The Multifunctional Applications and Research of Anion Exchange Membranes

About Anion Exchange Membrane

Anion exchange membrane (AEM) is essentially an alkaline electrolyte that has a selective permeability effect on anions, so it is also called an ion selective permeability membrane. Generally, cations such as -NH³⁺, -NR₂H⁺ or -PR³⁺ are used as active exchange groups, and OH^- is generated at the cathode as carriers, which move to the anode through the selective permeability of the anion exchange membrane.

Due to its excellent ionic conductivity properties, high selectivity and chemical stability, AEM has become a promising solution in various electrochemical technologies such as its applications in fuel cells, electrolyzers and alkali recovery processes.

Important Parameters of Anion Exchange Membrane

Various analytical and characterization methods can be employed to comprehensively evaluate the performance of AEMs. The chemical homogeneity, structure, stability and mechanical properties of AEM are several important parameters to evaluate its performance. [1] More detailed parameters include:

• Ion exchange capacity (IEC)
• Swelling ratio (SR)
• Water uptake (WU)
• Membrane moisture content (γ)
• Water contact angle (θ)
• Hydroxide conductivity (σ)
• Alkaline stability

Anion Exchange Membrane for Fuel Cells

In an alkaline membrane fuel cell (AEMFC), gaseous fuel is broken down into protons and electrons in a catalyst layer. Fuel crossover occurs in the opposite direction to the movement of OH^- ions, which greatly reduces electroosmotic resistance. The OH^- ions generated in the cathode reach the anode through the AEM, where they react with H₂ fuel to generate H₂O. This reaction is accompanied by the release of electrons, which pass through an external load to the cathode, thereby generating electricity. The humidity or water content of the membrane determines the mobility of OH^- anions through the membrane. [2]

Requirements for AEM membranes used in AEMFC:

• The good ability to conduct OH^- ions, and ionic conductivity ≥100 × 10^-3 S cm^-1.
• The chemical and mechanical stability in the AEMFC environment (temperature, humidity, alkali components, etc.).

Anion Exchange Membrane for Electrolyzers

Electrolyzers, which facilitate the conversion of electrical energy into chemical energy, play a crucial role in many clean energy applications such as hydrogen production and energy storage. AEM can be used in alkaline zero-gap water electrolyzers (WE). Thanks to the milder AEMWE conditions, costs can be effectively reduced by using only cheaper materials in the case of other components such as catalysts. For example, an IrO_2-based catalyst and a porous titanium transport layer (PTL) are used at the anode, and a carbon-supported Pt (Pt/C) catalyst and a porous carbon gas diffusion layer (GDL) are used at the cathode. [3]

Anion Exchange Membrane for Alkali Recovery

AEM can also be used to recover caustic soda from industrial waste through electrodialysis (ED). During base separation/recovery via ED, base instability of AEM may compromise the suitability of the process. Anuj K. Singh et al. reported a strategy using acrylonitrile-vinylimidazole copolymer (PVACN) as a new cross-linker to improve AEM stability. This PVACN-modified AEM can avoid Hofmann elimination reaction (E²) and exhibit alkali resistance and stability. [4]

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References

1. Hagesteijn K F L, et al. Journal of materials science, 2018, 53(16), 11131-11150.
2. Das G, et al. Polymers, 2022, 14(6), 1197.
3. Du N, et al. Chemical Reviews, 2022, 122(13), 11830-11895.
4. Singh A K, et al. Desalination, 2020, 494, 114651.