Enhancing Proton Conductivity in a 3D Metal–Organic Framework by the Cooperation of Guest [Me₂NH₂]⁺ Cations, Water Molecules, and Host Carboxylates

Abstract

A novel proton-conducting MOF material has been synthesized by using 3,3′,4,4′-biphenyltetracarboxylic acid (H4bptc) as organic ligands, which linked to four [Zn(COO)4]2– secondary building units and K ions through four carboxyl groups, and generated a 3D framework [Zn3K2(bptc)3(DMF)2][Me2NH2]4 (JLU-Liu44). The proton-conducting properties of JLU-Liu44 were investigated by impedance spectroscopy. JLU-Liu44 exhibits low conductivity under anhydrous conditions, but the proton conductivity under humidified conditions was significantly enhanced due to the cooperation of guest [Me2NH2]+ cations, water molecules, and host carboxylates. JLU-Liu44 exhibits high proton conductivity of 8.4 × 10–3 S cm–1 at ambient temperature (27 °C) and 98% relative humidity; it is comparable to the most reported proton-conducting MOFs materials under the same condition for powdered sample. The activation energy (Ea) obtained from an Arrhenius plot was 0.25 eV, indicating that the proton conduction behaviors occur through a Grotthuss mechanism.