Influence of Pt/C Mass Fraction on Catalyst Layer Architecture, Multiphase Transport, and Durability of CsH5(PO4)2-Doped Polybenzimidazole-Based High-Temperature Proton Exchange Membrane Fuel Cells

The catalyst layer (CL) structure critically governs Pt utilization, proton conduction, and oxygen transport in CsH5(PO4)2-doped polybenzimidazole (PBI) membrane-based high-temperature proton exchange membrane fuel cells (HT-PEMFCs). Herein, Pt/C catalysts with different Pt mass fractions (20, 40, and 60 wt %) are used to construct polytetrafluoroethylene (PTFE)-bonded catalyst layers while keeping the Pt loading, PTFE content, and phosphoric acid loading constant. Increasing the Pt mass fraction enlarges the Pt particle size from 1.57 ± 0.27 to 2.82 ± 0.55 nm and reduces the catalyst layer thickness from 98.9 to 41.8 μm. Pt-40 (40 wt % Pt/C) achieves the highest peak power densities of 1424 mW cm−2 under H2/O2 and 806 mW cm−2 under H2/air, owing to the balanced coupling among Pt accessibility, proton conduction, and oxygen transport. Although Pt-60 (60 wt % Pt/C) exhibits lower Tafel slope, charge-transfer resistance, and proton-transfer resistance, its performance is limited by severe oxygen mass-transport resistance, as supported by the distribution of relaxation times (DRT) analysis and reduced O2 mobility revealed by molecular dynamics simulations. After 5000 accelerated stress test (AST) cycles, Pt-60 shows the smallest apparent performance decay, which stems mainly from its initial mass-transport-limited state rather than inherent durability. This work clarifies the structure−transport−performance relationships of Pt/C-regulated catalyst layers in solid-acid-doped PBI membrane-based HT-PEMFCs.