Recent advances in anion-exchange membranes for electrolyzers, fuel cells and redox-flow batteries

Abstract

The performance of affordable electrochemical energy conversion and storage devices is primarily associated with developing efficient hydroxide-ion conducting anion-exchange membranes (AEMs) – a central component in these devices. Owing to their significantly lower cost compared to their proton-exchange membrane counterparts, AEM research has demonstrated significant progress in recent years in all aspects mainly in ionic conductivity and durability. In the last decade, remarkable advancements have been made in AEM research both in ionic conductivity and stability and in their successful implications for electrochemical devices. Therefore, in this review, we comprehensively discuss AEM structures, synthesis methods, key chemical and mechanical properties, and degradation mechanisms reported in the last eight years (2018–2025). We classified reported AEMs into four major categories based on cationic head groups: (i) quaternary ammonium, (ii) piperidinium, (iii) imidazolium and (iv) guanidinium. We covered AEM implementation in electrochemical devices – anion-exchange membrane water electrolyzers, anion-exchange membrane fuel cells, and vanadium redox-flow batteries which is highly useful for their practical applications. Finally, we present the significant progress made, remaining challenges and future directions to advance fast-growing AEM research for the development of affordable and sustainable energy devices.

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Neelima J.

Neelima J. is currently pursuing her PhD in the Department of Chemistry at the Vellore Institute of Technology (VIT), Vellore, India. She received her master’s degree in polymer science from the Cochin University of Science and Technology in 2021. Following that she worked as a project staff member at VIT on a DST-SERB funded project. Her broader research interest focuses on ion exchange membranes for electrochemical applications, polymer synthesis and sustainable materials for clean energy.

Ramesh K. Singh

Ramesh K. Singh is working as an Assistant Professor at the CO2 Research and Green Technologies Centre at the Vellore Institute of Technology, Vellore, India. He received his PhD degree from the Indian Institute of Technology Bombay, focusing on oxygen reduction reaction (ORR) electrocatalyst development for proton-exchange membrane fuel cell cathodes. He was a Postdoctoral Fellow at Ariel University, Israel, and the Technion-Israel Institute of Technology, Israel. His research interests are in developing electrode materials and electrolytes for electrolyzers, fuel cells, and metal–air batteries.

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1. Introduction

Electrochemical energy production, conversion and storage devices play a vital role in decarbonizing the energy sector and meeting sustainable development goals. Ion-conducting membranes are central components in these devices which help (i) electrically isolate the cell compartments,¹ (ii) facilitate the transport of selective masses,² and (iii) in selective conduction of anions from the cathode to the anode.^3,4 Typically, Nafion®, one of the most highly investigated and commercially available perfluorosulfonic acid-based proton conducting membranes, developed by Dupont, imparts a highly acidic environment in the devices.^5,6 The corrosive acidic environment demands expensive platinum group metal (PGM) catalysts for attaining high performance and stability. Contrary to this, the alkaline medium facilitates the use of low-cost membranes, PGM-free catalysts, and a less corrosive environment for the development of affordable devices.^7,8 Therefore, the search for economical and energy-efficient alkaline electrochemical energy devices particularly anion-exchange membrane water electrolyzers (AEMWEs),^9–13 anion-exchange membrane fuel cells (AEMFCs),^14–19 and vanadium redox-flow batteries (VRFBs)^20–24 has received significant research attention. In particular, the development of stable and low-cost alkaline anion-exchange membranes (AEMs) drives the development of these devices.²⁵ Typically, the AEM structure consists of an aliphatic/aromatic polymeric backbone on which the cationic head groups are attached. Ionic conductivity (IC), ion-exchange capacity (IEC), water uptake (WU), and swelling ratio (SR) are typically used to characterize AEMs and their performance in these devices.^26–28 Several breakthroughs have been achieved in recent years showing an IC > 100 mS cm⁻¹ which can be utilized for electrochemical applications and durability for >1000 h under alkaline conditions.^29–34 For instance, Varcoe and coworkers synthesized a high-density polyethylene (HDPE) polymer backbone with a trimethyl ammonium (TMA) cation-based AEM, by radiation grafting techniques with an IC of 214 mS cm⁻¹ at 80 °C and IEC of 2.44 mmol g⁻¹ with an alkaline stability of 500 h in a N₂ atmosphere with 100% relative humidity (RH).³⁵ Kohl and coworkers reported efficient water transport using a polynorbornene membrane with an IC of 212 mS cm⁻¹ at 80 °C and IEC of 3.88 mmol g⁻¹ with improved stability for 1000 h in 1 M NaOH at 80 °C.³⁰ In another interesting study, Yan and coworkers reported a poly(aryl piperidinium) AEM with an IC of 193 mS cm⁻¹ at 95 °C and IEC of 2.37 mmol g⁻¹ with 2000 h stability in 1 M KOH.³⁶ Thereafter, a series of durable poly(fluorenyl aryl piperidinium) AEMs synthesized by Lee and coworkers exhibited an IC of 208 mS cm⁻¹ and IEC of 2.81 mmol g⁻¹ with impressive ex situ durability for 2000 h in 1 M NaOH at 80 °C.²⁹ Furthermore, the same group reported a polyethylene piperidinium-based AEM with a significantly high IC of 354.3 mS cm⁻¹ at 80 °C and IEC of 4.38 mmol g⁻¹ with ionic crosslinkers, and a balance between their improved IC values along with less dimensional variation is achieved.³⁷ Xu and coworkers reported a combination of meta and para conformational isomers of a terphenyl AEM with an IC of 217 mS cm⁻¹ at 90 °C and IEC of 2.53 mmol g⁻¹ with very high alkaline stability for 8000 h in 1 M NaOH at 80 °C.³⁴ Sun and coworkers recently reported an AEM with a quinuclidinium cationic group on a polyaryl backbone exhibiting an IC of 186.54 mS cm⁻¹ at 80 °C and IEC of 2.66 mmol g⁻¹ with 2446 h of stability in 1 M KOH.³³ As of today, there are few commercial AEMs available such as Fumasep® (FAA-3), Sustainion® (X37-50), piperION® (Versogen), and Aemion™ (AF1-HNN8) which were formulated as a result of constant research.³⁸ These AEMs are featured in various electrochemical devices such as AEMWEs, AEMFCs and VRFBs showing high performance and stability for practical applications.^39,40

Several reviews for AEMs have been published showing significant progress in various aspects of AEMs and their applications in electrochemical devices.^8,41–44 For instance, Varcoe et al. comprehensively reviewed AEMs in electrochemical devices until 2014.⁷ You et al. reviewed cations and a broad set of backbones with typical synthetic methods for the development of AEMs for fuel cells in 2020.⁴¹ Song et al. reviewed structural descriptions of AEMs with stability enhancing strategies and proper synthesis insights in 2023.⁴² Recently, Park et al. reviewed aryl-ether free AEMs for electrochemical applications in 2024.⁴³ In summary, these reviews shed light on the various fundamental and applied aspects of AEMs in electrochemical devices.

AEM research is a fast-growing field and results need timely updates to showcase the advancement in research with the remaining challenges. In this review, the structure, synthesis methods and properties of AEMs reported in the last eight years (2018–2025) are covered, which widens the scope of previous reviews. A critical analysis of AEM properties – IC, IEC, WU, and SR – is presented as a function of cationic head groups mainly quaternary ammonium, piperidinium, imidazolium and guanidinium. We included their mechanical properties and various chemical degradation mechanisms which are essential to develop robust AEMs. Most importantly, AEMs incorporated in AEMWEs, AEMFCs and VRFBs are presented summarizing the development made in high-end electrochemical energy devices to unlock the cost and remaining challenges for their market-level deployment.

2. AEM structure

The structure of AEMs consists of a solid backbone polymeric skeleton on which positive functional groups are covalently bound, which transport negative anionic groups (Fig. 1).⁷

Fig. 1 Structure of the AEM, showing a polymeric backbone attached with positively charged cationic head groups represented in red balls along with counter anions in negative charge in blue colour.

The major function of an AEM is to conduct anions and it is impermeable to cations and other undesired ions and used as an ionic conductor. The backbone polymer structure provides mechanical, physical, and thermal stability to the AEM,⁴⁵ while cationic head groups are responsible for the exchange of anions at their functional site.^46,47 Numerous polymer backbones have been explored which include (i) polyolefin-based⁴⁸ (polyethylenes (PEs),^{32,35,49–51} polystyrenes (PS),^52,53 polynorbornene (PNB)⁵⁴ and polytetrafluoroethylene⁵⁵), (ii) polyphenylene-based, and (iii) polyarylene ether-based (polyether ether ketone,^56,57 ether sulfone, polyphenylenes,⁵⁸ polyphenylene oxide,^59–61 polyphenyl ether^62,63) (Fig. 2). We have classified the reported backbone polymers and cationic head groups based on their stability and IC, respectively (Table 1).^8,15,35,42

Fig. 2 Backbone polymers and cationic groups used for AEMs with a pie chart showing the percentage of studies on various cationic groups.

Table 1 Backbone polymers and cation head groups used for AEMs with their advantages and disadvantages

Backbone	Advantages	Disadvantages
Polyolefin	• Mechanically and chemically robust and flexible	Hydrophobic nature is challenging for functionalization
	• Widely available and cost-effective
Polyphenylene	• High thermal and chemical stability	• Limited processability due to rigidity
	• Rigid aromatic structure prevents the swelling issue	• Brittle and challenging functionalization
Polyarylene-ether	• Excellent thermal, mechanical and oxidative stability	Complex synthesis and prone to oxidative degradation
	• Flexible backbone structure
	• Easy processing

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Cation	Advantages	Disadvantages
Quaternary ammonium	• High IC	Prone to Hofmann elimination and nucleophilic substitution
	• Versatile functionalization
Piperidinium	• Improved alkaline stability	Complex synthesis and prone to ring-opening degradation
	• High IC
	• Resistant to Hofmann elimination
Imidazolium	• High mechanical properties and IC	Prone to nucleophilic attack and ring-opening degradation
	• Stable at high pH
Guanidinium	• High IC due to strong OH^− affinity	Challenging synthesis
	• Improved alkaline stability

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Cationic head groups such as (i) quaternary ammonium (QA) and its modified forms,^60,64–73 (ii) piperidinium,^{29,36,37,74–86} (iii) imidazolium,^{24,61,62,87–97} and (iv) guanidinium^98–101 were typically used for synthesis of AEMs. Among them, ammonium-based cationic head groups have been highly explored owing to their high IC, exceptional alkaline stability, and facile synthesis^{31,63,81,102,103} (Fig. 2 and Table 1). Apart from these above-mentioned groups, N-free cationic groups such as sulfonium,^104,105 phosphonium,¹⁰⁶ ligand–metal complexes,¹⁰⁷ and carbocation^{105,108–112} are also reported.

3. AEM synthesis

Various AEM synthesis methods such as grafting,^73,113,114 ring-opening metathesis polymerization (ROMP),^115–117 vinyl addition polymerization,^118–121 acid-mediated synthesis,^83,122 and the Suzuki–Miyaura coupling reaction^123,124 are typically used methods in the literature. Grafting of polymers has been considered an advanced technique that results in better properties and offers several chemical modifications for the polymer.¹²⁵ Grafting is further classified into four types: (i) radiation grafting (Fig. 3a and b)^{24,35,55,73,126–129} (ii) chemical grafting,¹³⁰ (iii) plasma grafting,^46,131 and (iv) atom transfer radical polymerization.¹³² Out of these, radiation grafting is the primary choice for developing a stable AEM. Three steps are followed in radiation grafting which include (i) irradiation to activate the polymer for initiating polymerization, (ii) grafting of monomers into the polymer substrate, and (iii) post-grafting functionalization (Fig. 3a and b). The ROMP method is typically recommended for cyclic alkane groups such as norbornene and its derivatives. The ROMP method was also followed in the synthesis of poly(bromopropyl norbornene)-block-poly(butyl norbornene)-based AEMs, which resulted in excellent alkaline stability and IC (Fig. 3c).¹¹⁶ Peltier et al. used ROMP for polyethylene-QA-based AEMs,⁴⁹ and Clark et al. used ROMP for the synthesis of olefin-based AEMs.¹¹⁵ Kim and coworkers used the ROMP method to synthesize piperidinium functionalized polyethylene backbone-based AEMs.³⁷

Fig. 3 (a) Radiation grafting method for synthesis of a low-density polyethylene (LDPE) or ethylene tetrafluoroethylene (ETFE)-based AEM, consisting of pre-irradiation, grafting and functionalization (adapted from ref. 114 Copyright 2024, Royal Society of Chemistry). (b) Synthesis scheme of an ETFE-N-methylpiperidine AEM by radiation grafting. Reprinted with permission.¹³⁴ Copyright 2022, Elsevier. (c) Synthesis of a cross-linked poly(bromo propyl norbornene) (XL20-rPNB-LY₁₀₀) AEM by the ROMP method. Reprinted with permission.¹¹⁶ Copyright 2019, American Chemical Society. (d) Synthesis of copolymers from polynorbornene by vinyl addition polymerization (adapted from ref. 119 available under CC-BY 4.0). (e) Synthesis of a polyarylene alkylene piperidinium-based AEM by super acid-mediated polyhydroxy alkylation and quaternization (adapted from ref. 86 avilable under CC-BY 3.0).

Vinyl addition polymerization is a well explored method for AEM synthesis.^118–121 For instance, a series of statistical copolymers of norbornene was synthesized by vinyl addition polymerization followed by functionalization with trimethyl ammonium and tetraaminophosphonium cations (Fig. 3d).¹¹⁹ The analysis of copolymerization kinetics results with the quaternary ammonium series shows sharp phase separation (insulating and ionic regions) with an increase in the ion concentration. A multiblock copolymer of polynorbornene by vinyl addition polymerization followed by copolymerization and functionalization showed high IC due to the improved increased surface to volume ratio and connectedness of ionic domains.¹²⁰ Another interesting vinyl addition polymerization study was carried out by Coates and coworkers, which focused on exploring how domain continuity influences the IC, water uptake and dimensional stability of polynorbornene based polymers functionalized with alkyl and benzyl substituents.¹³³

A series of vinyl addition polynorbornenes were synthesized with similar IEC and it was observed that 3D-co-continuous structured AEMs were retaining their domain continuity with superior morphological properties. ABC triblock terpolymers were identified to achieve morphology–performance relationships and high IC.¹³³ Saito and team synthesized a series of quaternized random copolymers of polynorbornene via vinyl addition polymerization to identify the impact of composition on AEM properties.¹¹⁸ The addition of n-hexylnorbornene into the structure of the copolymer improved the processability and robustness of AEMs for their implementation in electrochemical devices.

In addition to these methods, acid mediated techniques such as super acid condensation,^86,135,136 acid-catalyzed polycondensation,^33,135 acid-mediated Friedel–Crafts polycondensation,^29,70,83,137 and metal catalyzed reactions like the Suzuki–Miyaura¹²³ reaction are the other commonly used synthesis routes. Pan et al. followed the superacid-mediated polyhydroxy alkylation method to synthesize poly(arylene alkylene piperidinium) based AEMs with p- and m-terphenyl, biphenyl and fluorene groups followed by cationic functionalization.⁸⁶ This rational design strategy for monomer design significantly improved its IC (IC of 180 mS cm⁻¹ at 80 °C and stability for 480 h) (Fig. 3e). An acid catalyzed Friedel–Crafts alkylation of polystyrene-co-butylene-styrene (SEBS) followed by quaternary ammonium functionalization showed an elastomeric AEM with a triblock copolymeric structure.¹³⁸ A better control over the functionalization and tether length resulted in phase-separated morphology (IC of 65 mS cm⁻¹ at 80 °C and stability for 500 h). Turan and coworkers synthesized SEBS triblock copolymers via the thiol–ene click reaction followed by hydrogenation and subsequent functionalization at the rubbery side of SEBS which provided a mechanically robust AEM.¹³⁹ An excellent alkaline stability with lower WU that was attributed to the AEM suggests that this functionalization at the rubbery side of the polymer is a better material design strategy for novel AEM development (IC of 93 mS cm⁻¹ at 80 °C and stability for 672 h). Similarly, an acid-catalyzed Friedel–Crafts polymerization method for incorporating the quinuclidinium group into an aromatic polymer structure is also used, which showed remarkably improved conductivity and stability (IC of 186.54 mS cm⁻¹ at 80 °C with 2446 h of stability).³³

4. AEM properties

In this section, we critically analysed AEM properties such as IC, IEC, WU and SR, which play a crucial role in AEM performance. IC is calculated using eqn (1).^29,103,140where σ is ionic conductivity, R is resistance, A is the cross-sectional area of the AEM and L is the distance between the electrodes (voltage sensing probe in the conductivity cell).

IEC is estimated from conventional methods (Mohr's titrations³²) and ¹H NMR.^141,142 In Mohr's titration, the dry AEM in Cl⁻ form was soaked in 1 M NaNO₃ for ∼72 h and then these samples were back titrated with 0.01 M AgNO₃ solution using K₂CrO₄ as an indicator (eqn (2)).¹⁸

where V_AgNO3 is the volume of AgNO₃ used, M_AgNO3 is the molarity of AgNO₃, and W_dry is the weight of the dry AEM.

WU and SR of the AEM are calculated (eqn (3) and (4)) by gravimetrical weighing and dimensional measurement, respectively.^32,34,143 A minimum of three samples were typically measured to get their average dry weight (W_dry), wet weight (W_wet), dry length (L_dry) and wet length (L_wet).

4.1 Ionic conductivity and ion exchange capacity

IC is one of the key parameters which measure the conduction of anions from the cathode side of the electrode to the anode side and is an aggregate of properties such as IEC, WU, and the type of cationic head group. IEC implies more possible exchangeability in AEMs; it is a measure of the concentration of ion-conducting functional groups.^144–146 AEMs with their IC and IEC are categorized based on highly investigated cationic head groups–QA-based (Fig. 4a and b), piperidinium-based (Fig. 4c and d), imidazolium-based (Fig. 4e and f), and guanidinium-based (Fig. 4g and h). AEMs from all four categories typically have a linear correlation between IC and IEC (Fig. 4). In general, a high IEC value is accompanied by its corresponding high IEC value. In the case of QA-based cations, an IC value between 26 and 217 mS cm⁻¹ and IEC between 1.2 and 3.8 mmol g⁻¹ were achieved.^{16,30–33,35,49,50,54,68,71,72,114,116,123,129,137,143,147–180} For instance, Varcoe et al. used radiation grafting techniques to synthesize a tetramethyl ammonium (TMA) functionalized high-density polyethylene (HDPE) AEM [24] and achieved an IC of 214 mS cm⁻¹ at 80 °C and IEC of 2.44 mmol g⁻¹.³⁵ Mandal et al. synthesized a thin reinforced AEM [2] of polynorbornene with light crosslinking, which showed an IC of 212 mS cm⁻¹ at 80 °C and IEC of 3.84 mmol g⁻¹ with chemical stability for 1000 h.³⁰ Excellent stability (IC loss of <1.5%) was achieved by the light cross-linking which leads to better water management. Polynorbornene-based AEMs typically have high IEC. A maximum IEC value of 4.73 mmol g⁻¹ was achieved by Kohl and coworkers indicating a well-balanced water distribution inside the AEM [6] with an IC of 195 mS cm⁻¹ (Fig. 4a and b).¹¹⁶ A significantly high IC value is attributed to the nanomorphological microstructure of the precursor film which brought rapid water transport from the anode to the cathode. In another report, polycyclic derivatives of norbornene, dicyclopentadiene, and tricyclopentadiene by vinyl addition polymerization with TMA functionalization [48] showed an IC value of 212 mS cm⁻¹ at 90 °C with an IEC of 2.75 mmol g⁻¹.¹⁷⁵ The synthesis of triple cation functionalized polyolefins with QA [26] showed a hydroxide conductivity value of 201 mS cm⁻¹ at 80 °C with an IEC of 1.9 mmol g⁻¹ and improved alkaline stability for 1000 h.³¹ The second most investigated piperidinium cations (Fig. 4c and d) exhibited IC and IEC between 20 and 354 mS cm⁻¹ and ∼1–4.3 mmol g⁻¹, respectively.^{29,37,76,77,79,80,83,86,105,136,181–189} This group exhibited a higher average IC value compared to ammonium (Fig. 4a and b). For instance, Kim and coworkers synthesised a piperidinium-functionalized AEM (PEP80-20 PS) [78] with a polyethylene backbone with an excellent IC of 354.3 mS cm⁻¹ and IEC of 4.05 mmol g⁻¹ due to proper ionic cross-linking and its record IC was achieved at 80 °C.³⁷ However, the thermosetting nature of the AEM limits its practicability and large-scale production. In another study, Song et al. synthesized meta- and para-terphenyl piperidinium (MTCP-50) [95] AEMs with an IC of 217 mS cm⁻¹ at 80 °C and IEC of 2.53 mmol g⁻¹ with excellent alkaline stability for >8000 h.³⁴ The well-designed microporous chemical structure of the AEM contributed to high stability. In another study, a poly(fluorenyl aryl piperidinium)-based AEM [84] with an IC value of 208 mS cm⁻¹ at 80 °C with an IEC of 2.81 mmol g⁻¹ was reported.²⁹ The reported piperidinium cation with a rigid ether-bond free aryl backbone AEM [71] showed an IC value of 193 mS cm⁻¹ at 80 °C and IEC of 2.02 mmol g⁻¹ with improved chemical stability (300 h) and mechanical robustness.³⁶ A recent study showed tetraphenyl methane, functionalized with a 1-methyl-4-piperidone AEM [86], with an IC value of 142.71 mS cm⁻¹ at 80 °C and IEC of 2.77 mmol g⁻¹, attributed to the four-arm star structure with a high fraction of free volume.¹⁸⁹

Fig. 4 Various ammonium-based AEMs with their IC at 80 °C and IEC (a and b): [1] QA-PNB-vinyl,⁵⁴ [2]-QA-PNB-ROMP,³⁰ [3]-QA-BPPO,¹⁴⁷ [4]-PPO-QVBC,¹⁴⁸ [5]-QA-PNB-cast,¹⁹⁰ [6]-XL20-rPNB-LY100,¹¹⁶ [7]-BTMA-ETFE^#,⁷² [8]-VBC-TMA-ETFE,¹⁴⁹ [9]-QPC-TMA,¹⁶ [10]-TMA-PE,⁴⁹ [11]-QA-PE-ROMP,⁴⁹ [12]-HQPC-TMIM,¹⁵⁰ [13]-QA-comb-SEBS,¹⁵¹ [14]-QA-PEEK,¹⁵² [15]-TMA-PPPO-F,¹²³ [16]-QA-biphenylene,¹³⁷ [17]-QA-benzyl-ETFE,⁷³ [18]-TMA-F-benzene,¹²⁴ [19]-QA-spiro-PPO,¹⁵³ [20]-QA-Hex-Cross-PPO,¹⁵⁴ [21]-QA-phenyl-pip,¹⁵⁵ [22]-QA-spiro-PBI,⁶⁸ [23]-QA-F-arylene-ether,¹⁹¹ [24]-TMA-HDPE,³⁵ [25]-TMA-LDPE,¹⁵⁶ [26]-QA-Br-olefin,³¹ [27]-QA-dimethyl-PPO,¹⁵⁷ [28]-TMA-xanth-pip,¹⁵⁸ [29]-QA-styrene,¹⁵⁹ [30]-QA-istain,⁷¹ [31]-TMA-ETFE,¹²⁹ [32]-QA-PPO,¹⁶⁰ [33]-QPAFs,¹⁶¹ [34]-QA-Tdampb,¹⁶² [35]-QA-fluoro-alkylene,¹⁶³ [36]-TMA-BPEI,¹⁶⁴ [37]-QA-b-ethy-PS,¹⁶⁵ [38]-DVB-MPRD-VBC,¹⁶⁶ [39]-QA-Br-hex-TMA,¹⁶⁷ [40]-QA-PPO-cyclodextr,¹⁶⁸ [41]-QA-EDGE,¹⁶⁹ [42]-QSEBS-PPO,¹⁷⁰ [43]-TMA-EVA,⁵⁰ [44]-TMA-aryl-sulfo,¹⁷¹ [45]-GT-78-40,¹⁷² [46]-Poly(ary)-TMA,¹⁷³ [47]-QA-PPO,¹⁷⁴ [48]-PNBs-DCPD/TCPD,¹⁷⁵ [49]-QA-VAP-N-60,¹⁷⁶ [50]-PPI-30-C70-QA,³² [51]-VNB-alkyl-Br-NB,¹⁷⁷ [52]-PAQ-5,³³ [53]-L-50-30,¹¹⁴ [54]-GT75-5,¹⁷⁸ [55]-GT82-5,³⁰ [56]-QTAF-3.0,²⁴⁸ [57]-QA-COF,¹⁷⁹ [58]-MM-LPF-OH,¹⁴³ [59]-PHF10 TP 90-PVB,¹⁸⁰ [60] XL 100-SEBS-C5-TMA-0.8,¹³⁸ [61] h-SBS-TMA-1.83,¹³⁹ [62] PPS/mTPN/DABCO-Me,¹⁹² [63] pentablock-NMe3[OH],¹²⁰ [64] NB-5-Hex:NB-5-BuBr,¹¹⁹ [65] P8(TMA),¹³³ and [66] QPN.¹¹⁸ Various piperidinium cation-based AEMs with IC at 80 °C and IEC values (c and d): [67]-pheny-piperi,⁸³ [68]-methyl-Pip-aryl,¹⁸¹ [69]aryl-pip,⁸⁶ [70]-Pip-alkyl,⁸⁰ [71]-PAP-TP-85,³⁶ [72]-PDTP-PFTP,³⁶ [73]-Pip-PBN,¹⁸² [74]-Pip-Pva-Dibal-H,¹⁸³ [75]-cross-Pip-aryl,¹⁸⁴ [76]-Pip-SEBS,⁷⁷ [77]-Pip-fluorene,¹⁸⁵ [78]-PEP80-20 PS,³⁷ [79]-PEEK/PBI-pip,⁷⁶ [80]-PBI/PVBC-pip,¹⁸⁶ [81]-aryl-meth-pip,¹⁸⁷ [82]-PIS-Br-pip,⁷⁹ [83]-fluorene-pip,¹⁸⁸ [84]-fluoro-aryl-pip,²⁹ [85]-(PEI)I,¹³⁶ [86]-qPTTP-7,¹⁸⁹ [87]-poly-bi-ph-pip,¹²² [88]-PDTP-25,⁸³ [89]-PVA-FBMPx-TP,¹⁸³ [90]-PAP-ON-pF6,¹⁹³ [91]-PipPFTI-25,¹⁹⁴ [92]-QPTTP-5%,¹⁹⁴ [93]-PBP-M-35,¹²² [94]-PTP/TPB-0.5%,³⁴ [95]-MTCP-50*,³⁴ [96]-PNB-pip-34.¹²¹ Various imidazolium-based membranes IC at 80 °C and IEC (e and f): [97]-imida-Bppo,¹⁹⁵ [98]-imidaz-aryl,¹⁰³ [99]-imid-aryl,¹⁰³ [100]-BP-DFDPS,⁸⁸ [101]-imida-meth-sulft,⁹⁴ [102]-Br-meth-imidaz,¹¹³ [103]-PBI-imidaz,⁹³ [104]-PPO/PEG-imidaz,¹⁹⁶ [105]-aryl-imidaz,¹⁹⁷ [106]-H-rPBenzoNBD-CTPA,¹⁹⁸ [107]-Pol-benz-imid,¹⁹⁹ [108]-CL5-Ap,²⁰⁰ [109]-H-rPNB-T-triMph,²⁰¹ [110]-pyridi-benzimdaz,²⁰² [111]-ClmPEEK.⁹⁶ Various guanidinium-based membranes IC at 80 °C and IEC (g and h): [112]-Guan-quinox-PTFE,²⁰³ [113]-PAES-QA-guan,²⁰⁴ [114]-PPO-Benz-guan,²⁰⁵ [115]-PAEK-Guan,⁹⁸ [116]-PTFE-Bi-guan-sesquio,⁹⁹ [117]-PES-ethyl-guan,¹⁰¹ and [118]-PPO-Bi-guanidinium.¹⁰⁰ The AEMs are numbered in square brackets for clarity. All the IEC data were recorded at room temperature. ^#IC data was obtained at 60 °C and *IC data was obtained at 90 °C.

The third investigated imidazolium cations have IC between 20 and 356 mS cm⁻¹ and IEC between 1 and 4 mmol g⁻¹ (Fig. 4e and f).^{88,93,94,96,103,113,195–202} The linkage of two imidazolium diol monomers with unique designing strategies with crosslinking was reported with exceptional alkaline stability in a low-temperature range.⁹⁷ In imidazolium cations, most of the AEMs have IEC above 1.5 mmol g⁻¹, even though their IC average is <80 mS cm⁻¹. Thus, further modification of the AEM structure may lead to higher IC. A poly(arylene-imidazolium) based AEM was reported with a significantly high IC of 356.1 mS cm⁻¹ at 25 °C with thin-span channels in a reduced alkaline concentration.⁹⁷ The linkage of two imidazolium diol monomers with unique designing strategies with crosslinking was reported in this study for better properties in a low-temperature range. The crosslinked tri-imidazolium cationic cluster on a copoly(norbornene)s AEM [109] showed an IC value of 114.8 mS cm⁻¹ and IEC of 1.64 mmol g⁻¹ which is attributed to the better and efficient channel for anionic conduction provided by high self-assembly between the cation and backbone.²⁰¹ A benzimidazolium-based AEM [110] showed an IC of 111 mS cm⁻¹ at 80 °C and IEC of 3.97 mmol g⁻¹ which is attributed to proper crosslinking with the butylated pyridinium group and is found to be immensely effective towards enhancing various physical properties.²⁰² The fourth investigated cation is guanidinium, with IC between 36 and 81 mS cm⁻¹ and IEC between 1 and 2 mmol g⁻¹ (Fig. 4g and h).^{98–101,203–205} The uniform distribution of the charge on its central carbon molecules and nearby three nitrogen atoms is highly beneficial for stabilized charge delocalization.¹⁰⁰ The highest IC of 81 mS cm⁻¹ at 80 °C and IEC of 1.41 mmol g⁻¹ were exhibited by AEM [117] with ethylene oxide functionalized guanidinium cations on the polyether sulfone backbone.¹⁰¹ Among these four categories of cationic groups, guanidinium is the least explored group which indicates the further modification possibility of this group. The QA-based and piperidinium-based AEMs can be better choices with improved properties (IC and IEC) and stability.

4.2 Water uptake, swelling ratio, and thickness

WU, SR, and thickness are other important factors which affect the IC and stability of AEMs. For continuous hydroxide transport through AEMs, the initial factor is stability. Strong bonding between the backbone polymer and cation should be present even after long-term operation.^103,206 Owing to mainstream application of AEMs in AEMWEs, AEMFCs, and VFRBs demands proper management of water for its stable performance. One recent atomistic molecular dynamic (MD) simulation study by Clary et al. on various polyarylpiperidinium-terphenyl/biphenyl materials also emphasized the importance of the hydration factor in contributing to high IC and IEC for the AEM.²⁰⁷ Swelling is another factor that is associated with WU which affects the IC and stability of AEMs. A higher water imbibed membrane with an increased SR always tends to block the transport and decrease the IC and IEC; later it results in degradation of its structure due to the high pressure associated hydrogen production. Similarly, low AEM thickness is necessary to minimize the ohmic loss while higher thickness is essential to maintain mechanical stability. Hence AEMs need to be thin as well as thick to fulfill both criteria. Fig. 5 shows the WU (13–234%), SR (7–45%), and thickness (10–150 μm) of the reported AEMs. As one can see, the AEMs with higher WU are directly associated with the enhanced SR due to higher water absorption. AEMs with >100% WU showed a higher SR, but those having WU is <100% are very rarely prone to the swelling. The highest WU of 234% is reported for PPI-30-C70-QA AEM [50].³² Several AEMs such as TMA-LDPE [25],³⁵ QA-styrene [29],¹⁵⁹ TMA-EVA [43],¹⁵⁸ PPI-30-C70-QA [50],³² PDTP-PFTP [72]⁸⁶ showed an SR below 6%, even after ≥100% WU, implying their mechanical and chemical stability. Furthermore, AEMs with high thickness are less impacted by swelling and resulted in moderate WU.

Fig. 5 Plot of thickness, SR, and WU of reported AEMs. [13]-QA-comb-Sebs,¹⁵¹ [15]-TMA-PPO-F,¹²³ [16]-QA-biphenylene,¹³⁷ [17]-QA-benzyl-ETFE,⁷³ [20]-QA-Hex-cross-PPO,¹⁵⁴ [21]-QA-phenyl-pip,¹⁵⁵ [23]-QA-F-arylene-ether,¹⁹¹ [24]-TMA-HDPE,³⁵ [25]-TMA-LDPE,¹⁵⁶ [26]-QA-Br-olefin,³¹ [27]-QA-dimethyl-PPO,¹⁵⁷ [28]-TMA-xanth-pip,¹⁵⁸ [29]-QA-styrene,¹⁵⁹ [31]-TMA-ETFE,¹²⁹ [32]-QA-PPO,¹⁶⁰ [36]-TMA-BPEI,¹⁶⁴ [39]-QA-Br-hex-TMA,¹⁶⁷ [41]-QA-EDGE,¹⁶⁹ [43]-TMA-EVA,⁵⁰ [44]-TMA-aryl-sulfo,¹⁷¹ [50]-PPI-30-C70-QA,³² [51]-VNB-alkyl-Br-NB,¹⁷⁷ [52]-PAQ-5,³³ [57]-Qa-COF,³³ [58]-MM-LPF-OH,¹⁴³ [59]-PHF10 TP 90-PVB,¹⁸⁰ [60]-XL100-SEBS-C5-TMA-0.8,¹³⁸ [62]-PPS/mTPN/DABCO-Me,¹⁹² [67]-pheny-piperi,⁸³ [68]-methyl-Pip-aryl,¹⁸¹ [71]-PAP-TP-85,¹⁹² [72]-PDTP-PFTP,³⁶ [75]aryl-pip,⁸⁶ [80]-PBI/PVBC-pip,¹⁸⁶ [86]-qPTTP-7,¹⁸⁹ [87]-poly-bi-ph-pip,¹²² [88]-PDTP-25,⁸³ [90]-PAP-ON-pF6,¹⁹³ [91]-PipPFTI-25,²⁰⁸ [92]-QPTTP-5%,¹⁹⁴ [96]PNB-PIP-34,¹²¹ [98]-imidaz-aryl,¹⁰³ [99]-imda-aryl,¹⁰³ [106]-NBD-CTPA-MI-1.68,¹⁵⁴ [107]-polvinbenz-imid,¹⁹⁹ [108]-CL5-Ap,²⁰⁰ [109]H-rPNB-T-triMPh,¹⁵⁴ [111]-ClmPEEK,⁹⁶ [119]-GT64-15,¹⁹⁰ [120]- BG-PSF-1.40,¹⁹⁰ [121]-PVIm-C10-APS,²⁰⁹ and [122]-PPTQ-13BTD.²¹⁰

4.3 Mechanical properties

Mechanical properties of AEMs are crucial for their stability as these directly influence the device performance. A prolonged operation at high temperature and in high-alkaline environments requires strong and stable AEMs; otherwise it can lead to rupturing, tearing and loss of selectivity. Tensile strength (TS) measures how much force an AEM can withstand before breaking whereas elongation at break (EB) indicates the ability of the AEM to stretch without breaking. Storage modulus (SM) implies the stiffness/ability to resist deformation under stress. Improved TS, EB and SM are needed for robust AEMs. Most of the reported AEMs show TS < 50 MPa and an EB of 30%.^{33,122,143,174,186,193,197,208} Significantly a high TS of 114 MPa with an EB of 159% was achieved for a poly(aryl-co-aryl piperidinium) reinforced AEM with a three-layer crosslinking morphology.²¹¹ This reinforced AEM showed good mechanical stability. Similarly, a poly(aryl quinuclidinium) based AEM (PPTQ-13BTD) showed TS > 90 MPa with superior stability in a highly alkaline environment.²¹⁰ Olefin backbone polymers are chemically inert with good mechanical and thermal stabilities (PNB = T_g > 300 °C). SEBS backbone-based membranes are a better choice than PS due to their excellent film-forming properties and better phase separation.²¹² Pure aromatic groups are always a better choice for backbone polymers but their high rigidity causes brittleness and lower mechanical stability.^213,214 In another study, Fan et al. investigated a poly(aryl imidazolium)-based AEM which showed a tensile strength of 64 MPa and storage modulus of 1075 MPa, whereas for poly(fluorenyl aryl piperidinium) membranes it showed improved mechanical properties compared to the imidazolium group with a tensile strength of 84.5 MPa and storage modulus of >1500 MPa.¹⁰³ Kim and coworkers reported AEMs containing a series of aliphatic chain containing poly(diphenyl-terphenyl piperidinium) copolymers with excellent mechanical properties of a TS of >70 MPa, a storage modulus of 1800 MPa and an EB of 32%.⁸³ Achieving an optimal balance between the mechanical properties of AEMs with electrochemical properties is essential to ensure proper device performance. Further research efforts will enhance the mechanical properties of AEMs.

5. AEM degradation

The degradation of AEMs is one of the main challenges which hinders their long-term implementation into various electrochemical devices. Rupture of AEMs happens because of the strong activity between nucleophilic anions and cationic head groups under high pH conditions.²¹⁵ Exploring different backbones and cations, introduction of alkyl spacers^173,216,217 and multiple functionalities,^180,198,217 using composite membranes,¹⁷⁴ and phase separation^151,218 are different strategies used for AEM synthesis which parallelly contribute to degradation. The rate of degradation is highly influenced by the hydration number (λ). A low λ (λ ≤ 4) with increased temperature significantly degrades QA cations.²¹⁹ The major possible degradation mechanisms identified for AEMs are nucleophilic substitution (S_N2),^220,221 β-elimination,^222,223N-ylide intermediated rearrangements,²²³ heterocycle deprotonation,^224,225 dehydrofluorination,^226–228 and ring-opening reactions (Fig. 6).^229–231 The detailed AEM degradation mechanism is thoroughly explained by several groups which provides a detailed description of the major pathways.^{27,103,231–233} AEMs composed of QA-based functional groups are highly susceptible to Hoffmann elimination and nucleophilic substitution and display their decreased stability in alkaline environments.¹⁴³ For instance, You et al. investigated degradation mechanisms of various organic cations such as benzyl ammonium, tetraalkylammonium, spirocyclic ammonium, imidazolium, and other N-conjugated cations under 1 M and 2 M KOH/CD₃OH conditions.²³⁴

Fig. 6 Various AEM degradation mechanisms, (a) S_N2 nucleophilic substitution and β-elimination,²¹⁵ (b) three major degradation routes for dimethyl piperidinium (DMP) cations, (i) nucleophilic substitution of the methyl group, (ii) ring-opening elimination and (iii) ring-opening substitution,^7,155 (c) dehydrofluorination of the polymer backbone, (d) S_N2 aryl-ether cleavage of the polymer backbone and (e) N-ylide intermediate rearrangement.^27,155

As per their stability results, it was concluded that nucleophilic substitution is dominating in ammonium-based cations when benzyl groups are present; besides this, S_N2 and Hofmann elimination are followed. For non-imidazolium, nucleophilic addition to N-conjugated cations such as pyridinium and guanidinium is more challenging, along with hydrolysis and rearrangement mechanisms. They have found that the steric hindrance factor is highly favorable for improving the stability of these groups.

Yang et al. investigated high-temperature stability of metal–organic framework (MOF)-based AEMs emphasizing (i) cationic head group stabilization by resonance and steric protection and (ii) the difference in the stability between cations and the backbone polymer, and complexity when applied in devices.²³⁵ Moreover, Zeng et al. pointed out chemical stability as an urgent issue to be addressed; they suggested phase-separated morphology or a pore-filled composite structure for the AEM in order to clear out degradation issues.²³⁶ Xu and coworkers proposed a specific geometry for QA groups without β-H as an effective strategy to overcome cation degradation.²³⁷ Also, spirocyclic cationic groups are preferred as their β-H lies in the cyclic structure making them less susceptible to nucleophilic degradation.²³⁸

An interesting study by Willdorf-Cohen et al. was carried out on the stability comparison of a series of radiation-grafted AEMs such as poly(ethylene-co-tetrafluoroethylene)-based AEMs with trimethyl ammonium (ETFE-TMA) and triethyl ammonium (ETFE-TEA) functionalization (Fig. 7a).²³⁹ It was concluded that QA-based functional groups in low hydration and high pH environments have an accelerated degradation rate via the Hofmann elimination reaction, and also the TEA functionalization is highly susceptible to S_N2 and E₂ degradation compared to TMA (Fig. 7b). In order to block the degradation of cations, a maximum substitution at the β-position is preferred which is in agreement with the literature.^237,240 To support the effect of the hydration level on the stability of AEMs a conductivity comparison of OH⁻ for various periods of time with low hydration values is depicted (Fig. 7c and d).²³⁹ The results show that ETFE-TEA stabilized after seven days of soaking in KOH emphasizing the extensive degradation compared to ETFE-TMA which was stable up to an extended period of 28 days. This provides a proper indication of the degradation of AEMs caused by the water microsolvation effect. Also, it is observed that the covalent bond of attachment of QA groups to the polymer backbone has a negative impact on the stability which indicates a better design strategy for functionalization. By analysing major degradation pathways, a cationic charge neutralization mechanism is revealed involving the anion which is common in most of the studies.^241,242

Fig. 7 (a) Chemical structures of AEMs consisting of ETFE-TMA and ETFE-TEA along with their affecting degradation mechanisms, ETFE-TMA by nucleophilic attack (S_N2) and ETFE-TEA by Hofmann elimination (E₂). (b) QA decay in ETFE-TMA and ETFE-TEA as a function of time. (c) and (d) changes in the IC of respective groups before and after alkaline stability testing (adapted from ref. 239 avilable under CC-BY 4.0).

Elimination of ether-containing backbone polymers is preferred as they are prone to nucleophilic attack due to the electron-deficient ether groups.^243,244 Polyphenylene and olefins are better choices as they have in-built delocalized electron densities in their structure to block the OH⁻ attack.²⁴⁵ The IC retention of the qPTTP-7 AEM after the extended exposure in an alkaline environment up to 1000, 2000 and 3000 h of time was improved to 98, 93, and 86%, respectively (Fig. 8a).¹⁸⁹ Han et al. reported that the well-designed four arm star shaped structure with microphase separation, a rigid hydrophobic framework, lower transport resistance and a notable steric hindrance are the reasons for improved IC and stability of AEMs. The ¹H NMR spectra of the respective AEMs before and after the alkaline stability test proved S_N2 nucleophilic substitution degradation of the piperidinium groups. The increased signals at 0.85, 1.2, and 1.5 ppm of NMR spectra indicated degradation of piperidinium and formation of alkyl groups (Fig. 8b).¹⁸⁹ Wang et al. analysed the decrease in the IC value for three types of polybiphenyl piperidinium-based AEMs with different non-ionic pendant groups in their backbone (Fig. 8c).¹²²

Fig. 8 Stability and degradation analysis of various membranes: (a) conductivity retention of a qPTTP-7 (quaternized tetraphenyl methane-based poly(aryl piperidinium)) membrane at 80 °C. (b) ¹H NMR spectra of qPTTP-7 before and after the alkaline stability test (adapted from ref. 189 available under CC BY-NC 3.0). (c) OH⁻ conductivity retention of the polybiphenyl piperidinium (PBP) membranes in 1 M KOH at 80 °C. (d) Loss of piperidinium of the PBP membrane after different periods. Reprinted with permission.¹²² Copyright 2023, American Chemical Society.

After 700 h, the PBP-Br-29 AEM with the bromomethyl pendant group got broken, with an IC retention of 83.2% due to the electron withdrawing nature of the bromomethyl group. A better IC retention after 1200 h is found for the PBP-M-35 AEM with a value of 84.5% compared to PBP-B-30 with 81.4%, which is due to the methyl pendant group. The incorporation of non-ionic pendant groups into the polymeric structure greatly impacts the physiochemical properties of the AEM. The higher electron cloud density of the biphenyl group with piperidine cations showed excellent stability. The small sized methyl pendant groups with a lowered steric effect and electronic effect along with low molecular weight benefited in building a flexible AEM. The Hofmann elimination mechanism was identified as the major degradation pathway for piperidinium-based AEMs (Fig. 8d).¹²² This design strategy of copolymers with non-ionic pendant groups derived from ketone comonomers is proven to be an effective method to mitigate the degradation whereas a higher alkali concentration can further accelerate the degradation rate. The electrochemical and ex situ characterization study to analyse the instability of AEMs proved the requirement of an advanced ionomer design with different backbones to increase oxidative resistance of the AEM.²³³ This work showed the requirement of a supporting KOH electrolyte (instead of pure water) and addition of non-conducting binders and stabilizers with proper catalyst layer morphology for preventing oxidative degradation.

6. Electrochemical applications

AEM research showed significant progress in the cell performance for various electrochemical devices (AEMWEs, AEMFCs, and VRFBs) in the last decade. AEMWEs, AEMFCs, and VRFBs are promising candidates for cost-effective energy production and storage applications. In this section, the performance details of AEMs implemented in these devices are listed below with their specifications and features.

6.1 Electrolyzers

High-purity green H₂ production is primarily accomplished from AEMWEs (Fig. 9). The AEM serves as the central part of AEMWEs which conducts OH⁻ ions and separates H₂ and O₂ compartments.

Fig. 9 Schematic of an AEMWE showing the electrode reactions.

The AEMWE performance and durability with various AEMs are shown in Fig. 10 and summarized in Table 2. For instance, the combination of meta- and para-conformational isomers of terphenyl to synthesize an ultra-microporous AEM structure was successfully used (Fig. 10a and b).³⁴ The synthesised AEMs were able to achieve high IC and better water diffusivity (IC of 217 mS cm⁻¹ at 90 °C, an IEC of 2.53 mmol g⁻¹, a WU of 38.7%, and an SR of 6.7%) with remarkable stability for 8000 h in 1 M NaOH at 80 °C. A balanced molar ratio of two isomers of terphenyl such as m-terphenyl and p-terphenyl was used to produce an AEM with a uniformly distributed ultra-microporous structure (MTCP-50). They were able to achieve a high current density of 5.4 A cm⁻² at 1.8 V with 3000 h durability (0.015 mV h⁻¹ for the first 2500 h). Inter-connected ionic channels enabled fast hydroxide transport through the AEM with improved temperature stability. A stable poly(aryl-quinuclidinium) (PAQ) AEM with controlled molecular weight is synthesized, which showed a very high current density of 8 A cm⁻² at 2 V with stability >2446 h (Fig. 10c and d).³³

Fig. 10 Polarization and durability curves for various AEMWEs: (a) and (b) I–V curves of pure water feed of an MTCP-50 (meta-and para-terphenyl piperidinium) AEM based AEMWE and its long-term durability over 3000 h (adapted from ref. 34; available under CC-BY 4.0-). (c) and (d) Polarization and durability curve of a branched poly(aryl-quinuclidinium) AEM with stability over 2446 h. Reprinted with permission.³³ Copyright 2024, Wiley. (e) and (f) Linear sweep voltammetry curve and the durability result of a dibenzothiophene-terphenyl piperidinium based AEMWE device. Reprinted with permission.²⁴⁶ Copyright 2024, Wiley. (g) and (h) Polarization curve of a poly(p-terphenyl quinuclidinium hydroxide) (PPTQ-OH)-based AEM and its durability for 200 h. Reprinted with permission.²⁴⁷ Copyright 2023, Wiley.

Table 2 Comparative AEMWE performance matrix for Fig. 10 and 11^a

AEM	IC (mS cm^−1) at 80 °C	IEC (mmol g^−1)	Cell operating temperature (°C)	Current density (A cm^−2) at voltage (V)	Cell durability (h)	Reference
a NA: not available.
MTCP-50	217	2.53	90	5.4 at 1.8	3000	34
PPTQ-OH	139	2.83	80	1.94 at 2	200	247
PAQ-5	186	2.66	80	8 at 2	2446	33
Z-S-20	182	NA	80	7.12 at 2	650	246
L-50-30	123	1.69	60	1 at 1.78	100	114
QPTTP-5	164	2.84	80	1.5 at 2.2	500	194
QTAF-3.0	168	2.97	80	2 at 1.72	1000	248
PPTQ-13BTD	155	2.64	80	2.36 at 2	700	210

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Using DFT and accelerated aging analysis they proved the exceeding stability of the quinine ring structure along with the dominant S_N2 degradation mechanism. The durability evaluation of the AEM with PGM-free catalysts was conducted at different current densities of 1, 2 and 4 A cm⁻². Even under rigorous conditions it still operated for 524 h with 0.32 mV h⁻¹ voltage decay indicating the ability of the PAQ-5 AEM to operate at high current density. This AEM synthesized in this work has been claimed to be the first one that can operate for 500 h at 4 A cm⁻². A continuous hydroxide ion conducting channel is constructed using the sulphur enhanced bridging strategy in the study by Zheng et al. (Fig. 10e and f).²⁴⁶ The piperidinium and dibenzothiophene (Z-S-20) ion-dipole induced surface site hopping mechanism in the AEM (IC of 182 ± 28 mS cm⁻¹, an SR of 68.6%, and a WU of 246%) resulted in an enhanced Grotthuss mechanism with the help of sulphur atoms. Creating a hydrophilic region around the hopping site significantly enhanced the hydroxide transport mechanism with a remarkable current density of 7.12 A cm⁻² at 2.0 V over 650 h of operation. Furthermore, N-methyl quinuclidinium is one of the emerging cationic groups for AEM development due to its better stability (Fig. 10g and h),²⁴⁷ the AEM consisting of poly(aryl quinuclidinium) with N-methyl quinuclidinium as a cation (PPTQ-OH) (IC of 139.1 mS cm⁻¹, a WU of 25.95%, and an SR < 9.26%) showed 1800 h of stability in a highly alkaline environment (10 M NaOH). A current density of 1.94 A cm⁻² @2.0 V was achieved by incorporating this AEM. The higher alkaline stability of this cationic group is further investigated by many groups subsequently.^33,210

One of the latest comparative studies for radiation-grafted low-density polyethylene (RG-LDPE) and ethylene-tetrafluoroethylene (ETFE)-based AEMs by Biancolli et al. indicated that LDPE substrates might be more suitable for long-term efficient and durable AEMWE application than the ETFE-based AEM (Fig. 11a and b).¹¹⁴ A thickness in the range of 45 to 90 μm for the LDPE-AEM revealed better overall performance for AEMWEs with an achieved minimum IEC of 1.7 mmol g⁻¹. Further enhancing the thickness did not result in better performance due to the high IEC value, which showed lower hydrogen cross-over for the AEMWE. LDPE with 50 μm thickness showed an IC of 123 mS cm⁻¹ (IEC of 1.69 mmol g⁻¹) whereas ETFE with 25 μm thickness showed an IC of 145 mS cm⁻¹ (IEC of 1.75 mmol g⁻¹) and both are identified as promising candidates for AEMWEs. This study offers a better combination of thickness, IEC, and low hydrogen cross-over (2.7–3.2%) with good performance for AEMWEs. The durability test for these two AEMs put forth that in a >100 h period at a constant current density of 0.5 A cm⁻² the voltage increase for LDPE was only 71 μV h⁻¹ whereas for ETFE it was 300 μV h⁻¹ and proved LDPE as more durable for long-term application.

Fig. 11 (a) and (b) Performance and durability of AEMWEs containing an LDPE based AEM with varying thickness with 100 h of durability test. Adapted from ref. 114 Copyright 2024, Royal Society of Chemistry; (c) and (d) polarization and durability curves of the quaternized poly(p-terphenyl-tetraphenyl methane-piperidinium) (QPTTP) AEM with durability. Reprinted with permission.¹⁹⁴ Copyright 2024, Elsevier. (e) and (f) Performance and durability curve with the polyphenylene-based AEM (QTAF). Reprinted with permission.²⁴⁸ Copyright 2024, Wiley. (g) and (h) Polarization curve of the cross-linked poly(aryl N-methyl quinuclidinium) (PPTQ-13BTD) AEM at different temperatures and its AEMWE durability test. Reprinted with permission.²¹⁰ Copyright 2025, Elsevier.

Development of a rigid three-dimensional branched structure for AEMs with tetraphenyl methane and poly(aryl piperidinium) groups along with their subsequent quaternization (QPTTP) is reported (Fig. 11c and d).¹⁹⁴ The varying degree of branching was analyzed for its performance and it was concluded that this branched structure is able to provide micropores in the AEM morphology along with ionic group aggregation which altogether contributed to a phase-separated morphology. Due to these features, the QPTTP-5% AEM (IC of 164.70 mS cm⁻¹, a WU of 30.39%, an IEC of 2.84 mmol g⁻¹, and an SR of 8.53%) showed a current density of 1.5 A cm⁻² and 72.7% IC retention after 1920 h of soaking in 3 M KOH. Additionally, a performance decay rate of ∼0.54 mV h⁻¹ was reported for the QPTTP-5% AEM over 500 h. Synthesising a copolymer AEM referred to as a polyphenylene based AEM (QTAF-3.0) consisting of terphenyl-based hydrophobic monomers and fluorene based hydrophilic components on the polyphenylene backbone polymer was successfully carried out (Fig. 11e and f).²⁴⁸ The formation of a well-developed ionic channel inside the AEM due to the hydrophobic components provides relevant properties to the AEM (IC of 168.7 mS cm⁻¹, an IEC of 2.97 mmol g⁻¹, a WU of 128%, and an SR of 36.9%). The incorporation of QTAF-3.0 into the AEMWE resulted in a current density of 2 A cm⁻² at 1.72 V, and it operated continuously for 1000 h of duration at a current density of 1.0 A cm⁻². The possibility of improving the overall performance of a device by strategically altering the hydrophobic and hydrophilic components of its AEM structure is evident from this work.²⁴² One of the very recent studies with multi-cationic cross-linking showed good performance results for a quinuclidinium-based polyaryl AEM (PPTQ-13BTD) (IC of 155 mS cm⁻¹, an IEC of 2.64 mmol g⁻¹, a WU of 39.47% and an SR of 9.85%) (Fig. 11g and h).²¹⁰ A continuous operation for 700 h is achieved for the AEMWE equipped with the PPTQ-13BTD AEM with a current density of 1.02 A cm⁻² at 80 °C. Improved IC and high alkaline stability of this particular AEM at high pH and high temperature without any degradation for 2500 h were confirmed from its NMR data. The cross-linking strategy positively contributed to the mechanical stability (tensile strength > 90 MPa).²¹⁰ From the performance data of eight different AEMs, it can be summarized that MTCP-50 shows the best balance of IC, current density and exceptional cell durability. The highest current density is achieved for the PAQ-5 AEM with a moderate durability parameter. Most of the AEMs operate at 80 °C with voltage ranging from 1.72–2.2 V indicating different efficiency levels. These data demonstrate viability for hydrogen production using AEMWEs using the AEMs.

In addition to this, Jang et al. reported a low-cost large-sized three-cell stack AEMWE, which is composed of a copper-cobalt oxide anode, a polycarbazole (QPC-TMA)-based AEM, and a Pt/C cathode with high performance and durability (Fig. 12a–c).¹⁶

Fig. 12 Performance and durability curves of three-cell stacked AEMWEs: (a) graphical representation of three-cell stack arrangement, (b) polycarbazole-based (QPC-TMA) AEM incorporating water electrolyzer performance curve, and (c) durability tests. Reprinted with permission.¹⁶ Copyright 2022, American Chemical Society.

Long-term durability of 2000 h with an energy conversion efficiency of 75.6% with 0.0085 mV h⁻¹ degradation along with 40.4 L h⁻¹ of hydrogen production was achieved for this AEMWE. This three-cell stack dramatically enhanced its performance with increasing temperature and voltage. A successful incorporation of the QPC-TMA AEM improved performance with negligible chemical and physical degradation even after the durability test with a non-PGM based anode, which is the major significance of this work. The overall cell performance and durability of the AEMWE assembly are coupled with AEM properties.

6.2 AEMFCs

AEMFCs are direct electrochemical energy conversion devices which convert chemical energy of a fuel into electricity without any intermediate steps unlike internal combustion engines (Fig. 13). AEMFC performance has been improved significantly, showing remarkable research progress in the last few decades in both performance and durability of the cell. This is partly attributed to the development of robust and highly hydroxide-conductive AEMs along with several device engineering achievements.

Fig. 13 Schematic of an AEMFC showing the various components and the anode and cathode reactions.

We have included the results of AEMFCs showing the remarkable performance and durability and the results are presented in Fig. 14 and summarized in Table 3. For instance, Omasta et al. reported the importance of water management in achieving the record high performance of 1.9 W cm⁻² using an ETFE BTMA-AEM (IC of 132 mS cm⁻¹ at 80 °C and an IEC of 0.91 mmol g⁻¹).²⁴⁹ Wang et al. reported that switching the LDPE-backbone polymer to a HDPE-based backbone polymer for the AEM resulted in high peak power density (PPD) and a low degradation rate for the AEMFC.³⁵ A crystallinity of 57% and very minimum branching compared to LDPE (47% crystallinity) indicate significant improvement in the performance of the AEMFC (Fig. 14a and b). The nanomorphology/microstructure of HDPE-AEM enhanced the water transport mechanism which successively improved the performance. An IC of 214 ± 2 mS cm⁻¹ with a PPD of 2.55 W cm⁻² was achieved with 440 h of continuous operation for the AEM with 7% voltage decay (0.70 to 0.67 V).

Fig. 14 (a and b) AEMFC performance for a high density and low-density polyethylene radiation grafted AEM with a durability plot (adapted from ref. 35 available under CC-BY 3.0). (c and d) Performance and stability data of an HDPE-based AEM (adapted from ref. 250 available under CC-BY 4.0). (e and f) Polarization and durability curves of a polynorbornene based composite AEM (adapted from ref. 190 available under CC-BY 4.0). (g and h) Polynorbornene-tetramethyl-hexane diamine AEMs with three thickness values operated at 80 °C. Reprinted with permission.¹⁸ Copyright 2020, Wiley.

Table 3 Comparative AEMFC performance matrix for Fig. 14 and 15^a

AEM	IC (mS cm^−1)	IEC (mmol g^−1)	RH (%)	Cell temperature (°C)	Current density (A cm^−2) at cell potential	Peak power density (W cm^−2)	Cell durability (h)	Reference
a NA: not available.
HDPE-AEM	214	2.44	92	80	0.6 at 0.70	2.55	440	35
GT64-15	NA	3.28	NA	80	0.6 at 0.52	3.4	500	190
GT64-15	54.2	3.28	100	80	0.6 at 0.51	3.2	2000	18
qPTTP-7	142	2.77	100	80	0.2 at 0.76	1.37	150	189
PFTP-13	208	2.81	75/100	80	0.2 at 0.92	2.34	200	29
PTP/TPB-0.5%	126.4	2.75	90/95	80–100	0.6 at 0.95	2	195	252

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In another study, Peng et al. used this HDPE-AEM to understand the water dynamics of AEMFCs under various operating conditions (Fig. 14c and d).²⁵⁰ The operando neutron imaging and computed tomographic study suggested the requirement of a balanced condition in between electrode flooding and AEM degradation. Introduction of hydrophobic components into the respective sites resulted in a PPD of 2.35 W cm⁻². This observation of the water dynamics of AEM suggested the accumulation of water at the anode and severe ionomer swelling. HDPE-AEMs were stable for 1000 h with a voltage decay rate of 0.032 mV h⁻¹ (4.6%). A reinforced block copolymer of poly(norbornene) functionalized with a QA-group (GT64-15; 64% monomers and 15% N,N,N′,N′-tetramethyl-1,6 hexanediamine (TMHDA) cross-linker) with various percentages of cross-linking was investigated for AEMFCs (Fig. 14e and f).¹⁹⁰ The optimized operating conditions were able to deliver a significantly high PPD of 3.4 W cm⁻². GT64-15 showed minimalized WU (IEC of 3.28 mmol g⁻¹, a WU of 29% and an SR of 14) with good dimensional stability for 500 h without any major voltage loss. Later Mandal et al. modified the GT64-15 AEM with 82% halogenated monomer and 5 mol% cross-linker composition (GT82-5: an IC value of 212 mS cm⁻¹, an IEC of 3.84 mmol g⁻¹, and a WU of 122%) to alter and optimize the number of bound water and free-water molecules to enhance the durability from 500 h to 1000 h.³⁰ A further incorporation of the GT64-15 AEM for AEMFCs was carried out by Hassan et al. for identifying the influence of various ionomer compositions on the electrode configuration and ion transport pathways (Fig. 14g and h).¹⁸ They were able to achieve 2000 h of durability with 3.65% voltage loss when incorporated with a combination of GT78 hydrophilic ionomer in the anode side and GT32 hydrophobic ionomer in the cathode side. Furthermore, the same AEM was continued for long-term operation and its possible recoverable and non-recoverable degradation mechanisms affecting the cell were identified (Fig. 15a and b).²⁵¹

Fig. 15 (a and b) AEMFC performance data; a tetrablock poly(norbornene) copolymer based AEM for FC with performance and durability curves. Reprinted with permission.²⁵¹ Copyright 2022, American Chemical Society. (c and d) Four-arm star shaped poly(aryl piperidine) based AEM performance and durability (adapted from ref. 189 available under CC BY-NC 3.0). (e and f) Poly(fluorene-co-terphenyl N,N-dimethylpiperidinium) (PFTP-x) based AEM with 2000 h of durability (adapted from ref. 29; available under CC-BY 4.0). (g and h) High temperature performance of the poly(p-terphenyldimethylpiperidinium)-based AEM. Reprinted with permission.²⁵² Copyright 2024, Elsevier.

An improved operational durability of 3600 h was achieved without any kind of degradation affecting the AEM evidenced by its unaltered composition and insignificant increase in its H₂ cross-over. A 11.5% voltage loss for the cell is identified where the majority of the unrecoverable loss is confirmed from the cathode side.

Furthermore, it is essential to mention that water management is very important in AEMFCs; typically it is high at the anode where it is produced, while low at the cathode where it gets consumed.²⁴⁹ When water transport or cross-over from the anode to the cathode compartment is not proper or seems to be slow in rate, it leads to anode flooding and cathode drying. The accumulation of these water molecules in the catalyst pores can also block the supply of gases to the anode and cathode. So, there is a distinguishable gradient existing in water management inside AEMFCs. The concentration of the anionic group/hydroxides in the solution influences the stability, IC, and degradation of the AEM. A moderate concentration of hydroxide inside the system should be maintained which results in improved IC and catalyst utilization and prevents the dehydration of the AEM.²⁵³ Thus, water management of the cell remains a critical challenge.^206,207,254 For instance, Diesendruck et al. showed the role of water molecules in maintaining the stability of AEMs in AEMFCs.²⁵⁵ They proved that lower water content in the cathode makes hydroxide ions more nucleophilic towards the cation and enhances the kinetics of degradation. A PDTP-25 based AEM in situ durability test for AEMFCs showed significant voltage loss not because of membrane degradation but because of undiagnosed water management.⁸³

A structure-oriented study of a piperidinium-based AEM, a four-arm star-shaped co-polymerized terphenyl methane with 1-methyl-4-piperidine and p-terphenyl with subsequent quaternization for AEMFCs (IC 142.7 mS cm⁻¹ and an IEC of 2.77 mmol g⁻¹), showed a PPD of 1.37 W cm⁻² (Fig. 15c and d).¹⁸⁹ The free-volume space design of the four-arm shaped structure was advantageous for its conductivity, stability, and high PPD. This work provided a novel and effective strategy for designing AEMs for better performance. Usually in order to achieve a high IC for AEMs, there is always some compromise in the mechanical properties of AEMs. In this case the partial substitution of the monomers significantly helped block the tradeoff between these two factors. Another study on the aryl piperidinium group proved this group as a promising category for the development of stable high performance AEMs (Fig. 15e and f).²⁹ The analysis concludes that a poly(fluorenyl aryl piperidinium)-based AEM named PFTP-13 with a fluorene segment has a smaller dihedral angle and higher rotation energy barrier than biphenyl and terphenyl groups. This torsional rotation calculation indicated the simultaneous increase in the rigidity of the AEM with phase separated morphology, which altogether resulted in high IC and an ex situ dimensional stability for 2000 h with a PPD of 2.34 W cm⁻² for the AEM (IC of 208 mS cm⁻¹, an IEC of 2.81 mmol g⁻¹, a WU of 50%, and an SR of 16%). An improved design strategy for the AEM to manage its water dynamics through the perspective of a high temperature AEMFC (HT-AEMFC) was discussed recently by Dekel and team (Fig. 15g and h).²⁵² The poly(aryl piperidinium) (PAP) group due to its excellent performance towards electrochemical devices is chosen in this study for AEM synthesis. Here, the lightly cross-linked poly(p-terphenyl dimethyl piperidinium) (PTP) AEM exhibits an optimized PPD of 2 W cm⁻² over 195 h of time with only 4% voltage decay. A simplified water management mechanism due to the incorporation of the bulky rigid triphenyl benzene group arises because of the light branching, better chain entanglement, and high free volume inside the AEM with appropriate water kinetics. This work emphasized the requirement of balanced water management, mass transport, and thermal stability for an AEM which they successfully achieved from piperidinium-based groups (IC of 126.4 mS cm⁻¹ at 80 °C). Recently Batool et al. demonstrated the use of artificial intelligence (AI) in electrolyzers and fuel cells to study the selection of the material and degradation studies.²⁵⁶

Among these AEMs, GT64-15 offers a rare combination of high PPD and extraordinary durability marking outstanding long-life fuel cell operation. PTP/TPB-0.5% and PFTP-13 showed good PPD but moderate durability likely due to structural/chemical instability under high temperature operating conditions. Overall, this AEM has balanced IC, power output and operational stability, showcasing its potential for energy applications.

6.3 Vanadium redox-flow batteries

In this section, we focus on the role of the AEM for VFRBs which emerged as one of the most promising alternatives for large scale energy storage application due to their safety, rechargeability and stability (Fig. 16). The AEM serves as a separator in VRFBs with significantly low VO²⁺ ions crossover. Here we focus more on the AEM permeability and thickness rather than IC and IEC which are less important for this application. AEMs improve VFRB efficiencies by reducing VO²⁺ ion crossover. AEMs are frequently explored owing to their low permeability towards vanadium ions, which significantly improves the coulombic, voltage, and energy efficiencies.

Fig. 16 Schematic of a vanadium redox flow battery.

The performance of selected AEM-based VRFBs is compared in Fig. 17 and summarized in Table 4. For instance, a VRFB with cross-linked polyether ether ketone (ClmPEEK) and un-crosslinked polyether ether ketone (lmPEEK) AEMs has been studied in comparison to a Nafion® 212 membrane.⁹⁶ ImPEEK and CImPEEK showed a vanadium-ion (VO²⁺) permeability of 0.47 × 10⁻⁹ and 0.055 × 10⁻⁹ cm² s⁻¹, whereas Nafion® possessed 8.9 × 10⁻⁹ cm² s⁻¹, respectively (Fig. 17a). ClmPEEK was approximately two orders lower in vanadium ion permeability compared to Nafion®. The coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) at a current density of 160 mA cm⁻² of ClmPEEK were 99.5, 76.5, and 76.1%, whereas for Nafion® they were 95.8, 63.9, and 61.3%, respectively (Fig. 17b and c). This effectively reduces the cross-contamination of positive and negative electrolytes, minimizes the self-discharge and increases its efficiency. In another study, Zhang et al. introduced a positively charged pyridinium group into a hydrophobic structure to get a pyridinium functionalized adamantane AEM containing poly(aryl ether ketone) (Py-2ADMPEK).²⁵⁷ VRFBs fabricated using this AEM have CE, VE, and EE values of 98.3%, 89.7% and 88.2%, respectively, at current densities of 80 mA cm⁻² (Fig. 17d–f). The rigid hydrophobic polymeric structure of the AEM highly contributed to the performance and stability of the VRFBs for 2000 cycles. The fabrication of a cross-linked polyarylene ether sulfone-based AEM with a bi-guanidinium (BG-PSF) cationic group showed a CE, VE and EE of 99.8%, 86% and 85.8% at 80 mA cm⁻² current density with continuous operation for 200 cycles which indicates the stability of the AEM (Fig. 17g–i).²⁵⁹ A more stable microstructure with adequate cross-linking further increases the ion selectivity of the AEM. Another study by Ban et al. investigated a series of side chain modifications on various group functionalized fluoroaryl piperidine backbone polymers with a reduced area resistance (0.16 Ω cm²) and vanadium permeability of 8.63 × 10⁻⁹ cm² min⁻¹ (Fig. 17j).²⁵⁸ The microphase separated morphology of the AEM named ‘AMPFMP’ shown an excellent EE of 82.15% and CE of 99.23% compared to other branched groups and Nafion® 115 (EE of 73.7%) (Fig. 17k).²⁵⁸ A better control over the swelling and degradation of the AEM has been attributed to the amphiphilic structure with 1000 cycles of efficient performance (Fig. 17l).²⁵⁸ Among these AEMs, ClmPEEK demonstrated the lowest vanadium ion permeability which is highly beneficial for suppressing cross-over issues, whereas AMPFMP and Py2-ADMPEK exhibited higher permeability which can lead to more electrolyte mixing. A high coulombic efficiency for BG-PSF-1.40 reflects suppressed vanadium cross-over with good membrane selectivity. AMPFMP features the thinnest membrane (30 μm) and sustains 1000 cycles at 200 mA cm⁻² balancing permeability and long-term performance. These results support the implementation of AEMs in VRFBs. Therefore, the improved efficiencies of VFRBs with higher cycle numbers will lead to the development of affordable VRFBs. New development in AEMs for VFRBs should focus on design to further improve the cross-over of VO²⁺ ions.

Fig. 17 Performance analysis of vanadium redox flow batteries employing AEMs. (a) Permeability of VO²⁺ ions through cross-linked and non-cross-linked PEEK AEMs, (b) efficiency of a ClmPEEK AEM. (c) Efficiencies as a function of cycle number of VRFBs with ClmPEEK. Reprinted with permission.⁹⁶ Copyright 2021, American Chemical Society. (d) and (e) VO²⁺ permeability and efficiency plots before and after the oxidation test, respectively, on a pyridinium functionalized 2-adamantane containing poly(aryl ether ketone) (Py2-ADMPEK)-based AEM. (f) Efficiencies as a function of cycle number of a Py2-ADMPEK-3 AEM. Reprinted with permission.²⁵⁷ Copyright 2020, Elsevier. (g) VO²⁺ permeability through quaternized and bi-guanidinium cross-linked different poly(arylene ether sulfone) (BG-PSF) AEMs. (h) Efficiency of various BG-PSF AEMs and Nafion®115. (i) Efficiencies of the BG-PSF AEM in the VRFB as a function of cycle numbers. Reprinted with permission.²⁵⁹ Copyright 2022, Elsevier. (j) VO²⁺ permeability of a series of side-chain modified fluoropoly(aryl piperidone) AEMs compared to Nafion® 115, and (k) performance of AEMs at 120 mA cm⁻² compared with Nafion® 115. (l) Efficiencies of VRFBs as a function of cycle number. Reprinted with permission.²⁵⁸ Copyright 2024, Elsevier.

Table 4 Comparative VRFB performance matrix for Fig. 17^a

AEM	Vanadium permeability (cm^2 min^−1)	CE (%)	VE (%)	EE (%)	Current density (mA cm^−2)	Thickness (μm)	Life span (cycles)	References
a NA: not available.
ClmPEEK	3.3 × 10^−9	99.5	76.57	76.1	160	63	150	96
Py2-ADMPEK	0.34 × 10^−7	98.3	89.7	88.2	120	40–45	2000	257
BG-PSF-1.40	NA	99.8	86	85.8	80	47	200	259
AMPFMP	8.63 × 10^−9	99.23	NA	82.15	200	30	1000	258

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7. Summary and future perspectives

The AEM research area is one of the emerging fields that brought significant improvements in IC and stability which is highly important for their use in AEMWEs and AEMFCs. To date, AEMs have displayed IC higher than 200 mS cm⁻¹, durability of more than 8000 h, and a tensile strength of >100 MPa. In general, the two major components the backbone polymer and cation functional group are under constant investigation to make high performing AEMs. Various ammonium-based cations, piperidinium, imidazolium, and guanidium, are well-studied functional groups showing good results in terms of IC, IEC, WU and SR. Aromatic polymer-based AEMs have shown superior alkaline stability and mechanical integrity. The thickness of the AEM should be as minimum as possible to lower the ohmic resistance but at the same time, there shouldn't be any compromise in its mechanical and thermal properties. Despite all the significant research efforts on AEM cationic functional groups and backbone polymers, the major gaps regarding their durability and how to tackle them need significant research focus. A conventional design strategy has to be implemented along with a critical understanding of the degradation mechanism of AEMs. We highlighted AEM applications in AEMWEs, AEMFCs, and VRFBs. On a lighter note, for VFRBs, the permeability and the thickness of the AEM play a major role unlike other applications (AEMWEs and AEMFCs) discussed in this review. The improved performance of AEMs in these devices shows their promise for the practical implementation of large-scale deployment. Chemical and thermal stability are necessary for their implementation into these devices for long-term applications without catastrophic failure of the device. The following points are suggested for improving AEM research in the future.

(i) Piperidinium-based cations are emerging groups which show promising results (IC, IEC, WU, and SR). Category IV (guanidinium) cations need further work to establish their potential unlike other cations.

(ii) Implementing various performance enhancing strategies in designing AEMs, screening various functional groups and backbones and optimizing their physiochemical properties will be a better route to improve AEM properties.

(iii) Research work should focus on AEM degradation and strategies to improve the durability.

(iv) MD and DFT-based modeling have to be employed further to increase the understanding of hydroxide transportation and degradation mechanisms. These are highly recommended and necessary for the advancement of electrochemical devices.

(v) Conducting high-throughput screening of cationic groups using artificial intelligence/machine learning (AI/ML) techniques; employing in situ characterization methods (e.g., time-resolved fluorescence spectroscopy) to investigate water transport mechanisms will enrich the AEM reserach.

(vi) Future studies should focus on the understanding of the degradation of the AEM in electrochemical devices (AEMWEs, AEMFCs, and VFRBs) to improve the higher technology readiness level with affordable cost.

Author contributions

Conceptualization by R. K. S. and N. J.; data collection and visualization by N. J. and R. K. S.; formal analysis and validation by N. J. and R. K. S.; funding acquisition by R. K. S.; project administration and supervision by R. K. S.; writing – original draft by N. J.; writing – review & editing by N. J. and R. K. S.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this study are made available upon reasonable request.

Acknowledgements

The authors would like to greatly acknowledge the Anusandhan National Research Foundation (ANRF)- Start-up Research Grant (SERB Scheme) (SRG/2023/002041) and the VIT SEED Grant - RGEMS Fund (SG20220086). We also acknowledge Mr Jobel Jose for his help in creating the graphical abstract.

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