Multiphysics Modeling of Retained-Water Effects on Coupled Transport in PEM Fuel-Cell Catalyst Layers

External relative humidity specifies a boundary condition, but it does not uniquely determine the water retained within a fuel-cell catalyst layer after a high-humidity excursion. Here, we develop a three-dimensional, voxel-resolved multiphysics model in which a spatial retained-water field is carried from one humidity state to the next and coupled to oxygen, proton, and electron transport and local Pt reaction. At the same relative humidity, humidification and drying states can exhibit different water inventories, transport accessibility, and current, demonstrating path dependence within the sequential formulation. In the reference configuration, initial hydration lowers protonic losses, whereas continued water accumulation restricts oxygen-accessible pathways. The competition between these effects produces a maximum in the model-scaled current at RH ≈ 0.60, with a mean liquid-water saturation of ≈0.061. This maximum is specific to the reference configuration rather than universal. Across 13 distinct parameter configurations, the apparent peak shifts from RH ≈ 0.33 to 0.95, and in four configurations, it occurs at the upper boundary of the protocol. A cutoff-based fraction of threshold-qualified Pt sites also fails to provide a consistent surrogate for electrode current across regimes, showing that catalyst accessibility cannot be interpreted independently of the surrounding transport state. Together, these results distinguish imposed humidity, inherited water state, and catalyst accessibility as distinct coordinates of the model state and provide a falsifiable framework for investigating humidity-history effects in catalyst layers. Quantitative transfer to physical electrodes will require no-memory controls, protocol-sensitivity analysis, and independent experimental validation.