Catalyst contamination in PEM fuel cells

Abstract

PEMFC performance can be affected by contaminants in the fuel and air feed streams through catalyst poisoning processes. Performance degradation is dependent on the type of contaminants. At the anode catalyst layer, common contaminants other than CO are CO2, H2S, and NH 3. The effect of CO2 contamination on the anode catalyst layer is similar to that of CO. CO2 is reduced to CO, which strongly adsorbs to Pt, blocking H2 adsorption. The effect of H2S contamination is due to the formation of strongly adsorbed sulfur on the Pt surface, which permanently poisons the catalyst surface. High current density or low temperature operation of PEMFCs is more severely affected by H2S contamination than low current density or high temperature operation. The effect of NH3 contamination on the anode catalyst layer is due to its reaction with the bulk electrolyte membrane and the ionomer in the catalyst layer, reducing proton conductivity. However, NH3 seems not to poison the catalysts. Modeling of anode contamination based on the reaction kinetics provides an excellent fit to the experimental results. At the cathode catalyst layer, the common contaminants include SOx, NOx, H 2S, NH3, VOCs, and ozone. A trace amount of SOx in air can cause a significant performance decrease. Increases in SO x concentration accelerate the degradation. This degradation is due to the adsorbed sulfur on the Pt surface produced from Sox reduction, which not only poisons the catalyst but also changes the ORR mechanism. The fuel cell performance is only partly recovered after SOx contamination. NOx contamination of the cathode catalyst is also concentration dependent. Higher NOx concentration leads to more severe PEMFC performance degradation. The mechanism of NOx contamination is most possibly due to NOx reduction on the Pt surface, which competes with oxygen adsorption. PEMFC performance can be fully recovered in most cases after the NOx contaminants are eliminated. NH3 and H2S contamination of the cathode catalyst layer is similar to that of the anode catalyst layer. Civilian VOCs contamination of the cathode catalyst layer is not severe and is somewhat recoverable. However, battlefield contaminants such as chemical warfare gases are disastrous to PEMFCs, resulting in permanent performance degradation. The presence of ozone in air improves PEMFC performance. It is believed that adding a filter to the air is an effective way to mitigate cathode contamination. The presence of contaminants in both fuel and air results in an additive contamination effect: PEMFC performance degradation is the sum of the cathode and the anode degradation. © 2008 Springer-Verlag.