High proton conduction behavior in 12-connected 3D porous lanthanide–organic frameworks and their polymer composites

Two novel lanthanide–organic frameworks, {[Ln3(bpydb)3(HCOO)(OH)2(DMF)]·3DMF·xH2O}n (Ln = Eu3+ (1) and x = 2; Tb3+ (2) and x = 1; bpydbH2 is 4,4′-(4,4′-bipyridine-2,6-diyl) dibenzoic acid; DMF denotes N,N′-dimethylformamide), have been synthesized and structurally characterized. Topological analysis reveals that 1 and 2 are 12-connected frameworks with an fcu topology, which are based on rod-shaped {Ln6} as 12-connected nodes. AC impedance analysis shows that the proton conductivity of 1 and 2 reaches up to 1.7 × 10−4 S cm−1 at 325 K and 1.1 × 10−4 S cm−1 at 334 K, respectively, under 98% RH. Furthermore, the polymer composite membranes of poly(vinyl alcohol) (PVA) and 1 or 2 as fillers with different mass percentages (X%, with the composite membrane denoted as 1@PVA-X or 2@PVA-X) were fabricated. The proton conductivities of both 1@PVA-10 and 2@PVA-10 are higher than those of 1 and 2 under the same conditions, which can be interpreted that the PVA polymer can provide a continuous proton transfer pathway for 1 and 2. To the best of our knowledge, it is the first composite based on Ln–MOFs and a polymer as proton exchange membranes. This research may contribute to the further development of Ln–MOFs in the field of proton exchange membranes in fuel cells.