Abstract
The attack by HO• on the ionomer in the polymer electrolyte fuel cell (PEFC) can be mitigated by the incorporation of regenerative radical scavengers, such as cerium and manganese, into the membrane. The influence of the presence of the Ce and Mn ions on the reaction of HO• with the perfluoroalkylsulfonic acid (PFSA) ionomer was studied in the framework of a kinetic model. Scavenging of HO• by Ce3+ and Mn2+ leads to a decrease in the steady-state concentration of HO• and, therefore, the rate of ionomer attack, where Ce is more effective than Mn. The HO• scavenging yield was found to decrease with increasing reactive end-group concentration of the PFSA ionomer. The oxidized metal ions are rapidly reduced by both and HOO•, which restores the scavenger and ensures that the majority (>99.99%) of metal ions is in the reduced, “HO•-scavenging-active” oxidation state. This stabilization mechanism is not expected to be operational in hydrocarbon membranes, because HO• reacts with aromatic units far too rapidly. Also, the effectiveness of the mitigation mechanism is limited under the conditions of an ex situ Fenton test due to the abundance of H2O2, which is confirmed by experimental data reported in the literature.
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