Water Organization and Hydration‐Dependent Hydroxide Transport in Anion Exchange Membranes (AEM)

Hydroxide‐conducting anion exchange membranes (AEMs) are key components in alkaline fuel cells and other electrochemical devices, including water electrolyzers, CO 2 reduction systems, and redox flow batteries. Their development is constrained by a persistent trade‐off between ionic conductivity and dimensional stability, which is fundamentally governed by hydration. The quantity and organization of water within AEMs determine hydroxide mobility while influencing swelling, plasticization, and mechanical integrity. Despite extensive research, the relationship between hydration, transport mechanisms, and structural response remains insufficiently integrated. This review provides a mechanistic synthesis of hydration‐dependent hydroxide transport in AEMs, examining how water uptake and ion exchange capacity influence ion‐water coordination, ion‐cation interactions, and water‐domain connectivity. Evidence for vehicular and structural transport is critically evaluated using activation‐energy trends and hydration‐dependent performance data. The review further shows that conductivity gains at high hydration are often accompanied by disproportionate dimensional expansion arising from microstructural reorganization. A conceptual hydration‐window framework is proposed to link membrane chemistry, water organization, hydroxide transport, and structural stability, highlighting the importance of maintaining intermediate hydration to achieve high conductivity while preserving mechanical durability and guiding the design of robust hydroxide‐conducting AEMs.