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

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.