Sulfur-anchored, high-loading L10-PtCo intermetallic catalysts for durable heavy-duty PEM fuel cells

Proton exchange membrane fuel cells (PEMFC) for heavy-duty transport require cathode catalysts that combine high platinum utilization with exceptional resistance to electrochemical degradation, yet the failure mechanisms of intermetallic alloys under realistic operating conditions remain poorly resolved. In heavy-duty applications, high Pt loadings are further required to maintain thin catalyst layers, thereby enhancing mass transport and overall cell performance. Here we report a high-loaded (> 50 wt% Pt) L10-PtCo nanocatalyst on sulfurdoped carbon that couples strong metal-support interactions with an ordered intermetallic lattice to deliver both high oxygen reduction activity and outstanding stability under fuel-cellrelevant conditions. The catalyst reaches a mass activity of 1.28 A/mgPt at 0.9 V and, after 30,000 accelerated stress-test cycles, retains nearly its full electrochemically active surface area with only 2% loss, while substantially outperforming commercial PtCo/C and Pt/C benchmarks. In a 50 cm2 membrane electrode assembly, the catalyst translates this intrinsic reactivity into practical device performance. The sulfur-doped carbon-support also markedly suppresses carbon corrosion under start-up/shut-down-relevant conditions, limiting carbon mass loss to about 11%, versus 30% for PtCo/C. Operando small- and wide-angle X-ray scattering measurements in a working PEMFC, combined with online inductively coupled plasma mass spectroscopy, reveals the structural origin of this durability: suppressed Pt dissolution, delayed Co leaching, and mitigation of agglomeration. These measurements further uncover a coalescence-induced dealloying pathway and identify an anisotropic lattice evolution in the L10-core during potential cycling. These results provide mechanistic insight into degradation processes in intermetallic catalysts under operating conditions and establish design rules for durable PEMFC cathodes by stabilizing lattice structure and mitigating transition metal dissolution.