Quaternary Phosphonium‐Based Polymers as Hydroxide Exchange Membranes

Gu, Shuang; Cai, Rui; Luo, Ting; Jensen, Kurt; Contreras, Christian; Yan, Yushan

doi:10.1002/cssc.201000074

Cited by 167 publications

(111 citation statements)

References 30 publications

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“…A few other promising cations evaluated include pentamethylguanidium, quaternarized diazabicylco[2.2.2]octane, N,N,N,N-tetramethyl-hexanediammonium, 1-methylimidazoliium, and tris(2,4,6-trimethoxyphenyl) phosphonium (16,(24)(25)(26)(27)(28)(29)(30)(31)(32). Most of these cations exhibit some level of alkaline stability, but comparison of the reported alkaline stability data are difficult because most of the experiments performed were carried out under different conditions (e.g., alkali concentrations and temperature) and used different characterization methods to assess stability (e.g., 1D 1 H NMR, infrared spectroscopy, change in ion-exchange capacity, and change in ion conductivity).…”

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confidence: 99%

“…Both these reviews provide a comprehensive list of available hydrocarbon backbones for making AEMs, but they offer little information with regard to their suitability/stability in alkaline environments. Of the many available polymers for preparing AEMs, polysulfone (PSF) has been a very attractive candidate and is the most commonly used aromatic hydrocarbon backbone due to its excellent oxidative stability, ease of preparation into AEMs (ease of derivatization and tractability toward film formation), ability to be solubilized for making AEM electrode binders, low cost, and ready availability (2,3,32).…”

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confidence: 99%

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Two-dimensional NMR spectroscopy reveals cation-triggered backbone degradation in polysulfone-based anion exchange membranes

Arges

Ramani

2013

Proc. Natl. Acad. Sci. U.S.A.

427

295

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Anion exchange membranes (AEMs) find widespread applications as an electrolyte and/or electrode binder in fuel cells, electrodialysis stacks, flow and metal-air batteries, and electrolyzers. AEMs exhibit poor stability in alkaline media; their degradation is induced by the hydroxide ion, a potent nucleophile. We have used 2D NMR techniques to investigate polymer backbone stability (as opposed to cation stability) of the AEM in alkaline media. We report the mechanism behind a peculiar, often-observed phenomenon, wherein a demonstrably stable polysulfone backbone degrades rapidly in alkaline solutions upon derivatization with alkaline stable fixed cation groups. Using COSY and heteronuclear multiple quantum correlation spectroscopy (2D NMR), we unequivocally demonstrate that the added cation group triggers degradation of the polymer backbone in alkaline via quaternary carbon hydrolysis and ether hydrolysis, leading to rapid failure. This finding challenges the existing perception that having a stable cation moiety is sufficient to yield a stable AEM and emphasizes the importance of the often ignored issue of backbone stability.alkaline fuel cells | anion exchange membrane degradation | water electrolysis | quaternary benzyl ammonium cations T here have been intensive research efforts directed toward the development of anion exchange membranes (AEMs) for solidstate alkaline fuel cells (AFCs) and water electrolyzers in recent years (1-6). These devices permit the use of non-platinum-groupmetal electrocatalysts for the oxygen reduction/evolution and hydrogen oxidation/evolution reactions (7-11). Besides their use in AFCs, AEMs are highly relevant for other electrochemical energy conversion/storage devices such as redox flow batteries, electrodialysis stacks, and metal-air batteries (6,(12)(13)(14). The renewed interest in AFCs has led to many studies directed toward improving the hydroxide ion conductivity of AEMs, a property that is inherently limited by the lower intrinsic mobility of the hydroxide ion. Approaches have included investigating new fixed cation chemistries and/or modification of the polymer electrolyte membrane network (e.g., cross-linking, spacer chain pendants, and block copolymers) (15-17). These efforts have facilitated an order of magnitude gain in hydroxide ion conductivity (roughly 10) (15,18,19). AEMs have traditionally exhibited poor chemical stability in alkaline environments. The poor alkaline stability of AEMs is an important issue that has received much less attention than AEM ionic conductivity. This issue has limited the use of AEMs in applications that involve exposure to hydroxide ions, a potent nucleophile (12). The primary AEM degradation modes involve hydroxide ion attack on the fixed cation groups of AEMs, leading to Hoffman elimination (1, 10, 11), direct nucleophilic substitution (1-3), and chemical rearrangements induced through ylide intermediate formation (2,3,(20)(21)(22). Each of these degradation mechanisms results in rapid loss of ion exchange capacity, and hence ionic co...

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confidence: 99%

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confidence: 99%

Two-dimensional NMR spectroscopy reveals cation-triggered backbone degradation in polysulfone-based anion exchange membranes

Arges

Ramani

2013

Proc. Natl. Acad. Sci. U.S.A.

427

295

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“…27,28 In addition to the different chemical backbone structures of AEMs, the covalently attached cations on these polymers were derivatives of ammonium, 29,30 guanidinium, 31,32 phosphonium, [33][34][35] and imidazoilum [36][37][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.…”

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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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“…A low membrane resistance is particularly important to mitigate resistance-induced cell voltage loss, especially at large current densities. HEMs usually have lower ionic (hydroxide) conductivity than PEMs, largely because hydroxides have intrinsically lower mobility than protons (20.50 versus 36.25 × 10 4 cm 2 V 1 S 1 at 25°C) [5], even though hydroxide has the highest ionic mobility among all known anions. In comparison with ∼100 mS cm 1 of proton conductivity of a typical commercial PEM (Nafion 212, 50 μm) at 20°C, ∼50 mS cm 1 hydroxide conductivity would be expected for high-performance HEMs.…”

Section: High Hydroxide Conductivitymentioning

confidence: 99%

“…The basicity of trimethyl benzyl ammonium hydroxide has been found to be moderate in comparison with other cation hydroxides, leading to mild hydroxide conductivity at a given IEC. For example, the specific conductivity or the IEC-normalized hydroxide conductivity of trimethyl ammonium-functionalized polymer (19 mS g cm 1 mmol 1 [31]) is about half that of Cycloalkyl ammonium [20][21][22][23][24] the quaternary phosphonium one (39 mS g cm 1 mmol 1 [5]) (Table 6.2), but about twice that of the imidazolium one (8.4 mS g cm 1 mmol 1 [32]) where they use the same polysulfone matrix and have the similar homogeneous membrane structure. The tetraalkyl ammonium cationic functional groups have been observed to have very limited specific solubility in low-boiling-point watermiscible solvents, preventing them from being used as a high-performance solubilized ionomer for electrode applications.…”

Section: Tetraalkyl Ammoniummentioning

confidence: 99%