CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201403022

Alkaline Stability of Quaternary Ammonium Cations for Alkaline Fuel Cell Membranes and Ionic Liquids M. G. Marino* and K. D. Kreuer*[a] The alkaline stability of 26 different quaternary ammonium groups (QA) is investigated for temperatures up to 160 8C and NaOH concentrations up to 10 mol L1 with the aim to provide a basis for the selection of functional groups for hydroxide exchange membranes in alkaline fuel cells and of ionic-liquid cations stable in basic conditions. Most QAs exhibit unexpectedly high alkaline stability with the exception of aromatic cations. b-Protons are found to be far less susceptible to nucleophilic

attack than previously suggested, whereas the presence of benzyl groups, nearby hetero-atoms, or other electron-withdrawing species promote degradation reactions significantly. Cyclic QAs proved to be exceptionally stable, with the piperidine-based 6-azonia-spiro[5.5]undecane featuring the highest half-life at the chosen conditions. Absolute and relative stabilities presented herein stand in contrast to literature data, the differences being ascribed to solvent effects on degradation.

Introduction There is considerable interest in the application of anion exchange membranes (AEM) as separator materials in alkaline polymer electrolyte fuel cells (AFC).[1–3] Not only can they mediate the electrochemical reactions at the electrodes, among other advantages the high pH value at which they operate allows the use of non-noble metal catalysts, promising a severe cost reduction for fuel-cell technology.[4–8] Unfortunately, the replacement of the well-established proton exchange membranes (PEM) by AEMs in their hydroxide form (hydroxide exchange membrane: HEM) is hampered by several stability issues. Apart from the fact that the OH in HEMs is readily converted into carbonates when in contact with ambient CO2,[9, 10] the molecular structure of the membrane itself decomposes due to the presence of the highly nucleophilic OH . Decomposition typically starts at temperatures above T = 60 8C and leads to a reduction of ion exchange capacity (IEC), conductivity, and mechanical strength.[8, 11–17] Base-induced decomposition of QAs is also an issue with their use as cations in ionic liquids (ILs). While ILs are generally stable under reducing and oxidizing conditions, their low base stability[18] is preventing them from being used as solvent for base-catalyzed reactions.[19] The primary reasons for degradation are considered to be nucleophilic substitution and b-elimination at the quaternary ammonium (QA) through reaction with OH (Figure 1) and reasonably so, as QAs are used as leaving groups in organic synthesis.[20] Depending on the structure of the QA, other degra[a] M. G. Marino, Dr. K. D. Kreuer Max Planck Institute for Solid State Research Heisenbergstraße 1 70569 Stuttgart (Germany) E-mail: [email protected] [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201403022.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 1. Degradation of QAs through nucleophilic substitution and b-elimination.

dation mechanisms are known to occur as well, such as ortho substitution, rearrangement reactions with benzyl ammonium compounds, or Stevens rearrangement in the absence of bprotons.[21–23] Even decomposition of the polymer backbone has been observed.[24, 25] Although alternative cationic species such as phosphonium or sulfonium groups have been shown to be less stable than QAs with similar substituents,[26] some potentially viable highly sterically shielded phosphonium groups may exist. However, their synthesis is highly complex,[27, 28] which is the reason why current research efforts mainly focus on QAs as anion exchanging groups or cations for ionic liquids. Unfortunately, the results of available studies on the stability of QAs are hard to compare because reactivity may be significantly affected by the chemical environment of the QA. This includes solvent, solvation number, how the QA is attached to the backbone (in the case of membranes), degree of functionalization, and water content. Control of these factors may allow to resolve the controversy regarding relative AEM stabilities, in which aromatic functional groups have been found to be more[29–31] and less[13, 16, 17, 32–34] stable than alkyltrimethylammonium. Herein, this complexity was removed by studying a series of prototypical QA groups in their simple salt form in aqueous soChemSusChem 2014, 7, 1 – 12

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CHEMSUSCHEM FULL PAPERS lution under identical conditions (hydration number, OH concentration, temperature). This made it possible to comparatively assess the alkaline stability of different QA molecular structures, which include aromatic cations, alkyl chains of various lengths, aliphatic heterocycles, and QAs with removed or rotationally inhibited b-protons. Benzylic groups containing electron-withdrawing and -donating functionalization were also included to mimic the effects of tethering to a polymer. Approaches similar to the one in this work have been followed before, most notable in the work of Bauer et al., who compared the stability of a number of QAs in 2 m KOH solution at T = 160 8C.[12] However, their experiments were carried out in glycol instead of water, as disclosed in the diploma thesis in which the results were first published;[35] this is likely to have contributed to the considerably different results obtained in our study. Because not all experiments described in the thesis were reported in the article and vice versa, we have compiled the complete data set in the Supporting Information. As will be shown below, the choice of solvent has a large impact on reaction rates[20, 36] and, in the case of glycol, this leads to degradation rates that are orders of magnitude higher than those in water.

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Figure 2. Decomposition of BTM at 160 8C in water (*, 4 m NaOH), glycol (~[12] and ~, 2 m NaOH), and a 50:50 wt % mixture of both (&, 2 m NaOH). This demonstrates the large influence of the solvent on the degradation rate and that the experiments by Bauer et al.[12] were performed in glycol.

Results and Discussion In the following, the influences that govern the alkaline stability of quaternary ammonium groups are discussed. First, external factors such as temperature and solvation are considered before the effects of various features of the molecular structure are examined. Ion solvation and temperature The resistance of a quaternary ammonium (QA) to nucleophilic attack by OH depends not only on its molecular structure but also on external factors such as temperature, solvent, and degree of solvation. These factors influence the overall QA decomposition rate and sometimes even the preferred reaction pathway if more than one is available.[20, 36, 37] This signifies the importance of comparing the alkaline stability of different QAs at the same temperature and solvation number and in the same solvent, preferably that in which they are intended to be used—water in the case of aqueous HEM fuel cells. To provide an idea to what extend degradation rates are affected by the choice of the solvent, Figure 2 shows the degradation of benzyltrimethylammonium (BTM) in NaOH solutions of water, glycol, and a 50:50 wt % mixture of both. Measuring QA decompositions in glycol clearly underestimates their stability in aqueous media and similar deviations are expected for other solvents. This is an important reason why the results of the stability experiments carried out by Bauer et al.[12, 35] in glycol cannot be easily translated into QA stabilities in aqueous media. How strongly the degree of solvation affects degradation rates can be inferred from the half-life of BTM and tetramethylammonium (TMA) in varying NaOH concentrations (Figure 3).  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 3. Dependence of TMA (~) and BTM (*) half-life on NaOH concentration at 160 8C.

Between 6 and 4 m NaOH (l = [H2O]/[NaOH] = 8.6 and 13.2) the half-life drops by 70 % for both QAs; this trend continues with decreasing water content, ultimately leading to instantaneous decomposition when approaching the pure, solvent-free hydroxide form.[38] The increase of the degradation rates with temperature is very high, corresponding to activation enthalpies far beyond 100 kJ mol1 (  1 eV) in 10 m NaOH (Figure 4). These trends illustrate the importance of keeping HEMs sufficiently hydrated at all times and especially at elevated temperatures to minimize degradation. The results also provide an explanation for the recent controversy regarding the stability of various QAs in membranes, as the water uptake and with it the degree of solvation is determined by a number of factors, which may have been varied. Apart from temperature, these factors include ion exchange capacity, degree of crosslinking, chemical nature of the backbone, and relative humidity (water ChemSusChem 0000, 00, 1 – 12

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Figure 4. Temperature dependence of TMA (~) and BTM (*) half-life in 10 m NaOH. The respective activation enthalpies for the overall decomposition process correspond to 157 and 120 kJ mol1, as shown in the inset.

activity). Even more complexity is added if the degradation experiments are carried out in aqueous NaOH solutions. For our study, the standard conditions for the stability test of QA salts were a NaOH concentration of 6 mol L1 (corresponding to a hydration number of l  8.6) and a temperature of T = 160 8C. This hydration number falls into the range frequently reported for HEMs (l = 6–15),[39–43a] and the relatively high temperature was selected to accelerate degradation rates to manageable half-lives below 100 h. The QA salts used in this study exhibit varying solubility, causing some constraint on the starting concentration, and come with one of three different anions (Cl , Br , or I) based on commercial availability and preparation route. To demonstrate that neither of these factors affect reactivity, the degradation data of the simplest QA (TMA), which is used as a benchmark throughout this work, is given in Figure 5 for the three halide forms and different starting concentrations. Nevertheless, we have limited QA starting concentrations for all experiments to below 0.11 mol L1 to minimize possible effects arising from the use of different anions and interactions between cations (e.g., dimerization, micelle formation).

Molecular structure Benzylic cations One of the common methods to functionalize commercially available polymers (e.g., polyphenylsulfones such as Udel and Radel)[43b,c] with QAs is through chloromethylation and subsequent treatment with trialkylamine.[41] This preparation route attaches the QA groups to the backbone via a benzylic carbon. As opposed to SO3H cation-exchanging groups, which are frequently tethered to similar backbones in analogous reactions, QAs immobilized in such a way are prone to degradation in the presence of OH counterions. This can be attributed to the nature of the alkaline degradation, which is caused by the electrophilic character of the QA itself that may be further en 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 5. Decomposition of TMA with Cl (*), Br (*), and I (*) as anion at 160 8C in 6 m NaOH. The decay with t1/2 = 62 h follows a pseudo-first-order kinetic and is independent of halide counter ion and starting concentration. Empty symbols correspond to the respectively anion on the logarithmic scale.

hanced through the presence of nearby electron-withdrawing (I) groups. The benzyl group exhibits such a I effect, facilitating the nucleophilic attack of OH . Additionally, its aromatic p-system stabilizes radicals and carbanions at the benzylic site, promoting additional degradation pathways[20, 36, 37] that might also prove to be detrimental under fuel-cell conditions. The electron-withdrawing property of the benzyl group can be increased or decreased by introducing appropriate substituents at the aromatic ring with substantial effects on the halflife of the corresponding QA in alkaline conditions. This is shown in Table 1 for three benzylic QA salts containing an electron-donating (OMe, + I), an electron-withdrawing (NO2, I), and no substituent. Although electron-donating functionalization counteracts the destabilizing effect of the benzyl group to some extent, the methoxy-functionalized benzyl group still only has a half-life of 27 % of that of TMA. Whenever a methyl group is replaced by a benzyl group in a given QA, its half-life decreases by about an order of magnitude, suggesting that nucleophilic attack occurs primarily at the benzyl group. The only exception seems to be molecules that contain ethyl groups, which are even more reactive due to highly facile b-elimination. In any case, benzyl functionalities appear to be unsuitable as substituent or as link between QAs and the polymer backbone of any HEM. Aromatic cations The idea behind the use of aromatic cations as QAs in HEMs is to increase alkaline stability by decreasing the charge density at any atom through delocalization of the positive charge, with imidazolium and guanidinium structures most frequently used in this approach. Main drawbacks of this strategy are its constraint to planar geometries, lacking virtually any steric shielding, as well as the increased acidity of nearby protons and radical stabilization induced by the extended p-system (similar to the benzyl groups). ChemSusChem 0000, 00, 1 – 12

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Table 1. Half-life of QA compounds at T = 160 8C in 6 m NaOH with electron-withdrawing and electron-donating substituents compared to a neutral BTM, BTE, and the TMA benchmark (entries 1–5) and DABCO- (entries 6 and 7) and piperidinium-based QAs (entries 8 and 9) with and without a benzylic group.

1 m NaOH for 50 h, although its monobenzylated counterpart decomposed rapidly under the same conditions.[48] With a few arguable exceptions[29–31] the available literature does not support the claim that aromatic QAs are suitable functional groups for HEM applications. All aromatic QAs investigated in this work proved to be highly susceptible to degradation by OH and decomposed far more rapidly than any of the aliphatic QAs (Table 2). Most aromatic QAs decompose virtually instantaneously at T = 160 8C in 6 m NaOH and even exhibit very high decomposition rates at RT. It is worth noting that TMI is considerably less stable than TMA or BTM.

Abbreviation[a]

Half-life [h]

1

TMA

61.9

2

MBTM

16.6

3

BTM

4.18

Table 2. Half-lifes of BTM and various aromatic QA compounds at different temperatures in 6 m NaOH.

4

BTE

0.68

Entry QA

Abbreviation[a] Half-life [h]

T [8C]

5

NBTM

0.66

1

BTM

4.18

160

6

MAABCO

13.5

2

PhTM

0.14

160

7

BAABCO

1.4

3

TMI

too short to measure 160

8

DMP

87.3

4

DTG

too short to measure 160

9

BMP

7.3

5

MOI

too short to measure

60

6

BMI

too short to measure

25

7

BP

too short to measure

25

Entry

QA

[a] TMA: tetramethylammonium; MBTM: 3-methoxy-benyzl trimethyl ammonium; BTM: benzyltrimethylammonium; BTE: benzyltriethylammonium; NBTM: 3-nitrobenyzltrimethylammonium; MAABCO: 1-methyl-4-aza1-azonia-bicyclo[2.2.2]octane; BAABCO: 1-benzyl-4-aza-1-azonium-bicyclo[2.2.2]octane; DMP: N,N-dimethylpiperidinium; BMP: N-benzyl-N-methylpiperidinium.

The question is whether the increased stability due to delocalization can compensate the drawbacks. Literature data, for example, on imidazolium-based QAs suggest that QAs are unstable in the presence of bases, even at relatively mild conditions.[18] Similarly, methylated polybenzimidazole is known to decompose very rapidly in KOH solutions[44] and decomposition rates of phenyl or anilinium-based QAs were found to be orders of magnitude faster than comparable aliphatic QAs.[45, 46] A recent study investigated imidazolium cations with various substituents and found that the alkaline stability is governed by “a complex relationship between sterics, electronics, and competing reactions”, but even the most stable imidazolium compound [1,2,3-trimethylimidazolium (TMI)] was reported to have a slightly lower half-life than an aliphatic neopentylbased QA.[47] Furthermore, dibenzyl tetramethylguanidinium (DTG) was shown to be stable at mild conditions of T = 60 8C in  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

[a] PhTM: phenyltrimethylammomnium; TMI: 1,2,3-trimethylimidazolium; DTG: 1,1-dibenzyl-2,2,3,3-tetramethylguaninidinium; MOI: 1-methyl-3-octylimidazolium; BMI: 1-benzyl-3-methylimidazolium; BP: N-benzylpyridinium.

The results demonstrate that delocalization alone cannot stabilize a QA sufficiently, at least not without additional steric shielding and/or increase of its electron density by several electron-inducing substituents. Simple aromatic structures are, therefore, expected to decrease alkaline stability and should be avoided in HEMs. Spacer chains Attaching QA groups to a polymer via a long hydrocarbon spacer chain instead of using a direct benzylic tether has been shown to improve alkaline stability.[32, 49] Interestingly, alkyl chain substituents have also been reported to increase QA stability even when benzylic tethers are involved.[50] ChemSusChem 0000, 00, 1 – 12

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CHEMSUSCHEM FULL PAPERS Because benzylic protons next to a QA are highly reactive (see the section on benzylic cations), enhanced stability is expected by increasing the distance between QA and the benzyl group. The reason for the superior base resistance of benzyl group-attached QAs with an alkyl-chain substituent is, however, less clear. A systematic look at the alkaline degradation of QAs with alkyl chains reveals that the additional steric strain on the decomposition reactions caused by longer chains diminishes for chain lengths beyond the propyl group. This is because the additional + I effect of longer chains is negligible, whereas steric and stereoelectronic factors cause the chain to orient (or “grow”) away from the nitrogen atom.[36, 51] This is evident by the fact that solvated molecules with a hydrophobic chain and a charged head group (surfactants) stretch out and may even form micelles.[52, 53] Any stability trend directly depending on the chain length should be easily discernible in a systematic study, but unfortunately a more complex relationship is observed (Figure 6). The shortest possible side chain is the methyl group in TMA, which exhibits a relatively high half-life of 62 h with nucleophilic substitution being the only possible degradation mechanism. Adding only one additional CH2 group to create an ethyltrimethylammonium (ETM) group decreases half-life by a factor of 20 due to considerably faster b-elimination. This b-elimination proceeds most rapidly with a freely rotating CH3 end group devoid of steric interference, which becomes apparent when another CH2 group is added to create a propyltrimethylammonium (PTM) group. The propyl chain causes strain on the steric requirements of the elimination reaction, increasing the transition state energy and with it the half-life compared to ETM. The next data point is that of a hexyltrimethylammonium (HTM) with six carbon atoms, which exhibits basically the same half-life as PTM within the margin of error. However, beyond

Figure 6. Half-life of alkyltrimethylammonium functional groups containing one alkyl chain of varying length in 6 m NaOH at T = 160 8C (*) and a neopentyl chain at the same conditions (&). Corresponding reaction free energy barrier DG# data (*) was calculated by using the Eyring equation. DFT data by Long et al. (~)[61] follows the same qualitative trend, but absolute values differ by about 50 kJ mol1.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org HTM the half-life surprisingly decreases again, dropping down to ETM values between 10 and 16 carbon atoms, which suggests that more than steric and inductive factors are involved. Perhaps the same cause results in a decrease in the QA halflife in membranes when compared to their simple-salt counterparts.[47, 54, 55] As in these experiments the OH concentration was always the same, different hydration numbers can likely be excluded as an explanation. A possible cause could be that the elimination products from longer chains are removed more quickly from the reaction equilibrium due to an increasing tendency for phase separation with the aqueous phase. A potential back reaction that can occur in the presence of a strong base such as OH[56, 57] may be thus inhibited. A more probable reason is the formation of micelles, which also increases in tendency with chain length and which is known to influence the rate of chemical reactions[58] and also potentially lead to decreased dissociation.[52, 53, 59, 60] A lower degree of dissociation would increase the local OH concentration around the QA and thus cause faster degradation (see the section on ion solvation and temperature). Beyond a chain length of six, micelle-induced decomposition may well be the dominating factor and similar effects induced by the polymer backbone are potentially responsible for accelerated decomposition in HEMs. Although no data between HTM (six carbons atoms) and PTM (three carbon atoms) were measured, it is quite likely that a stability maximum (or plateau) exists in this region, where belimination is inhibited to the largest possible extend by steric and inductive factors while chain length-induced accelerated decomposition does not yet dominate. This is supported by previous experimental and theoretical studies, which showed that the elimination rate in QAs decreases with chain lengths up to around four carbon atoms, whereas substitution at the methyl group remains comparatively unaffected.[61–64] Such a maximum was also calculated for the reaction free energy barriers DG# by Long et al.[61] (unfortunately only calculated up to HTM) although micelle formation was not taken into account. By calculating DG# from the degradation rate constants using the Eyring–Polanyi equation for solutions (see the Supporting Information) a direct comparison with the data by Long et al.[61] was possible, showing that the data follow the same relative trend, with absolute values differing by about 50 kJ mol1 (12 kcal mol1).[65, 66] The maximum half-life of side chain-containing QAs with optimal length is considerably less then that of TMA, suggesting that their primary degradation pathway is b-elimination, which is to some extend suppressed by the chain itself. Removal of b-protons may then conceivably lead to higher resistance against nucleophilic attack; however, this prediction could not be confirmed experimentally as neopentyltrimethylammonium (NTM), which contains no b-protons, decomposed far more rapidly than the PTM analogue with an equally long main chain (see Table 3 and Figure 6). This implies other degradation mechanisms (e.g., Stevens rearrangement) proceed more rapidly in NTM compared to the elimination in PTM.

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Table 3. Half-life of alkyltrimethylammonium cations with varying sidechain length and a neopentyl-based QA at T = 160 8C and in 6 m NaOH, ordered with respect to main-chain length. Abbreviation[a]

Half-life [h]

1

TMA

61.9

2

ETM

2.8

3

PTM

33.2

4

NTM

20.7

5

HTM

31.9

6

OTM

12.7

7

DTM

4.4

Entry

8

QA

HexDTM

1.9

[a] ETM: ethyltrimethylammonium; PTM: propyltrimethylammonium; NTM: neopentyltrimethylammonium; HTM: hexyltrimethylammonium; OTM: octyltrimethylammonium; DTM: decyltrimethylammonium; HexDTM: hexadecyltrimethylammonium.

The degradation half-lives of such comparatively simple alkyltrimethylammonium compounds show that alkaline stability does not necessarily follow an easily predictable trend. With this in mind, it is definitely worth investigating QAs based on molecular structures that have been rather neglected so far, such as aliphatic heterocycles. Heterocycles Comparatively little research has been performed on aliphatic heterocyclic QAs in the field of AEMs, the only investigated compounds being 1,4-diazabicyclo[2.2.2]octane (DABCO) based-cations.[12, 67, 68] With the use of DABCO, the goal is to reduce the reactivity of the b-protons by locking them into a non-anti-periplanar position with respect to the nitrogen atom. This is expected to decrease the b-elimination rate, which is facilitated in an antiperiplanar conformation.[20, 36, 37] However, b-elimination must not be considered on its own, as competing reactions (e.g., substitution) and—as mentioned above—factors such as steric properties of the reactants, solvent, base-strength, leaving group, and temperature influence preferred reaction pathway and resulting product composition. This complexity makes it difficult to predict the relative alkaline stability of structurally disparate QAs. In spite of their b-protons being constraint, 1-benzyl-4-aza-1azonium-bicyclo[2.2.2]octane (BAABCO) and 1-methyl-4-aza-1azonium-bicyclo[2.2.2]octane (MAABCO) for example, proved  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

to be noticeably less stable than their non-cage-like counterparts BTM and TMA (Table 4). Reasons for the increased reactivity are probably the additional electron-withdrawing nitrogen atom in close proximity to the positive charge[49, 69] and ring strain (compare bicyclo[2.2.2]octane).[70, 71] Apparently, the stabilizing effect of the constrained geometry is more than compensated. Removing the additional heteroatom while retaining the rotational inhibition might prove beneficial, and a molecule such as quinuclidine having the same cage-like structure as DABCO, including the detrimental ring strain but without the additional nitrogen atom, may exhibit an increased alkaline stability; however, the extent to which this occurs is hard to predict. Because the synthesis of that molecule is not trivial, simpler alternatives that fulfill the mentioned criteria have been examined. Just as in DABCO, cations with 6- and 5-membered [N,N-dimethylpiperidinium (DMP)] and [N,N-dimethylpyrrolidinium (DMPy)] contain b-protons with the CC bond rotationally restricted by the ring geometry. In the strain-free 6-membered Table 4. Half-life of cyclic QAs and TPA (entries 1–6), the open-chain analog to ASU at 160 8C in 6 m NaOH. DABCO-based cations are compared with non-cage-like analogs (entries 7–10). Abbreviation[a]

Half-life [h]

1

ASU

110

2

DMP

87.3

3

DMPy

37.1

4

ASN

28.4

5

TPA

7.19

6

BMP

7.26

7

TMA

61.9

8

MAABCO

13.5

9

BTM

4.18

10

BAABCO

1.38

Entry

QA

[a] ASU: 6-azonia-spiro[5.5]undecane; DMP: N,N-dimethylpiperidinium; DMPy: N,N-dimethylpyrrolidinium; ASN: 5-azonia-spiro[4.4]nonane; TPA: tetrapropylammonium; BMP: N-benzyl-N-methylammonium.

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Figure 7. Newman projection of DMP and DMPy. DMP contains two protons that can readily rotate into an anti-periplanar position marked with an asterisk. In DMPy, on the other hand, such a conformation is unfavorable due to ring strain.

www.chemsuschem.org degradation pathways (Figure 10) predominates in an N,N-dimethylammonium heterocycle. For DMP this is substitution at the methyl groups whereas both ring-opening substitution and methyl substitution seem to occur in DMPy (see the Supporting Information); however, in alkaline methanolic solutions at T = 130 8C ring-opening substitution appears to dominate.[45, 73, 74] Both spiro compounds, ASU and the pyrrolidinebased 5-azonia-spiro[4.4]nonane (ASN), decompose by ringopening substitution at the a-carbon (see the Supporting Information). In light of this it is unlikely that the cage-like quinuclidine is of higher stability than ASU as it exhibits significant ring strain,[70, 71] which probably increases its reactivity in a similar way observed for DMPy compared to DMP. Without precise knowledge of the reason for the high alkaline stability of heterocycles, the ring itself definitely makes the QA exceedingly resistant against alkaline degradation. This becomes immediately apparent by direct comparison with an open-chain analogue of ASU, namely tetrapropylammonium (TPA), which has a comparatively short half-life of 7.8 h compared to the 110 h of ASU. The stability of the 6-membered ring structure might conceivably be increased even further by attaching suitable sub-

ring, b-protons can easily move into an anti-periplanar position (Figure 7), which is not the case for the strained 5-membered ring. Although it is likely that this geometry results in a comparatively high half-life of 37 h for DMPy, unexpectedly the 6-membered ring in DMP displays the highest resistance against alkaline decomposition of all compounds investigated so far (t1/2 = 87 h), surpassing even the TMA benchmark (Table 4). The higher alkaline stability compared to TMA strongly points towards a reduced degradation susceptibility of the 6-membered ring in comparison to the methyl groups. Indeed, 1H NMR spectroscopy confirmed that substitution at the methyl groups is the primary reaction pathway (see the Supporting Information), which is even more surprising as elimination reactions are generally favored at elevated temperatures compared to substitution.[20, 72] If the methyl groups are the primary cause for degradation in DMP, replacing them with another 6membered ring should further increase alkaline stability. Therefore, 6-azonia-spiro[5.5]undecane (ASU), consisting of two 6-membered rings with a central nitrogen cation, was synthesized and found to have the longest half-life (t1/2 = 110 h) of all compounds ex- Figure 8. The transition state for the elimination of a b-proton in DMP or ASU requires energetically expensive bond angles and lengths. This is difficult to visualize on paper amined in this study (Table 4). and most easily seen by using a molecular construction kit. Although an anti-periplanar b-proton is readily available twice in DMP and even four times in ASU, they seem to have a reduced propensity for both elimination and substitution. The resistance against elimination is probably caused by the geometric constraint of the ring on the elimination transition state, which requires unfavorable bond angles and lengths (Figure 8). Similarly, ring-opening substitution also requires disadvantageous angles, as the preferred trigonal bipyramidal SN2 transition state cannot be achieved otherwise (Figure 9). Whereas other approaches attempt to stabilize the QA by avoiding vulnerable groups with, for example, b-protons, the heterocyclic approach with ASU relies on a severe decrease of their reactivity (i.e., increase of activation energy). Reduced propensity for b-elimination in such 5and 6-membered cyclic QA compounds has been observed before, but without having AEMs or ILs in mind.[45] Product analysis revealed disfavored elimination, with the rate of the ring-opening substitution Figure 9. Sketch of nucleophilic substitution at the DMP cation. The trigonal bipyramidal related to ring strain.[45, 73, 74] transition state prefers in-plane and out-of-plane angles of 1208 and 908, respectively. In The balance between ring strain and transition- the ring-opening substitution these angles can only be acquired through distortion of state energy determines which of the three possible the 6-membered ring.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 10. The three possible degradation pathways for a DMP cation: a) nucleophilic substitution at the methyl group, b) ring-opening elimination, and c) ring-opening substitution. For DMP (a) is the dominating pathway, whereas both (a) and (c) occur in DMPy. See also references by Lillocci et al.[45, 73, 74]

stituents to appropriate positions in the ring (e.g., 3, 5, or 4 position) to further increase the energy of the transition state and potentially block OH approach. Nevertheless, care has to be taken to avoid placing a substituent at the wrong position as this can be highly detrimental. This is the case, for example, if methyl groups are attached to the 2 and 6 positions in the DMP ring (i.e., N,N,2,6-tetramethylpiperidium). Freely rotating CH3 groups with easily accessible b-protons then lead to rapid ring-opening elimination,[74] increasing the overall degradation rate by orders of magnitude because the molecular structure basically contains ethyl groups and the reaction can proceed as in an open-chain QA. Based on the data presented in this study, the structure of a comparatively base stable HEM may consist of ASU repeating units or ASU attached to a hydrocarbon backbone without any benzylic or other electron-withdrawing groups present (Figure 11).

Conclusions The degradation rate of quaternary ammonium (QA) groups increases dramatically both with OH concentration and temperature, demonstrating the importance of sufficient hydration of hydroxide exchange membranes (HEMs) in alkaline fuel cells for achieving long half-life times. The most stable QA found herein is 6-azonia-spiro[5.5]undecane (ASU), in spite of four readily available b-protons in antiperiplanar position for b-elimination. This piperidinium-based cation demonstrates high resistance against both nucleophilic substitution and elimination in alkaline conditions and at elevated temperature. As main reason for this finding we suggest unfavorable bond angles and lengths in the reaction transition states. While 5- and 6-membered aliphatic heterocycles were surprisingly stable, inherent ring strain or another heteroatom near the positive charge increases reactivity. Benzylic carbon atoms are comparatively vulnerable to nucleophilic attack, especially if electron-withdrawing substituents are attached to the aromatic ring, which further increases  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 11. Two examples of potential ASU-based polymers. x depicts a suitable linking group.

reactivity. Electron-donating substituents decrease the reactivity of the benzylic group, but stability remains comparatively low; thus, benzylic groups should be avoided in HEMs. This is even more true for QAs based on aromatic groups. These decompose far more rapidly than any aliphatic QA, probably because of the nearly complete lack of steric shielding, which cannot be sufficiently compensated by the stabilizing effect achieved through charge delocalization. QAs with freely accessible ethyl groups are very reactive due to fast b-elimination, but for slightly longer alkyl chains this effect is less significant. The chain length to obtain maximum stability is between three and six carbon atoms, beyond which the stability decreases with increasing chain length.

Experimental Section Materials Reinecke’s salt, benzyl chloride (99 %), 1,4-diazabicyclo[2.2.2]octane (DABCO,  99 %), tetramethylammonium chloride (TMA, > 98 %), tetramethylammonium iodide (TMAI, 99 %), benzyltrimethylammonium chloride (BTM, 97 %), benzyldodecyldimethylammonium chloride (BDDM,  99 %), hexadecyltrimethylammonium bromide (HTM, > 99 %), dihexadecyldimethylammonium bromide (DHDM, 97 %), 1benzyl-3-methylimidazolium chloride (BMI,  97 %), 1-methyl-3-octylimidazolium chloride (MOI,  97 %), benzyl chloride (99 %), methyl iodide (99 %), 1,2-dimethylimidazole (98 %), piperidine (99 %), pyrrolidine (99 %), 1,5-dibromopentane (97 %), 1,4-dibromobutane (99 %), and bromoethane (98 %) were acquired from Sigma–Aldrich and tetramethylammonium bromide (TMABr, 98 %) from Alfa-Aesar. Diethylether (> 99.8 %), ethanol (> 99.8 %), and acetonitrile (> 99.5 %) were supplied by Roth. All chemicals and solvents were used as received without further purification.

Reaction vessels The strongly alkaline NaOH solutions corrode glass containers within minutes, especially at elevated temperatures, resulting in white precipitate that increases the viscosity of the solution over time. To avoid contact of alkaline solution and glass, Teflon inserts were manufactured to cover the inside of the glass cylinders. The glass cylinders themselves were prepared from particularly thick glass to resist the increased pressure at T = 160 8C. ChemSusChem 0000, 00, 1 – 12

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Determination of the quaternary ammonium concentration A NaOH solution of set concentration containing the quaternary ammonium salt (~ 0.1 mol L1) was prepared, filled into a pressuretight glass flask with a Teflon insert (V  3.5 mL), and heated to the desired temperature in a silicone oil bath while stirring (see the Supporting Information). At defined times the flask containing the sample was cooled down by tap water flow for about 180 s, then cleaned of residual silicone oil and water with a paper towel. After carefully opening the container, the reaction mixture was removed (200 mL, the exact sample mass was determined gravimetrically) and stored at T = 8 8C until at least five samples were collected for quantitative QA concentration analysis. Freshly prepared Reinecke’s salt (RS; 5 mm, 5 mL) solution was added to each sample and shaken for 30 s. The precipitate of the RS-QA complex was then filtered using a 0.7 mm Chromacol glass syringe filter (some samples required a 0.45 mm PTFE filter to reliably remove all particles). Disposable BRAND UV micro cuvettes (12.5  12.5  45 mm, 70 mL, light path 10 mm, center height 8.5 mm) were filled with the filtrate and measured using an Uvikon XL UV-Vis spectrometer (BioTek Intruments) between 350–800 nm with 800 nm chosen as the point of zero absorption for all samples. The concentration of the leftover ammonium [QA] was calculated by using Equation (1): ½QA ¼ ½RS0 ½RS

ð1Þ

where [RS]0 is the concentration of the freshly prepared RS solution and [RS] the measured concentration after precipitation and filtration. [RS] was determined through its undisturbed absorption signal at 520 nm, using a previously recorded calibration curve according to the Lambert–Beer law (Figure 12 b).

Figure 12. a) UV/Vis-spectra of different Reinecke’s salt solutions of BAABCO, BMP, and BTM (see Table 1) in 6 m NaOH corresponding to some of the data points in the calibration curve (b).

Reinecke’s salt solution The dark red Reinecke’s salt (168 mg, 0.5 mmol) was dissolved in deionized water (100 mL) to create a 5 mm violet solution. To accelerate the dissolution process, solutions were sonicated three times for 10 s. The RS solutions were always freshly prepared and only used up to 2 h after preparation as slow decomposition with a half-life of  300 h occurred, initially forming a green solution and then a green precipitate of probably Cr2O3.

create an almost saturated solution to which an excess of Et2O (approx. 80 mL) was added to precipitate a snow-white solid. This solid was filtered and washed with Et2O, n-heptane, and Et2O again (the product appeared to be slightly soluble in n-heptane). The solid was dried in vacuo at T = 80 8C over night to yield 4.49 g of the product (77 % yield). 1H NMR (300 MHz, D2O, 25 8C): d = 3.48 (t, 8 H), 1.91 (m, 8 H), 1.75 ppm (m, 4 H).

Synthesis To avoid an exceedingly long experimental section only the synthetic routes for ASU and MAABCO are provided. The synthesis of the various other QA groups followed a similar procedure (a nucleophilic substitution reaction of an amine with a halocarbon) and can be found in the Supporting Information together with 1H NMR spectra of all synthesized compounds.

6-Azonia-spiro[5.5]undecane bromide (ASU) In a three-neck flask a mixture of 1,5-dibromopentane (5.75 g, 25 mmol) and K2CO3 (4.2 g, 30 mmol) in acetonitrile (50 mL) was heated to reflux. Piperidine (2.14 g, 25 mmol) in acetonitrile (10 mL) was added dropwise using a dropping funnel under reflux, and the mixture was refluxed over night. Afterwards the solvent was evaporated using a rotary evaporator. EtOH was added in excess (approx. 70 mL) to the residual solid, and the mixture was filtered to remove K2CO3. The solvent was partially evaporated to  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

1-Methyl-4-aza-1-azonium-bicyclo[2.2.2]octane chloride (MAABCO) DABCO (1.45 g, 14 mmol), methyl iodide (0.85 mL, 14 mmol), and K2CO3 (0.7 g, 15 mmol) in EtOH (50 mL) were stirred at RT for 3 days. The mixture was filtered to remove K2CO3, and the solvent was evaporated using a rotary evaporator. The white solid was washed with Et2O and n-heptane and dried in vacuo at T = 80 8C over night, resulting in 2.56 g of the white product (78 % yield). 1 H NMR (300 MHz, D2O, 25 8C): d = 3.24 (t, 6 H), 3.44 (t, 6 H), 3.10 ppm (s, 3 H).

Supporting Information The supporting information contains figures of all raw data points that were used to determine QA half-lives. In addition, more details of the experimental setup and as well as 1H NMR spectra of all synChemSusChem 0000, 00, 1 – 12

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CHEMSUSCHEM FULL PAPERS thesized compounds including detailed descriptions of the synthesis are included, and also the original data by Bauer et al.[12, 35]

Acknowledgements The authors thankfully acknowledge the financial support by the Bundesministerium fr Wirtschaft und Technologie (BMWi) under the contract number 03ET2004A and the Bundesministerium fr Bildung und Forschung (BMBF) under the contract number 03SF0473, Energie Baden-Wrttemberg (EnBW) and FuMA-Tech. We thank Dominik Samuelis for carefully reading the proofs. Keywords: alkaline fuel cells · alkaline stability · anion exchange · membranes · ionic liquids [1] D. R. Dekel, ECS Meeting Abstracts 2012, MA2012-02, 1368, URL http:// ma.ecsdl.org/content/MA2012-02/13/1368.abstract [2] S. Gottesfeld, ECS Meeting Abstracts 2012, MA2012-02, 1498. URL http:// ma.ecsdl.org/content/MA2012-02/13/1498.abstract. [3] J. R. Varcoe, P. Atanassov, D. R. Dekel, A. M. Herring, M. A. Hickner, P. A. Kohl, A. R. Kucernak, W. E. Mustain, K. Nijmeijer, K. Scott, T. Xu, L. Zhuang, Energy Environ. Sci. 2014, 7, 3135 – 3191. [4] M. Cifrain, K. Kordesch, Hydrogen/oxygen (air)fuel cells with alkaline electrolytes in Handbook of Fuel Cells, Wiley, Hoboken, NJ, 2010. DOI: 10.1002/9780470974001.f104013. [5] J. Larminie, A. Dicks, Fuel Cell Systems Explained, Wiley, Hoboken, NJ, 2003. [6] G. Mulder, Fuel cells—alkaline fuel cells—overview in Encyclopedia of Electrochemical Power Sources (Ed.: J. Garche), Elsevier, Amsterdam, 2009, pp. 321 – 328. [7] A. Appleby, Fuel cells—overview—introduction in Encyclopedia of Electrochemical Power Sources (Ed.: J. Garche), Elsevier, Amsterdam, 2009, pp. 277 – 296. [8] Y. Ye, Y. A. Elabd, Chemical Stability of Anion Exchange Membranes for Alkaline Fuel Cells, Chap. 15, pp. 233 – 251. URL http://pubs.acs.org/doi/ abs/10.1021/bk-2012-1096.ch014. [9] H. Yanagi, K. Fukuta, ECS Trans. 2008, 16, 257 – 262, 8th Symposium on Proton Exchange Membrane Fuel Cells, Honolulu, HI, OCT, 2008. [10] M. Marino, J. Melchior, A. Wohlfarth, K. Kreuer, J. Membr. Sci. 2014, 464, 61 – 71. [11] M. J. Hatch, W. D. Lloyd, J. Appl. Polym. Sci. 1964, 8, 1659 – 1666. [12] B. Bauer, H. Strathmann, F. Effenberger, Desalination 1990, 79, 125 – 144 Proceeding of the 6th Symposium on Synthetic Membranes in Science and Industry. [13] T. Sata, M. Tsujimoto, T. Yamaguchi, K. Matsusaki, J. Membr. Sci. 1996, 112, 161 – 170. [14] G. Couture, A. Alaaeddine, F. Boschet, B. Ameduri, Prog. Polym. Sci. 2011, 36, 1521 – 1557. [15] G. Merle, M. Wessling, K. Nijmeijer, J. Membr. Sci. 2011, 377, 1 – 35. [16] C. G. Arges, J. Parrondo, G. Johnson, A. Nadhan, V. Ramani, J. Mater. Chem. 2012, 22, 3733 – 3744. [17] D. Chen, M. A. Hickner, ACS Appl. Mater. Interfaces 2012, 4, 5775 – 5781. [18] S. Chowdhury, R. S. Mohan, J. L. Scott, Tetrahedron 2007, 63, 2363 – 2389. [19] U. Frohlich, S. Huq, S. Katdare, R. F. Lukasik, E. Bogel, N. V. Plechkova, K. R. Seddon, M. J. Earle (University of Belfast), EP 2319621 (A2), 2011. [20] M. Smith, J. March, March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, Wiley, Hoboken, NJ, 2007. [21] F. N. Jones, C. R. Hauser, J. Org. Chem. 1962, 27, 1542 – 1547. [22] S. H. Pine, The Base-Promoted Rearrangements of Quaternary Ammonium Salts, Wiley, Hoboken, NJ, 2004. [23] T. S. Stevens, E. M. Creighton, A. B. Gordon, M. MacNicol, J. Chem. Soc. 1928, 3193 – 3197. [24] C. G. Arges, V. Ramani, Proc. Natl. Acad. Sci. USA 2013, 110, 2490 – 2495. [25] A. Amel, L. Zhu, M. Hickner, Y. Ein-Eli, J. Electrochem. Soc. 2014, 161, F615 – F621.

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Received: September 19, 2014 Published online on && &&, 0000

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FULL PAPERS M. G. Marino,* K. D. Kreuer* && – && Alkaline Stability of Quaternary Ammonium Cations for Alkaline Fuel Cell Membranes and Ionic Liquids Twice thwarted: The hydroxide-induced decomposition of 26 different quaternary ammonium (QA) groups reveals a piperidine-based heterocycle and its spiro compound as the most stable QAs. Their ring structure makes them highly resistant against both elimination and

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substitution reaction This is counterintuitive in view of the common assumption that avoidance of b-protons is essential for reducing degradation rates. This is fundamentally intriguing as well as relevant for the development of OH exchange membranes and ionic liquids.

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