Comparative Studies on the Proton Conductivities of Hafnium-Based Metal-Organic Frameworks and Related Chitosan or Nafion Composite Membranes.

Hafnium (Hf)-based UiO-66 series metal-organic frameworks (MOFs) have been widely studied on gas storage, gas separation, reduction reaction, and other aspects since they were first prepared in 2012, but there are few studies on proton conductivity. In this work, one Hf-based MOF, Hf-UiO-66-fum showing UiO-66 structure, also known as , was synthesized at room temperature using cheap fumaric acid as the bridging ligand, and then imidazole units were successfully introduced into to obatin a doped product, . Note that both and demonstrate excellent thermal, water, and acid-base stabilities. Expectedly, the maximum proton conductivity (σ) of (1.46 × 10 S·cm) is nearly 4 times greater than that of (3.98 × 10 S·cm) under 100 °C and 98% relative humidity (RH). To explore their possible practical application value, we doped them into chitosan (CS) or Nafion membranes as fillers, namely, , , and (, , and are the doping percentages of MOF in the membrane, respectively). Intriguingly, it was found that and indicated the highest σ values of 1.73 × 10 and 2.14 × 10 S·cm, respectively, under 100 °C and 98% RH and also revealed a high σ value of 4.87 × 10 S·cm under 80 °C and 98% RH, which showed varying degrees of enhancement compared to the original MOFs or pure CS and Nafion membranes. Our study illustrates that these Hf-based MOFs and related composite membranes offer great potential in electrochemical fields.