Theoretical Examination of the Hydroxide Transport in Cobaltocenium-Containing Polyelectrolytes

Polymers incorporating cobaltocenium groups have received
attention
as promising components of anion-exchange membranes (AEMs), exhibiting
a good balance of chemical stability and high ionic conductivity.
In this work, we analyze the hydroxide diffusion in the presence of
cobaltocenium cations in an aqueous environment based on the molecular
dynamics of model systems confined in one dimension to mimic the AEM
channels. In order to describe the proton hopping mechanism, the forces
are obtained from the electronic structure computed at the density-functional
tight-binding level. We find that the hydroxide diffusion depends
on the channel size, modulation of the electrostatic interactions
by the solvation shell, and its rearrangement ability. Hydroxide diffusion
proceeds via both the vehicular and structural diffusion mechanisms
with the latter playing a larger role at low diffusion coefficients.
The highest diffusion coefficient is observed under moderate water
densities (around half the density of liquid water) when there are
enough water molecules to form the solvation shell, reducing the electrostatic
interaction between ions, yet there is enough space for the water
rearrangements during the proton hopping. The effects of cobaltocenium
separation, orientation, chemical modifications, and the role of nuclear
quantum effects are also discussed.