Materials Sciences and Applications, 2010, 1, 285-291
doi:10.4236/msa.2010.15042 Published Online November 2010 (
Copyright © 2010 SciRes. MSA
PWA Doped SiO2 PEG Hybrid Materials of
Class II
S. Grandi, P. Mustarelli, A. Carollo, C. Tomasi, E. Quartarone, A. Magistris
Department of Physical-Chemistry “M. Rolla”, INFM and I.E.N.I. C.N.R. Department of Pavia, Pavia, Italy.
Received July 6th, 2010; revised August 31st, 2010; accepted September 10th, 2010.
Sol-gel is a promising technique for the synthesis of organic-inorganic hybrid materials of class II. One of the most
interesting applications for these hybrid materials is as solid polymer electrolytes (SPEs). In particular, when doped
with proton species they have potential applications in fuel cells. In this paper SiO2–PEG1500 hybrids of class II were
prepared with different contents of SiO2 and phosphotungstic acid. The influence of the SiO2 content in the matrix has
been studied. The samples were investigated by thermal analysis (TGA and DSC), X-ray diffraction, infrared spectros-
copy (IR), scanning electron microscopy (SEM) and Impedance Spectroscopy.
Keywords: Hybrid Materials, Sol-Gel, Protonic Conduction, Phosphotungistic Acid, Polymer Electrolyte
1. Introduction
This Organic-inorganic hybrids have important applica-
tions in several technological fields, and sol-gel synthesis
allows nanoscale mixing of organic and inorganic parts.
Two major classes of organic-inorganic materials, related
to the kind of molecular bonds/interactions, have been
described [1]: 1) materials of class I where organic mole-
cules, prepolymers or even polymers are embedded in an
inorganic matrix; 2) materials of class II where the inor-
ganic and organic parts are connected by covalent bonds.
Among the various methods used to form covalently
bonded organic-inorganic hybrids [2], the following two
are widely employed. In the first one sol-gel monomers
containing Si-C bonds are directly used as sol-gel pre-
cursors. The Si-C bonds are preserved in the sol-gel hy-
drolysis and polycondensation, and the organic groups
remain attached to the silica skeleton. A second method
consists in functionalising the gel network by organic
reactive groups and, after gelation, in attaching organo-
functional groups by impregnation. By this way, however,
organo-functional groups remain generally bonded to the
porous surface only.
Organic-inorganic systems of class II as hosts for pro-
tons were synthesised in 1999 by Honma et al. [3,4].
Typically, 3-isocyanatopropyltriethoxysilane reacted
with polyethylenoxide (PEO) and then the obtained pre-
cursor condensed following a typical sol-gel procedure.
In some cases other molecules were added in the sol-gel
synthesis, such as monophenyltriethoxysilane or mono-
phenyltrimethoxysilane. Protons were supplied by
monododecylphosphate (MDP) or phosphotungstic acid
(PWA). The authors did not study the influence of dif-
ferent content of silica in the skeleton and found that the
presence of PWA led to a better conductivity of the sam-
ples [5]. Lin and co-workers synthesised a series of hy-
brid membranes adopting a similar route but employing
the 4-dodecylbenzene sulfonic acid [6] or PWA [7] as a
proton carrier. According to various fabrication strategies,
they also investigated the compatibility of these mem-
branes with electrodes for proton conducting fuel cells
[8]. PWA exhibits very large proton conductivity at
around 0.18 S/cm when crystals are hydrated [9]. How-
ever, PWA alone cannot be directly employed in a fuel
cell because of its solubility in water. In order to prepare
a membrane for a PEMFC operating at intermediate
temperature PWA must be dispersed in a polymeric ma-
trix with good thermal stability up to the desired tem-
perature. In fact, relatively high temperatures could solve
problems of CO poisoning, as well as the passivation of
the Pt electrocatalysts. For these reasons, in the last years
some authors focussed their attention on hybrids of
SiO2-PEO (or PPO or PTMO) doped with PWA [10-13]
and obtained thermally stable (up 200°C) membranes
with good conductivity (~10-2 S/cm). It was found that
there is no leaching of the PWA and that the presence of
PWA Doped SiO2 PEG Hybrid Materials of Class II
Copyright © 2010 SciRes. MSA
water, stable up to 150°C, is needed for the proton con-
ductivity of the membrane. Stangar et al. [14] proposed a
hybrid membrane of class II starting from 3-isocyanato-
propyltriethoxysilane and poly(propylene glycol)bis(2-
aminopropyl ether) doped with PWA or silicotungstic
acid (SiWA). Thermal stability of these hybrids did not
exceed 150°C. Both proton donors (PWA and SiWA)
allow these membranes to reach conductivity up to 10-3
S/cm. They later reported [15-16] that the chain length of
poly(propylene glycol) is important for a high and stable
doping of PWA and SiWA. In particular, molecular
weights (MW) in the range 2000-4000 gave good results,
whereas MW = 230 did not lead to good samples from
the point of view of conductivity. In fact, they reported
that the amidonium [C(OH)=NH+] ions play an important
role in the immobilisation of the negatively charged
Keggin ions inside the sol-gel host. In a subsequent work
[16] they reported high proton conductivity (0.2 S/cm)
above 90°C at saturated humidity and good thermal sta-
bility up to 150°C. They did not report anything about
the content of silica and the relationship with and con-
ductivity. Tadanaga et al. [17] presented a hybrid system
starting from 3-glycidoxypropyltrimethoxysilane and
tetramethoxysilane (TMOS) but with orthophosphoric
acid as a proton donor. Employing the same proton donor,
Huang et al. [18] prepared hybrid membranes with 3-gly-
cidoxypropyltrimethoxysilane, 3-aminopropyltriethoxy-
silane and tetraethoxysilane (TEOS). Although these
syntheses were very suitable for varying the silica con-
tent by varying the TMOS or TEOS fraction, they did not
prepare different compositions in order to verify its effect
on conductivity. More recently, Lakshminarayana and
Nogami studied a series of novel fast proton conductive
hybrid membranes prepared by sol-gel, and doped with a
mixture of PWA and phosphomolybdic acid (PMA)
[19,20]. The authors started from 3-glycidoxypropyltri-
methoxysilane (GPTMS), 3-aminopropyltriethoxysilane
(APTES), and TEOS as precursors and obtained ther-
mally stable membranes (up to 200–300°C) with conduc-
tivity values that in some cases could attain 10-2 10-3
At our knowledge, up to now, no specific studies re-
garding the relationship between silica content in the
skeleton and electric conductivity were carried out.
However, some previous works devoted to different hy-
brid membranes based on mercaptopropylmethyldi-
methoxysilane (MPDMS) and TEOS [21-22] showed the
influence of the amount of SiO2 contained in the skeleton
on the electric conductivity. For these reasons the present
work is dealing with the preparation and characterisation
of hybrid materials of class II similar to those discussed
by U.L. Stangar [14,15]. In particular, the attention is
focused on the relationship betweenthe above two pa-
2. Experimental
2.1. Materials
The Si-C precursor (P):
3-isocyanatepropyltriethoxysilane (ABCR) and poly-
(ethylene glycol) bis(3-aminopropyl) terminated (Aldrich,
MW ~ 1550) (PEG1500-NH2 terminated) were mixed in a
molar ratio of 2:1. A 5% excess of isocyanate was added
in order to prevent reactivity with water. Then tetrahy-
drofuran (THF, 1 g for every gram of PEG) was added.
The mixture was stirred for 7 hours at about 70°C. Then
the THF was evaporated and a viscous precursor was
obtained. The reaction reported in scheme 1 below oc-
curred. Completion of the reaction was verified by thin
liquid chromatography in CHCl3: MeOH = 85:15 treated
by KMnO4. RF of P is 0.05, while RF of PEG1500-NH2
terminated is 0.49. The formation of Si-C precursor was
also checked by routine IR spectroscopy (not reported) in
nujol. The reaction was nearly complete.
Group A samples (P: SiO2 = 1:1 mol) (7.5 wt% SiO2):
1 g of the precursor was mixed with 3.5 ml of HCl 0.01N
and with 0.85 ml of ethanol (EtOH), then 0.36 ml of a
mixture of pre-hydrolysed TEOS (1 g TEOS, 2 ml HCl
0.01 M, 0.5 ml EtOH) was added. The sol was mixed
with a solution of phosphotungstic acid (PWAH2O, Al-
drich) in 8.8 ml of HCl, 0.01 M and 7.4 ml of EtOH. The
amounts of the added PWA were the 0, 5, 10, 30, and
60% of the total weight (SiO2 + precursor). The sol was
poured into polyethylene moulds and gelation occurred
in about one day for the sample without PWA and in
about a week for the others. The gels were dried in an
oven at 35°C for 7 days and at 40°C for 7 days, in order
to obtain dry membranes. The samples were named A0,
A5, A10, A30, A60, according to the amount of PWA.
Group B samples (P: SiO2 = 1:2 mol) (10 wt% SiO2):
these membranes were prepared following the above
recipe except the addition of 0.72 ml of hydrolysed
TEOS. Samples containing 0% (sample B0), 30% (B30),
and 60% (B60) of PWA were synthesized. Gelation oc-
curred in 1 day for sample B0 and 7 days for sample B30.
In the case of sample B60, gelation was forced by a
quicker evaporation of the solvent at 30-35°C and it re-
quired 14 days.
EtO (CH2)3N=C=O NH2(CH2)3O(CH2
O)34 O(CH2)3NH2
EtO (CH2)3N
Scheme 1
Group C samples (P: SiO2 = 2:1 mol) (6.2 wt% SiO2):
PWA Doped SiO2 PEG Hybrid Materials of Class II
Copyright © 2010 SciRes. MSA
in this case, only samples with 0% PWA (C0), 30% (C30)
and 60% (C60) were produced. The synthesis is similar
to that of sample A, but for the addition of 0.18 ml of
hydrolysed TEOS. With this addition, the gelation time
was about 7 days for C0 and C30. The gelation of C60
took place after 13 days.
Group D samples (P: SiO2 = 1:4 mol) (13.3 wt% SiO2):
membranes containing 0% (sample D0), 30% (D30) and
60% (D60) of PWA were synthesized. The procedure is
similar to the previous ones, but for the addition of 1.44
ml of hydrolysed TEOS. Gelation of these samples oc-
curred in 2 days for D0 and D30. In the case of D60 the
gelation occurred in 3 days.
Sample E0 (P: SiO2 = 1:5 mol) (17 wt% SiO2): the
procedure is similar to the previous ones, but for the ad-
dition of 1.8 ml of hydrolysed TEOS. Gelation of this
sample occurred in 3 days.
Sample F60 (P: SiO2 = 1:9 mol) (23.8 wt% SiO2): the
procedure is similar to the previous ones, but for the ad-
dition of 3.24 ml of hydrolysed TEOS and the PWA
doping of 60%. Gelation occurred in 2 days.
Leaching test: a fragment of the membrane (about 0.5
g) was dipped in 50 ml of a solution of water and
methanol 1 M for 4 days. The membrane was dried at
room temperature and weighed before and after the test.
The liquid was evaporated and the residue was compared
to the difference of weight and analysed by IR spectros-
2.2. Measurements
Thermogravimetric analyses were performed by a TGA
2950 (TA Instruments) under dry N2 flow, at a heating
rate of 5°C/min, from room temperature up to 550°C.
Differential scanning calorimetry measurements were
carried out by means of a 2910 DSC (TA Instruments)
under N2 purge at a rate of 5°C/min in the same tem-
perature range. IR reflectivity spectra were obtained be-
tween 600 and 4000 cm-1 by using a FT-IR 410 JASCO
spectrometer. The samples were crushed and finely
mixed with KBr (about 5 wt% of sample), and the spec-
tra were obtained by subtraction with a blank one (pure
KBr). The signals were averaged over 512 scans, with a
resolution of 2 cm-1 and a scan velocity of 2 mm/sec.
Spectra were reported as transmittance mode. Room
temperature X-ray diffraction patterns were collected on
all the samples using a Bruker D8-Advance diffractome-
ter, employing Cu(k) radiation. The measurements were
performed in a 2θ range from 3° to 70° with a scan step
of 0.02° and a fixed counting time of 5 s for each step.
Scanning electron microscopy was performed by means
of a variable pressure Scanning Electron Microscope
VEGA TS 5136 (Vega-Tescan) using an accelerating
voltage of 20 kV and achieving magnifications ranging
from 100 to 10000 with a spot size of about 1 m. EDS
maps were taken on an area of 181 × 232 m2.
The proton conductivity was measured by means of
the impedance spectroscopy technique, using a frequency
response analyser (FRA Solartron 1255) over the fre-
quency range 1 Hz – 1 MHz. The membrane was sand-
wiched between two electrodes and the humidity control
was performed under dry nitrogen atmosphere and
checked by a thermo-hygrometric probe (Rotronic). The
impedance scans were performed with the following
protocols: 1) at room temperature ranging from 0 to 100
% RH; 2) by increasing the temperature from room tem-
perature to 80°C at the constant moisture level of 40%
RH; 3) at 80°C ranging from 0 to 50% RH. The films
were allowed to equilibrate 3-4 hours at each moisture
level before of the measurements.
3. Results
All the samples were characterised by TGA and DSC
measurements. Figures 1 and 2 show the TGA and DSC
thermograms of samples A0, B0, C0 and E0, respectively.
The doped samples exhibit similar behaviours, almost
independent on the amount of PWA and they are not
reported. Figure 3 reports the TGA and DSC thermo-
grams of samples B0 and B0I.
The leaching test was very useful in order to under-
stand if at the operative conditions, i.e., in a fuel cell, the
proton species in the membrane are lost since they are
usually water-soluble as in our case. The tests showed
that the weight loss of the membranes after leaching is
between 3-5%wt. The oleaginous residue -R from
evaporation was analysed by IR spectroscopy. The spec-
tra reported in Figure 4 showed that the residue is
PEG1500-NH2. It was lost because it is not polymerised in
the hybrid matrix. On the contrary, PWA was not re-
leased from the membrane and this is a very good result
that confirms that the membrane could be suitable for a
T ( ?C )
0100 200 300 400 500
%Wt A.U.
A0 (P:SiO2 = 1:1)
B0 (P:SiO2 = 1:2)
C0 (P: SiO2 = 2:1)
E0 (P: SiO2 = 1:5)
20% wt
Figure 1. TGA thermograms of samples A0, B0, C0, E0.
The thermograms are shifted for a better comprehension.
T (°C)
PWA Doped SiO2 PEG Hybrid Materials of Class II
Copyright © 2010 SciRes. MSA
T (°C)
0100 200 300 400
Heat Flow (W/g) - A.U.
A0 (P:SiO
= 1:1)
B0 (P:SiO
= 1:2)
C0 (P: SiO
= 2:1)
E0 (P: SiO
= 1:5)
0.3 W/g
Figure 2. DSC thermograms of samples A0, B0, C0, E0. The
thermograms are shifted for a better comprehension.
T (°C)
100 200 300 400 500
Heat flow (W/g)
hybrid of class II
- - - - hybrid of class I
Figure 3. TGA and DSC thermograms of sample B0I of
class I (dashed line) and B0 of class II (continuous line).
%T (arbitrary)
Residue R
Figure 4. IR spectra of the residue R, PEG1500-NH2 and
fuel cell application.
All XRD patterns of the samples, not reported, did not
show any peaks related to crystalline phases and there-
fore all the samples are amorphous. It was interested to
verify that these systems do not crystallise even after one
year, in this sense these materials are very stable. As a
matter of fact PEG based membranes easily undergo
crystallisation in some months with loss of conductivity
as reported in ref. [23].
Selected SEM micrographs were presented: in our
opinion sample B60 shows the most significant features.
Figure 5 shows the SEM image in backscattering mode
while Figures 6 and 7 show the C and Si-W maps by
means of EDS microanalysis. See the description in the
Figure 5. SEM micrographs (backscattering mode) of sam-
ple B60.
Figure 6. SEM-EDS maps (backscattering mode) of C of
sample B60.
Figure 7. SEM-EDS maps (backscattering mode) of Si and
W of sample B60.
T (°C)
T (°C)
PWA Doped SiO2 PEG Hybrid Materials of Class II
Copyright © 2010 SciRes. MSA
discussion paragraph.
4. Discussion
From Figure 1 it is worth noting a good thermal stability
at least up to 300°C, despite of an initial weight loss at
T<130°C that is ascribed to the removal of adsorbed wa-
ter and ethanol, as confirmed by the small endotherms in
Figure 2. The following continuous weight decrease
(130 < T < 300°C) can be chiefly assigned to the release
of water H-bonded to the silica network, and to the evo-
lution of some non-reacted 3-isocyanatepro-pyltriethox-
ysilane (b.p. 238°C) since there is always about a 5% of
excess of it. In particular, the TGA curve of sample B0
shows a greater loss due to not completed condensation
reaction of the precursor, as suggested by the tiny endo-
thermic peak observed at 50°C in Figure 2, which can be
ascribed to the melting of residual PEG1500-NH2 termi-
nated. Between 350 and 420°C the decomposition of the
organic-inorganic skeleton occurs. The one-step weight
losses of Figure 1 correspond to two exothermic features
in the DSC curves. This complex behaviour is reasonably
due to the almost concurrent occurring of several proc-
esses: 1) breaking of the ureic bonds, 2) rearrangement of
the strained PEG chains, and 3) breaking of the polymer
bonds with evolution of gaseous by-pro- ducts. Similar
thermal behaviours were recently reported for hybrids
prepared with PEG200 [24]. The present results, however,
confirm that the thermal stability of these hybrid systems
is rather good compared to the literature data.
It is interesting to investigate the thermal stability of
these class II systems in comparison to those ones of the
corresponding class I. In order to do this, we have com-
pared the sample B0 with a hybrid, called B0I, with the
same composition, in which the organic part is not cova-
lently bonded to the silica matrix. The sample B0I was
synthesised following the same recipe adopted for the
class II samples, except that the precursor P was replaced
by the required amount of PEG1500-NH2 terminated. In
Figure 3 the DSC of sample B0I exhibits an endotherm
at about 50°C that is ascribed to the melting of non-
bonded PEG1500-NH2 terminated, and another one in the
range 350 420°C that is due to the PEG degradation, as
suggested by the TGA loss in the same temperature range.
The thermal behaviour of the sample B0 has been previ-
ously reported. It can be concluded that the thermal sta-
bility of the class I sample is similar or even better than
that of the corresponding of class II. However, it should
be pointed out that the hybrids of class I display very low
mechanical properties, and tend to crack by simple hand
manipulation. Therefore, class II hybrids are preferable
for applications in fuel cells.
XRD diffraction patterns showed that all the samples
do not contain crystalline phases even after one year,
about that it was reported by Stangar et al. [15] that
PWA easily crystallizes following agglomeration. The
absence of crystalline reflections in the XRD patterns,
however, does not allow to directly inferring that PWA is
homogeneously distributed. As a matter of fact, Figure 5
shows the SEM image (backscattering mode) of the sam-
ple B60. Here the white and grey areas represent the in-
organic and organic phases, respectively, and it is clear
that the sample is not homogeneous on a 100 m scale.
Further information on the composition of the inorganic
phase can be obtained from EDS microanalysis. Figures
6 and 7 show the C and Si-W maps of sample B60, re-
spectively. It should be pointed out that signals of silicon
and tungsten fall at the same energy and therefore are
superimposed in the map. Since Figures 6 and 7 look
like one the negative of the other, we can conclude the
carbon phase did not mix very well with the inorganic
part and therefore PWA has more affinity with the sili-
cate-rich phase than with the polyethylene chains. A
similar phase separation was also observed by Honma et al.
[13] even if their materials were characterized by smaller
agglomerates (50 nm). The same authors reported that
condensation of the cross-linking silicate precursors oc-
curred at PWA cluster surfaces, because of a particular
affinity between SiO2 and PWA itself [5]. The existence
of a peculiar affinity between PWA and the Si-based
precursors makes it advisable to vary the Si-W ratio in
order to optimize the transport properties of these hy-
About transport properties, from Figure 8 it is possible
to see that the addition of PWA causes an increase of the
conductivity of about two orders of magnitude from 0 to
60 wt%. Values around ~10-4 S/cm are reached for 60
wt% of PWA. It is interesting to note that the best or-
ganic-inorganic matrix is that of type B, and as expected
the conductivity is nearly linear with the amount of PWA.
A similar behaviour is for the matrix C too whereas ma-
PWA (wt %)
0 10203040506070
80 C , RH=40% (S/cm)
A (P:SiO2 = 1:1)
B (P:SiO2 = 1:2)
C (P:SiO2 = 2:1)
D (P:SiO2 = 1:4)
Figure 8. Conductivity at 80°C and RH = 40% vs. the
amount of PWA.
PWA Doped SiO2 PEG Hybrid Materials of Class II
Copyright © 2010 SciRes. MSA
trices of type A and D do not have such a trend. There-
fore the amount of SiO2 into the matrix, i.e., the ratio P:
SiO2, has a not linear effect on the conductivity at a
given PWA content. This point will be further addressed
in the following. In the Figure 9 is it possible to see that
the conductivity increases of one or two orders of mag-
nitude when the RH changes from 0 to 50%, reaching
values of ~10-4 S/cm at RH = 50% in the case of the sam-
ple B60 (P:SiO2 = 1:2). As already stated the best sam-
ples are B60 and C60 (P:SiO2 = 2:1), while raising the
amount of silica, sample D (P:SiO2 = 1:4) and sample F
(P:SiO2 = 1:9) do not improve the conductivity. An ex-
ception to the general trend of conductivity increase is
given by sample A60 (P:SiO2 = 1:1) that should have an
intermediate behaviour between samples B60 and C60
but on the contrary it is characterized by a very small
slope on the conductivity values vs. RH%.
In order to detect the influence of the silica content on
the conductivity the Figure 10 is pregnant. From the
analysis of this Figure 10, it is not possible to detect a
common trend. The conductivity of the membranes with
0 and 30 wt% of PWA does not seem to be remarkably
affected by the silica content. In contrast, in the case of
0 102030405060
s80 (S cm-1)
Figure 9. Conductivity at 80°C vs. RH% of samples A60-
SiO2 (wt%)
0510 15 20 25 30
s80 (S/cm)
60 %PWA
30 % PWA
0 wt% PWA
Figure 10. Plots of the conductivity behaviour at 80°C and
RH = 40% as a function of the SiO2 content for samples
A0,A30,A60, series B,C, D and samples E0 and F60.
the sample with the highest PWA concentration, a drop
in conductivity is observed above 13 wt% of SiO2. This
can be due to a dilution effect (if considering SiO2 as the
less conductive species) as it happens in Li-based com-
posite membranes [25], or to the formation of more
Si-rich agglomerates, as probed by the SEM analysis
previously discussed.
About the last group of results (Figure 11) it is possi-
ble to verify that the samples display a reasonable Arrhe-
nian behaviour in the explored temperature range, from
which it has been possible to extract the activation ener-
gies, Ea, for the proton conductivity. The activation en-
ergies of the most promising matrices (A,B and C) vs.
PWA content are reported in Figure 12. A non-linear
behaviour of Ea vs. the PWA content is clearly seen,
with a minimum for 30 wt%.
It also seems that the addition of silica determines a
decrease of the activation energies (compare series 1:2
and 2:1) and again, series B seems to be the best candi-
date for a fuel cell application.
5. Conclusions
Thermally stable hybrid membranes of class II were ob-
1/T (103 K-1)
2.6 2.8 3.0 3.2 3.4 3.6
s (S cm-1)
Figure 11. Arrhenius plots of samples A60 and B60 at 40
PWA (wt%)
0 10203040506070
Ea (eV)
0.6 A (P:SiO2 1:1)
B (P:SiO2 1:2)
C (P:SiO2 2:1)
Figure 12. activation energies vs. PWA content for series
A,B and C.
PWA Doped SiO2 PEG Hybrid Materials of Class II
Copyright © 2010 SciRes. MSA
tained by means of a sol-gel route also for high PWA
doping. The membrane proton conductivity depends on
both the humidity and the PWA content. From this study
it seems there is not a marked influence of the amount of
silica in these systems, however it is not advisable to
exceed the 13% wt of SiO2 in the hybrid matrices be-
cause there is a drop in conductivity. The main perspec-
tive at the moment is to set up particular equipments to
perform conductivity measurements at higher tempera-
tures and humidity in order to check whether these mate-
rials are real candidates for PEMFC applications.
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