Carbon encapsulation dynamics for the solid-state synthesis of high-loading sub-3 nm PtNi alloy electrocatalysts

Developing techniques to synthesize Pt alloy nanoparticles (NPs) with small, uniform sizes while suppressing agglomeration and metal leaching is essential for fuel cell applications. Herein, we fabricated highly durable, high-loading PtNi alloy catalysts (45 wt%) via scalble solid-state synthesis and evaluated their structural and electrochemical properties. Ball-milled Pt(acac)₂ and Ni(acac)₂ precursors with carbon supports were thermally treated to synthesize sub-3 nm PtNi alloy NPs. These NPs were encapsulated in thin carbon shells formed via thermal decomposition of ligands, ensuring stable dispersion even at high-metal loading. In-situ XRD and TEM revealed a dynamic carbon shell formation mechanism: carbon atoms were absorbed into NPs during high-temperature Pt lattice expansion and released during cooling to form shells—elucidated in real time for the first time. Half-cell tests showed Pt₃Ni@C/C and PtNi@C/C exhibited 1.9-fold and 1.7-fold higher mass activity than Pt/C. After 90k AST cycles, Pt₃Ni@C/C exhibited only 11.3 % electrochemical surface area (ECSA) loss and 18.9 % mass activity (MA) loss, outperforming Pt/C, which showed 37.8 % ECSA and 41.2 % MA losses after 30k cycles. Single-cell tests confirmed a voltage loss of only 19 mV at 0.8 A cm⁻², meeting the DOE 2025 target (<30 mV). These results highlight the potential of solid-state synthesis for practical fuel cell catalysts.