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JOURNAL OF
ELSEVIER
Journal of Electroanalytical Chemistry 385 (1995) 269-271
Short communication
Electric resistance in a Nafion ® membrane exposed to air after a step
change in the relative humidity
Franti~ek Opekar, Daniel Svozil
UNESCO Laboratory of Environmental Electrochemistry, Department of Analytical Chemistry, Charles University,Albertov 2030, 128 40 Prague 2,
Czech Republic
Received 16 September 1994
Keywords: Air exposure
1. Introduction
Solid polymer electrolytes in the form of membranes
are often used in industrial electrolysers, fuel cells and
solid-state chemical sensors. Perfluorosulphonic membranes known under the name Nation ® (DuPont) are
the most common. The physico-chemical properties of
solid polymer electrolytes are strongly dependent on
the amount of water contained within the membrane
[1]. The relationship between the water content in the
membrane and the electric resistance or conductivity of
the membrane is usually obtained from a measurement
of the resistance of a membrane sample containing a
defined amount of water, under conditions ensuring
that Nafion ® does not exchange water with the environment for the time of measurement. The resultant
membrane resistance values then correspond to certain
equilibrium water contents [2,3].
The present paper follows the changes in the resistance of a Nation ® membrane during the process in
which one equilibrium water content changes into another owing to exchange of water between Nafion ®
and ambient air of a certain relative humidity (RH).
The air R H is changed stepwise and the time dependence of the Nation ® resistance is monitored. The
conductivity electrodes formed directly on the Nation ®
membrane surface make possible continuous resistance
monitoring.
2. Experimental
The test membrane was Nafion ® 117 (Aldrich, Cat.
No. 29, 256-7). A 12 × 12 mm piece of the membrane
was first cleaned by leaching for 24 h in ethanol and
0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved
SSDI 0 0 2 2 - 0 7 2 8 ( 9 5 )03 767-1
then was boiled in water for 45 min; in this way, the
Nafion ® was converted into the expanded form [4].
Conductivity electrodes 10 mm in diameter were prepared on both sides of the membrane by depositing
platinum chemically from a solution of chloroplatinic
acid by the action of an alkaline hydrazine solution
[5,6]. After the electrode preparation, the membrane
was converted into the hydrogen cycle by boiling it for
30 min in 1 mol 1-1 sulphuric acid and then was stored
in water for 24 h. The roughness factor of electrodes
prepared in the above way is very high, between 10 2
and 10 3, so that the electrodes behave like platinized
platinum electrodes. Chemically deposited platinum is
porous (these electrodes are often used in gas sensors),
so that it does not prevent transport of water vapour
between Nation ® and its environment.
Platinum leads were pressed on to the conductivity
electrodes by spring clamps and the membrane was
hung in a vessel above a saturated solution of a salt,
creating an atmosphere of a defined R H (see Fig. 1).
The following salts were used (the R H values are given
in parentheses): C H 3 C O O K (22%), MgCI 2 (33%),
Na2Cr20 7 (52%) and NaC1 (75%). The clamp with the
leads was fixed on the vessel lid. By transferring the lid
from one vessel to another, it was then possible to
transfer the test membrane relatively rapidly (within
i - 2 s) from air with one R H value into air with
another R H value.
The time dependence of the resistance on a change
in the R H was found from the voltage drop across the
membrane through which a constant alternating current (5 kHz, 360 /zA) was passed. The alternating
voltage was converted into a d c signal that was then
treated and displayed by a computer.
The following procedure was used: the test mere-
F. Opekar, D. Suozil/ Journal of Electroanalytical Chemistry 385 (1995) 269-271
270
800
6
600
1 ~
7
\
_LL
output
o
-~.
400
n-
/
200
2
/
3
\
-
-
w
I
I
2000
4000
6000
Time/s
Fig. 1. Apparatus for continuous measurement of Nation ® resistance. 1 = Nation ® membrane with conductivity electrodes; 2 =
vessel with air of a defined RH; 3 = saturated salt solution; 4 = clamp
with leads; 5 = transferable lid; 6 =constant-current source; 7 =
voltage meter and ac-dc converter.
brane was stored in the vessel containing air with a
certain RH value for a time required to attain an
equilibrium amount of water in the membrane and,
consequently, a constant resistance. The membrane
was then rapidly transferred to a vessel containing air
with another RH value, simultaneously monitoring the
time dependence of its resistance. All the measurements were performed at 30°C.
Fig. 2. Time dependence of the Nation ® membrane resistance on a
step change in the RH of air from (A) 33 to 52% and (B) from 52 to
33%.
Further, it follows from Table 1 that:
(a) with the same initial RH value, the initial rate of
change in the resistance decreases with decreasing RH
difference, for both loss and uptake of water;
(b) the rate of the change in the membrane resistance is a function not only of the absolute magnitude
of the change in the RH, but also of the initial RH
value;
Table 1
Characteristics of the time dependences of the Nation ® membrane
resistance on a step change in the air relative humidity
RH change
Initial slope a
12 s - 1
75
22
75
33
75
52
52
22
52
33
33
22
1.6
-95.7
0.2
--8.0
0.05
-0.32
1.8
--35.1
0.4
--2.9
2.0
--14.0
3. Results and discussion
On transferring the membrane from an environment
with a higher R H into one with a lower RH, Nation ®
loses water and its electric resistance increases, and
vice versa. The time dependence of the resistance after
a step change in the R H is given in Fig. 2, from which
it can be seen that the change from lower R H (33%) to
higher R H (52%) is much faster than the opposite
change; this occurs with all the R H changes studied
(see Table 1). The rate of change in the membrane
resistance is expressed in Table 1 in terms of the initial
slope of the resistance-time dependence and in terms
of the times required to attain 63.2% ("time constant"),
95% and 100% of the steady-state resistance.
~
~
"-0
~
~
~
~
~
~
~
~
~
22
75
33
75
52
75
22
52
33
52
22
33
Time required to attain
percentage of steady-state
value/s
63.2%
95%
100%
4000
50
1860
70
580
130
2800
140
1200
160
2200
450
20000
120
4700
220
4500
400
6500
340
4400
520
3600
900
- b
350
-- b
480
- b
1100
- b
850
- b
1300
-- b
1350
a The minus sign denotes a decrease in resistance.
b Time of steady-state establishment is of the order of tens of hours.
F. Opekar, D. Svozil /Journal of Electroanalytical Chemistry 385 (1995) 269-271
(c) steady-state resistance values are attained relatively rapidly on uptake of water by the m e m b r a n e ,
whereas the changes on loss of water are so slow after
the attainment of about 95% of the steady-state resistance that the stationary value is reached after more
than 24 h.
The resistance of a Nation ® m e m b r a n e depends on
the water content in the Nation ® and the rate of the
resistance change depends on the velocity of water
transport between Nation ® and its environment. It has
been shown [7] that the water entering Nation ® is
consumed primarily in the solvation of the ions present
in the membrane, within an R H range from 14 to 75%,
i.e. a range similar to that studied in the present work
(see Table 1). The electrostatic interactions between
hydrated counterions and the sulpho groups fixed to
the hydrocarbon skeleton of Nation ® affect not only
the Nation ® resistance, but also the transport of water
between Nation ® and the ambient atmosphere. Hydrophilic ions very readily include water in their solvation shells and thus facilitate entry of water into Nafion ®. When the water content decreases, these ions
form immobilized ion pairs that hinder transport of
water from Nation ®.
The rate of water transport is undoubtedly also
affected by the cluster structure of Nation ®. When the
R H of air changes close to Nation ® membrane, be it to
higher or lower values, then the surface of the polymer
is in contact with the ambient air. On a decrease in the
water content in the polymer, the clusters in the surface layer decrease in size, are reorganized and the
connecting channels are narrowed [8]. The surface
layer is thus more difficult to penetrate and the transport of water from the polymer bulk to its surface is
hindered. However, on entry of water into Nation ® the
clusters are increased by the water molecules accepted
and the connecting channels are widened, enhancing
the water transport. Therefore, the processes determining the electric resistance of Nafion ~ simultaneously influence the rate of water transport between
Nation ® and its environment.
The above reasons cause the rate of the m e m b r a n e
resistance change to be slower when the Nation ® loses
water, compared with the opposite process. The in-
271
creasing hindrance to water transport is manifested in
long times required to attain an equilibrium water
concentration and a steady-state resistance. The opposite 'process is faster by several orders of magnitude,
owing to facilitated water transport.
As the resistance of a Nation ® m e m b r a n e is primarily determined by the surface layers of Nation ® when
the R H of ambient air changes, it is impossible to
establish unambiguously the relationship between the
changing resistance and the actual water content within
the polymer. However, when studying the influence of
the ambient humidity on the properties of Nation ®, it
must be taken into consideration that, under given
conditions, water uptake by Nation ® is much faster
than transport of water from Nation ®. This phen o m e n o n may be important, e.g. when correcting the
response of electrochemical gas sensors employing Nafion ® as a solid electrolyte for fluctuations in the relative humidity of the test medium.
Acknowledgement
This work was supported by the G r a n t Agency of
the Czech Republic, G r a n t No. 2 0 3 / 9 3 / 0 0 5 0 .
References
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Membranes, ACS Symposium Series No. 180, American Chemical Society, Washington, DC, 1982.
[2] G. Pourcelly, A. Oikonomou, C. Gavach and H.D. Hurwitz, J.
Electroanal. Chem., 287 (1990) 43.
[3] T.A. Zawodzinski, M. Neeman, L.O. Sillerud and S. Gottesfeld,
J. Phys. Chem., 95 (1991) 6040.
[4] R.S. Yeo, J. Electrochem. Soc., 130 (1983) 533.
[5] H. Nakajima, Y. Takakuwa, H. Kikuchi, K. Fujikawa and H. Kita,
Electrochim. Acta, 32 (1987) 791.
[6] F. Opekar, J. Electroanal. Chem., 260 (1989) 451.
[7] T.A. Zawodzinski, T.E. Springer, F. Uribe and S. Gottesfeld,
Solid State Ionics, 60 (1993) 199.
[8] T.D. Gierke and W.Y. Hsu, in A. Eisenberg and H.L. Yeager
(eds.), Perfluorinated Ionomer Membranes, ACS Symposium Series No. 180, American Chemical Society, Washington, DC, 1982,
p. 283.