The effect of free carboxylic acid groups on the proton conductivity of a series of UiO-66-Ce(IV) metal-organic frameworks

Although Ce(IV)-based MOFs have similar structural characteristics to Zr(IV)-based MOFs, the research about proton conductive properties for the Ce(IV)-based MOFs is very limited. Herein, three Ce-MOFs, UiO-66-Ce, UiO-66-COOH-Ce, and UiO-66-(COOH)2-Ce built by similar carboxylate derivatives (1,4-benzenedicarboxylic acid (BDC), benzene-1,2,4-tricarboxylic acid (BDC-COOH) and benzene-1,2,4,5-tetracarboxylic acid (BTEC)) with similar structures are presented. Their excellent structural stabilities, including thermal and water stability, have been fully examined. Subsequently, their H2O-mediated proton conductivities (a) at variable relative humidities (RHs) and temperatures have been determined and compared. The results denoted that their a values could be boosted with the rising RH and temperature. More interestingly, under 100 degrees C/98% RH, the best a values of them can be sequentially enhanced being 1.63 x 10(-5), 2.25 x 10(-4), and 1.10 x 10(-3) S cm(-1), respectively, showing a positive correlation with free -COOH units within the frameworks. Such comparative studies provide new inspiration for the fabrication of Ce(IV)-based MOFs with high stability and excellent a values, which would be used in energy and related fields.

Automated summary

In this study, three Ce-MOFs with similar structures were constructed and their structural stabilities and proton conductive properties in water were investigated. The results showed that the proton conductivity of the MOFs increased with higher relative humidity and temperature. MOFs with more free -COOH groups exhibited better proton conductivity. These comparative studies provide new inspiration for the fabrication of Ce(IV)-based MOFs with high stability and excellent proton conductivity.