High molecular weight, quaternary ammonium-tethered poly(biphenyl alkylene)s without alkaline labile C−O bonds were synthesized via acid-catalyzed polycondensation reactions for the first time. Ion-exchange capacity was conveniently controlled by adjusting the feed ratio of two ketone monomers in the polymerization. The resultant anion exchange membranes showed high hydroxide ion conductivity up to 120 mS/cm and excellent alkaline stability at 80°C. This study provides a new synthetic strategy for the preparation of anion exchange membranes with robust fuel cell performance and excellent stability.T he synthesis of robust, highly anion-conductive polymers has been a subject of intense research because of the great potential of anion exchange membranes (AEMs) for applications in fuel cells, electrolysis, water treatment, and other electrochemical energy conversion and storage technologies. 1,2 Compared with acidic proton exchange membrane fuel cells, alkaline fuel cells offer the significant advantages of faster kinetics for the oxygen reduction reaction and the option to use earth-abundant transition metals (e.g., nickel) as electrocatalysts. 3,4 Thus, AEM fuel cells, which use an AEM as the solid electrolyte, are significantly less costly than Nafionbased proton exchange membrane fuel cells. Unfortunately, most current AEMs lack sufficient ion conductivity. Furthermore, their poor chemical and mechanical stabilities under alkaline conditions, particularly above 80°C, have been major barriers to the adoption of AEM fuel cells as reliable clean energy conversion technology.In recent years, a variety of AEMs with main polymer chain structures ranging from polysulfones, 5,6 poly(phenylene oxide)-s, 7 poly(phenylene)s, 8 poly(benzimidazolium)s, 9 poly(arylene ether ketone)s, 10,11 and poly(arylene ether sulfone)s 12 have been investigated. These hydroxide ion conducting polymers are generally prepared by attaching pendant quaternary ammonium (QA) groups to premade polymer backbone chains. Aromatic polymer backbones, which typically contain aryl ether bonds, are postfunctionalized via chloromethylation or benzylic bromination followed by quaternization with trimethylamine ( Figure 1a). Although the synthetic process is simple, the chloromethylation reaction often requires toxic reagents, long reaction times, and extensive optimization to reach a desired degree of functionalization. Side reactions (e.g., gelation) frequently occur over prolonged reaction times, making it difficult to achieve an ion-exchange capacity (IEC) above 2.5 mequiv/g. 13 Furthermore, these postfunctionalization approaches allow the installation of only benzyltrimethylammonium as a QA in AEMs. After screening a variety of model QAs, we recently reported that compared with benzyltrimethylammonium a long alkyl-tethered QA (e.g., hexyltrimethylammonium) has comparable or better thermochemical stability under alkaline conditions. 14 The primary role of the polymer backbone in AEMs is to provide mechanical stability via entanglements of polymer chains...

Three novel fluorene-based polymers with pendant alkyltrimethylammonium groups were synthesized and characterized. The polymers were soluble in dimethylformamide, and dimethyl sulfoxide at room temperature and methanol at 40°C while remaining insoluble in water. The polymeric membranes were transparent and flexible and exhibited hydroxide ion conductivities above 100 mS/cm at 80°C. The results of 1 H NMR and titration measurements demonstrated an excellent chemical stability of the synthesized polyfluorene, even after treatment in 1 M NaOH solution at 80°C for 30 days. The results of this study suggest a feasible approach to the synthesis and practical applications of a new class of alkaline anion exchange membranes. D ue to increasing demands for clean energy technology worldwide, fuel cells are attracting significant attention as environmentally friendly power generators that can replace fossil fuel-based generators. 1−6 Among fuel cell types, proton exchange membrane fuel cells (PEMFCs), which use a solid polymer membrane as the electrolyte, have been the most extensively explored because they have high power density, high energy conversion efficiency, low start temperature, and no pollutant emission. 7 However, the high costs of noble metal catalysts (e.g., platinum) and perfluorosulfonated polymer electrolytes have hindered the wide adoption of PEMFCs as a viable commercial technology.Alkaline anion exchange membrane (AEM) fuel cells are an attractive alternative to PEMFCs because they can potentially use less expensive nonprecious metal electrocatalysts. One of the major challenges in AEM fuel cells is finding suitable hydroxide ion conducting polymeric membranes that maintain robust mechanical properties, chemical stability, and moderate water swelling while providing high hydroxide ion conductivity. 8−10 A variety of AEMs containing quaternary ammonium (QA) cationic groups based on polysulfones, 11−13 polyphenylenes, 14,15 polystyrenes, 16−18 polyethylenes, 7,19,20 and poly-(phenylene oxide)s 21−23 have been studied as AEM materials. These membranes are typically prepared via the chloromethylation of aromatic polymer backbones, followed by substitution of the chloromethyl group with trimethylamine to form QA groups. However, chloromethyl methyl ether, the reagent most commonly used to introduce a chloromethyl group to polymers, is carcinogenic, and precise control of the degree and location of functionalization is usually difficult. Therefore, the preparation of AEMs via alternative synthetic routes that avoid the use of the toxic reagents is desirable.Despite the importance of AEMs in fuel cells, strategies to produce high-performance AEMs remain under-developed. Poor chemical stability of AEMs under high-pH conditions at elevated temperature is one of the most critical issues that limit the practical use in fuel cells. Particularly, robust stability above 80°C is highly desired because the CO 2 solubility in water greatly diminishes above 80°C, preventing carbonate/ bicarbonate formation, and fuel cell ...

A new design concept for ion-conducting polymers in anion exchange membranes (AEMs) fuel cells is proposed based on structural studies and conformational analysis of polymers and their effect on the properties of AEMs. Thermally, chemically, and mechanically stable terphenyl-based polymers with pendant quaternary ammonium alkyl groups were synthesized to investigate the effect of varying the arrangement of the polymer backbone and cation-tethered alkyl chains. The results demonstrate that the microstructure and morphology of these polymeric membranes significantly influence ion conductivity and fuel cell performance. The results of this study provide new insights that will guide the molecular design of polymer electrolyte materials to improve fuel cell performance.

This pilot study suggested that anodal tDCS on the leg motor cortex in the impaired hemisphere may facilitate the effect of RAGT on functional ambulation in chronic stroke patients. Larger clinical trials will be needed to confirm the effect of RAGT combined with tDCS in chronic stroke patients based on the present study.