Potential dependence of sulfur dioxide poisoning and oxidation at the cathode of proton exchange membrane fuel cells

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to investigate changes in the cathode after the introduction of SO2. The decay in performance of the proton exchange membrane fuel cell (PEMFC) was ascribed to the increasing of the charge transfer resistance (Rct) caused by the loss of the electrochemical surface area (ECA). The results show that the oxidation and adsorption behaviors of SO2 depended closely on the potential. Adsorbed sulfur began to be oxidized over 0.9 V and could be oxidized completely with CV maximum potentials up to 1.05 V or higher. At about 0.65 V, the adsorbed SO2 was probably in a molecular state, which could be reduced in the range of 0.65–0.05 V. Some SO2 molecules occupied initially two Pt sites through S and O. One of the two sites could be released after the reduction. The increasing of ECA due to reduction could lessen the impact of SO2 on the PEMFC performance at voltages below 0.65 V.