Ether Cleavage Decreases the Ion Exchange Capacity of Anion Exchange Membranes

Anion exchange membranes (AEMs) are integral to fuel
cells and
water electrolysis systems but suffer from poor durability under alkaline
conditions. Ether cleavage is an important failure pathway of poly(arylene
ether)-based AEMs that compromises both mechanical stability and ion
transport. While this degradation pathway is often studied in terms
of polymer fragmentation, the role of newly formed hydrophilic groups
has been largely overlooked. We show that polymer scission leads to
reduced mechanical rigidity, while the introduction of hydrophilic
groups partially mitigates this loss. Under alkaline conditions, phenoxide
groups formed during ether cleavage neutralize the polymer cations,
leading to a previously unreported loss of ion exchange capacity (IEC).
This IEC loss mechanism exacerbates the reduction in ionic conductivity,
emphasizing the severity of ether cleavage as a degradation pathway.
Recognizing that ether cleavage introduces significant chemical changes
beyond polymer fragmentation provides critical insights into its interplay
with other degradation mechanisms, such as the direct reduction of
cationic sites by E2 and SN2, and provides molecular-level
interpretations for the concurrent effects of polymer scission and
increased hydrophilicity on membrane performance.