Engineering crystallinity and hydrated ion clusters via branched design for anion exchange membranes in water electrolysis

The shortage of anion exchange membranes (AEMs) with both high hydroxide conductivity and stable physicochemical properties remains a major impediment to the development of high-performance AEM water electrolysis (AEMWE) technology. Herein, we designed a series of branched AEMs with specific spatial configurations at the molecular level to tackle such a dilemma. The core of rational design incorporates rigid, non-rotatable, single-bonded branched monomers and spirobisindane-co-terphenyl structures to modulate branched rotational freedom and microphase separation. The low rotational freedom branched structure improves local chain stacking, enhances the crystallinity, and forms a dense network of interconnected micropores. Furthermore, the delicate design regulates the hydrated ionic cluster aggregation state, reducing the OH⁻ diffusion barriers within the polymer networks. Well-designed AEMs exhibit a low swelling ratio (< 18.0%) even with high water uptake (94.2%–101.3%) at 80 °C while possessing high conductivity (165.4 mS·cm⁻¹) and stabilizing in 1 M KOH for 1200 h. Impressively, the AEM was used to construct an IrO₂ anode AEMWE cell, which exhibits a performance of 4 A·cm⁻² at 2.0 V and more than 500 h of stable operation at 1 A·cm⁻². This study provides insights into the design of high-performance AEMs for energy conversion devices.