Fluorene-Based Hydroxide Ion Conducting Polymers for Chemically Stable Anion Exchange Membrane Fuel Cells

Abstract

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 ...

Configuring Anion‐Exchange Membranes for High Conductivity and Alkaline Stability by Using Cationic Polymers with Tailored Side Chains

“…Hydroxide ion conductivity of AEMs based on different polymers carrying QA or imidazolium groups via alkyl spacer units: PPO‐7Q, gQAPPO, PFBFF, and PAES containing fluorene units . The respective IEC values are given within parentheses, and the conductivity of a PPO with QA groups directly on the backbone (PPO‐1Q) is shown for comparison …”

“…PFs are fully aromatic and conjugated polymers usually associated with electroactive and photoactive materials. Lee et al have synthesized PFs with QA groups located on hexyl spacers using Pd catalyzed Suzuki coupling of premade bromoalkylated monomers, followed by quaternization using trimethylamine (designated PFBFF, IEC = 2.9 mmol g −1 , Scheme b) . These polymers, having the side chain configuration shown in Scheme e, were not soluble in water, but dissolved in, e.g., dimethylformamide at room temperature.…”

Robust Hydroxide Ion Conducting Poly(biphenyl alkylene)s for Alkaline Fuel Cell Membranes

“…Although AEMs prepared from conjugated aromatic polymers can achieve good alkaline stability due to the absence of aryl ether bonds and a low degree of swelling in water, if polymer backbones are too rigid, the resulting AEMs are rather brittle. 17 The use of expensive palladium catalysts for the polymerization reaction also limits the scaling-up of materials.…”

Structure–Property Relationships in Hydroxide‐Exchange Membranes with Cation Strings and High Ion‐Exchange Capacity

“…[22][23][24][25][26] For example, Marinoe tal. The alkaline stability of CS-n was investigated in 1 m NaOH at 80 8C( FigureS10).…”