High Proton Conduction Behavior of a Water-Stable Cadmium Organic Framework and Its Polymer Composite Membranes

Abstract

A water-stable metal-organic framework material of [Cd2 (pbdc)(H2O)3](1) has been synthesized based on H4pbdc (H4pbdc = 5-phosphonobenzene-1,3-dicarboxylic acid) ligands and Cd2+ ions. The Cd2+ ions are interacted with phosphonate and carboxylate groups to form infinitely extending one-dimensional chains, which are then bridged by pbdc ligands to generate a three-dimensional structure. AC impedance study found that the proton conductivity of compound 1 is 2.32 × 10−4 S·cm−1 at 333 K and 98% RH. Many MOF-polymer composite materials have excellent properties (such as heat resistance, high proton conductivity, etc.) and can be used in fuel cells and other fields. Thus, compound 1 was added to the CS matrix to prepare a series of composite membrane 1@CS-X (X = 1%, 3%, 5%, and 7% wt), which have good thermal stability and high proton conductivity. Under the same conditions, the effects of the 1@CS-X composite membranes on proton conduction are better than that of 1, which is caused by the hydrogen bond network constructed between CS and 1, and that provides a continuous channel for proton transfer. Strikingly, the proton conduction value of 1@CS-3 reached 1.10 × 10−2 S·cm −1 353 K and 98% RH, which is much better than many recently reported MOF-polymer composites.

Comparative study on proton conductivity of a polypyridinyl multicarboxylate-based hydrogen-bonded organic framework and related chitosan composite membrane

“…Although the optimal σ value of the CS/HOF-4 hybrid membrane is smaller than that of some CS/MOF composite membranes, 16,21,22 it is comparable to the values of some composite membranes 17–19 and greater than those of some other composite membranes under similar test conditions 14,20,31,32 (see Table 1). It should be noted that the composite membrane prepared by us represents the first time that a HOF has been introduced into the CS membrane, which is relatively rare.…”

“…[11][12][13] It has been proved that the above methods can effectively combine the advantages of other materials with CS and expand the application of CS membranes in the field of proton conduction. [14][15][16][17][18][19][20] The resulting CS-based composite membranes with other fillers such as functionalized nanotubes, 15,16 graphene oxide, 17 sulfonated SiO 2 , 18 heteropolyacids, 19,20 and metal-organic frameworks 14,21,22 usually have excellent separation characteristics and physical and chemical stability. Unexpectedly, hydrogen-bonded organic frameworks (HOFs), which are promising crystalline proton-conductive materials, are rarely seen as additives in proton-conductive CS-based composite films.…”