SOFT MATTER AND BIOMATERIALS 108 WATER PROFILE DETERMINATION IN A RUNNING PEMFC BY SMALL-ANGLE NEUTRON SCATTERING. G. Gebel 1 , O. Diat 1 , S. Escribano 2 , R. Mosdale 2 1 DRFMC/SI3M/Groupe Polymères Conducteurs Ioniques 2 DTEN/SCSE/Laboratoire Hydrogène et Piles à Combustible CEA Grenoble, 17, rue des martyrs - 38054 GRENOBLE cedex 9, France A fuel cell is an electrochemical system producing a DC current from two half chemical reactions: an H 2 (the fuel) oxydation at the anode and the reduction of the O 2 at the cathode as indicated in figure 1a. Proton exchange membrane fuel cell (PEMFC) is a cell using a proton conducting polymeric membrane as a solid electrolyte separating both electrodes (and so both gases) and allowing the proton conduction between the anode and the cathode. These membranes require a minimum of water content to exhibit a sufficient conductivity and this water has to be controlled whatever the temperature and current density conditions during the cell operation. Then, the management of the water around the cell and more especially between both electrodes is a major challenge for PEMFC applications. When the cell is operating, water is produced at the cathode. Therefore, under stationary conditions and depending on the current density, an excess of water accumulates at the cathode and has to be removed. Nevertheless, it exists always a water concentration profile across the membrane, which is partially counterbalanced by water back-diffusion and reinforced by water electro-osmosis (see Fig. 1b). This concentration profile can also be reduced using humidified inlet gases especially at the anode. Several attempts to experimentally determine these profiles during fuel cell operation have been published. However the experimental constraints [1- 4] 1-4 lead to operate in conditions, which were not representative of the actual fuel cell operation. Figure 1. a) Scheme of a fuel cell and b) illustration of the water management in an operating fuel cells The determination of the water concentration profile across the Nafion 1 membrane and under fuel cell operation can be achieved using small-angle neutron scattering (SANS) technique. Indeed, both the shape and the level of scattered intensities of Nafion spectra are very sensitive to the water content. Moreover, it is possible to use a specially designed fuel cell which is almost transparent to neutrons and allows mainly the observation of the membrane contribution to the SANS spectra. In a previous study, we have demonstrated the feasibility of such experiments but with some poor fuel cell performances due to i) electrodes which were not appropriate (chemically deposited platinum) ii) a cell design which did not allow an optimized gas flow on the membrane and iii) a control of the cell temperature which were non- existent. In the present study, a new bench and a new cell were designed (see Fig. 2) to operate at 80°C and under pressure, to control the gas flow, and to measure the gas outlet humidities. Figure 2. Porous gas distributors and current collectors are either in an titanium/zirconium alloy (48/52) which was grated and sintered or in highly porous steel (90% porosity). Quartz windows were to insure each compartment to be airtight and the transparency to neutrons. The temperature was measured through a thermocouple and adjusted through a temperature controller. Two different types of electrodes were used either a spray (Sm) or a hot pressed Etek ® electrode (Etn) on membrane. In the case of Etn electrodes the diffusion layer has been torn away in order to deposit the active layer on the membrane and to avoid the strong scattering of the carbon tissue. 1 Nafion ® is a perfluorinated membranes purchased from du Pont de Nemours Company