Anion exchange membranes based on novel quaternized block copolymers for alkaline direct methanol fuel cells

Lee, Hsing-Chieh; Liu, Kun-Lin; Tsai, Li‐Duan; Lai, Juin‐Yih; Chao, Chi-Yang

doi:10.1039/c3ra47886f

Cited by 28 publications

(24 citation statements)

References 39 publications

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“…This block copolymer demonstrates that high conductivity can be achieved at low water contents due to the phase-separated structure. Kim and co-workers 30 demonstrate higher ion conductivity in a block copolymer compared to a random copolymer at a similar composition. These results are similar to earlier findings from our laboratory described below.…”

Section: ■ Introductionmentioning

confidence: 96%

Bromide and Hydroxide Conductivity–Morphology Relationships in Polymerized Ionic Liquid Block Copolymers

Meek

Sharick

et al. 2015

Macromolecules

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A polymerized ionic liquid (PIL) diblock copolymer, poly (MMA-b-MEBIm-Br), was synthesized at various compositions from an ionic liquid monomer, (1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium bromide) (MEBIm-Br), and a nonionic monomer, methyl methacrylate (MMA), via the reverse addition−fragmentation chain transfer (RAFT) polymerization technique. A hydroxide-conducting PIL diblock copolymer, poly(MMA-b-MEBIm-OH), was also prepared via anion exchange metathesis of the bromide-conducting block copolymer. In a former study, the conductivity and morphology of the bromide-and hydroxide-conducting PIL diblock copolymer were examined at one fixed PIL composition: 17.3 mol %. In this study, additional PIL compositions of (6.6, 11.9, and 26.5 mol %) were explored to fully understand the previous unusual conductivity results. Both bromide and hydroxide conductivities were higher in the PIL block copolymer at PIL compositions of 11.9, 17.3, and 26.5 mol % compared to the PIL homopolymer under the same experimental conditions, even though the homopolymer possessed a higher water and ionic content compared to the block copolymers. These unusual results suggest that the confinement of the PIL microdomain within the block copolymer morphology enhances ion transport compared to its predicted value. Morphology factors (or normalized ionic conductivity, f) were as high as >3 at some conditions, which is much higher than the maximum theoretical limit for randomly oriented lamellar domains (f = 2/3). Application of percolation theory revealed a 3−4-fold enhancement of conductivity when comparing the inherent conductivity to the measured PIL homopolymer conductivity. Both morphology factor analysis and percolation theory corroborate with the absolute conductivity results and the hypothesis that PIL domain confinement in PIL block copolymers enhances conductivity over its bulk properties. ■ INTRODUCTIONPolymerized ionic liquid (PIL) block copolymers are a relatively new class of polymer electrolytes, where the cation of the ionic liquid is covalently attached to one of the polymers in a block copolymer and the benefits of both PILs and block copolymers are combined. PILs possess unique properties, such as high solid-state ionic conductivity, high chemical, electrochemical, and thermal stability, and a widely tunable chemical platform, where significant changes in physical properties have been observed with subtle changes in chemistry. 1−5 When PILs are incorporated into the block copolymer, the resulting PIL block copolymer can possess orthogonal properties, such as high modulus (from the nonionic polymer) and high conductivity or transport (from the ionic polymer or PIL) through the self-assembly of two distinct polymers into welldefined nanostructures of long-range order with tunable morphology and domain size. 1,3,6−10 This has promoted the investigation of PIL block copolymers as thin films for organic electronic devices, 11−14 gas permeation membranes for CO 2 separations, 15,16 and solid-state polymer electrolytes for use in...

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Section: ■ Introductionmentioning

confidence: 96%

Bromide and Hydroxide Conductivity–Morphology Relationships in Polymerized Ionic Liquid Block Copolymers

Meek

Sharick

et al. 2015

Macromolecules

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“…The decrease in the N/C ratio can imply on decomposition of the cationic groups by the main degradation mechanisms. 68 The data for the AMINOETHER cross-linked membrane indicated that high pH environment led to more severe degradation in the stability of cationic groups with an electron-donating crosslinker.…”

Section: Surface Images Of the Membranes-mentioning

confidence: 99%

Characterization and Chemical Stability of Anion Exchange Membranes Cross-Linked with Polar Electron-Donating Linkers

Amel

Smedley

Dekel

et al. 2015

J. Electrochem. Soc.

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The effect of the cross-linker chemical structure on the properties and chemical stability of anion exchange membrane is the focus of this study. Two different cross-linkers were investigated, one with linear hexyl chain between crosslinking sites, and the other, ether in the center of the alkyl linker. These two cross-linkers have a fundamental difference in their polarity and hydrophilicity. The ether-containing cross-linker is more polar and therefore will improve membrane's water uptake and conductivity. Swelling and conductivity measurements were performed at various temperatures for both types of samples. While water uptake and conductivity were found to be higher for the ether-based cross-linker, degradation measurements indicated that the membrane that is cross-linked with electron-rich linker degraded in hydroxide faster than the alkyl linker at temperature of 60 • Fuel cell technology has been recognized as a promising clean energy conversion technology in next-generation energy systems. Fuel cells have been undergoing revolutionary developments in the last few decades. [1][2][3] Among the different types of fuel cell systems, proton exchange membrane fuel cells (PEMFC) are the most investigated and developed for low-temperature operation in portable and automotive environments. 4,5 Although there has been great progress in improving the performance and lifetime of PEMFCs and large-scale commercial applications are on the horizon, the largest barriers to wide-spread fuel cell technology are materials availability and cost. Among the limitations of PEMFCs, the dependence on noble metal catalysts is a critical concern. For example, the use of Nafion, the state of the art acidic membrane, generally allows only platinum group metals to be used as stable catalysts for long-term use, thus considerably increasing the cost of PEMFC technology. To overcome the requirement of precious metal catalysts, anion exchange membrane fuel cell (AEMFC) technology has begun to attract recent attention. This new effort focused on AEMFCs is due to several potential advantages that anion exchange membranes hold over their acidic membrane counterparts. In AEMFCs, oxygen reduction kinetics is improved in the high pH environment of the membrane and the use of non-precious metals as electrocatalysts greatly reduces the cost of fuel cell devices.6-8 A highlyconductive, chemically robust anion exchange membrane (AEM) is a crucial component of the fuel cell as it acts as a barrier between the fuel and oxidant streams while simultaneously transporting anions from the cathode to the anode. and imidazoilum 36-38 groups. It is known that the quaternary ammonium moiety is susceptible to hydroxide attack, resulting in degradation of alkaline membrane at elevated temperature under alkaline conditions, but clear pathways for degradation of polymer backbones and other functional groups of AEMs including non-ammonium cations has not been extensively pursued. In general, the degradation pathways for the tetra-alkyl quaternary ammonium cations...

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“…Common approaches to facilitate phase separation in AEMs are the concentration of ions into single units in statistical copolymers, 28,29 stiff side groups 30 or block copolymers. 20,[31][32][33][34] However, those approaches may lead to limitations in ion dissociation, and thus conductivity, because of the close proximity of the ions to the polymer structure. 28 The…”

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confidence: 99%

Synthesis and Alkaline Stability of Solubilized Anion Exchange Membrane Binders Based on Poly(phenylene oxide) Functionalized with Quaternary Ammonium Groups via a Hexyl Spacer

Parrondo

Jung

Wang

et al. 2015

J. Electrochem. Soc.

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Anion exchange membranes (AEMs) with hexyl pendant chains were synthesized by Friedel-Crafts acylation of 6-bromo-1-hexanoyl chloride on poly(phenylene oxide) (PPO), followed by the reduction of the ketone with triethylamine, and quaternization with a tertiary amine (trimethylamine, quinuclidine, or 1-methylimidazole). 1 H-NMR, 13 C-NMR, COSY and HMQC NMR spectroscopies were used to prove the formation of the brominated polymers and to assess the alkaline stability of the AEMs. The alkaline stability of trimethylamine (TMA) based AEMs was evaluated using two separate, complementary experiments. In the first experiment, the AEMs were dissolved in 1 M NaOD+DMSO-d6 and kept at 60 • C. 1 H NMR spectra revealed signs of AEM degradation (same qualitative result for all the cationic groups) after 2 days in alkaline solution. The ion exchange capacity of TMA-C6-PPO, as calculated by integration of the corresponding NMR peaks, decreased 39% after 30 days (from 1.8 ± 0.1 to 1.1 ± 0.1 mmol/g). In the second experiment, TMA-C6-PPO AEM was immersed in nitrogen degassed 1 M KOH at 60 • C for 30 days. The ion exchange capacity of this AEM decreased 33% confirming previous results. The finding that PPO-based AEMs with hexyl spacers degrade in alkali suggests the hypothesis that alkyl spacers can be used to increase the alkaline stability is not valid for all backbones and conditions. © The Author There is an increasing interest in alkaline stable anion exchange membranes (AEMs), and solubilized AEM binders for applications in alkaline membrane fuel cells (AMFCs) and solid-state alkaline water electrolyzers.1-3 Operating in alkaline conditions presents distinct and attractive advantages to AMFCs and solid-state alkaline water electrolyzers, in comparison to acidic proton-exchange membrane fuel cells and water electrolyzers. Under alkaline conditions, it is possible to achieve faster oxygen reduction kinetics, more efficient water management, there is greater flexibility in the choice of fuels, and more importantly, presents the possibility of using nonprecious-metal electrocatalysts.1,4 On the other hand, the hydrogen oxidation/evolution reaction is more sluggish in alkaline media. 5-10The majority of the AEMs evaluated and reported in the peer-reviewed literature comprise polymers functionalized with benzyl trimethylammonium groups or, similar cation groups attached to the benzyl carbon of polymer backbones like polyphenylene, 11,12 polyphenylene oxide, 13,14 polysulfone, 15-20 and polyetheretherketone. [21][22][23] The reasons for this structure are the preference for polyaromatic backbones (due to its better chemical stability when compared with aliphatic backbones) and the favored functionalization routes using bromination or chloromethylation. Several recent reports have shown that the benzyl substituted quaternary ammonium cation degrades significantly faster than other cations with alkyl groups under mild alkaline conditions. 24,25 Most of the quaternary ammonium salts evaluated experimentally by Marino and Kreuer 24 and ...

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Anion exchange membranes based on novel quaternized block copolymers for alkaline direct methanol fuel cells

Cited by 28 publications

References 39 publications

Bromide and Hydroxide Conductivity–Morphology Relationships in Polymerized Ionic Liquid Block Copolymers

Bromide and Hydroxide Conductivity–Morphology Relationships in Polymerized Ionic Liquid Block Copolymers

Characterization and Chemical Stability of Anion Exchange Membranes Cross-Linked with Polar Electron-Donating Linkers

Synthesis and Alkaline Stability of Solubilized Anion Exchange Membrane Binders Based on Poly(phenylene oxide) Functionalized with Quaternary Ammonium Groups via a Hexyl Spacer

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