One of the focus areas in proton exchange membrane fuel cell (PEMFC) research is the development of novel catalysts with improved activity for the oxygen reduction reaction (ORR), aiming to reduce the total noble metal loading on the cathode. Based on theoretical calculations, alloys composed of Pt and Y were proposed as possible alternative¹ to the well-known Pt transition metal alloy catalysts based on Ni² or Co.³ Subsequently, Pt₃Y bulk alloys,¹ as well as unsupported Pt_xY_z nanoparticles⁴ were prepared and tested as model systems in a rotating disk electrode (RDE) setup, showing promising ORR activity (~670 µA cm^-2_Pt). However, synthesis of carbon supported Pt-Y alloy catalysts remains challenging due to the rather non-noble nature of yttrium compared to platinum, so that commonly used wet chemical methods cannot be used. Furthermore, until now, carbon supported Pt_xY_z alloys have not been tested in actual PEMFCs due to the difficulty in scaling up the synthesis of carbon supported Pt_xY_z nanoparticles.

In this study, we present the preparation of a Pt_xY_z/C by reduction of an yttrium precursor on a commercial Pt/C catalyst. After physical (transmission electron microscopy, X-ray diffraction) and chemical characterization (elemental analysis), the ORR activity of the resulting Pt_xY_z/C catalyst was tested by RDE measurements, and subsequently integrated into the cathode of a membrane electrode assembly (MEA). In terms of surface area normalized ORR activity, the tested catalyst was indeed found to be approximately 2-3x superior compared to pure Pt/C. Furthermore, the rather large particle size of the Pt_xY_z nanoparticles provided a significant enhancement of the stability towards voltage cycling, based on an accelerated stress test (AST) consisting of triangular voltage scans between 0.6 and 1.0 V at 50 mV s^‑1 in a single-cell PEMFC (for details see Harzer et al.)⁵. Even though the initial mass activity of Pt_xY_z/C was approximately the same as that of the commercial Pt/C catalyst, due to the large particle size of the developed catalyst, Pt_xY_z/C retained ~90% of its initial activity, whereas Pt/C lost more than 70% in the same testing protocol over 30000 cycles. Hence, the H₂/air performance of the Pt_xY_z/C catalyst at end-of-test (EOT) was significantly superior compared to commercial Pt/C, as shown in Figure 1. The results of this study were further compared to a heat treated Pt/C catalyst (Pt/C-HT) with a similar average particle size compared to Pt_xY_z/C, which therefore also showed a high stability in the AST, on the expense of a low H₂/air performance, as shown in Figure 1.

Acknowledgments

The authors of this work would like to give special thanks to Umicore AG & Co KG who supported this research financially and scientifically. The efforts by the elemental analysis laboratory at the Technical University of Munich are greatly acknowledged. Furthermore, we would like to direct our thanks to Benjamin Strehle for his efforts to aid in the analysis of the X-ray diffractograms.

References

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