Quaternary Ammonium‐Free Membranes for Water Electrolysis with 1 m KOH

Abstract

While anion exchange membrane (AEM) water electrolysis has many advantages, its commercialization is impeded by the low alkaline stability of most AEM, due to the fragility of quaternary ammonium groups. Ion solvating membranes (ISM) can be an alternative, but so far require high alkaline concentrations. Here, it is shown that sulfonation of polybenzimidazole results in ISM which swell strongly in 1 m KOH. Crosslinking with dibromoxylene controls the swelling, and after activation conductivities of >100 mS cm−1 can be reached in 1 m KOH. Stability in 1 m KOH at 80 °C is excellent: Conductivity remains unchanged and tensile strength and Young's modulus remains high over the test period of a half year. In an electrolyzer operating with 1 m KOH feed solution at 80 °C, a stable performance is achieved for over 500 h test without failure, suggesting that the high alkaline stability observed in ex situ tests is also achieved in the electrolyzer.

Separators and Membranes for Advanced Alkaline Water Electrolysis

“…In a very recent work, Dayan et al showed that sulfonated p -PBI absorbs so large amounts of electrolyte that it needs to be cross-linked to control the swelling. These membranes survived 6 months in 1 M KOH at 80 °C without loss of mechanical properties, showed a conductivity of >100 mS cm –1 at room temperature in 1 M KOH, and showed a very stable performance in an AEMWE over the tested 500 h. This work shows that the problem of low alkaline stability of AEMs could simply be solved by substituting AEMs with ISMs …”

“…This opens the field for membranes which are not or less ion-selective. Membranes which have features of proton exchange membranes and ISM, for example sulfonated PBI, can become a field of future research. , Such membranes are basically ISM and potentially will have higher conductivity than AEM or traditional ISM like KOH doped m -PBI, and avoid quaternary ammonium groups. In principle, aromatic sulfonic acid groups can be substituted by hydroxides, but at least on a preparative scale, very harsh reaction conditions are required for this degradation. , This potential degradation pathway will need to be investigated in future research.…”

“…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. , Furthermore, it was recently shown that an ion solvating membrane (ISM) based on sulfonated p -PBI (i.e., a quaternary-ammonium-free membrane) has a conductivity of >100 mS cm –1 in 1 M KOH solution . For these reasons, this current review will focus not only on porous diaphragms and ISM designed for traditional AWE, but will also discuss advances in the field of AEM.…”

Addressing the Challenge of Electrochemical Ionomer Oxidation in Future Anion Exchange Membrane Water Electrolyzers

“…The current technical challenge involves developing more-alkaline-stable membranes and advanced nanoparticle deposition that enable commercially viable performance and long-term electrolyzer operation (>10,000 h). Several recent papers reported sulfonation of ion-solvating PBI membranes enhances hydroxide conductivity by absorbing more KOH without a degradation penalty, offering new opportunities to develop high-performance AEMWEs. , …”

In-Situ Gradient Cross-Linking in Anion Exchange Membranes for Water Electrolyzers

“…To address the energy problem effectively, it is imperative to develop efficient and clean energy sources. Hydrogen is a highly promising option due to its high energy density, zero carbon emissions, and compatibility with renewable energy sources. − The anion exchange membrane water electrolyzer (AEMWE) is a promising hydrogen production technology that combines the advantages of alkaline water electrolyzer and proton exchange membrane water electrolyzer to achieve high hydrogen conversion efficiency while using non-precious-metal catalysts. − However, as a key component, anion exchange membranes (AEMs) lack in-depth research in water electrolyzers.…”