The application of proton exchange membrane fuel cells (PEMFCs) in light-duty vehicles (LDV) has dominated the fuel cell research field for the past decades. To reduce the LDV FC system cost to 30 $/kW by 2025 has driven the search for highly active cathode catalysts that could substitute the costly state-of-the-art Pt-alloy catalysts or reduce the cathode loadings to < 0.125 mg_Pt/cm².^{1, 2} As novel ORR catalyst continue to emerge, theoretical calculations have turned the attention on Pt-rare earth (RE) alloys, such as platinum-yttrium alloys that were suggested to have superior activity for the oxygen reduction reaction (ORR) and superior long-term stability compared to conventional carbon-supported platinum catalyst (Pt/C). ³ Based on these findings, many attempts have been made to produce high electrochemical surface area (ECSA) Pt-RE alloy catalysts.⁴ In recent years, Hu, et al. introduced a new synthesis procedure that utilizes a carboiimide-assisted synthesis for the preparation of Pt‑RE/C catalysts with an unprecedentedly high ECSAs in the range of ~50-60 m²/g_Pt.⁵ With such high ECSA Pt_xY/C catalysts, good PEMFC performance would be expected, but to date the ORR activity and stability of such Pt_xY/C catalysts has not been demonstrated in a PEMFC environment.

In view of this, we reproduced the synthesis reported by Hu et al. using a Ketjen Black (KB) carbon support. The resulting Pt_xY/KB catalyst was characterized by X-ray diffraction, X-ray spectroscopy (EDS) in scanning transmission electron microscopy (STEM), and elemental analysis, indicating the formation of a Pt-rich_shell/Pt₃Y_core structure that was caused by the acid washing procedure introduced before the electrochemical characterization.⁵ In our present study, two different Pt/KB catalysts were selected as reference materials: i) a Johnson Matthey reference catalyst consisting of small well-dispersed Pt nanoparticles on a KB support (Pt/KB‑JM); ii) an in‑house synthesized Pt/KB catalyst (Pt/KB‑syn.) with a similar particle size distribution and ECSA as the in-house synthesized Pt_xY/KB catalyst (Pt_xY/KB‑syn.). The ECSA (via CO-stripping voltammetry) and the ORR mass activity at 0.9 V vs. RHE (i^mass_{0.9 V}) of these three catalysts were first evaluated by rotating disk electrode (RDE) measurements in 0.1 M HClO₄ at 25 °C. The catalysts were also incorporated as cathode catalysts in membrane electrode assemblies (MEAs), first determining their ECSA (by CO-stripping) and then their H₂/O₂ and H₂/air performance in a 5 cm² single-cell PEMFC hardware at 80 °C under differential flow conditions, including a quantification of the ORR mass activity at 0.9 V vs. RHE at 100 kPa O₂.

As can be seen in Figure 1, the relative ECSA and ORR mass activity values of the three different catalysts obtained in the RDE and the PEMFC configuration were essentially identical when evaluated at the same ionomer to carbon ratio (I/C = 0.7, g/g). Furthermore, a comparison of the RDE ORR activity at two different I/C ratios showed that the Pt_xY/KB‑syn. and Pt/KB‑syn. catalysts were more susceptible to ionomer poisoning when compared to the Pt/KB‑JM catalyst. Following the approach taken in our previous publications, the voltage-cycling stability of the synthesized catalysts was also evaluated using an accelerated stress test (AST) consisting of a triangular potential scan between 0.6 and 1.0 V_RHE at 50 mV/s up to 30000 cycles.⁵ Overall, the RDE and PEMFC data showed that the here synthesized carbon-supported Pt-rich_shell/Pt₃Y_core catalyst does not shown an enhanced ORR activity nor a long-term stability benefit when compared to a Pt/KB catalyst with the same ECSA, contrary to what had been hypothesized earlier.³

References

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Acknowledgment

This work has been supported by the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No. 826097 (GAIA). This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe Research.