A review of the synthesis and characterization of anion exchange membranes

Abstract

This review highlights advancements made in anion exchange membrane (AEM) head groups, polymer structures and membrane synthesis methods. Limitations of current analytical techniques for characterizing AEMs are also discussed. AEM research is primarily driven by the need to develop suitable AEMs for the high-pH and high-temperature environments in anion exchange membrane fuel cells and anion exchange membrane water electrolysis applications. AEM head groups can be broadly classified as nitrogen based (e.g. quaternary ammonium), nitrogen free (e.g. phosphonium) and metal cations (e.g. ruthenium). Metal cation head groups show great promise for AEM due to their high stability and high valency. Through ''rational polymer architecture'', it is possible to synthesize AEMs with ion channels and improved chemical stability. Heterogeneous membranes using porous supports or inorganic nanoparticles show great promise due to the ability to tune membrane characteristics based on the ratio of polymer to porou2s support or nanoparticles. Future research should investigate consolidating advancements in AEM head groups with an optimized polymer structure in heterogeneous membranes to bring together the valuable characteristics gained from using head groups with improved chemical stability, with the benefits of a polymer structure with ion channels and improved membrane properties from using a porous support or nanoparticles. AbbreviationsAAEM Alkaline anion exchange membrane AEM Anion exchange membrane AEMFC Anion exchange membrane fuel cell AEMWE Anion exchange membrane water electrolysis CEM Cation exchange membrane DSC Differential scanning calorimetry IEC Ion exchange capacity (mmol/g) IEM Ion exchange membrane PEMFC Proton exchange membrane fuel cell

Density functional theory study on the degradation of fuel cell anion exchange membranes via removal of vinylbenzyl quaternary ammonium head group

“…Quaternary ammonium (QA)-based anion exchange membranes (AEMs) have been studied extensively for fuel cell and electrolyser application due to their ease of functionalisation and high hydroxide ion conductivity. [1][2][3] Commonly employed cationic head groups based on QA are benzyl-trialkylammonium such as functionalised with trimethyl amine (TMA), QA with bridged bicyclic amines such as 1,4-diazabicyclo[2.2.2] octane (DABCO) and 1-azabicylo[2.2.2]octane (ABCO), and cycloaliphatic QA such as N-methylpiperidine (NMP). However, QA-functionalised AEMs have poor stability in high pH conditions due to base-induced degradation mechanisms, namely, Hofmann elimination, nucleophilic substitution, and ylide formation pathway.…”

“…However, QA-functionalised AEMs have poor stability in high pH conditions due to base-induced degradation mechanisms, namely, Hofmann elimination, nucleophilic substitution, and ylide formation pathway. [2][3][4][5] Richard Espiritu and John Lester Tan contributed equally to this work.…”

Multi‐walled carbon nanotubes incorporation into cross‐linked novel alkaline ion‐exchange membrane for high efficiency all‐solid‐state supercapacitors

“…Overall, many studies have demonstrated that hydroxide conductivity and other performance characteristics of the AEM membranes require further improvement. 26,33,34 Recently, many researchers have sought to enhance the physicochemical properties of the polymer electrolyte membranes by adding fillers such as ionic liquids and inorganic nanoparticles [35][36][37][38][39] (ZrO 2 , 40 SiO 2 , 39 and CNTs [41][42][43][44] ). Wen et al 45 found that addition of ionic liquids in polymer films did enhance the OH − conductivity and performance in SC.…”

Alkali‐doped hyperbranched cross‐linked polybenzimidazoles containing benzyltrimethyl ammoniums with improved ionic conductivity as alkaline direct methanol fuel cell membranes

“…Anion exchange membranes (AEMs), one of the crucial components of ADMFCs, participate in anion transport and fuel/oxidant separation. Now, series of polymers like polyethers, polyketone, polysulfones, polyaromatics with aryl ether‐free backbones, etc have been developed as AEMs, among which polybenzimidazoles (PBIs) have been regarded as one of the desirable alkaline polymer electrode membrane matrixes because PBIs perform high mechanical properties and good alkali resistance even at an elevated temperature . Moreover, the nitrogen including ‐NH‐ and ‐N= in the polymer scaffold can absorb and interact with the hydroxide ions, which are beneficial for the ions conduction .…”

“…Now, series of polymers like polyethers, 2-6 polyketone, 7,8 polysulfones, [9][10][11][12][13][14] polyaromatics with aryl ether-free backbones, 15 etc have been developed as AEMs, among which polybenzimidazoles (PBIs) have been regarded as one of the desirable alkaline polymer electrode membrane matrixes because PBIs perform high mechanical properties and good alkali resistance even at an elevated temperature. [16][17][18][19][20][21][22][23][24][25][26] Moreover, the nitrogen including -NH-and -N= in the polymer scaffold can absorb and interact with the hydroxide ions, which are beneficial for the ions conduction. 17,27 However, compared to acid-doped PBI membranes, the alkali-doped PBI membranes displayed lower ionic conductivity, because the diffusion coefficient of hydroxide ions is only one quarter of that of protons, suggesting that a high alkali-doped PBI membranes are been expected.…”