Perfluorosulfonimide as an Additive in Phosphoric Acid Fuel Cell

The addition of a 0.5 weight percent (w/o) of the perfluorosulfonimide (PFSI), CF/sub 3/SO/sub 2/N(H)SO/sub 2/C/sub 4/F/sub 9/, as an additive to 85% H/sub 3/PO/sub 4/ in hydrogen-oxygen fuel cell results in a 70 mV increase in the potential of the oxygen cathode (high surface area, 10% Pt on Vulcan XC-72) at current densities up to 500 mA cm/sup -2/ at 70/sup 0/C. This improvement is proposed to be due to the enhanced oxygen reduction kinetics caused by a physically adsorbed layer of the PFSI on the catalyst with the fluorocarbon tail toward the surface. The adsorbed layer creates a dry cave environment adjacent to the electrode surface which favors the competitive adsorption of oxygen on the catalyst surface. This adsorbed layer apparently does not interfere with water or proton transfer from or to the interface. Flooding of the gas structure of the porous cathode has not been observed at 0.5 w/o of the PFSI but has been observed at higher concentrations.