Much
research has focused on the stability of substituted ammonium
salts in anion-exchange membranes (AEMs). While cation chemistry dictates
AEM stability, chemical degradation has been recently shown to be
significantly influenced by the hydration level at which the AEM operates.
At low hydration, it is now known that almost every quaternary ammonium
may suffer significant decomposition. In this work, we use molecular
dynamics simulations to explore the behavior of three common quaternary
ammonium cations with stoichiometric hydroxide concentration and at
very low hydration. We find that water preferentially solvates hydroxide
anions and hence when water is present in sufficient amount (more
than four water molecules per ion pair), stability of the cations
is expected to significantly improve. However, lower amounts of water
result in the formation of isolated molecular clusters and ammonium
hydroxide pairing that lead to degradation of the cation. The composition
and size of the water–hydroxide–cation clusters that
form is shown to be significantly affected by the cation chemistry.
Implications of the observed behavior are discussed in view of recent
experimental results on the differences in stability of these cations.
This study highlights, for the first time, the crucial importance
of studying hydroxide–water–cation interactions at low
hydration levels.
Colloidal protein-protein interactions (PPIs) of attractive and repulsive nature modulate the solubility of proteins, their aggregation, precipitation and crystallization. Such interactions are very important for many biotechnological processes, but are...
Currently, there
are two main challenges in state-of-the-art anion-exchange
membrane fuel cells (AEMFCs)first, cation degradation in the
presence of hydroxide anions; second, carbonation process during AEMFC
operation. Both degradation and carbonation processes lead to a significant
decrease in the ionic conductivity of the anion exchange membranes
(AEMs), and, in turn, in the AEMFC performance. In this work, we use
molecular dynamics simulations to bring first insights into the contributing
factors that lead to changes in the degradation of quaternary ammonium
cations due to the presence of carbonate anions. Focusing on low hydration
levels, we explore the behavior of benzyltrimethylammonium cation
(BTMA+) in the presence of a mixture of hydroxide and carbonate
anions at different water:cation ratios. Water is shown to have a
stronger affinity toward carbonate than hydroxide. Thus, the introduction
of carbonate anions effectively lowers the concentration of free hydroxide
anions and thereby decreases the conductivity of the AEM. Lower hydration
of the hydroxide anion, in turn, leads to higher coordination of hydroxide
compared with carbonate around BTMA+, hence increasing
the probability of degradation of the cation. Nonetheless, carbonate
competes with hydroxide in its interaction with cation, leading to
approximately 20% reduction in hydroxide coordination around the BTMA+ when carbonate is present. We examine in detail these two
competing factorssteric shielding of BTMA+ by carbonate
and effectively lower hydration of the hydroxidewhich are
critical for understanding the effect of carbonate on the stability
of quaternary ammonium cations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.