The diffusion mechanism of water in conductive metal-organic frameworks

Water is one of the most important guest molecules in metal-organic frameworks (MOFs) since it often serves as a solvent for ions and other molecules. Studying the diffusion mechanism of water molecules in conductive MOFs (c-MOFs) is fundamental to harnessing the potential of c-MOFs in designing next generation energy storage devices. In this work, using molecular dynamics simulations, we show that water follows the Fickian-type of diffusion mechanism in different types of c-MOFs. We investigate the effect of the stacking and metal center type on the water diffusion coefficient in c-MOFs. Water in c-MOFs with eclipsed stacking is shown to have 21.5% higher diffusion coefficient than in c-MOFs with slipped-parallel stacking, and 4-8% higher diffusion coefficient than in bulk water. The physical reasons behind the reduced water diffusion coefficient in slipped-parallel stacking c-MOFs are the higher number of hydrogen bonds near the inner surface and the zig-zag geometry. This work provides a molecular insight into the water dynamics and water structure inside multiple types of c-MOFs.