Hybrid Nafion–silica membranes doped with

Solid State Ionics 145 Ž2001. 101–107
www.elsevier.comrlocaterssi
Hybrid Nafion–silica membranes doped with heteropolyacids for
application in direct methanol fuel cells
P. Staiti, A.S. Arico,
` V. Baglio, F. Lufrano, E. Passalacqua, V. Antonucci )
Institute CNR for Transformation and Storage of Energy, Via Salita S. Lucia sopra Contesse 39 98126, St. Lucia, Messina, Italy
Received 25 September 2000; received in revised form 12 January 2001; accepted 17 January 2001
Abstract
Nafion–silica composite membranes doped with phosphotungstic and silicotungstic acids have been investigated for
application in direct methanol fuel cells at high temperature Ž1458C.. The phosphotungstic acid-based membrane showed
better electrochemical characteristics at high current densities with respect to both silicotungstic acid-modified membrane
and silica–Nafion membrane. A maximum power density of 400 mW cmy2 was obtained at 1458C in the presence of
oxygen feed, whereas the maximum power density in the presence of air feed was approaching 250 mW cmy2 . The results
indicate that the addition of inorganic hygroscopic materials to recast Nafion extends the operating range of a direct
methanol fuel cell. Operation at high temperatures significantly enhances the kinetics of methanol oxidation. q 2001
Elsevier Science B.V. All rights reserved.
Keywords: Phosphotungstic acid; Silicotungstic acid; Composite Nafion membranes; Direct methanol fuel cells
1. Introduction
Heteropolyacids ŽHPAs. have been largely employed as catalysts in heterogeneous reactions due to
their unique structural and chemical properties. They
are ionic solids based on high molecular weight
anions having the general formula wX x M nO y x 3y with
x F n. These materials are proton conductors with
high intrinsic conductivity. In recent years, significant efforts have been addressed to evaluate such
compounds as both electrolyte and surface promoters
for low temperature fuel cells w1–4x. Phosphotungstic
acid ŽPWA. was successfully employed as an elec-
)
Corresponding author. Tel.: q39-090-624-234; fax: q39-090624-247.
E-mail address: [email protected] ŽV. Antonucci..
trolyte in H 2 –O 2 fuel cells w1x, as well as a surface
promoter for CO-electro-oxidation w2x, whereas,
silicotungstic acid has shown interesting properties
as a promoter in the electrochemical oxidation of
methanol w3x.
One of the main drawbacks of DMFCs relies on
the slow methanol oxidation kinetics. For this reason, an increase in the operation temperature of the
DMFC from 90 to about 1508C is highly desirable.
In this regard, we have recently developed a composite Nafion–silica membrane capable of water retention at high temperatures. This avoids the loss of
ionic conductivity up to 1458C w5,6x. In the present
study, in order to further increase the level of water
retention at high temperatures, two heteropolyacids
ŽPWA and SiWA. were impregnated onto the silica
particles, and the resulting materials were loaded in
the recast Nafion. Similar materials formed by Nafion
0167-2738r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 2 7 3 8 Ž 0 1 . 0 0 9 1 9 - 5
P. Staiti et al.r Solid State Ionics 145 (2001) 101–107
102
and silicotungstic acid with or without thiophene
were recently prepared by Tazi and Savadogo w7,8x.
They reported some interesting characteristics of the
materials, such as conductivity, water adsorption,
and performance in H 2 –O 2 fuel cells, but they do
not report the behavior of their membranes at temperatures above 1008C. In the present paper, the
heteropolyacid-modified membranes have been tested
in a direct methanol fuel cell in a range of temperatures from 90 to 1458C, and the results have been
compared with that obtained by a fuel cell using a
bare silica–Nafion recast membrane.
2. Experimental
2.1. Preparation of the membranes
Three different membranes were prepared having
the following compositions: Ž1. Nafion–silica; Ž2.
Nafion–silica–phosphotungstic acid; Ž3. Nafion–
silica–silicotungstic acid, they are indicated in the
text as NAS, NA30P and NA45S, respectively. The
membranes were prepared by using a proprietary
procedure w5x. With respect to the previous work w5x,
Aerosil 200 silica from Degussa was substituted in
the present work with Cabosil silica from Cabot.
Both silica compounds have an average particle size
of 7 nm and an amorphous structure. An appropriate
amount of Nafion ionomer Ž5% wtrwt, Aldrich. was
mixed with SiO 2 ŽCab–O–Sil, EH-5, Cabot. or
PWArSiO2 or SiWArSiO2 powders in an ultrasonic bath for 20 min. This solution was cast in a
Petri dish and heated at 808C for 20 min. The recast
composite Nafion films were detached from the Petri
dish by addition of distilled water and allowed to dry
for 15 h at room temperature. Afterwards, they were
cut to obtain a regular shape and inserted between
two polytetrafluoroethylene ŽPTFE. foils and hotpressed at low pressure and increasing temperatures.
The final treatment was at 1608C for 10 min. The
final membranes contained 3% wtrwt of silica w5x.
Heteropolyacid-modified silica powders were composed of 30% wtrwt phosphotungstic acid ŽPWA.
on the silica and 45% silicotungstic acid ŽSiWA. on
silica w9x. The amount of heteropolyacid loaded on
silica was limited by the uptake characteristics of
silica. The selected amounts correspond to uniform
distribution of the heteropolyacid on the silica surface. X-ray diffraction patterns of the powders consisting of 30% PWA and 45% SiWA revealed an
overlapping between the typical crystalline structure
of heteropolyacid and the amorphous structures of
silica ŽFig. 1.. A chemical interaction between the
heteropolyacids and the silica was detected by IR
analysis and reported in a previous paper w9x. The
thickness of membranes was about 80 mm. The
membranes were purified by using the usual procedure w5x.
Fig. 1. Cu K a X-ray diffraction patterns of bare Ža. and PWA impregnated Žb. Cab–O–Sil silica powders.
P. Staiti et al.r Solid State Ionics 145 (2001) 101–107
2.2. Membrane and electrodes (M & E) assembly
The catalyst employed for methanol oxidation
was 60% Pt–Ru Ž1:1.rC ŽE-Tek., whereas a 30%
PtrC ŽE-Tek. was used for oxygen reduction. The
reaction layer, for both anode and cathode, was
prepared by direct mixing in an ultrasonic bath a
suspension of Nafion ionomer in water with the
catalyst powder, the obtained paste was spread on
carbon cloth backings. The platinum loading for all
the electrodes Žanodes and cathodes. used in the
experiments was 2.2 " 0.2 mg cm2 . The M & E assembly was manufactured by pressing the electrodes
onto the membrane at 1308C and 50 atm.
2.3. Single cell experiments
Each M & E assembly was loaded into a single-cell
test fixture ŽGlobe Tech. which was connected to an
HP 6060B electronic load. An aqueous solution of
methanol, as well as humidified oxygen were preheated at 858C and fed to the cell. Different operating temperatures for the cell were settled in the range
from 90 to 1508C. The anode back-pressure was
varied between 1 and 3 atm as the temperature was
increased from 90 to 1508C. Whereas the cathode
103
compartment back-pressure was maintained constant
at 4 atm. Methanol–water solutions of different concentrations were used in order to evaluate the appropriate concentration of fuel which allows the acquisition of the best cell performance. The inlet fluxes
from anode and cathode compartments were measured on-line.
3. Results and discussion
3.1. X-ray diffraction analysis
XRD patterns obtained for different membranes
evidenced only slight differences in the structural
characteristics ŽFig. 2.. The bare Nafion membrane
shows a main peak related to the hexagonal structure
of Nafion at about 2 u s 188, which overlaps with the
X-ray scattering from the amorphous region of the
membrane at lower Bragg angles w5x. More detailed
information on these aspects are derived from a peak
profile analysis carried out by using the Marquardt
algorithm w5x ŽFig. 3..
The addition of silica and heteropolyacid modifies
the relative ratio between the crystalline and amorphous structures of cast Nafion with respect to bare
Fig. 2. Cu K a X-ray diffraction patterns of Nafion 117 Ža., silica- Žb., silica–PWA- Žc. and silica–SiWA- Žd. modified recast Nafion
membranes in protonic form and dry state.
104
P. Staiti et al.r Solid State Ionics 145 (2001) 101–107
Fig. 3. X-ray diffraction profiles for Nafion 117 Ža. and silica–PWA-modified recast Nafion Žb. membranes in protonic form and dry state.
The measured data are shown by dots, whereas amorphous Žlower Bragg angles. and crystalline scatterings are the deconvoluted profiles.
Nafion. Yet, due to the low loading of silica and
heteropolyacid in the membranes Ž3% silica, less
than 1.5% heteropolyacid., it is unlikely that the
inorganic materials are responsible for such a significant modification in the Nafion structure. More probably, the above changes were caused by the different
thermal treatments to which the modified membrane
had been subjected.
3.2. Polarization tests
All the M & E assemblies equipped with the various membranes were capable of operation at 1458C.
Fig. 4 shows that the cell resistances in the various
systems are almost unvaried in the range between 90
and 1458C, even if a slight loss of conductivity is
observed at 1458C. Yet, this does not affect signifi-
Fig. 4. Variation of cell resistance with temperature during DMFC operation Žcurrent interrupter method. for the various M & E assemblies
equipped with different membranes.
P. Staiti et al.r Solid State Ionics 145 (2001) 101–107
105
Fig. 5. Polarization and power density curves for the various M & E assemblies equipped with different membranes. Operating conditions:
1458C, oxygen feed, methanol 2 M.
cantly the cell performance due to the fact that the
enhancement of methanol oxidation kinetics plays a
prevailing role.
The polarization curves obtained for the fuel cells
equipped with the different membranes and operating under the same conditions in the presence of
oxygen feed at cathode and 2 M methanol solution at
anode are reported in Fig. 5. Similar electrochemical
characteristics are observed up to 0.5 A cm2 . The
PWA-based membrane shows lower mass-transport
limitation with consequent higher cell voltages at
high current densities. The best electrochemical performance is obtained with the PWA-based membrane, which gives a maximum power density of 400
mW cm2 at current density of about 1.4 A cm2
under oxygen feed operation at 1458C. Maximum
power density of 340 mW cm2 is obtained from the
fuel cell which uses the silica-modified membrane,
whereas a lower performance was achieved with the
SiWA-based membrane. It is clear that a positive
Fig. 6. Comparison of the polarization and power density curves in the presence of air and oxygen feed for the M & E assembly equipped
with the PWA-modified membrane at 1458C. Operating conditions: oxygen feed, 2 M methanol; air feed, 1M methanol.
106
P. Staiti et al.r Solid State Ionics 145 (2001) 101–107
Fig. 7. Influence of operating temperature on the DMFC polarization characteristics for the M & E assembly equipped with the
PWA-modified membrane in the presence of oxygen and 2 M methanol feed.
effect on cell performance is given by the addition of
phosphotungstic acid to the silica–Nafion membrane. Since the enhancements were observed at high
current densities, one may ascribe, at a first instance,
this behavior to a better ion transport through the
membrane under such conditions. But such conjectures are not supported by the analysis of variations
of cell resistance with temperature ŽFig. 4.. In a
previous paper w10x, we have evidenced a promoting
effect of phosphotungstic acid on the kinetics of
electrochemical reduction of oxygen. This was
mainly attributed to the high oxygen solubility in the
PWA electrolyte w10x. For what concerns a possible
promoting effect on methanol oxidation, it is known
that CO-like products, which form at the anode
during methanol oxidation reaction giving rise to a
strong chemisorption on surface catalyst, are better
removed in the presence of the heteropolyacid w2x.
In the presence of oxygen, the best fuel cell
performances were obtained, for all the membranes,
with 2 M MeOH solution feed at the anode. When
air is fed to the cathode compartment, the best cell
performance is obtained with 1 M MeOH solution
feed at the anode. This occurs because the effect of
methanol crossover on cathode polarization is more
significant in the presence of air as oxidant. The
maximum power density obtained in air with the
PWA-based membrane is 250 mW cm2 at 1458C
ŽFig. 6., and 210 mW cm2 with the Nafion–SiO 2
membrane at the same temperature. The fuel cell
operating with the SiWA-based membrane showed
lower electrochemical performances when both air or
oxygen are fed to the cathode.
The effect of temperature on electrochemical polarization characteristics of a fuel cell operating with
the PWA-based membrane is reported in Fig. 7. The
voltage losses decrease progressively as the cell temperatures increase in the range from 90 to 1458C, in
both activation and diffusion control regions. A maximum of fuel cell activity is recorded at 1458C.
Experiments carried out at 1508C gave rise to an
unstable electrochemical behavior. It is pointed out
that the voltage gain at very low current densities Ž50
mA cm2 . passing from 90 to 1458C is about 100
mV, indicating the strong activation nature of the
methanol electro-oxidation process.
4. Conclusions
Heteropolyacid-modified silica–Nafion recast
composite membranes showed suitable properties for
operation at 1458C in a direct methanol fuel cell.
Phosphotungstic acid-based membrane performed
better than both bare Nafion–silica and Nafion–
silica–silicotungstic acid membranes under oxygen
P. Staiti et al.r Solid State Ionics 145 (2001) 101–107
and air operation. Similar cell resistance values were
observed for these M & E assemblies; this evidence
indicates that the enhancement effect produced by
phosphotungstic acid is not related to an increase of
electrolyte ionic conductivity, but more likely, it
relies on its promoting behavior w9,10x. Further studies are in progress to better focus these aspects.
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