Probing the effect of ionomer swelling on the stability of anion exchange membrane water electrolyzers

Anion exchange membrane water electrolyzers (AEMWEs) have drawn increased attention in recent years due to the potential of efficient, low-cost, green hydrogen production. AEMWEs typically require supporting electrolytes to reduce cell voltages and improve long-term stability. Here, we show that using 1 M KOH electrolyte at 60 degrees C, a hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI) catalyst coated membrane exhibits negligible degradation over 100 h at 1 A cm-2 and cell voltage of 1.74 V. Reducing the KOH concentration to 0.1 M or pure water results in substantially decreased voltage stability, which is hypothesized to be due to increased swelling of the ionomer in the catalyst layer and/or the membrane. We therefore examined an AEMWE system incorporating a crosslinked derivative of the HMT-PMBI membrane which exhibited 4 times lower volumetric swelling. No enhancement in the voltage stability was observed under pure water feed, suggesting that controlling membrane swelling does not impact durability in this instance. HMT-PMBI was then derivatized via benzylation, in order to maintain its solubility for ink formulation, yet decreasing its swelling by increasing its hydrophobicity. In contrast to the membrane, a reduction in ionomer swelling in the catalyst layer enhanced the lifetime of the AEMWE system four-fold operating with pure water.