Effect of SO2 on the Performance of the Cathode of a PEM Fuel Cell at 0.5-0.7 V

The effect of 1 ppm SO2 in air on the performance of the proton exchange membrane (PEM) fuel cell cathodes was studied at operating voltages of 0.5, 0.6, and 0.7 V at 80 degrees C and 100% relative humidity. The coverage of the cathode catalyst (50% Pt/Vulcan carbon) with sulfur species theta(S) after exposure to SO2 in air at different cell voltages is evaluated from the hydrogen desorption region on cyclic voltammetry (CV) curves and from ohmic-resistance-corrected polarization curves. The theta(S) calculated from CVs after exposure to SO2 in air is compared to those of cathodes exposed to SO2 in nitrogen. The theta(S) decreases as the cathode potential increases in both air and nitrogen. The cathodes poisoned in air have 2.5 times lower sulfur coverage than those poisoned in nitrogen. The cells poisoned at 0.6 V have the highest performance vs those poisoned at 0.5 and 0.7 V at constant current density; even though the sulfur coverage is lowest in the cell poisoned at 0.7 V, adsorbed hydroxy groups and sulfate/bisulfate ions on the platinum surface are likely to impede the oxygen reduction reaction at 0.7 V. The sulfate/bisulfate ions can be removed from the surface by holding the cathode at its open-circuit voltage in nitrogen. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3236650] All rights reserved.