Molecular Mechanisms of Water-Mediated Proton Transport in MIL-53 Metal-Organic Frameworks

Metal-organic frameworks have recently been proposed as promising proton conducting materials for application in fuel cell technologies. Here, molecular dynamics simulations are used to reveal the microscopic mechanisms associated with water-mediated proton transport in the MIL-53 materials as a function of temperature, water loading, and pore size. The structure of the hydrated proton is found to resemble that of a distorted Zundel complex when the framework closes into a narrow-pore configuration. A transition to Eigen-like structures is then observed at higher water loading when the pores open as a result of the breathing effect. Although the free-energy barriers to proton transfer at room temperature are lower than in bulk water; proton transport in MIL-53 is largely suppressed, which is attributed to the low water mobility inside the pores. Faster proton diffusion is found at higher temperature, in agreement with conductivity measurements.