Abstract
The use of metal-organic frameworks (MOFs) as proton conductors for practical applications is challenging, owing to the difficulties in obtaining a MOF with the desired conductivity levels and stability, and the complications of incorporating a polycrystalline powder into a device. This thesis examines these challenges, and addresses methods that could be used to improve the suitability of MOFs for commercial applications. First, an overview of the difficulties in preparing, measuring, and interpreting the data obtained from alternating current electrochemical impedance spectroscopy of MOFs is presented. Next, an examination of three potential methods of improving the performance of a proton conducting MOF, trisodium 1,3,5-trihydroxy-2,4,6-benzene trisulfonate (Na3Pgs), will be discussed. First, ligand doping of the Na3Pgs system, with 1,3,5 benzenetriphosphonic acid, was performed via a novel isomorphous ligand replacement method. The conductivity increased by an order of magnitude to (2.1 ± 0.5) x 10^-2 S/cm, which is the highest conductivity reported for any MOF system. Secondly, loading heterocycle guests into Na3Pgs improves the conductivity, but controlled loading is a relatively unexplored area, and was examined using Na3Pgs loaded with 1,2,4-triazole (Tz). It was found that Tz is incorporated into the system as the triazolium cation (HTz), giving the general formula [Na(1-x)(HTz)x]3Pgs, and that this kinetically controlled loading is affected by many synthetic parameters, but precise control over such systems is achievable. Finally, as a proof-of-principle, Na3Pgs was mixed with Nafion®, to examine whether a MOF mixed-matrix membrane could be used in a fuel cell. Homogeneous membranes were formed, and conductivity is improved under anhydrous conditions above 150°C, but a more stable MOF is required before commercial applications can be considered.
