Desalination 200 (2006) 636–638

Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy.

0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved.

A quartz crystal microbalance study of water vapor sorption in a short side-chain PFSI membrane

Yusuke Yamamoto, Maria Chiara Ferrari, Marco Giacinti Baschetti*, Maria Grazia De Angelis, Giulio Cesare Sarti

Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali, Università degli Studi di Bologna, Viale Risorgimento 2, 40136 Bologna, Italy

Tel. +39 051 2090408; Fax +39 051 6347788; email: marco.giacinti@mail.ing.unibo.it

Received 27 October 2005; accepted 3 March 2006

1. Introduction

Perfluorosulfonic acid ionomer (PFSI)membranes are commonly used as electrolytesin proton-exchange membranes fuel cells(PEMFCs): they consist of a hydrophobic poly-tetrafluoroethylene backbone and perfluorovinylether side chains terminated by a sulfonic acid(SO3H) group. The PFSI membrane studied inthis work is a new material produced by SolvaySolexis S.p.A., under the name of Hyflon Ion®,and contains a short side chain, that gives it alower equivalent weight (860 g(pol)/mol(SO3H))at similar molar content of sulfonic comonomerwith respect to similar commercial membranessuch as Nafion® from Dupont.

The study of water transport properties throughthe membrane is essential for controlling its fuelcell performance. Many studies available inthe literature refer to extruded films, while veryfew data can be found on cast membranes. Ithas been shown that such films may have verydifferent behavior with respect to the previousones [1]. In the present work, cast films ofHyflon Ion® have been tested to determine the

water vapor solubility as a function of activity,in view of its use in fuel cell applications. Thedata have been obtained with a quartz crystalmicrobalance (QCM), at different temperatures,and have been compared with similar dataobtained for the extruded films with a differenttechnique.

2. Experimental

The QCM system has become a popularmethod for the study of processes characterizedby very small mass variations in the sample dueto its high sensitivity, in the order of nanograms,and very fast response. The method consists inthe measurement of the resonant frequencyshift, DF, of the quartz crystal that is linear withthe mass change Dm according to Sauerbrey’sequation [2]:

(1)

where n is the order of the harmonic, F0 the basefrequency of the crystal, A the area of the elec-trode, rQ and mQ the density and shear modulusof the crystal, respectively.

*Corresponding author.

D DFnF

Am

Q Q

= - 2 02

1 2 1 2r m/ /

doi:10.1016/j.desal.2006.03.452

Y. Yamamoto et al. / Desalination 200 (2006) 636–638 637

The specimens used for the present studyhave been prepared by casting from a suspension(about 5 w%) of the H-type perfluorosulfonateionomer in a solution of water, methanol, dime-thylacetamide (DMA) and H Galden (R) B, a per-fluorinated solvent produced by Solvay Solexis.The membranes were cast directly on quartzcrystal oscillator at atmosphere and then treatedunder vacuum and at high temperature (about160oC) for 1 h to completely remove the DMAand stabilize the film [3]. The samples weightwas equal to about 0.6 mg and they were about1.5 mm thick.

Prior to each experiment, the membranes werefurther annealed at 120°C overnight to removethe residual water. Each experiment consisted ina first sorption run followed by a desorption anda re-sorption after equilibration for a night undervacuum at the experimental temperature. Withthis protocol, sorption isotherms for water havebeen obtained at 40°C, 50°C, 60°C and 70°C.

3. Results and discussion

The sorption isotherm of a cast film at 50°Cis reported as a function of the activity in Fig.1,where we also plotted the data obtained in aprevious study [4] for extruded membranes of a

similar material, of equivalent weight equal to860 g(pol)/mol(SO3H), which was dried undervacuum at 50°C before the experiment. It can beseen that the cast samples are in general charac-terized by a lower solubility value with respectto extruded films. Moreover, the general shapeof the sorption isotherm seems to be different inthe two cases: for extruded materials it shows adual mode behavior in almost the entire activityrange, while for the cast films it is first concaveto the activity axis and then the concavitychanges for higher activity. The solubility isalmost unaffected by temperature and it showsjust a slight increase going from 40°C to 70°Cas it can be seen in Fig. 2.

4. Conclusions

The solubility of water vapor in cast films ofHyflon Ion® have been investigated through aQCM technique at different temperatures andcompared with similar data obtained forextruded films. Even considering deviations dueto the different experimental techniques, the castand extruded membranes show important differ-ences not only in the value of the solubility butalso in the shape of the sorption isotherms. In all

0

0.02

0.04

0.06

0.08

0.1

0.12

0Activity (P/P*)

Mas

s up

take

(g H

2O/g

pol

)

Pressure decay technique,extruded membrane

QCM, 50°C, cast membrane

0.2 0.4 0.6 0.8

Fig. 1. Water vapor sorption in extruded and castperfluorosulfonic acid ionomer (PFSI) membrane at 50°C.

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0Activity (P/P*)

Mas

s up

take

(g H

2O/g

pol

)

50°C 40°C60°C 70°C

0.2 0.4 0.6 0.8

Fig. 2. Comparison between mass uptake in castperfluorosulfonic acid ionomer (PFSI) membranes atdifferent temperatures.

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638 Y. Yamamoto et al. / Desalination 200 (2006) 636–638

cases, the temperature seems to have only a slighteffect on the solubility; in both the extruded andcast films the solubility increases with increasingtemperature.

These results suggest some kind of differencein the structure of the different polymer mem-branes, dependent on the preparation procedure,which affects the transport properties of theinitial material. Further studies are ongoing tounderstand the reason for this behavior.

References

[1] P. Krtil, A. Trojanek and Z. Samec, Kinetics ofwater sorption in nafion thin films — Quartz

crystal microbalance study, J. Phys. Chem. B, 105(2001) 7979–7893.

[2] G. Sauerbrey, Verwendung von Schwingquarzenzur Wägung dünner Schichten und zurMikrowägung, Z. Phys., 155 (1959) 206–222.

[3] R.B. Moore, C.R. Martin, Procedure for pre-parino solution cast perfluorosulfonated Ionomerfilms and membranes, Anal. Chem. 58 (1986),2569–2570.

[4] M.G. De Angelis, S. Lodge, M.G. Baschetti,G.C. Sarti, F. Doghieri, A. Sanguineti andP. Fossati, Water sorption and diffusion in ashort-side-chain perfluorosulfonic acid ionomermembrane for PEM-FCs: effect of temperatureand pre-treatment, Desalination, 193 (2006)398–404.

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