WATER PROFILE DETERMINATION IN A RUNNING PROTON-EXCHANGE MEMBRANE FUEL-CELL USING SMALL-ANGLE NEUTRON-SCATTERING

A new method of water profile determination across the electrolyte of a running H-2/O-2 proton exchange membrane fuel cell (PEMFC) is descri bed. This method, involving in situ small-angle neutron scattering (SA NS) technique is based on the modification of the internal structure o f the membrane depending on its water content. Prior to the actual exp eriments, reference spectra have to be recorded in all ranges of water concentration. The profile is then constructed by considering the mem brane as a stack of thin polymer layers, each of them representing one single water content. The observed spectrum for the operating PEMFC i s, thus considered as a sum of the reference spectra. The calculated c oefficient affecting each spectrum is directly correlated to the thick ness of the corresponding hydration layer. With this information and t he recording of the electrochemical performance of the PEMFC, the temp erature and the relative humidity of the gases entering or leaving the cell, it is possible to back calculate the quantity and the origin th e water found in the membrane. These SANS experiments showed the inver sion of the water concentration profile depending on whether the H-2 s ent to the anode was dry or not; it is also showed that even a dried o ut membrane remains conductive, due to the treatment undergone before being in the PEMFC.