Two water-rich amphiphilic and hydrophilic coordination polymers: syntheses, structures and proton conduction behaviors in Nafion composite membranes

Abstract

Amphiphilic {[Cd₃(L)₂(phen)₂(H₂O)₆]·2H₂O·2DMF}_n (1) (H₃L = 1,3,5-benzenetricarboxylic acid and phen = phenanthroline) and hydrophilic [Zn₃(L)₂(H₂O)₁₂]_n (2) were obtained via hydrothermal reactions. Compound 1 is less hydrophilic due to its hydrophobic phen ligands. Hydrogen bonds extend along one direction in compound 1 and two directions in compound 2; thus, the hydrogen bond continuity of compound 1 is less than that of compound 2. The proton conduction behaviors of the two compounds as fillers of Nafion composite membranes were studied. The results show that the optimal doping amounts of 1 and 2 are 5% and 3%, respectively, and the proton conductivities of 1/Nafion-5 and 2/Nafion-3 composite membranes are about 29% and 60% higher than the proton conductivity of the pure Nafion membrane, respectively. The relationships between the structures and properties have been explored; the inherent hydrogen bonding chains or networks in the crystal structures as well as the additional continuous hydrogen bonds formed between the hydrophilic groups on the surface of their particles and the –SO₃H groups of Nafion can provide more proton transfer pathways. The higher doping amount of compound 1 was attributed to its hydrophobic phen ligand, which can promote its compatibility and interactions with the Nafion membrane and reduce aggregation.