Stability of Poly(2,6-dimethyl 1,4-phenylene)Oxide-Based Anion Exchange Membrane Separator and Solubilized Electrode Binder in Solid-State Alkaline Water Electrolyzers

Jung

et al. 2015

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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 ...

Section: 3mentioning

confidence: 97%

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

Jung

et al. 2015

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“…35% after holding the cell at a constant voltage of 0.55V for 12 hours. Fuel cell performance with SEBS-based AEMs resulted in peak power densities of 300 mW/cm 2 at 70 • C. Anion exchange membranes (AEMs) are a promising technology for alkaline membrane fuel cells (AMFCs), 1-6 redox flow batteries (RFBs), 7-13 alkaline water electrolyzers (AWEs) [14][15][16] and reverse electrodialysis (RED) cells. 17,18 The AEMs commonly reported in peer reviewed papers mostly contain quaternary ammonium fixed cation groups, mainly in the form of benzyl trimethylammonium cations.…”

mentioning

confidence: 99%

Anion Exchange Membranes Based on Polystyrene-Block-Poly(ethylene-ran-butylene)-Block-Polystyrene Triblock Copolymers: Cation Stability and Fuel Cell Performance

Ramani

2017

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A series of polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS)-based anion exchange membranes (AEMs) were synthesized via chloromethylation of SEBS followed by quaternization with trimethylamine (TMA). SEBS-based AEMs functionalized with TMA + cations exhibited a hydroxide ion conductivity of 136 mS/cm at 70 • C. This membrane exhibited a phaseseparated morphology with wide and interconnected ionic channels as observed by atomic force microscopy. Poly (phenylene oxide) (PPO)-based AEMs with quaternary benzyl-trimethylammonium (TMA + ) and quaternary benzyl-tris(2,4,6-trimethoxyphenyl) phosphonium (TTMPP + ) and PPO-based AEMs with hexyl pendant chains were also synthesized and evaluated as binders in AMFC electrodes. PPO-based AEMs functionalized with TTMPP + demonstrated the best ex situ alkaline stability, losing only 9% of their ion-exchange-capacity after 30 days in 1M KOH (at 60 • C). The best in situ stability was achieved by SEBS-based MEAs (as membrane and as binder in the electrodes); The peak power density dropped approx. 35% after holding the cell at a constant voltage of 0.55V for 12 hours. Fuel cell performance with SEBS-based AEMs resulted in peak power densities of 300 mW/cm 2 at 70 • C. Anion exchange membranes (AEMs) are a promising technology for alkaline membrane fuel cells (AMFCs), 1-6 redox flow batteries (RFBs), 7-13 alkaline water electrolyzers (AWEs) [14][15][16] and reverse electrodialysis (RED) cells. 17,18 The AEMs commonly reported in peer reviewed papers mostly contain quaternary ammonium fixed cation groups, mainly in the form of benzyl trimethylammonium cations.14,19-40 However, it has been shown that quaternary ammonium-based AEMs are sensitive toward Hofmann elimination 20,41 and direct nucleophilic elimination reactions 42,43 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, 41,44,45 benzimidazolium, 3,45,46 guanidinium, 15,47,48 phosphonium 8,9,49 and metal cations 50,51 have been proposed and investigated. Phosphonium-based AEMs were investigated by Gu et al. 52 Gu et al. attached benzyl-tris(2,4,6-trimethoxyphenyl) phosphonium (TTMPP + ) onto polysulfone backbone to make AEMs, and found them to be alkaline stable; There was no ionic conductivity fade after immersion in 1M KOH for 30 days at room temperature. The bulky phosphonium-based AEMs remained stable in alkali because of the steric hindrance effect that protects the core phosphorus atom and the α-carbons against hydroxide ion attack. There is no possibility of β-elimination reaction because of the lack of hydrogen atoms in β-carbons. However, very few researchers have investigated in situ stability of bulky phosphorus-based AEMs. It is essential to correlate the ex situ stability results obtained in immersion alkaline stability tests with in situ stability in a functional H 2 /O 2 fuel cell.Hibbs 53 has shown that poly(phenylene)-bas...

“…Additional details of the application of the method can be found in our previous papers. 3,6,10,14,47 Results and Discussion Figure 1 shows the scheme for the synthesis of modified imidazolium cation groups: 1-heptyl-2-methyl-imidazole (2MI-C7) and 1-dodecyl-2-methyl-imidazole (2MI-C12). The formation of 2MI-C7 was confirmed by the 1 H-NMR and 2D NMR (Correlation Spectroscopy; COSY) spectra as shown in Figures S1 and S2 49 The protons initially at 3.4 ppm (in the spectrum of 1-bromoheptane) shifted, after reaction with 2-methylimidazole, to ca.…”

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

confidence: 99%

“…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][4][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.…”

mentioning

confidence: 99%

“…8 The most commonly used AEMs reported in the literature contain quaternary ammonium groups, primarily in the form of benzyl trimethylammonium cations. 6, 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][39][40] and metal cations 41 have been proposed and investigated.…”

mentioning

confidence: 99%

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

Ramani

2016

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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 ...