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A Brief Introduction to Anion Exchange Membrane for AEM Elect
Anion exchange membrane (AEM) has attracted much attention in recent years as a key component of low-cost water electrolysis systems, which can be combined with renewable energy and become a sustainable alternative to fossil fuels for hydrogen production. Developing potential materials for the production and characterization of AEM is an important step towards its commercialization and has a competitive advantage in the hydrogen production industry.
The main difference between the mature alkaline water electrolysis technology (AWE) and AEMWE is that although the separator used to separate two half cells in AWE allows ion transport across the system, it is non-conductive and its conductivity depends on the alkaline hydroxide solution filling the porous separator; The polymer film of AEMWE has a non porous structure and intrinsic anionic conductivity. It should be pointed out that, unlike proton exchange membrane (PEM) electrolysis cells that only use polymer electrolytes, many AEMWE systems additionally introduce liquid electrolytes (such as KOH or K ₂ CO3 solutions) to enhance reaction kinetics. Liquid electrolytes can increase the local pH value at the catalyst electrolyte interface, forming additional electrochemical interfaces. Due to the resolution of the main technical bottleneck of AWE, the research attention of AEMWE in the industrial and academic fields has significantly increased in recent years.
A large number of studies, papers, and reviews have been published on the development of alkaline membranes, covering different application scenarios (such as electrodialysis, electrodialysis reversal, desalination, deionization, etc.) and multidimensional research perspectives (including independent components, materials, processes, and operating conditions, etc.). Most of the developed membrane materials have been optimized for scenarios with mild chemical environments (low pH and operating temperature). However, there is still a lack of research on the synthesis of hydroxide ion exchange membranes (AEMs) specifically designed for water electrolysis. In alkaline electrolysis cells, the core function of the anion membrane is to drive hydroxide ions (OH ⁻) to migrate from the cathode to the anode, while blocking electron conduction and preventing cross permeation of gases generated by electrochemical reactions in both reaction chambers. An ideal anion membrane should possess the following characteristics:
◎ High OH ⁻ conductivity
◎ High chemical stability (alkali resistance, oxidation resistance, thermal stability)
◎ Excellent mechanical performance under humid environment and high pressure difference
◎ Low gas permeability
This type of membrane is typically composed of a polymer backbone (providing mechanical and thermal stability) and positively charged functional groups (introduced through backbone modification or side linking branches). These functional groups promote anion migration, dominate the ion exchange capacity, ion conductivity, and migration number of the membrane, thereby achieving zero pole spacing structure design and pressure differential operation mode.
So far, various anion exchange membrane (AEM) main chain materials have been studied and developed, including polystyrene (PS), polyphenylene oxide (PPO), polysulfone (PSF), fluorinated polymers, ethylene tetrafluoroethylene copolymers (ETFE), polyetherimide (PEI), polyvinyl alcohol (PVA), polyetheretherketone (PEEK), polyaryletherketone (PAEK), and polycarbazole (PCz). The most widely studied functional groups include: - NH ∝⁺, - RNH ₂⁺, - R ₂ NH ⁺= R ₂ N ₂⁺, - R ∝ P ⁺, - R ₂ S ⁺, quaternary ammonium salts (QA), and tertiary amines (such as BTMA). Among them, cyclic aliphatic quaternary ammonium salts (QA) exhibit excellent alkali resistance.
As mentioned in previous literature, the main challenges faced by anion exchange membranes are:
1) Its chemical and thermal stability are relatively low. This is because under alkaline conditions, both the main chain and functional groups of the membrane will degrade due to the erosion of hydroxide ions (OH ⁻). The degradation hazards of polymer main chains include chain breakage, reduced molecular weight, and significantly increased membrane brittleness. The main chain containing aromatic ether groups (such as PEEK, polyethersulfone (PESU), and polyphenylene oxide (PPO), which are common in low-cost and readily available polymers) is particularly prone to such degradation. Therefore, materials containing aromatic ether groups should be avoided.
2) Ionic conductivity limitation. Compared with proton exchange membranes (PEM, such as Nafion), anion exchange membranes have lower ion conductivity because the migration rate of OH ⁻ is much lower than that of H ⁺.
At present, a few alkaline solid polymer films have been commercially applied