Previous tests revealed that SO 2 contamination in proton exchange membrane fuel cells (PEMFCs) resulted in a two-stage degradation in cell voltage. Unfortunately, the effect of SO 2 on the cell cathode reactions and the corresponding SO 2 contamination mechanism are still unknown. In this paper, the effect of SO 2 contamination on oxygen reduction reactions (ORR) was analyzed using electrochemical impedance spectroscopy (EIS) through an equivalent electrical circuit model (ECM). The ORR parameters extracted from the ECM suggested that during the first poisoning stage, the SO 2 adsorption increased the charge transfer resistance of the ORR, causing the initial cell performance degradation. During the second poisoning stage, SO 2 adsorption may assist in shifting the ORR from the "direct four electron pathway" to the "series two electron pathway." This shift resulted in a decrease in the current efficiency of the ORR, which then resulted in the subsequent degradation of cell performance. The increase in the fluorine emission rate, as detected by ion chromatography, was indirect evidence for the ORR pathway shift during the second poisoning stage and the decrease in current efficiency.Proton exchange membrane fuel cells (PEMFCs) are considered to be one of the most promising clean energy technologies and have been shown to act as a suitable power source for transportation and stationary applications. This fuel cell technology offers high efficiencies, high power densities and zero emissions when operated with pure hydrogen as the fuel and clean air as the oxidant. 1, 2 Unfortunately, trace sulfur dioxide (SO 2 ) from air pollution, i.e., vehicle exhaust and industrial emissions, is known to detrimentally affect PEMFC performance and may cause permanent damage to membrane electrode assemblies (MEAs). [3][4][5][6][7][8] In previous work, 9-11 the effect of airborne SO 2 on PEMFC performance and the performance recovery were investigated in detail at different operating conditions. The results showed a two-stage degradation in the cell voltage during SO 2 exposure. The SO 2 poisoning was determined to consist of two processes: an irreversible process, which was dependent on sulfur species coverage, but independent of SO 2 concentration, and a reversible process, which was dependent on the cathode potential and SO 2 concentration. An inflection point at a cathode potential of about 0.69-0.70 V vs. reversible hydrogen electrode (RHE) indicated the transition from the irreversible to reversible process. The cell performance loss caused by the reversible process was recovered in-situ with neat H 2 /air operation. The irreversible process, however, required a driven-mode potential (0.08-1.2 V vs. RHE), scanning for recovery after the adsorbed sulfur species were reduced to 'S-zero' (S 0 ). Additionally, the reversible poisoning process was affected by cell operating conditions, such as cell temperature, operating current density and SO 2 concentrations.However, the performance degradation mechanisms within PEMFCs caused by...
