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