  Anion exchange membranes are used in alkaline fuel cells and offer a
promising alternative to the more expensive proton exchange membrane fuel
cells. However, hydroxide ion conductivity in anion exchange membranes is low,
and the quest for membranes with superior ion conductivity, mechanical
robustness, and chemical stability is ongoing. In this study, we use classical
molecular dynamics simulations to study hydroxide ion transport and mechanical
properties of eight different hydrated polyolefin-based membranes, to provide a
molecular-level understanding of the structure-function relationships in these
systems. We examine the microstructure of the membranes and find that polymers
with narrow cavity size distribution have tighter packing of water molecules
around hydroxide ions. We estimate the self-diffusion coefficient of water and
hydroxide ions and find that water molecules have a higher diffusion than
hydroxide ions across all systems. The trends in hydroxide diffusion align well
with experimental conductivity measurements. Water facilitates hydroxide
diffusion, and this is clearly observed when the hydration level is varied for
the same polymer chemistry. In systems with narrow cavities and tightly bound
hydroxide ions, hydroxide diffusion is the lowest, underscoring the fact that
water channels facilitate hydroxide transport. Finally, we apply uniaxial
deformation to calculate the mechanical properties of these systems and find
that polymers with higher hydration levels show poor mechanical properties.
Atomistic molecular dynamics models can accurately capture the trade-off
between hydroxide transport and mechanical performance in anion exchange
membranes and allow us to screen new candidates more efficiently.
