UTILITY OF NEUTRONS AND BACKGROUND OF THE PROPOSERS

Neutrons are a very relevant and non-intrusive probe to quantify the water content in the various components of a fuel cell, especially during operation:

i) The metallic materials constituting the fuel cell are transparent to neutrons

ii) Theneutrons are extremely sensitive to protons due to the exceptional incoherent cross section of hydrogen atoms with respect to any other atomic species. Therefore, the neutrons probe quite exclusively the properties of protons contained in the ionic phase of the membrane (i.e.charge carriers and hydration protons)and/or in the liquid water located in the components of the MEA and in the gas distribution channels.

Our group has developed since several years a method based on Small angle neutron scattering (SANS). It is the only method that allows to quantify simultaneously the local water content in the membrane (via the direct relationbetween microstructure and water content of perfluorosulfonated ionomers) and the amount of liquid water outside this component (producing an incoherent proton scattering signal) [16]. Accordingly, the operando technique has provided, up to now, invaluable insights into the water distribution in the working FC [17-26]. A PEMFC transparent to neutrons can be built using standard MEA materials and without significant modification of the state-of-art design (Figure 4a). SANS measurements have thus been carried out under representative conditions of gas stoichiometry, temperature (from -10 to 80 °C), current density (up to 800 mA.cm^-2), pressure (up to 3 bars) and with relative humidity (RH) of fed gases varying from 0% to 100% [20,22,23,25] (Figure 4b).

The local water content in an operatingfuel cell has been probed every 30 seconds in front of the rib and the channel of the bipolar plate. These results are currently under publication [26].

Figure 4. (a) 25 cm² single cell developed at CEA for SANS experiments on D22 beamline at ILL (Picture of the single cell at -10°C inserted). (b) SANS spectra recorded in different areas of an operating PEMFC (30 seconds, sub-millimeter resolution) and used to determine water repartition at 80°C_1.6 bar H2/Air 60/40%RH_St. 1.2/2 counter-flow.

Date: 2015-01-11; view: 885