Microscopic insights into UiO-66@proton exchange composite membrane by molecular dynamics simulation

Abstract

UiO-66 as one of the known metal-organic frameworks (MOFs) has been recognized as highly promising dopants for enhancing the proton conductivity of proton exchange membrane (PEM) owing to the large pore volume and structure tunability. Despite the numerous experimental reports on MOF-doped PEMs, their increased proton conductivity is commonly ascribed to enhanced water uptake. The underlying mechanisms from a microscopic perspective remain elusive. Therefore, this work explores the microstructure and water diffusion dynamics within composite membranes to decipher their mechanisms involved in proton conductivity using molecular dynamics (MD) simulations. Four types of composite membranes based on two representative MOFs i.e. UiO-66 and UiO-66-NH₂ and two PEMs including Nafion and Dow, respectively, were taken into account. It is revealed that the UiO-66-NH₂ doped Nafion composite membrane exhibits the highest water uptake among the four composite membranes resulting from the super hydrophilicity of UiO-66-NH₂. Besides, the more concentrated distribution of sulfonic groups near the water-PEM interface and the higher interface roughness of UiO-66-NH₂ doped Nafion lead to more water molecules surrounding its sulfonic groups that are favorable for the proton dissociation from sulfonic groups. Furthermore, the increased water channel connectivity of MOF-doped membranes that promotes proton transport through water via the Grotthuss mechanism demonstrates one of the mechanisms for increased proton conductivity. On the other hand, although the reduced lifetime of the hydrogen bond network and the enhanced water diffusion coefficient within MOF-doped membranes manifest the favorable proton transfer via the Vehicle mechanism. Overall, UiO-66-NH₂ doped Nafion membranes exhibiting the highest water channel connectivity and water diffusion coefficients demonstrate the greatest potential of UiO-66-NH₂ doping in advancing the proton conductivity. These findings provided microscopic insights into understanding the improved proton conductivity mechanism of MOF doped PEMs, and the approaches developed in this work may be extended to other composite membranes.