Calculation and Measurement of Salt Loading in Metal-Organic Frameworks

The inclusion of salts within the pores of metal- organic frameworks (MOFs) has been shown to increase the ionic conductivities of the salt ions relative to that of the bulk salt, raising the possibility that salt/MOF composites could be used as solid electrolytes. The present study explores the mechanism by which the inclusion of ionic liquids (ILs) occurs in MOFs and the mass loadings that can be achieved. Simulated annealing simulations and cyclic differential scanning calorimetry (DSC) experiments are used to investigate the loading of tetraethylammonium bis-(trifluoromethylsulfonyl)-imide ([NEt4][TFSI]) salt in three different UiO-6x MOFs: UiO-66, UiO-67, and a defective form of UiO-66 containing missing linkers. Good agreement is achieved between the predicted loadings and those measured experimentally. An analysis of the spatial distribution of salt ions within the MOF pores as a function of loading shows that repulsive anion-anion and cation-cation electrostatic interactions contribute to the upper limit of loading. The surface area and void fraction of the MOFs approach zero at the maximum loading, suggesting that the ions completely fill the MOF pores. The absence of free volume at the maximum loading is proposed as an explanation for the decrease in conductivity observed in prior experiments; hence, conductivity will be maximized for loadings that do not fill the MOF pores. Finally, the surface area and pore volume of the empty MOFs are shown to be good predictors of the maximum salt loading.

Automated summary

The inclusion of salts in metal-organic frameworks (MOFs) has been shown to increase ionic conductivities, suggesting their potential use as solid electrolytes. This study explores the mechanism and maximum loadings of including ionic liquids (ILs) in MOFs. The results suggest that electrostatic interactions contribute to the upper limit of loading and that conductivity is maximized for loadings that do not completely fill the MOF pores. The surface area and pore volume of empty MOFs are good predictors of the maximum salt loading.