Microphase water control utilizing highly hydrophilic anion-exchange ionomers

Abstract

In anion-exchange membrane water electrolysis (AEMWE), the dry-cathode condition can simplify balance-of-plant systems and improve hydrogen purity, but it is prone to cathode drying, which leads to polarization loss. By optimizing the hydrophilicity of ionomers, the water uptake and retention capacity of the catalyst layer can be enhanced. Here, a three-dimensional (3D) ionomer, Pip(BPF-BPN)/Trip-x is developed, for AEMWE, a promising green hydrogen technology. The ionomer integrates triptycene and bisphenol fluorene units to increase free volume and enhance mass transport, while phenolic hydroxy groups establish hydrogen-bonding networks that enhance hydrophilicity and water mobility. This purposefully-structured ionomer facilitates efficient water/gas management within the catalyst layer. The ionomer shows excellent properties, including 53.6% higher water uptake, hydroxide conductivity of 188 mS cm⁻¹ at 80 °C, and <8% cation degradation after 1440 hours in alkaline conditions, compared to that without phenolic hydroxy groups. An AEMWE using this ionomer in the cathode achieves 6.1 A cm⁻² at 2.0 V (80 °C), outperforming the commercial PiperION-A40. These findings underscore a molecular design strategy that integrates 3D architecture and hydrophilic modification to enhance ion transport and device durability, thereby offering valuable insights for the advancement of next-generation AEMWE technology.