Superprotonic Mixed Matrix Membranes Based on Anionic MOFs Containing Dimethylammonium Cations for Sustainable Energy Devices

High proton conductivity and structural robustness are critical for advancing proton exchange membrane fuel cells (PEMFCs) in sustainable energy technologies. Here, we present a pH-controlled synthetic strategy that yields two cadmium-based metal-organic frameworks (MOFs), Cd-FAIA-1 (neutral) and Cd-FAIA-2 (anionic), both derived from a common amide-functionalized tetra-carboxylate linker. Structural analysis shows that Cd-FAIA-1 adopts a layered framework incorporating water molecules, whereas Cd-FAIA-2 forms a porous network hosting both water molecules and dimethylammonium (Me₂NH₂⁺) cations. These structural distinctions enable solvent-assisted proton conduction, with conductivities of 3.64 × 10^-4 S·cm^-1 for Cd-FAIA-1 and 8.76 × 10^-3 S·cm^-1 for Cd-FAIA-2 at 80 °C and 98% relative humidity (RH). The superior performance of Cd-FAIA-2 stems from the Me₂NH₂⁺-assisted proton transport. To boost practical applicability, Cd-FAIA-2 was incorporated into a polyvinylpyrrolidone-polyvinylidene fluoride (PVP-PVDF) matrix to fabricate mixed matrix membranes (MMMs). Remarkably, the membrane with a 60 wt % MOF loading (Cd-FAIA-2@MMM-60) delivered a superprotonic conductivity of 3.21 × 10^-2 S·cm^-1 under identical conditions─surpassing the pristine MOF by a wide margin. This improvement is attributed to cooperative interactions among Me₂NH₂⁺ cations, water molecules, amide groups, and the polymer network. Furthermore, the gram-scale synthesis of these MOFs highlights their promise for integration into sustainable electrochemical devices, particularly fuel cells.