Investigating the Influence of Pore Sizes in Metal-organic Frameworks (MOFs) on Electrochemical Properties

Abstract Three distinct MOFs; MOF-199, MOF-5, and UiO-66 - each having varying pore diameters and unique structural characteristics were synthesized and their molecular structures are confirmed using FT-IR, PXRD and FESEM techniques. Continuing the experiments, conductivity of MOFs were measured by EIS. The results indicate that UiO-66 exhibits the highest conductivity value, with a value of 2.32 × 10-5 Scm-1. Upon evaluating the effect of pore diameters with conductivity, different salt percentage were added into the MOFs to obtain the MOF-based free-standing electrolytes. Results obtained shows that different MOFs display varied conductivity values that consistently increase upon the addition of salt. However, each MOF demonstrates a unique optimized percentage of salt—was determined to be 10%, 60%, and 40%, specifically for MOF-199, MOF-5, and UiO-66, respectively and did not show significant correlations with the pore sizes of the MOFs. The highest conductivity value was observed for UiO-66 with 10% NaCl, reaching up to 7.35 × 10-5 Scm-1. 10% NaCl was substituted with 10% NaOH to evaluate the effect of different salt. Results obtained shows show minimal changes with a conductivity value of 4.61× 10-5 Scm-1. CV were conducted on the MOFs and their free-standing electrolytes showing leaf-shaped cyclic voltammogram demonstrate promising enhanced capacitive behaviour. Comparing the cyclic voltammograms of UiO-66 with 10% NaCl and 10 % NaOH separately, no significant changes observed confirming the conducting species in the free-standing electrolyte systems to be the Na+. Dielectric calculations were performed and agree well with the experimental values. BET results obtained for MOF-199 and UiO-66, along with their optimized MOF-based free-standing electrolytes, show that the surface area as well as the pore volume decreased, indicating that the pores were being occupied by Na+, thereby increasing the conductivity. In contrast to these two systems, MOF-5 displays unique properties where the surface area and pore sizes extended. Overall, results obtained through out this research supports their potential application of MOFs as hybrid electrolyte materials in the future.