Alkaline Stable Cross-Linked Anion Exchange Membrane Based on Steric Hindrance Effect and Microphase-Separated Structure for Water Electrolyzer

Abstract

Anion-exchange membranes (AEMs) with high ion conductivity often encounter challenges associated with excessive water uptake and dilution effects, which can impact alkaline stability and mechanical properties. The preferred polymer backbone for this purpose is polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS), renowned for its ether-free backbone and excellent chemical stability in alkaline environments. Our focus was on cross-linked SEBS (X-SEBS-BC) AEMs with an environmentally friendly synthesis method in the presence of SnCl4 at mild conditions. The study delves into using a cross-linking agent, 1,4-diazbicyclo[2.2.2]octane (DABCO), to form ion-conducting channels and address alkaline stability concerns. This is achieved through the rigid cage-like structure of DABCO, which hinders syn-periplanar conformational changes and protects against the degradation of SN2 and ylide reactions. A detailed investigation was carried out in terms of conductivity, water uptake (WU), swelling ratio (SR), and alkaline stability of the X-SEBS-BC membrane. X-SEBS-BC membranes exhibit low swelling, high conductivity, and significant chemical stability in alkaline environments. Well-defined microphase-separated morphology of membranes is studied using TEM which can support the ion transport channel of membrane. The alkaline stable X-SEBS-BC-0.81 membrane was not degraded by 3 M KOH for 30 days at 30 °C and degraded by 3.3% in a 1 M KOH environment for 600 h at 50 °C. The performance of the water electrolysis cell with an X-SEBS-BC-0.81 demonstrated exceptional performance (0.95 Acm−2 at 2.0 V), which is 82% higher than that of the commercial FAA-3-50 membrane. The study demonstrates that the X-SEBS-BC membrane is an effective strategy for the preparation of AEMs with enhanced properties, particularly in the context of water electrolysis.

Keywords: chloromethylation, cage structure, Anion-exchange membrane, phase separation, Electrolysis