Monolayer LDH nanosheet / quaternized polymer composite membranes with enhanced anion conductivity and alkaline durability for water electrolysis

Abstract

Anion exchange membranes (AEMs) with high ion conductivity and alkaline resistance are of importance for achieving high performance and long-term durability of anion exchange membrane water electrolyzers (AEMWEs) and anion exchange membrane fuel cells (AEMFCs). In the present work, we have developed a series of composite membranes composed of inorganic two-dimensional (2D) layered double hydroxide nanosheets (LDH NS), with intrinsic alkaline stability and high hydroxide ion conductivity (approaching 10 -1 S cm -1 ), and our original partially fluorinated, quaternized polymer-based copolymers (QPAF-4) via a facile solution-casting process. The obtained organic/inorganic composite membranes (QPAF-4/LDH NS) exhibited excellent mechanical properties, characterized with a maximum elongation over 300%. A high hydroxide ion conductivity (102.0 mS cm -1 ) was achieved on an optimized percentage of LDH NS at 3 wt.%. The hydroxyl groups on the surface of the LDH NS provided strong hydrogen bonds and highly dense Grotthuss conduction sites, which were responsible for good compatibility with the QPAF-4 matrix, contributing to high mechanical strength and high ion conductivity. Furthermore, the LDH nanosheets mitigated the chemical degradation of the ammonium groups of QPAF-4. As a result, a high retention of 80% conductivity was realized for the composite AEMs under harsh alkaline conditions (8 M KOH at 80 o C for 1,000 h) compared to only 43% for the pristine QPAF-4 membranes under the same conditions. Furthermore, an alkaline water electrolysis cell was constructed by combining the optimized QPAF-4/LDH NS composite membrane with an Ni 2/3 Fe 1/3 LDH-rGO anode catalyst, which delivered a current density of 1 A cm -2 at 1.61 V (76.4% voltage efficiency), outperforming state-of-the-art AEM electrolysis systems.