Chitosan Composite Membrane with Efficient Hydroxide Ion Transport via Nano‐Confined Hydrogen Bonding Network for Alkaline Zinc‐Based Flow Batteries

The development of membranes with rapid and selective ionic transport is imperative for diverse electrochemical energy conversion and storage systems, including fuel cells and flow batteries. However, the practical application of membranes is significantly hindered by their limited conductivity and stability under strong alkaline conditions. Herein, a unique composite membrane decorated with functional Cu²⁺ cross-linked chitosan (Cts-Cu-M) is reported and their high hydroxide ion conductivity and stability in alkaline flow batteries are demonstrated. The underlying hydroxide ions transport of the membrane through Cu²⁺ coordinated nano-confined channels with abundant hydrogen bonding network via Grotthuss (proton hopping) mechanism is proposed. Consequently, the Cts-Cu-M membrane achieves high hydroxide ion conductivity with an area resistance of 0.17 Ω cm² and enables an alkaline zinc-based flow battery to operate at 320 mA cm^-2, along with an energy efficiency of ≈80%. Furthermore, the membrane enables the battery for 200 cycles of long-cycle stability at a current density of 200 mA cm^-2. This study offers an in-depth understanding of ion transport for the design and preparation of high-performance membranes for energy storage devices and beyond.