Designing Proton Conductivity in a Metal-Organic Framework from a Molecular Scale

Abstract

Two design strategies were investigated to enhance proton conductivity of a proton conducting MOF named β-PCMOF2. First design strategy was isomorphous ligand replacement where an entire C3-symmetric trisulfonate ligand was substituted with a C3-symmetric tris(hydrogen phosphonate) ligand to yield PCMOF2½, which had its proton conductivity raised 1.5 orders of magnitude, to 2.1 × 10-2 S cm-1 at 85 °C and 90% relative humidity compared to the parent material, while maintaining the parent MOF structure. To further enhance the proton conductivity of PCMOF2½, isomorphous ligand replacement was paired with heterocycle doping. Seven new PCMOFs were synthesized and investigated. One resulting material, PCMOF2½(Pyrazole), had its proton conductivity raised 1.9 orders of magnitude compared to the parent material, to 1.1 × 10-1 S cm-1 at 85 °C and 90% relative humidity, while maintaining the parent MOF structure. In addition, the exact mechanism of isomorphous ligand replacement synthesis was elucidated to be a thermodynamically driven solid state reaction.