Stability of a variety of quaternary ammonium groups was systematically studied for development of robust alkaline exchange membranes for fuel cells.

Anion exchange membranes are an important component in alkaline electrochemical energy conversion and storage devices, and their alkaline stability plays a crucial role for the long-term use of these devices. Herein, a systematic study was conducted for the analysis of polymer backbone chemical stability in alkaline media. Nine representative polymer structures including poly­(arylene ether)­s, poly­(biphenyl alkylene)­s, and polystyrene block copolymers were investigated for their alkaline stability. Polymers with aryl ether bonds in their repeating unit showed poor chemical stability when treated with KOH and NaOCH3 solutions, whereas polymers without aryl ether bonds [e.g., poly­(biphenyl alkylene)­s and polystyrene block copolymers] remained stable. Additional NMR studies and density functional theory (DFT) calculations of small molecule model compounds that mimic the chemical structures of poly­(arylene ether)­s confirmed that electron-withdrawing groups near to the aryl ether bonds in the repeating unit accelerate chemical degradation. Results from this study suggest that the use of all-carbon-based polymer repeating units (i.e., polymers not bearing aryl ether bonds) can enhance long-term alkaline stability of anion exchange membranes in electrochemical energy devices.

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 chemically stable and elastomeric triblock copolymer, polystyrene-b-poly­(ethylene-co-butylene)-b-polystyrene (SEBS), was functionalized with various benzyl- and alkyl-substituted quaternary ammonium (QA) groups for anion exchange membrane (AEM) fuel cell applications. Synthetic methods involving transition metal-catalyzed C–H borylation and Suzuki coupling were utilized to incorporate six different QA structures to the polystyrene units of SEBS. Changes in AEM properties as a result of different QA moieties and chemical stability under alkaline conditions were investigated. Anion exchange polymers bearing the trimethylammonium pendants, the smallest QA cation moiety, exhibited the most significant changes in water uptake and block copolymer domain spacing to offer the best ion transport properties. It was demonstrated that incorporating stable cation structures to a polymer backbone comprising solely C–H and C–C bonds resulted in AEM materials with improved long-term alkaline stability. After 4 weeks in 1 M NaOH at 60 and 80 °C, SEBS-QA AEMs remained chemically stable. Fuel cell tests using benzyltrimethylammonium-containing SEBS (SEBS-TMA) as an AEM demonstrated excellent performance, generating one of the best maximum power density values and lowest ohmic resistance with low Pt catalyst loaded electrode reported thus far. Both polymer backbone and cation functional group remained stable after 110 h lifetime test at 60 °C.

Elastomeric anion exchange membranes (AEMs) were prepared by acid-catalyzed Friedel–Crafts alkylation of the polystyrene block of polystyrene-b-poly­(ethylene-co-butylene)-b-polystyrene (SEBS) using bromoalkylated tertiary alcohols and triflic acid as a catalyst, followed by amination with trimethylamine. This simple one-step bromoalkylation allowed convenient control of both the degree of functionalization and cation tether length by changing the molar ratio and the structure of the bromoalkylated tertiary alcohol. The resulting quaternary ammonium-functionalized ionic triblock SEBS copolymers showed a microphase-separated morphology on the 35 nm length scale. A series of AEMs with different ion exchange capacities and ion tether lengths were systematically investigated by comparing swelling and anion conductivity. Because the SEBS AEMs showed high swelling and low dimensional stability in water due to the rubbery nature of SEBS, the hard segment PS units were cross-linked by 1,6-hexanediamine for practical use. The cross-linking of SEBS AEMs reduced water uptake significantly (e.g., 155% vs 28%) and enhanced their mechanical properties. Because the backbone of the SEBS AEMs are composed of all carbon–carbon bonds, they showed good alkaline stability, preserving their IEC and OH– conductivity after testing in a 1 M NaOH solution at 80 °C for 500 h. Alkaline membrane fuel cell performance was evaluated with the cross-linked SEBS AEM, and a peak power density of 520 mW/cm2 was achieved at 60 °C under H2/O2 conditions.