Mn Doping to Break the Trade-Off of Ordering Degree and Particle Size for PtCo Intermetallic Catalysts for Heavy-Duty Fuel Cells

Developing effective strategies for designing highly ordered PtCo intermetallic catalysts is essential to enhance catalytic activity and stability for oxygen reduction in proton exchange membrane fuel cells (PEMFCs). Annealing at an elevated temperature (>900 °C) is recognized as a favorable approach to enhancing ordering. However, such higher annealing temperatures would make it challenging to maintain a small particle size due to the thermodynamic driving force (high surface energy). Here, to break the trade-off between ordering degree and particle size, we investigate a Mn-assisted strategy for synthesizing highly ordered L1₀-PtCo nanoparticles with controlled particle size. Mn, with low surface energy (1.55 J m⁻²), was doped into PtCo intermetallics to simultaneously reduce the surface energy and increase the phase transition driving force, which likely mitigates particle coarsening (<4 nm) and improves the ordering degree (increased by 50%). The developed L1₀-Pt₅₀Co₄₀Mn₁₀/NC catalyst was systematically studied in membrane electrode assemblies (MEAs) under HDV conditions using both the traditional accelerated stress test (AST) protocol and the Million Mile Fuel Cell Truck (M₂FCT) AST protocol, demonstrating promising performance and long-term durability. The MEA achieved high initial current densities of >1.65 A cm⁻² at 0.7 V, with less than 25% performance decay after 90,000 or 500-h AST cycles. Detailed electrochemical analysis and extensive post-microscopy characterizations provide possible mechanisms to elucidate catalyst/MEA degradation under two AST protocols. The AST under H₂-N₂ conditions (protocol 1) led to greater particle dissolution, whereas Ostwald ripening was dominant under H₂-air conditions (protocol 2). Computational modeling suggests that water molecules can stabilize additional Pt adatoms on Pt surfaces and accelerate Ostwald ripening under the H₂-air AST.