Free volume, gas permeation, and proton conductivity in MIL-101-SO3H/Nafion composite membranes

Abstract

A series of MIL-101-SO₃H/Nafion composite membranes was synthesized. They show an improved proton conductivity, due to the abundance of SO₃H groups, which fosters proton conduction by binding the water molecules and enabling a larger number of conducting sites. Gas (including water vapor, hydrogen, and oxygen) permeability, crystallinity, and free volumes of the MIL-101-SO₃H/Nafion composite membranes were investigated, as well as their correlation. By increasing the MIL-101-SO₃H content, the gas permeability of the membranes significantly decreases, since the crystalline region is larger and the water-bearing MIL-101-SO₃H particles are efficient barriers for the gas molecules. The gas permeation in the composite membranes is a very complex process and the results indicate no simple linear relation between the gas permeability and the free volume size (V_FV), or between the gas permeability and the crystallinity. Moreover, it is very interesting to observe that the influence of V_FV on the gas permeability is closely related to the size of the particular gas molecules: the larger the size of the gas molecules, the larger the free volume needed to achieve their rapid diffusion in the membrane. The results suggest the presence of a threshold value for V_FV, which depends on the size of the gas molecules: when V_FV is lower than this value, the gas molecules cannot easily jump through neighboring free volumes to a neighboring site, and, as a result, the permeability drops quickly.