Two nonporous MOFs with uncoordinated carboxylate groups: Fillers for enhancing the proton conductivities of nafion membrane

Abstract

Two nonporous MOFs [Ni(L)0.5 (Mbpy)(H2O)2]2 (1) and [Cd(H2L)(Mbpy)]n (2) (H4L = 5,5’-(butane-1,4-diylbis (oxy)) diisophthalic acid, Mbpy = 5,5′-dimethyl-2,2′-bipyridine) were obtained through hydrothermal reactions under different pH values reaction condition. Structure analyses reveal that there are uncoordinated deprotonated and protonated carboxylate groups in compounds 1 and 2, respectively, which are confirmed by IR spectra. The XRD and TG studies indicate that both of compounds exhibit good water and thermal stability. The proton conductivities of the Nafion membrane doped by compounds 1 and 2 have been studied. It was revealed that compound 2 can enhance the proton conductivity of the composite membrane about 29% higher than that of pure Nafion. The water uptakes of 1/Nafion and 2/Nafion composite membrane are similar and slightly higher than that of pure Nafion membrane. The apparently high proton conductivity of 2/Nafion membrane should be attributed to the high proton density of compound 2 framework, which is resulted by the protonated uncoordinated carboxylate acid groups.