Low-Pt proton exchange membrane fuel cells (PEMFCs) require cathode catalyst layers (CCLs) capable of mitigating coupled performance-durability trade-offs under variable-humidity operating conditions. Here, a mechanism-guided through-plane ionomer-gradient design is developed for a low-Pt CCL (0.1 mg cm−2) by integrating a three-dimensional multiphase PEMFC model, an agglomerate submodel, and regional sensitivity analysis. The analysis reveals relative humidity (RH)-dependent functional demands along the CCL thickness, motivating an asymmetric dual-segment power-law profile (ADSPLP) to parameterize the ionomer-to-carbon (I/C) ratio distribution using six physically interpretable variables. Single-factor analyses clarify how these variables reshape local transport and reaction distributions, followed by surrogate-assisted multi-RH robust optimization using 50% and 100% RH as boundary scenarios. The results show that dry operation benefits mainly from membrane-side proton-access enhancement that suppresses ohmic loss, whereas fully humidified operation requires stronger oxygen access on the microporous-layer side to alleviate concentration loss. Both single-sided improvements, however, tend to shift the high-reaction region away from the membrane side and increase reaction non-uniformity. The optimized ADSPLP achieves performance close to that of the RH-specific optima by mitigating the dominant bottlenecks at the dry and humid extremes, while preserving balanced proton and oxygen access to deliver the largest gain at intermediate humidity. Relative to the corresponding best uniform I/C designs, it increases peak net power by 3.30%–4.39% across 50%–100% RH while effectively limiting the growth in reaction non-uniformity. This study provides a mechanism-guided gradient-CCL optimization strategy and design guidance for humidity-adaptive low-Pt membrane electrode assemblies.
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