Separators and Membranes for Advanced Alkaline Water Electrolysis

Our plain-English summary of Separators and Membranes for Advanced Alkaline Water Electrolysis, by D. Henkensmeier, W.C. Cho, P. Jannasch, et al (Chemical Reviews, 2024).

The study

• Authors: D. Henkensmeier, W.C. Cho, P. Jannasch, J. Stojadinovic, Q. Li, D. Aili, J.O. Jensen
• Journal: Chemical Reviews
• Published: 2024;124(10):6393-6443
• Type of article: Review
• Cited by: 66 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Traditionally, alkaline water electrolysis (AWE) uses diaphragms to separate anode and cathode and is operated with 5-7 M KOH feed solutions. The ban of asbestos diaphragms led to the development of polymeric diaphragms, which are now the state of the art material. A promising alternative is the ion solvating membrane. Recent developments show that high conductivities can also be obtained in 1 M KOH. A third technology is based on anion exchange membranes (AEM); because these systems use 0-1 M KOH feed solutions to balance the trade-off between conductivity and the AEM's lifetime in alkaline environment, it makes sense to treat them separately as AEM WE. However, the lifetime of AEM increased strongly over the last 10 years, and some electrode-related issues like oxidation of the ionomer binder at the anode can be mitigated by using KOH feed solutions. Therefore, AWE and AEM WE may get more similar in the future, and this review focuses on the developments in polymeric diaphragms, ion solvating membranes, and AEM.

In plain English

This article was a review of materials that separate the two electrodes in alkaline water electrolysis, a method of producing hydrogen. It was not a study of asbestos exposure or human health, and it reported no data on workers or disease. The authors reviewed roughly the last 10 years of published work on polymer-based porous diaphragms, ion solvating membranes and anion exchange membranes. Asbestos appeared only as background. The authors wrote that chrysotile had served as a porous separator in industrial alkaline electrolyzers for the past 50 years, and that it resists strong alkali at low temperatures but fails above 85 °C. They also wrote that asbestos has been banned from commercial use in more than 60 countries because of health hazards when the fibrils are airborne and inhaled. The best-known replacement is Zirfon, a composite of polysulfone and zirconium oxide. Across published cells, those with coated nickel electrodes reached up to 1.9 V (plus or minus 0.1 V) at 1 ampere per square centimetre. The authors concluded that ion solvating membranes are a promising alternative and that reliable methods to predict membrane lifetime are lacking.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Review of published research (chemistry and engineering) on separators and membranes for alkaline water electrolysis; not an epidemiological or health study
• Who was studied: No human or animal population; the review covered published literature on electrolyzer separator materials from roughly the last 10 years (for example, literature since 2011 on imidazolium-based ionenes and since 2015 on AEMs prepared by polyhydroxyalkylation)
• Exposure: No human exposure was studied. Chrysotile asbestos is discussed only as the historical separator material in industrial alkaline electrolyzers, which polymeric diaphragms (such as polyphenylene sulfide felts and Zirfon) replaced after asbestos bans
• Main results:
  • The authors described chrysotile as resistant to strong bases such as KOH at low temperatures. Above 85 °C its silica component dissolves, which leads to diaphragm failure.
  • As background (not a finding of this review), the authors stated that asbestos has been banned from commercial use in more than 60 countries because of health hazards when the fibrils are airborne and inhaled.
  • Zirfon, the most commonly known replacement porous separator, has a thickness of 500 ± 50 μm, a porosity of 55 ± 10% and a pore diameter in the range of 150 ± 50 nm.
  • Among published cells with diaphragms and uncoated nickel electrodes, average performance was 2.3 ± 0.1 V at 1 A cm−2 (64% efficiency). Cells with coated (advanced) electrodes reached a maximum of 1.9 ± 0.1 V at 1 A cm−2 (77% efficiency).
• The authors' conclusion: The authors concluded that ion solvating membranes are emerging as a promising alternative to anion exchange membranes and porous diaphragms. They also concluded that little is known about membrane lifetime under real operating conditions and that reliable methods to predict lifetime are lacking.
• Limitations the authors state:
  • The authors noted that differences in cell setups limit how far performances measured by different research groups can be compared.
  • The authors stated that little is known about membrane lifetime under real operating conditions.

Topics

Medical terms in this study, explained in our Lexicon: Diaphragm, State-of-the-Art Defense.

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