High Protonic Conductivity of Three Highly Stable Nanoscale Hafnium(IV) Metal–Organic Frameworks and Their Imidazole-Loaded Products

Attracted by the exceptional structural rigidity and inherent porous structures of the Hf-based metal-organic frameworks (MOFs), we adopted a rapid synthesis approach to preparing three nanoscale MOFs, Hf-UiO-66 (1), Hf-UiO-66-(OH)2 (2), and Hf-UiO-66-NH2 (3), and systematically explored the water-assisted proton conductivities of the original ones and the post-modified products. Interestingly, the proton conductivities (σ) of all three MOFs exhibit significant temperature and humidity dependence. At 98% RH and 100 °C, their optimal σ values can reach up to 10-3 S·cm-1. Consequently, imidazole units are loaded into 1-3 to obtain related MOFs, Im@1, Im@2, and Im@3, and the σ values of the imidazole-loaded products are boosted to 10-2 S·cm-1. Note that these modifications not only do not change the frameworks of the pristine MOFs but also do not affect their high chemical and water stability. The proton-conductive mechanisms of these MOFs before and after modification have been thoroughly discussed based on structural analyses, N2 and H2O vapor adsorptions, and activation energy values. The excellent structural stability as well as the durability and stability of their proton conduction ability indicate that these MOFs can be used in the field of fuel cells and so on.