CharacterisationofFuelCellMembranes asaFunctionofDryingbyMeans ofContactAngleMeasurements H.-P.Brack 1,2 *,M.Slaski 1 ,L.Gubler 1 ,G.G.Scherer 1 ,S.Alkan 1 ,andA.Wokaun 1 1 PaulScherrerInstitut,ElectrochemistryLaboratory,CH-5232VilligenPSI,Switzerland 2 Presentaddress:ETHZürich,AbteilungfürBetriebs-undProduktionswissenschaften,D-GESS,Weinbergstrasse43,CH-8006Zürich, Switzerland Received25.09.03,inrevisedform25.12.03,accepted27.02.04 1 Introduction It is well known that the membrane-electrode interfacial properties are quite important in determining PEFC perfor- mance [1, 2]. This importance results from the fact that the electrochemical oxidation and reduction reactions only occur in the three phase boundary consisting of the ion-conductor (membrane), electronically conducting catalyst (platinum on porous carbon electrode), and gaseous H 2 or O 2 . Our labora- tory has recently described how the electrochemical interface of MEAs based on radiation-grafted membranes can be improved significantly by hot-pressing the MEA and also, to a lesser extent, by Nafion  -impregnation of the radiation- grafted membrane prior to hot-pressing [3]. Our other contri- bution to this volume examines, in more detail, how the for- mation of optimised MEA interfaces characterised by low charge transfer resistances can be favourably influenced by hot-pressing of the MEA [4]. In particular, the effects of the Nafion  -impregnation of membranes prior to hot-pressing and the water swelling of the membrane immediately prior to the hot-pressing process are investigated in that work. In particular, it is reported that in many cases the influence of water content and swelling appears to dominate over that of either Nafion  -impregnation of the membrane or even hot- pressing. Water may influence both the surface properties of ion- containing polymers such as hydrophilicity and rheological properties including flow. For example, Zawodzinski and co- workers have reported that the surfaces of perfluorinated ionomers, like a Nafion  membrane, become more hydropho- bic as the membranes dry out [5, 6]. It is well known that sur- face and wetting properties influence the adhesive and bond- ing properties of materials [7], so, it would be useful to understand the influence of the membrane water content on these properties. In particular, adhesive forces between two materials can even be calculated from their surface energy properties. Measurements with at least two or more liquids are typically required for the determination of surface ener- gies [7], and unfortunately only contact angles of water on perfluorinated membrane surfaces were measured in the work of Zawodzinski et al. Nonetheless, their relatively sim- ple measurements of the hydrophilicity of membrane sur- faces can still be quite useful in understanding the water ± [ * ] Corresponding author, hans-peter.brack@hispeed.ch Abstract In another paper in this volume, it is demonstrated that the electrochemical interface in MEAs, and thus the polarization performance of the resulting fuel cells, can be improved by optimising the hot-pressing procedure in the MEA prepara- tion. In particular, the extent of drying of the membrane dur- ing MEA preparation was shown to be critical. In the present investigation, the effect of the drying process, and thus water content, on the hydrophilicity, wetting, and surface energies of some fuel cell membranes is examined. Wetting and surface energies are well known to influence the bond- ing behaviour of materials. Conclusions about how mem- brane drying and changes in water content influence mem- brane bonding and the relative importance of these surface effects are drawn. Keywords: Membrane, MEA, Interface, Contact Angle, Wetting FUEL CELLS 2004, 4, No. 3  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 141 ORIGINAL RESEARCH PAPER DOI: 10.1002/fuce.200400018