Asymmetric Interfacial Demulsification Creates Functional Wettability Gradients in Microporous Layers for PEM Fuel Cells

ABSTRACT Precise regulation of coupled liquid‐water and gas transport in microporous layers (MPLs) remains a major barrier to robust proton exchange membrane fuel cell (PEMFC) operation across wide temperature and humidity windows. Here, we report a scalable asymmetric interfacial demulsification strategy for constructing MPLs with a continuous through‐thickness wettability gradient (Gi‐MPL). The Gi‐MPL integrates a more hydrophilic catalyst‐layer‐facing region with a hydrophobic bulk, thereby promoting directional water transport while preserving gas‐accessible pathways. Structural characterization reveals a clear sulfur‐content gradient across the MPL thickness, and transport measurements show anisotropic liquid‐water breakthrough behavior consistent with directional transport asymmetry. In membrane electrode assemblies, the Gi‐MPL substantially outperforms a homogeneous control MPL over a broad operating window. At 80°C, it delivers a peak power density of 2.2 W cm −2 and reduces the concentration‐polarization loss by approximately 80% at 3 A cm −2 . More notably, under harsh operation at 105°C, the Gi‐MPL also maintains a peak power density of 1.4 W cm −2 , together with a clear advantage in durability. Multiscale simulations further suggest that the wettability gradient enhances liquid‐water permeability under weak driving forces. These results establish through‐thickness wettability engineering as an effective materials strategy for high‐current‐density PEMFC operation.