Decoupling the Contributions of Different Instability Mechanisms to the PEMFC Performance Decay of Non-noble Metal O2-Reduction Catalysts

Non-noble metal catalysts (NNMCs) hold the potential to replace the expensive Pt-based materials currently used to speed up the oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC) cathodes, but they feature poor durability that inhibits their implementation in commercial PEMFCs. This performance decay is commonly ascribed to the operative demetallation of their ORR-active sites, the electro-oxidation of the carbonaceous matrix that hosts these active centers, and/or the chemical degradation of the ionomer, active sites, and/or carbon support by radicals derived from the H2O2 produced as an ORR byproduct. However, little is known regarding the relative contributions of these mechanisms to the overall PEMFC performance loss. With this motivation, in this study, we combined four degradation protocols entailing different cathode gas feeds (i.e., air vs N2), potential hold values, and durations to decouple the relative impact of the above deactivation mechanisms to the overall performance decay. Our results indicate that H2O2-related instability does not depend on the operative voltage but only on the ORR charge. Moreover, the electro-oxidation of the carbon matrix at high potentials (which for the catalyst tested herein triggers at 0.7 V) seems to be more detrimental to the NNMCs' activity than the demetallation occurring at low potentials.

Automated summary

Non-noble metal catalysts (NNMCs) have the potential to replace expensive Pt-based materials in proton exchange membrane fuel cells (PEMFCs), but their poor durability hinders their commercial implementation. The degradation mechanisms include operative demetallation, electro-oxidation of the carbonaceous matrix, and chemical degradation caused by H2O2 radicals. However, the relative contributions of these mechanisms to overall performance loss are not well understood. In this study, degradation protocols were used to determine that H2O2-related instability is dependent on the ORR charge, while electro-oxidation of the carbon matrix at high potentials is more detrimental to NNMCs' activity than demetallation at low potentials.