributed to an excessive membrane swelling[4], which is expected for SPEEK too on the basis of the hydration data ofFig. 6. In contrast, a homo-geneous increase of proton conductivity up to 90% RH is observed for the composite, without degradation at high humidity.Fig. 13 shows an Arrhenius plot of proton conductivity for both materials under 90% RH. As previously shown, the degradation of conductiv-ity is observable already at 100◦C for SPEEK, whereas it is seen at 120◦C for the composite. This underlines an important advantage of such composites for higher humidity/higher temperature opera-tion. The activation energy, around 30 kJ/mol, is in good agreement with previously published results[36].
4. Conclusions
We have studied composites of SPEEK with
surface-functionalized TiO2, one with hydrophilic
(tri(hydroxymethyl)-propane) and one with hydrophobic molecules (silicone oil). Composites with hydrophilic TiO2
show large agglomeration of oxide particles and an inhomoge-neous microstructure, but the agglomeration of titania particles together with segregation of sulfonic acid groups gives high proton conductivity. The composites with hydrophobic TiO2 present a
very homogeneous microstructure with well dispersed oxide nanoparticles. However, this composite presents relatively low proton conductivity. It is clear that further fine-tuning of the surface treatment is worthwhile to get an optimal compromise between high conductivity and homogeneous microstructure.
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