Sengupta S, Lyulin AV. Molecular Modeling of Structure and Dynamics of Nafion Protonation States.
J Phys Chem B 2019;
123:6882-6891. [PMID:
31306017 PMCID:
PMC6691399 DOI:
10.1021/acs.jpcb.9b04534]
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Abstract
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We
present the results of the atomistic molecular dynamics modeling of
different protonation states of Nafion at varying hydration levels.
Previous experiments have shown that the degree of deprotonation (DDP)
of the sulfonic acid groups in a Nafion membrane varies significantly
upon hydration. Our goal is to provide insights into the effects of
variable protonation states and water content on the internal structure
and vehicular transport inside the Nafion membrane. The Nafion side
chain lengths showed a weak increasing trend with increasing DDP at
all hydration levels, exposing more of the sulfonic acid groups to
the hydrophilic/water phase. The water-phase characteristic size/diameter
decreased with increasing DDP, but, interestingly, the average number
of water molecules per cluster increased. The probability of water–hydronium
hydrogen bond formation decreased with increasing DDP, despite an
increase in the total number of such hydrogen bonds. The water diffusion
was largely unaffected by the state of deprotonation. In contrast
to that, the hydronium ion diffusion slowed down with increasing DDP
in the overall membrane. The hydronium ion residence times around
the sulfonic acid group increased with increasing DDP. Our simulations
show a strong connection between the morphology of the water domains
and protonation states of Nafion. Such a connection can also be expected
in polyelectrolyte membranes similar to Nafion.
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