A 2D Mg(II)-MOF with High Density of Coordinated Waters as Sole Intrinsic Proton Sources for Ultrahigh Superprotonic Conduction

As an important type of crystalline solid proton-conducting material, metal-organic frameworks (MOFs) have received immense attention in the scientific community. In general, such MOFs are designed with intrinsic (H3O+, NH4+, Me2NH2+, H2PO4–, OH– located in pores or dangling −SO3H, −COOH, and −PO3H2 groups as integral parts of the backbone) or extrinsic proton sources (imidazole, triazole, histamine, H3PO4, H2SO4 as guests). It has long been known that metal ions enhance the acidity of coordinated H2O molecules; however, such metal ion-bound water molecules as sole intrinsic proton sources for the construction of MOF-based conductors are largely unexplored. By exploiting such a design principle, herein, we report the proton-conduction properties of Mg-OBA MOF with the molecular formula {[Mg2(OBA)2(H2O)6]·2H2O}n [OBA = 4,4′-oxybis(benzoic acid)], where half of the octahedral coordination sites around Mg(II) centers are occupied by water molecules. The 2D layered structure with hydrophilic interlayer spaces within the framework having abundant water coordination as the sole proton sources makes extended H-bonding pathway for efficient protons migration and thus exhibiting ultrahigh conductivity with the value of 1.27 × 10–2 S cm–1 at 80 °C and 95% relative humidity (RH). The activation energy (Ea = 0.13 eV) obtained is one of the lowest of MOF-based proton conductors suggesting facile proton transfer. Besides, this material is quite robust and easy to scale up in the aqueous medium.