Water Uptake in an Anion Exchange Membrane Based on Polyamine: A First-Principles Study

An atomistic level study of a single monomer of polyamine
interacting
with water molecules and hydroxide anions (OH–)
was carried out to investigate the role of the polyamine structure
in the hydrated morphology of anion exchange membranes (AEMs) for
alkaline fuel cells and its influence on ionic conductivity and chemical
stability. DFT calculations were performed to find the ground state
of the system, studying the interactions of the solvent species with
three different regions of the polymerthe amine functional
group, the backbone, and the carbonyl group. The hydrophilic/hydrophobic
behavior of each segment was determined, with calculated binding energies
and Bader charge analysis providing a more quantitative analysis of
the interactions and activation and reaction energies computed to
investigate the chemical degradation mechanism. The results show the
tendency of both OH– and water molecules to form
water clusters in the proximity of the ionized amine group. As such,
these regions constitute the preferential pathway for ionic conductivity.
Besides, the essential role of the water content is pointed out, not
only to enhance conductivity but also to reduce degradation in an
alkaline environment. The present work provides a baseline to assess
the impact of polymer chemistry on the ionic conductivity of the membrane
and acts as the first step for the development of high-performance
AEMs and for an improvement of the overall performance of the fuel
cell.