Tuning the Hydrophilicity of Nd-MOF Fillers from 3D Frameworks to 2D Layers for High-Performance Proton Exchange Composite Membranes

Abstract Metal–organic frameworks (MOFs) attract wide interest for proton exchange membranes (PEMs) owing to structural tunability and proton-conducting capability. Nevertheless, how MOF-filler hydrophilicity governs proton conductivity and optimal doping loading in Nafion composites is insufficiently understood. Herein, three Nd-based MOFs with varied hydrophilicity are synthesized: one 3D Nd-L1 and two 2D layered Nd-L2 and Nd-L3. Nd-L3 possesses the highest surface hydrophilicity despite limited hydrophilic groups, originating from evenly distributed coordinated water on its 2D layers. Within the Nafion matrix, composite-membrane proton conductivity positively follows MOF hydrophilicity. Under 363 K and 100% RH, Nd-L3/Nafion-0.3% delivers 0.261 S cm–1, 1.89-fold higher than pure Nafion. Conversely, the optimal doping content correlates negatively with hydrophilicity; the highly hydrophilic Nd-L3 is prone to aggregation, leading to the lowest optimal doping content. The structure–property relationship analysis indicates that 2D layered structures facilitate the exposure of hydrophilic sites on the surface, effectively constructing continuous hydrogen-bonding networks that promote proton conduction. This work elucidates the critical role of MOF hydrophilicity in balancing proton conductivity and filler dispersion, offering a new strategy for designing high-performance PEMs.