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

Abstract

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 ...

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

“…14, 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 solution was titrated with 0.1 M potassium thiocyanate (0.1 M KSCN, standard solution) until the color changed from a light orange to a medium-dark orange color (equivalence point). A control sample was prepared by mixing 20 mL of 1 M NaNO 3 with 5.0 mL of 0.1 M AgNO 3 and 2-3 drops of 11 wt% of Fe(NO 3 ) 3 . The control was also titrated with 0.1 M KSCN and the difference in volume used to titrate the control solution and the sample solution was used for calculating the IEC (see Equation 1):…”

Polystyrene-Block-Poly(ethylene-ran-butylene)-Block-Polystyrene Triblock Copolymer Separators for a Vanadium-Cerium Redox Flow Battery

“…The chemical and mechanical stability and high conductivity of SEBS-based AEMs make them promising separator candidates for electrode-decoupled RFBs. There is a growing interest in using anion exchange membranes (AEMs) as separators in alkaline membrane fuel cells (AMFCs) [1][2][3] and in other energy conversion and storage systems such as redox flow batteries (RFBs), 4-6 alkaline water electrolyzers (AWEs) 7-9 and reverse electrodialysis (RED) cells. …”

“…There is a growing interest in using anion exchange membranes (AEMs) as separators in alkaline membrane fuel cells (AMFCs) [1][2][3] and in other energy conversion and storage systems such as redox flow batteries (RFBs), [4][5][6] alkaline water electrolyzers (AWEs) [7][8][9] and reverse electrodialysis (RED) cells. 10 RFBs are promising candidates for large-scale energy storage systems since the capacity, power and energy density parameters can be designed independently and easily modified even after installation.…”

Multi‐walled carbon nanotubes incorporation into cross‐linked novel alkaline ion‐exchange membrane for high efficiency all‐solid‐state supercapacitors

“…The currently used ion‐exchange polymer electrolytes respectively are proton‐exchange membranes (PEMs) and anion‐exchange membranes (AEMs) . Because commercial PEMs are expensive and it is difficult to overcome their tendency to corrode, AEMs have recently attracted much research attention AEMs have recently attracted much research attention . For example, Xiong et al synthesized quaternary poly(vinyl alcohol) AEM (QAPVA) and obtained the OH − conductivity of 7.34 × 10 −3 S cm −1 but did not study thermal stability, alkaline stability, and other properties of this membrane.…”