Influence of magnetic agglomeration and multiphase composition on fuel cell PtCo3 alloy catalyst

Pt-Co alloys with high Co content are promising low-Pt catalysts for proton exchange membrane fuel cells (PEMFCs) but suffer from magnetic agglomeration and phase segregation. Here, Pt-Co catalysts with different Co contents (Pt: Co = 3:2, 1:1, and 1:3) were prepared to clarify mechanisms. Among them, PtCo 3 exhibited strong ferromagnetism (Hc > 300 Oe) and severe ink agglomeration (600-700 nm), which deteriorated catalyst layer fabrication. The magnetic agglomeration showed an inverse parabolic dependence on annealing temperature, governed by multiphase composition. With increasing temperature, PtCo 3 underwent three structural stages: insufficient alloying, complete alloying, and phase segregation. Complete alloying and reaching the Curie temperature point, the magnetic properties decrease. Whereas Co-rich phase segregation restored magnetic properties and intensified agglomeration. This work establishes the relationship among multiphase evolution, magnetic agglomeration, and electrochemical performance. The optimized PtCo 3 annealed at 700-750 °C achieved 0.341 A/mg Pt mass activity and 0.988 mA/cm 2 specific activity.