Investigating Degradation Mechanisms in PtCo Alloy Catalysts: The Role of Co Content and a Pt-Rich Shell Using High-Energy Resolution Fluorescence Detection X-ray Absorption Spectroscopy.

Low Pt-based alloy catalysts are regarded as an efficient strategy in achieving high activity for the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). However, the desired durability for the low Pt-based catalysts, such as the PtCo catalyst, has still been considered a great challenge for PEMFCs. In this study, we investigate sub-2.5 nm PtCo alloy catalysts with varying Co content and PtCo@Pt core-shell (CS) nanostructure catalysts obtained through a simple displacement reaction. The PtCo@Pt_H catalysts showed a high mass activity (MA) of 1.46 A/mg at 0.9 V and 14% MA loss after 10k accelerated degradation test (ADT) cycles, which suggested the improved stability compared with PtCo catalysts (52% MA loss). To clarify the degradation mechanism, high-energy resolution fluorescence detection X-ray absorption spectroscopy (XAS) was applied in addition to conventional advanced measurement techniques, including conventional XAS, to analyze the electronic state and structure changes during operation potentials. We found that introducing Co improves the catalysts' activity mainly from the strain effect, but an excessive amount of Co leads to increased Pt-oxidation, which accelerates the degradation of the catalysts. The PtCo@Pt_H catalyst shows high tolerance to Pt-oxidation, benefiting both the stability and activity. Our findings demonstrate an in-depth understanding of the degradation mechanism and the importance of designing PtCo CS nanostructures with optimal Co content for enhanced performance in PEMFCs.