Transport of hydronium ions inside poly(styrene-co-divinyl benzene) cation exchange membranes

The electric field-induced transport of hydronium ions in sulfonated poly(styrene-co-divinyl benzene) membranes has been investigated using atomistic Molecular Dynamics simulations. The diffusive behavior has been found to increase with the electric field, even though the diffusion coefficient is higher in the direction of the applied electric field than in the perpendicular directions. This anisotropy, which has been also detected in the velocities of the hydronium ions, is particularly pronounced for electric fields higher than 1.0 V/nm. Indeed, at such high electric fields the simulated systems are not able to reach the normal diffusive regime. The highest protonic conductivity has been obtained at 0.03 V/nm. At higher electric fields, the protonic current density increases too slowly (i.e. for electric fields ≤0.7 V/nm) or too rapidly (i.e. electric fields >0.7 V/nm) resulting in a reduction of the conductivity or in significant structural deformations of the membrane, respectively. Finally, the interactions between the hydronium ions and water molecules contained in the hydrated membranes have been analyzed and discussed considering the influence of the applied electric field. Specifically, results have allowed us to discuss the existence of different types of hydration shells, which differ in the number of water molecules contained in the shell and their exchangeability, and residence times.

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► The anisotropy in the diffusive behavior increases with the electric field.
► The conductivity is affected by the protonic current density and the electric field.
► Hydration shells around the cations remain unaffected by electric fields lower than 1.0 V/nm.
► High electric fields affect the number and exchangeability of molecules in the hydration shells.