Unveiling the recent progress in anion exchange membranes and their composites for fuel cells and water electrolyzer applications

Surface modification of anion-exchange electrodialysis membranes to enhance anti-fouling characteristics

<p indent=0mm>Alkaline anion-exchange membrane fuel cells (AAEMFCs) have attracted worldwide interest due to their advantages including fast oxygen reduction kinetics, high compatibility with non-precious-metal catalyst and low cost. As one of the key components in AAEMFCs, the performance of alkaline anion exchange membranes (AAEMs) directly affects the power output and durability of the fuel cells. During fuel cell operating, AAEMs require high ionic conductivity, excellent dimensional and chemical stability to ensure high efficiency and outstanding durability. However, it is still difficult for any type of the AAEMs to meet all these requirements. This monograph summarizes recent development around the world for AAEMs, especially for the trade-off effect between ionic conductivity and stability of AAEMs as well as the proposed strategies for this issue. The charge carrier in AAEMs is OH^-, and it has a lower transporting efficiency owing to its lower mobility, higher dependence on water molecular and the blocking of many hydrophobic domains in AAEMs. The improvement of ion-exchange capacity (IEC) by increasing the grafting degree (GD) of cationic functional groups can, to some extent, solve this issue. however, a high GD always bring the following negative issues: (1) Excessive swelling of AAEMs and significant reducing in the dimensional stability of membranes; (2) the increase of OH^- concentration accelerates the kinetics of nucleophilic substitution and Hofmann elimination, leading to the degradation of cationic groups; (3) the enhanced polarization of the cationic groups and the hydrophilicity of the main chain enable the polymer backbone susceptible to nucleophilic attack by OH^-, resulting in the degradation of the membrane, and even short-circuit of the fuel cells. In order to solve these issues, various of polymer chain architectures have been designed and regulated. To balance the ionic conductivity and the dimensional stability in AAEMs, double, triple and multi-cations are grafted on one site of polymer backbone to achieve a sufficiently high IEC at relatively low GD. Another realistic strategy is constructing 3D anion channels by the segregated hydrophilic/hydrophobic phase. The alkali stability of the cationic groups is affected by many factors including field effects, steric effects and conformation of substituent groups, and so on. The improvement of chemical stability of AAEMs has been another formidable scientific challenge. Researchers reduce the kinetics of the nucleophilic substitution and elimination reactions, and improve the basic stability of the cationic groups by modulating the structure of substituent groups such as the introduction of electron-donating groups, increased steric hindrance, and adequate hydration of OH^-. Among various cationic groups, the piperidinium-based cations show high resistance against both nucleophilic substitution and elimination in alkaline conditions and at elevated temperature. Furthermore, the polymer backbones without ether band and electron-withdrawing groups have been synthesized and exabit highly resistant to alkali hydrolysis. Recently, new strategy for constructing ordered ion channels in AAEMs by novel porous materials such as metal-organic frameworks (MOF), Trögers base, and macrocyclic crown ether compounds, provide for efficient ionic transport. Additionally, highly stable metal complexes have been used as cationic group in AAEMs. These new trends will open up an exciting opportunity to design high-performance AAEMs. The appearance of highly stable AAEMs enables the AAEMFCs to be operated at 80°C, and the cell works stably in a period of study over <sc>100 h.</sc> Although this progress is encouraging, there still remains work for improving the cell performance and stability. A <sc>1000 h</sc> of stable operation at elevated temperatures will be the next mission for AAEMFCs. In addition, there are some fundamental issues necessary to explore, such as the transport mechanism of OH^- in the membrane, the molecular interaction of polyelectrolytes and their self-assembly mechanism in solution and film formation, the mechanism of the influence of morphology on the chemical stability of AAEMs, and the factors affecting the long-term stability of AAEMFCs. These scientific issues will be the focus of future research. The development of AAEMFCs is on its way, and it calls for more efforts in fundamental study, polymer chain architectures and morphology designing, and fuel cell engineering to make it viable.

Self-crosslinked blend alkaline anion exchange membranes with bi-continuous phase separated morphology to enhance ion conductivity

Alkaline anion exchange membrane (AEM) is the key material for anion exchange membrane fuel cells (AEMFC), which can potentially outperform proton exchange membrane fuel cells due to facile oxygen reduction kinetics and wide catalyst choices. Quaternary ammonium (QA) AEMs have been shown to be low in alkaline stability, and their fabrication often involves carcinogenic chemicals. Therefore, recent years have witnessed emerging research efforts in developing non-QA AEMs. As part of these efforts, this work reports imidazolium AEMs that were fabricated via functionalization of chloromethylated polysulfone with methylimidazole. The membrane exhibited ion exchange capacity, conductivity and thermal stability that are comparable or superior to those of conventional membranes. A H2/O2fuel cell using the synthesized membrane yielded a peak power density of 16 mW cm−2. Although this result is not high and the membrane stability is still an issue, the potential of using imidazolium AEM in a fuel cell is seen. This work opens up a new route for non-QA AEM design and fabrication. Based on the chemistry involved, it is anticipated that different chloro- or bromomethylated aromatic polymers and imidazole derivatives may be used to optimize membrane conductivity and stability, thus offering the possibility to fabricate high performance AEM.

Preparative chromatography is widely used in the downstream purification of biopharmaceutical products. Replacement of resins by membranes as chromatographic supports, overcomes many of the limitations associated with resin-based chromatography such as high-pressure drops, slow processing rates due to pore diffusion and channeling of the feed through the bed. In particular, adsorptive membranes may be ideally suited for virus capture. Virus capture is critical in a number of applications. In gene therapy and vaccine production, large-scale purification of virus vectors is often essential. In the manufacture of biopharmaceuticals, validation of virus clearance is critical. Here results for purification of Aedes aegypti densonucleosis virus (AeDNV) using anion and cation exchange membranes are presented. AeDNV is a non-enveloped, single-stranded mosquito-specific parvovirus. Virus particles are around 20 nm in size. AeDNV could find potential applications in integrated vector-borne disease control programs. In addition, capture of parvovirus for validation of virus clearance in the manufacture of biopharmaceuticals is of commercial importance. By adjusting the pH of the feed stream, AeDNV particles may be adsorbed by both anion and cation exchange membranes. However, strongly basic anion exchange membranes were the most effective in adsorbing AeDNV particles. Adsorption and subsequent elution of AeDNV by anion exchange membranes leads to significant virus concentration. Dynamic and static capacities for anion exchange membranes were similar. Further, a sharp elution curve was obtained suggesting that pore diffusional resistances are insignificant. The adsorption of AeDNV particles by anion exchange membranes may be described by a linear isotherm.

Fuel Cells have received a lot of attention in the area of clean renewable power sources because of their high efficiency, low emission, and the wide range of potential applications such as automobiles, portable devices etc. The state-of-the-art commercial proton exchange membrane fuel cell (PEMFC) systems are based on Nafion® as an electrolyte. Recently, Anion Exchange Membrane (AEM) based fuel cells have received much attention from researchers not only due to possibility to use non-precious catalyst [1] but also because of other advantages in comparison to the PEMFC systems such as the fast kinetic of the oxygen reduction reaction and lower cost by using non-precious metals as electrocatalysts [2, 3]. However there are still many challenges to develop commercial AEMFC systems including lower power density, higher degradation rate and more complicated water management in comparison to the PEMFC [4-6]. There are different possibilities to improve the system like using highly active catalysts, optimization of the operating parameters, and development of different AEM materials [4]. One of the main challenges is to produce an AEM which possess high ionic conductivity as well as chemical, thermal, and mechanical stability. The increased number of active groups in the membrane structure usually leads to a decrease of the mechanical properties. Most of the developed and reported AEMs show poor dimensional stability with their presence of functional groups which lead to strong swelling and brittleness of the membrane after drying [7, 8]. The chemical degradation is also a known effect in the alkaline membranes and limits its long term stability [5, 8-10]. To avoid these disadvantages of AEMs, there are several promising methods reported in the literature. Some promising methods for the modifications are pore-filling technique [11], chemical crosslinking [12], reinforcement [13] and the synthesis of porous membranes [14, 15]. The goal of this research work is to develop AEMs with high ionic conductivity without sacrificing their physical and chemical properties. It has been considered that the formation of pores leads to the improvement of the water content and thus the transport and mobility of the anion during operation. With the use of a commercial anion exchange polymer and different types of additive porous AEMs are developed. The modified porous AEM relevant properties were studied and also compared with a commercially available AEM. Ionic conductivity was measured with four-probe electrodes as a function of temperature and relative humidity by using electrochemical impedance spectroscopy. The thermal behaviour and the released substances in the steps of degradation for the modified membranes were studied with a thermogravimetric system coupled to a GC/MS system. In addition, other properties such as mechanical properties, water uptake and swelling behaviour were also studied and reported. For future research, the modified membranes will be used for the production of alkaline electrolyte membrane fuel cells. X. Deng and H. Tüysüz, ACS Catal. 4 (10), 3701-3714 (2014). I. Katsounaros, S. Cherevko, A. R. Zeradjanin and K. J. J. Mayrhofer, Angew. Chem. Int. Ed. 53 (1), 102-121 (2014). K. Tammeveski, I. Kruusenberg, L. Matisen, Q. Shah and A. Kannan, in 3rd CARISMA International Conference (Copenhagen, Denmark, 2012), pp. 1. S. Gottesfeld, D. R. Dekel, M. Page, C. Bae, Y. Yan, P. Zelenay and Y. S. Kim, J. Power Sources 375, 170-184 (2018). T. J. Omasta, L. Wang, X. Peng, C. A. Lewis, J. R. Varcoe and W. E. Mustain, J. Power Sources 375, 205-213 (2018). D. R. Dekel, J. Power Sources 375, 158-169 (2018). Y. Zhao, H. Yu, F. Xie, Y. Liu, Z. Shao and B. Yi, J. Power Sources 269 (0), 1-6 (2014). A. Amel, S. B. Smedley, D. R. Dekel, M. A. Hickner and Y. Ein-Eli, J. Electrochem. Soc. 162 (9), F1047-F1055 (2015). Z. Yang, J. Ran, B. Wu, L. Wu and T. Xu, Current Opinion in Chemical Engineering 12, 22-30 (2016). V. J. Bharath, R. Jervis, J. Millichamp, T. P. Neville, T. Mason, B. Tjaden, P. R. Shearing, R. J. C. Brown, G. Manos and D. J. L. Brett, Int. J. Hydrogen Energy 42 (9), 6243-6249 (2017). S. Y. Chen, C. C. Han, C. H. Tsai, J. Huang and Y. W. Chen-Yang, J. Power Sources 171 (2), 363-372 (2007). Y. Luo, J. Guo, C. Wang and D. Chu, Electrochem. Commun. 16 (1), 65-68 (2012). H. Tang, M. Pan, F. Wang, P. K. Shen and S. P. Jiang, J. Phys. Chem. B 111 (30), 8684-8690 (2007). E. Vijayakumar and D. Sangeetha, RSC Advances 5 (53), 42828-42835 (2015). H. Zarrin, M. Fowler and Z. Chen, ECS Trans. 50 (2), 2083-2089 (2013)

Comparison of the performances of dibenzofuran- and dibenzothiophene-based poly(terphenyl piperidinium) anion exchange membranes for water electrolysis

Hydrogen production using alkaline membrane water electrolysis has recently attracted interest as an alternative to traditional liquid alkaline water electrolysis, proton exchange membrane water electrolysis, and solid-oxide water electrolysis (1). Alkaline membrane electrolyzers provide an efficient, modular, and reliable method to produce hydrogen from water and renewable electricity sources. One advantage arises from the fact that the alkaline environment facilitates better oxygen evolution reaction (OER) kinetics and allows the use of non-platinum group catalysts for the OER (2) (Of course, one must acknowledge that this is countered by the more sluggish HER in alkaline media). In a solid-state alkaline membrane water electrolyzer, the anion exchange membrane (AEM) has two functions: 1) it acts as an impermeable barrier to the fuel and oxidant gases; and 2) conducts the hydroxide ions from the cathode, where they are generated by the electrochemical reduction of oxygen, to the anode. In this work we will discuss our latest findings on AEM degradation under alkaline conditions encountered during AEM water electrolyzer operation. Despite the promising results reported in terms of hydroxide ion conductivities and electrolyzer performance, there is a general understanding that the AEM membranes and solubilized AEM binders do not yet have the necessary alkaline stability for practical applications. It is accepted that the mechanisms of AEM degradation under alkaline conditions are related to the common and well-reported modes under which the fixed cation groups degrade. Quaternary ammonium groups can degrade under alkaline conditions through: 1) Hoffman elimination, where the quaternary cation is cleaved, leaving as tertiary amine, and resulting in the formation of an alkene at the carbon where the ammonium was bonded (requires the presence of alpha and beta hydrogen); 2) a direct nucleophilic reaction where the cation is completely cleaved, resulting in the formation of a tertiary amine and an alcohol at the carbon where the ammonium was bonded to the polymer backbone; and 3) through another nucleophilic substitution reaction where the adjacent organic moiety to the inorganic atom (usually a methyl or alkyl group) is cleaved resulting in the formation of a tertiary amine and an alcohol. There are other less frequently observed degradation pathways, that involve the formation of ylide intermediates, known as Sommelet–Hauser and Stevens rearrangements. All these mechanisms involve the presence of a strong base and cause the formation of tertiary amines with the subsequent loss of ion exchange capacity and ionic conductivity. However, cation degradation alone cannot account for all the membrane deterioration encountered during AMFCs operation. It has been observed that the AEM membranes and solubilized AEM binders suffer degradation that affects the integrity of the polymer backbone. Postmortem inspection of the MEAs and probing via 1D and 2D NMR spectroscopy confirmed backbone hydrolysis. The degradation is especially intense in the anode side of the electrolyzer. In an experiment with two AEM separators placed together, we observed preferential thinning of the membrane in contact with the anode. We also investigated the prevailing hypothesis that the backbone hydrolysis was triggered by the presence of quaternary ammonium cations in close proximity to the aromatic rings (ether hydrolysis leading to chain scission) (3). We evaluated the alkaline stability of AEMs with six carbon pendant chains tethered to the polyphenylene backbone and derivatized with trimethylamine (TMA) and quinuclidine (ABCO). We found that such AEMs underwent chemical degradation under alkaline conditions yielding similar products encountered in AEMs without pendant chains. In an attempt to explain the preferential degradation and thinning of the membrane at the anode of an operating electrolyzer, we studied the effect of oxygen on AEM degradation in alkali. We compared the AEM degradation of PPO-based AEMs in oxygen-saturated 1M KOH and nitrogen-degassed 1M KOH. The presence of oxygen accelerated the degradation of the AEMs. It was found that the PPO-TMA membranes lost 50% of their ion exchange capacity after 30 days immersed in oxygen-saturated 1M KOH (at 60°C). When the membranes were kept in nitrogen-degassed 1M KOH they only lost 20% of their IEC under similar conditions. This is clear evidence that oxygen, and probably reactive oxygen species (superoxide), are actively involved in the alkaline degradation of the PPO-TMA+ AEMs. NMR spectra of degraded AEMs as well as other evidence of preferential degradation in the presence of oxygen will be presented and discussed.

Hybrid organic-inorganic anion-exchange pore-filled membranes for the recovery of nitric acid from highly acidic aqueous waste streams

Low cost hydrogen production by anion exchange membrane electrolysis: A review

Surface modification of anion exchange membrane using layer-by-layer polyelectrolytes deposition facilitating monovalent organic acid transport

Anion exchange membrane fuel cells (AEMFCs) are attractive alternatives to proton exchange membrane fuel cells due to using non-noble metal catalysts and faster cathode reaction kinetics. Anion exchange membranes (AEMs) are one of the key materials composed of AEMFCs. The hydroxide conductivity of AEMs has been able to meet operating requirements of fuel cells. Although the hydroxide conductivity is no longer a problem, the stability of AEMs is still a notable challenge. This work mainly introduces the development of alkaline stability of AEMs. AEMs mainly consist of functional groups, side chains and polymer backbones. The degradation of functional groups causes by attacking of hydroxide ion through Hofmann elimination, nucleophilic substitution and ylide-intermediate degradation pathways. Numerous functional groups have been developed in order to improve the alkaline stability of AEMs. The introductions of appropriate electron donors and steric shielding are effective ways to weaken the attack of hydroxide ion. Developing new functional groups is a very important way. In addition, the development of novel stable groups is an important way for long-lived AEMs. For example, hetero-cycloaliphatic quaternary ammonium cations (QAs) are proved to be especially stable in both model compound studies and even AEM studies. The most commonly studied AEMs were synthesized by chloromethylation/bromination of the aromatic backbones, followed by a Menshutkin reaction to introduce QAs. Benzylic groups, electron-withdrawing groups, are prone to degrade by nucleophilic substitution on the benzylic carbon/α-carbon, Hofmann elimination and N-ylide intermediate formation in the presence of hydroxide ion. Therefore, the tethering linkage between the polymer backbone and functional groups is quite important. Recent studies have mainly introduced long side chains between functional groups and polymer backbones, leaving functional groups away from the benzene ring of the polymer backbone. The all-alkyl, amine- and ether-containing branched-chain structures are proved to be stable. Poly(arylene ether)s have been extensively studied in AEMs due to the advantages of good thermal stability, high mechanical property and easy modification. Frequently employed polyaromatic electrolytes are quaternized poly(aryl ether sulfone)s, which are prepared via the nucleophilic polycondensation reaction. The introduction of functional groups leads to degrade of these main chains containing aryl ether bonds, which are unable to avoid in the polycondensation reaction. It is necessary that main chains are devoid of aryl ether bonds so as to develop AEMs with excellent alkaline stability. Diels-Alder reaction, coupling reaction and acid-catalyzed Friedel-Craft polycondensation are effective methods for synthesizing aromatic backbones devoid of aryl ether bonds. In addition, aliphatic backbones, such as poly(ethylene-co-tetrafluoroethylene), are proved to have good stability in alkaline conditions, so its modification for AEMs has also become an important research direction. At present, the alkaline stability of AEMs has made great progress. Three major aspects have been studied in an effort to develop stable AEMs: (1) screening stable functional groups such as five- or six-membered cyclic amines, substituted imidazoles; (2) developing effective linkages between functional groups and polymer backbones; (3) designing aryl ether-free aromatic backbones and stable aliphatic backbones. The designed AEMs can remain for hundreds of hours without degradation in alkaline solution, which is a significant advancement in increasing alkaline stability of AEMs.

Multilayered surface modification of anion exchange membrane by MoS2 flakes for improved antifouling performance

Modification and properties characterization of heterogeneous anion-exchange membranes by electrodeposition of graphene oxide (GO)

Enhanced diffusion dialysis performance of cross-linked poly(aryl piperidine) anion exchange membranes by thiol-ene click chemistry for acid recovery