A two-step strategy for the preparation of anion-exchange membranes based on poly(vinylidenefluoride-co-hexafluoropropylene) for electrodialysis desalination

Highly conductive anion exchange membranes with low water uptake and performance evaluation in electrodialysis

A facile approach to fabricate composite anion exchange membranes with enhanced ionic conductivity and dimensional stability for electrodialysis

Innovative low-energy enrichment of sulfuric acid using PVDF-HFP anion exchange membranes with acid-blocking properties

Functionalization of rubbery domains in SBS triblock copolymers for anion exchange membrane applications with enhanced robustness

Benzylmethyl-containing poly(arylene ether nitrile) as anion exchange membranes for alkaline fuel cells

Anion-exchange membrane with ion-nanochannels to beat trade-off between membrane conductivity and acid blocking performance for waste acid reclamation

Poly(arylene ether sulfone)s ionomers containing quaternized triptycene groups for alkaline fuel cell

<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.

In this study, bipolar membrane electrodialysis was proposed to directly convert L-ornithine monohydrochloride to L-ornithine. The stack configuration was optimized in the BP-A (BP, bipolar membrane; A, anion exchange membrane) configuration with the Cl− ion migration through the anion exchange membrane rather than the BP-A-C (C, cation exchange membrane) and the BP-C configurations with the L-ornithine+ ion migration through the cation exchange membrane. Both the conversion ratio and current efficiency follow BP-A > BP-A-C > BP-C, and the energy consumption follows BP-A < BP-A-C < BP-C. Additionally, the voltage drop across the membrane stack (two repeating units) and the feed concentration were optimized as 7.5 V and 0.50 mol/L, respectively, due to the low value of the sum of H+ ions leakage (from the acid compartment to the base compartment) and OH− ions migration (from the base compartment to the acid compartment) through the anion exchange membrane. As a result, high conversion ratio (96.1%), high current efficiency (95.5%) and low energy consumption (0.31 kWh/kg L-ornithine) can be achieved. Therefore, bipolar membrane electrodialysis is an efficient, low energy consumption and environmentally friendly method to directly convert L-ornithine monohydrochloride to L-ornithine.

Preparation of acid block anion exchange membrane with quaternary ammonium groups by homogeneous amination for electrodialysis-based acid enrichment

Mechanism underlying anion-sieving in poly(ionic liquid)-based membrane: Effective acid recovery from engineering waste

Preparation of anion exchange membrane by efficient functionalization of polysulfone for electrodialysis

While research and development has been focused primarily on proton exchange membrane (PEM) devices in recent decades, devices based on anion exchange membranes (AEM) have garnered increased interest as a potential cost-saving alternative. To be viable for use in electrochemical devices, AEMs must have good long-term alkaline and thermal stability, high hydroxide ion conductivity, lower water uptake, high molecular weight, and low oxygen or fuel permeability. Polar groups in the polymer backbone are susceptible to nucelophilic attack by the hydroxide ion, which is a major concern for the long-term stability of these materials. In the past, AEMs containing aryl ether bonds degraded under alkaline conditions. Alternatively, materials with a fully hydrocarbon polymer backbone have been shown to provide a route to low-cost and chemically resilient AEMs. In this study, several new classes of AEMs with fully hydrocarbon backbones were synthesized and characterized. Multiblock copolymers with tethered quaternary ammonium conducting groups were chosen to promote phase segregation and enhance the ionic conductivity. The AEMs were made from vinyl addition and ring-opening metathesis polymerization (ROMP) of norbornenes. These results were compared to a previous study involving AEMs made from partially fluorinated poly(arylene ether)s. The mechanical and thermal stability, ionic conductivity, long-term alkaline stability of all classes of polymers are discussed in detail in the context of performance in an AEM fuel cell or electrolyzer. Figure 1

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.

<p indent=0mm>Among various types of fuel cells, alkaline anion exchange membrane fuel cells (AEMFCs) have attracted enormous attention as clean and highly efficient energy conversion devices for vehicles and portable electronic applications. As the key component of AEMFCs, anion exchange membranes (AEMs) act both as a barrier to separate the fuel and an electrolyte to transport hydroxide anion. To fulfill the application of AEMFCs, an AEM should possess good thermal stability, good mechanical properties, high conductivity and excellent alkaline stability. The lack of commercially available AEMs with excellent alkaline stability is limited the application of AEMs in the AEMFCs. In the recent years, a variety of AEMs based on poly(sulfone)s, poly(styrene)s, polypropylene, poly(arylene ether)s, poly(phenylene)s, poly(phenylene oxide)s and poly(olefin)s, have been synthesized for the application of AEMs. Most of these AEMs showed a similar chemical structure that a polymer backbone with with pendent quaternary ammonium cations. However, quaternary ammonium cations are unstable in alkaline environment due to the Hofmann degradation, the nucleophilic substitution and the ylide reaction, even in high temperature. Therefore, various types of AEMs based on some potential alternative cations, including imidazolium, guanidinium, phosphonium, metal-cation, pyridinium, tertiary sulfonium, and pyrrolidinium cations have been prepared and investigated in the last few years. Recently, different quaternary ammonium small molecules were synthesized and their alkaline stability was thoroughly investigated by Marino and Kreuer. Their study showed that spirocyclic quaternary ammonium have good stability in alkaline condition. To date, there is few work on spirocyclic quaternary ammonium-based AEMs, and to understand the properties of this type of AEMs, a systematic study is needed. In the present work, <italic>N</italic>,<italic>N</italic>-diallylpyrrolidinium bromide [DAPy][Br] was synthesized and used as hydrophilic phase in the polymeric membranes. The purity and chemical structure of [DAPy][Br] were confirmed by ¹H NMR measurements. However, poly(<italic>N</italic>,<italic>N</italic>-diallylpyrrolidinium bromide), the homopolymer of [DAPy][Br], shows poor film forming properties, and it is very soluble in water. Therefore, styrene and acrylonitrile were chosed to synthesize the copolymers with [DAPy][Br] due to the ease of processing, good mechanical properties and chemical resistance. A mixture of styrene/acrylonitrile, [DAPy][Br], divinylbenzene and benzoin ethyl ether was stirred to obtain a homogeneous solution. Then, the mixture was casted onto a glass mold irradiation with UV light. The prepared spirocyclic quaternary ammonium-based membranes were immersed in <sc>1 mol/L</sc> KOH solution at 60°C for <sc>24 h</sc> to convert the anion of membrane from Br^- to OH^-. Then the membranes was immersed in deionized water and washed with deionized water until the pH of residual water was neutral. In summary, a series of spirocyclic quaternary ammonium-based AEMs were successfully designed and prepared via a simple synthetic strategy. The resultant AEMs demonstrated great potential for alkaline anion exchange membrane fuel cell applications based on its good thermal stability, sufficient mechanical properties and high conductivity. The ion exchange capacity can be controlled by change the mixture ratio, and the swelling ratio, water uptake and ion exchange capacity of the spirocyclic quaternary ammonium-based membranes increase with the increasement of the content of [DAPy][Br] in the membranes. The transparent and mechanically robust spirocyclic quaternary ammonium-based AEMs show high conductivity up to <sc>7.29×10^-2 S/cm</sc> at 80°C, and all the membranes showed conductivity up to <sc>1×10^-2 S/cm</sc> at room temperature. In addition, all the AEMs synthesize in this work showed excellent long-term alkaline stability at elevated temperature. The spirocyclic quaternary ammonium-based AEMs with an aliphatic backbone exhibit high alkaline stability, which will open up new prospects for the preparation of AEMs with excellent alkaline stability and high conductivity.