XPS AND FE-SEM ANALYSIS OF PLATINUM ELECT ROCATALYSTS IN A PHOSPHORIC-ACID FUEL-CELL

The operational change in platinum particles sizes of the electrocatal ysts in a PAFC (Phosphoric Acid Fuel Cell) was evaluated using the met hod of Kerkhof. The particle size determined by X-ray photoelectron sp ectroscopy (XPS) almost agreed with that by X-ray diffraction (XRD). T he small difference between the results of two methods might be attrib utable to the broad distribution in the platinum particle size and/or the difference of the surface sensitivity of XPS and XRD. Furthermore, the morphologies of the platinum particle at the surface and in the i nside of the electrode after cell operation were observed using fielde mission scanning electron microscope (FE-SEM). The particle size at th e surface was larger than that in the inside of the electrode. In part icular, the clustered particles were only observed at the surface. Suc h clustered platinum particles at the surfaces showed the effect of th e electrolyte attack or ''surface corrosion'' and were considered to b e formed through the coalescence mechanism. On the other hand, the num ber of platinum particles was scarce in the inside of electrode (catho de), which indicated the dissolution of platinum from the cathode. Thu s, the present study supports the idea that the particle growth is pro ceeded by the dissolution-redeposition process and/or coalescence proc ess.