Grafting Free Carboxylic Acid Groups onto the Pore Surface of 3D Porous Coordination Polymers for High Proton Conductivity

Development of proton
conduction materials with a high performance
via functionalization of porous coordination polymers (PCPs) is challenging
yet promising. Herein, a facile and efficient method to synthesize
multicarboxylate-group-functionalized rigid ligands has been developed.
Interestingly, when utilizing these ligands to prepare PCPs, the peripheral
carboxylate groups of the ligands selectively coordinate to the metal
ions for constructing the PCP frameworks while ortho-dicarboxylic acid groups on the central benzene ring remain free
in the pores. Single-crystal X-ray diffractions and various gas sorption
analyses reveal the characteristics of high-density free carboxylic
acid groups in the pores of these PCPs, not only forming hydrogen-bond
clusters and three-dimensional hydrogen-bond networks potentially,
but also stimulating the flexibility of these very robust PCP frameworks.
Based on this unique structure, the PCP functionalized with four free
carboxylic acids on every ligand (TJU-102) exhibits a high proton
conductivity of 5.26 × 10–2 S cm–1 at 90 °C and 98% relative humidity. The activation energy for
proton conduction of TJU-102 is ca. 0.59 eV, suggesting that the conduction
mechanism is predominantly vehicular.