Activity and degradation of Pt–Co and Pt–Ni alloy catalysts for application in high-temperature PEM fuel cells

In the emerging hydrogen energy economy, proton-exchange membrane fuel cells (PEMFCs) serve as a key enabling technology, yet their cost is among other things dominated by platinum group metals-based cathode catalysts. This paper is focused on investigation of intermetallic Pt–Co and Pt–Ni nanoparticles supported on carbon (Ketjen black, reduced graphene oxide) as low-Pt-load candidates for high-temperature PEMFCs (HT-PEMFCs) operated at elevated temperature ∼180 °C in the presence of concentrated phosphoric acid. Catalytic activity toward the oxygen reduction reaction (ORR) was quantified by rotating electrode measurements (exchange current densities, Tafel slopes), and stability was probed by leaching in 97.6 wt% H$_3$PO$_4$ at 180 °C followed by post-exposure characterisation. A suite of techniques – XAS, XRD, TEM/EDS, XRF, Raman spectroscopy and ICP-OES – was used to study changes in composition and structure during degradation. All alloy catalysts showed in HClO$_4$ at 25 °C higher ORR activity than commercial Pt/C. However, exposure to concentrated H$_3$PO$_4$ at 180 °C caused electrochemically active surface area loss, reduced ORR activity and supported Pt crystallite growth, Co/Ni dissolution, and surface reorganisation. Comparatively, reduced graphene oxide-supported catalyst was more resistant to ripening and dealloying than its Ketjen black analogue, and Pt–Ni alloy was more stable than Pt–Co. Overall, the results disentangle the roles of the carbon support and alloy composition and outline activity – stability trade-offs that guide the design of low-Pt loading cathodes for HT-PEMFCs.