Alkaline stability of poly(phenylene)‐based anion exchange membranes with various cations

Hibbs, Michael

doi:10.1002/polb.23149

Cited by 260 publications

(264 citation statements)

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“…Membrane densities and volumetric IEC v (meq/cm 3 ) were determined according to previously reported methods 41 based on membrane water uptake using the following equation. …”

Section: ■ Experimental Sectionmentioning

confidence: 99%

Highly Stable, Anion Conductive, Comb-Shaped Copolymers for Alkaline Fuel Cells

et al. 2013

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To produce an anion-conductive and durable polymer electrolyte for alkaline fuel cell applications, a series of quaternized poly(2,6-dimethyl phenylene oxide)s containing long alkyl side chains pendant to the nitrogen-centered cation were synthesized using a Menshutkin reaction to form combshaped structures. The pendant alkyl chains were responsible for the development of highly conductive ionic domains, as confirmed by small-angle X-ray scattering (SAXS). The combshaped polymers having one alkyl side chain showed higher hydroxide conductivities than those with benzyltrimethyl ammonium moieties or structures with more than one alkyl side chain per cationic site. The highest conductivity was observed for comb-shaped polymers with benzyldimethylhexadecyl ammonium cations. The chemical stabilities of the comb-shaped membranes were evaluated under severe, accelerated-aging conditions, and degradation was observed by measuring IEC and ion conductivity changes during aging. The comb-shaped membranes retained their high ion conductivity in 1 M NaOH at 80°C for 2000 h. These cationic polymers were employed as ionomers in catalyst layers for alkaline fuel cells. The results indicated that the C-16 alkyl side chain ionomer had a slightly better initial performance, despite its low IEC value, but very poor durability in the fuel cell. In contrast, 90% of the initial performance was retained for the alkaline fuel cell with electrodes containing the C-6 side chain after 60 h of fuel cell operation. ■ INTRODUCTIONFuel cells are efficient energy conversion devices that generate electric power from energy-dense chemical fuels and have been attracting attention as a clean energy technology. 1−3 The most widespread low-temperature fuel cell technologies are based on sulfonated polymer membranes to separate the oxidant and fuel chambers and conduct protons between the anode and cathode of the cell. 4−6 These proton exchange membrane fuel cells (PEMFC) principally employ Nafion, a product from DuPont, as the sulfonated polymeric proton conductor. PEMFCs require platinum or other precious metal-based catalysts because these are the only types of metal catalysts stable under the low pH and electrochemical conditions of an operating cell. The expensive Pt-based catalysts, combined with perfluorinated membranes and corrosion-resistant cell hardware increase the cost of PEMFC devices beyond what is currently commercially viable. 7,8 Alkaline fuel cells (AFCs) have received significant interest in recent years relative to acidic fuel cells, 9−14 because of advantages when operating under alkaline conditions, which include enhancement of the electrode reaction kinetics, especially at the cathode, and the catalysts are not subjected to corrosion at high pH. 9,12,13 Consequently, non-noble metals or inexpensive metal oxides can be used as catalysts to greatly reduce the cost of the device. 13−16 In addition, high energy density liquids and gases such as ethanol, hydrazine, and ammonia can be adopted as fuels. 17,18 Alkaline fuel ...

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“…Membrane densities and volumetric IEC v (meq/cm 3 ) were determined according to previously reported methods 41 based on membrane water uptake using the following equation. …”

Section: ■ Experimental Sectionmentioning

confidence: 99%

Highly Stable, Anion Conductive, Comb-Shaped Copolymers for Alkaline Fuel Cells

et al. 2013

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“…Hibbs 45 pointed out that the ketone group in PPO-KC6Br promotes rapid AEM degradation in alkaline solutions. To increase the stability of our AEMs, the ketone group in PPO-KC6Br was reduced with triethylsilane in the presence of trifluoroacetic acid in 1,2-dichoroethane ( Figure S11 shows the 1 H NMR spectrum of the reduced polymer, PPO-C6Br).…”

Section: Reduction Of the Ketone Group In Ppo-kc6br (Ppo-c6brmentioning

confidence: 99%

“…45 The following AEMs were synthesized using this modified platform: PX1, PX12, PXC7 and PXC12. Figure 3 shows the synthesis scheme for imidazolium-based AEMs prepared with the spacer chain (PX1, PX12, PXC7 and PXC12).…”

Section: Reduction Of the Ketone Group In Ppo-kc6br (Ppo-c6brmentioning

confidence: 99%

Alkaline Stability of Poly(Phenylene Oxide) Based Anion Exchange Membranes Containing Imidazolium Cations

Wang

Parrondo

Ramani

2016

J. Electrochem. Soc.

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Two modified imidazole bases (1-heptyl-2-methyl-imidazole and 1-dodecyl-2-methyl-imidazole) were synthesized, each with a long alkyl chain (7 or 12 carbon) attached to the N-3 position. Anion exchange membranes (AEMs) were prepared with imidazolium cations derived from these bases that were either grafted directly onto the benzyl position of poly(phenylene oxide) (PPO) or affixed to the PPO using a hexyl spacer chain. First, we reacted 1-methylimidazole with brominated PPO and with PPO with a hexyl spacer chain. By comparing the alkaline stability of the resultant AEMs, we demonstrated that a hexyl spacer chain could improve AEM alkaline stability substantially. Second, by comparing the alkaline stability of PPO-based AEMs obtained by the reaction of brominated PPO with 1-methylimidazole and 1,2-dimethylimidazole, we showed that C-2-substituted (with a methyl group) imidazolium-based AEMs were much more stable in alkali than C-2-unsubstituted imidazolium-based AEMs. Finally, by investigating the alkaline stability of AEMs synthesized by reaction of brominated PPO with 1,2-dimethylimidazole and modified imidazole bases (with 7 or 12 alkyl carbon chain at the N-3 position), we demonstrated that the increase in length of a long alkyl chain affixed to the N-3 position decreases the alkaline stability of the resultant imidazolium-based AEMs. There is a growing interest in using anion exchange membranes (AEMs) as separators in alkaline membrane fuel cells (AMFCs) 1,2 and in other energy conversion and storage systems such as redox flow batteries (RFBs), 3-5 alkaline water electrolyzers (AWEs) 6,7 and reverse electrodialysis (RED) cells. 8 The most commonly used AEMs reported in the literature contain quaternary ammonium groups, primarily in the form of benzyl trimethylammonium cations.6,9-30 However, it had been shown that quaternary ammonium-based AEMs are sensitive toward Hofmann elimination 10,31 and direct nucleophilic elimination reactions 32,33 that result in loss of ion exchange capacity (IEC) and ionic conductivity. To resolve the stability problem inherent to quaternary-ammonium-group-containing AEMs, several alternative cations, including imidazolium, 31,34,35 benzimidazolium, 36 guanidinium, 37 phosphonium 38-40 and metal cations 41 have been proposed and investigated. Imidazolium-based AEMs have drawn researchers' interests mainly because of their high hydroxide-ion conductivities. 42 However, it has been reported that imidazoliumbased AEMs degrade under alkaline conditions at temperatures below 80• C. Ye and co-workers investigated the degradation mechanism of imidazolium-based AEMs at several relative humidities, temperatures and pH. They found that imidazolium-based AEMs were chemically stable at low temperatures, but relatively unstable at higher temperatures in alkaline conditions (degradation commences at 80• C when [KOH] >1 M). They postulated the following degradation mechanism: Ring-opening of the imidazolium ring was triggered by the nucleophilic attack of OH -on the imidazolium ring at the ...

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“…A reaction degree of 70%-80% can be easily achieved without the risk of crosslinking. Therefore, many studies have been focused on the synthesis of benzylmethyl-containing PAE precursors with special structures for bromomethylations [28,29].…”

Section: Introductionmentioning

confidence: 99%