Oxidative instability of ionomers in hydroxide-exchange-membrane electrolyzers

Abstract

Hydroxide exchange membrane (HEM) electrolyzers can produce green H2 with only earth-abundant catalysts and electrolyte-free (nominally pure) water feed, significantly decreasing system cost and complexity. However, HEM technology suffers from short lifetimes, attributed in part to poor stability of anion-exchange polymers used in the membrane and catalyst layers. Here, we use electrochemical analysis and ex-situ characterization techniques to study anion exchange polymer degradation in electrolyzers. Using multiple ionomers, catalyst layer additives, and electrolyte feed, we show anode ionomer oxidation is the dominant degradation mechanism for all HEM-based electrolyzers tested. We show improved device stability using oxidation-resistant catalyst layer binders and offer new design strategies for advanced ionomer and catalyst layer development.

Separators and Membranes for Advanced Alkaline Water Electrolysis

“…This is due to (a) the increasing conductivity when KOH is added to water, up to about 5–7 M, (b) a trade-off between high mechanical strength (low swelling) and efficient hydroxide transport in ionomer binder systems, and (c) the anodic oxidation of anion conducting polymers, which is a major reason for AEM WE degradation. The latter is slowed when KOH hinders direct contact between the polymer and the catalyst particles by participating in the formation of the double layer …”

“…The latter is slowed when KOH hinders direct contact between the polymer and the catalyst particles by participating in the formation of the double layer. 26 Based on these facts, we expect that the increasing stability of AEM, and the improved performance when the feed solutions contain KOH, will shift the research direction away from pure water to KOH-containing feed solutions. In this light, it could even be that future WE systems will use KOH concentrations between the upper 1 M KOH limit for AEM WE followed by most researchers today, and the 5−7 M used in traditional alkaline systems.…”