Initial conditioning of a polymer electrolyte fuel cells: The relationship between microstructure development and cell performance, investigated by small-angle neutron scattering

Real time in-situ observation by small-angle neutron scattering (SANS) was performed to observe the development of microstructure in a polyelectrolyte membrane packed in a polymer electrolyte fuel cell (PEFC) during initial conditioning. By utilizing the excellent penetrability of neutrons, an operating fuel cell was studied. SANS originating from the membrane was analyzed to determine the water cluster radius (R-c) and its number density (rho(c)). We found that R-c increased and rho(c)decreased, as the load current was increased during initial conditioning. The maximum R-c achieved at high current density became larger as the conditioning cycle proceeded. In this process, two water clusters combined and the connectivity of proton conduction channels was enhanced. As a consequence, was reduced ohmic loss, which was detected by using a special segmented electrode cell covering the cell section corresponding to that observed by SANS.