Imidazolium Cations with Exceptional Alkaline Stability: A Systematic Study of Structure-Stability Relationships

Article abstract of DOI:10.1021/jacs.5b02879

Highly base-stable cationic moieties are a critical component of anion exchange membranes (AEMs) in alkaline fuel cells (AFCs); however, the commonly employed organic cations have limited alkaline stability. To address this problem, we synthesized and characterized the stability of a series of imidazolium cations in 1, 2, or 5 M KOH/CD₃OH at 80 °C, systematically evaluating the impact of substitution on chemical stability. The substituent identity at each position of the imidazolium ring has a dramatic effect on the overall cation stability. We report imidazolium cations that have the highest alkaline stabilities reported to date, >99% cation remaining after 30 days in 5 M KOH/CD₃OH at 80 °C.

Full text of DOI:10.1021/jacs.5b02879

Article
pubs.acs.org/JACS
Imidazolium Cations with Exceptional Alkaline Stability: A Systematic
Study of Structure−Stability Relationships
Kristina M. Hugar, Henry A. Kostalik, IV, and Geoffrey W. Coates*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States
S
* Supporting Information
ABSTRACT: Highly base-stable cationic moieties are a critical
component of anion exchange membranes (AEMs) in alkaline fuel
cells (AFCs); however, the commonly employed organic cations
have limited alkaline stability. To address this problem, we
synthesized and characterized the stability of a series of
imidazolium cations in 1, 2, or 5 M KOH/CD3OH at 80 °C,
systematically evaluating the impact of substitution on chemical
stability. The substituent identity at each position of the
imidazolium ring has a dramatic effect on the overall cation
stability. We report imidazolium cations that have the highest
alkaline stabilities reported to date, >99% cation remaining after
30 days in 5 M KOH/CD3OH at 80 °C.
polystyrenes,14 and various aliphatic backbones.15 Despite
INTRODUCTION
■
exhibiting high initial conductivity, numerous studies, such as
The environmental and financial implications of our near
exclusive dependence on fossil fuels have expedited research
efforts to develop more effective methods of extracting the
those by Boncella and co-workers,16e have demonstrated that
ammonium cations degrade rapidly under fuel cell operating
energy stored in chemical bonds.1 Fuel cells have emerged as
conditions, limiting their utility and making the improvement
of AAEM stability a critical priority.16 Of note, the alkaline
attractive electrochemical conversion devices due to their high
stability of membranes composed of a variety of polymer
energy density and their ability to produce energy more cleanly
and efficiently as compared to conventional systems, such as
internal combustion engines.2 In particular, proton exchange
membrane fuel cells (PEMFCs) have been useful in many
commercial applications.3 However, widespread production is
limited by the cost and durability of the materials, specifically
the platinum electrodes and electrolyte membrane.4 To address
these challenges, alkaline fuel cells (AFCs) have been
investigated, which operate by transporting hydroxide ions
through the electrolyte under basic conditions.5 At elevated pH,
oxygen reduction is more facile and lower overpotentials are
required, enabling the use of non-noble metal catalysts in
AFCs.6 Indeed, the earliest examples of commercial fuel cells
used aqueous potassium hydroxide solutions as the electrolyte
medium to facilitate anion conduction. Unfortunately, the
performance of these early fuel cells was compromised by
exposure to carbon dioxide, a common component of feedstock
gases, which reacts with hydroxide to produce carbonate salts.7
To overcome this issue, alkaline anion exchange membranes
(AAEMs), which are generally comprised of organic cations
covalently linked to a polymer backbone, are employed to
prevent the formation of mobile salts and retain the conductive
organic cation/hydroxide species.8
backbones was followed using 1H NMR spectroscopy by Nunez
̃
and Hickner.16a The disadvantages of using ammonium cations,
particularly the ubiquitous benzyl trimethylammonium
(BTMA) cation, have spurred investigations into the stability
of other positively charged moieties in the presence of
hydroxide, such as guanadinium,17 phosphonium,18 diazabicy-
clooctane-based (DABCO),19 benzimidazolium,20 morpholi-
nium,21 pyridinium,22 pyrrolidinium,23 metal organic frame-
works (MOFs),24 and ruthenium25 cations (Chart 1). Marino
and Kreuer recently described a class of quaternary spiro-
ammonium compounds that exhibited improved alkaline
stability as compared to the acyclic counterparts.19a Polymers
containing base-stable cationic groups are also useful in other
applications, including electrolysis,26 gas separation,27 desalina-
tion28 and as stimuli-responsive materials.29 Our group
reported a polyethylene membrane containing a tetrakis-
(dialkylamino)phosphonium cation that exhibited excellent
stability;30 however, the synthesis of this cation requires several
difficult multistep reactions. Ideally, the best candidates for
practical fuel cell devices are cations that are easy to make and
incorporate into polymers, while maintaining optimal con-
ductivity and stability.
Tetraalkylammonium cations have been appended to various
polymer architectures to prepare AAEMs, including perfluori-
nated membranes,9 aromatic polysulfones,10 poly(arylene
ethers),11 poly(arylene ether ketones),12 polyphenylenes,13
Received: March 19, 2015
© XXXX American Chemical Society
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between the organic cation and reactive counteranions.
Accordingly, recent studies on polymers containing imidazo-
liums with methyl groups in the C2 position suggested that this
substitution improved alkaline stability compared to unsub-
stituted imidazoliums.39 The numbering system of the
imidazolium cation is described in Scheme 1. The attenuation
in reactivity is attributed to steric factors, where nucleophilic
addition and subsequent ring-opening of the heterocycle are
hindered by the C2-methyl group (Scheme 1a).
Chart 1. Selected Cations Investigated for Use in Alkaline
Anion Exchange Membranes (AAEMs)
Several researchers have explored the effect of C2
substitution on alkaline stability.40 Specifically, studies on a
related class of class of compounds, benzimidazolium cations,
by Beyer,20c Holdcroft,20e and co-workers highlight the
importance of bulky substituents at the C2 position. Density
functional theory (DFT) calculations by Pivovar,41a Ramani,41b
and others41c,d predict that substitution at the C4 and C5
positions will improve stability. These claims are supported
experimentally in original work conducted by Wang et al.,40e
which investigated imidazoliums with C4,5 methyl groups.
Presumably, C4,5 substitution improves imidazolium stability
by preventing deprotonation reactions (Scheme 1b). The
degradation pathways discussed thus far only involve reactions
directly at the ring positions (Scheme 1a and b); however,
reactions with the peripheral substituents are also predicted
(Scheme 1c and d). In fact, deprotonation of the peripheral
substituents containing α-hydrogens is readily observed in basic
solutions that contain protic deuterated solvents, as evidenced
by hydrogen/deuterium exchange. Imidazolium reactivity is
easily regulated by substituent variation, and eliminating the
sites of vulnerability prevents degradation. The synthetic
convenience, simplistic modification, and resonance stabiliza-
tion of imidazoliums make them attractive targets, and we
hypothesized that these features would enable the creation of
cations with exceptional base stability.
Imidazoles are a class of organic compounds that are
amenable to synthesis because they are prepared by a modular
route, with easily modified substituents, and are readily
converted to the cationic form via alkylation. Additionally,
they are stabilized by charge delocalization like the virtually
inert tetrakis(dialkylamino)phosphonium cations. Researchers
have attached N-methyl31 or N-alkyl32 benzyl imidazoliums to
polymers and investigated them as alternatives to ammonium
cations, and while these cations transport hydroxide sufficiently,
the chemical stabilities of unsubstituted imidazoliums are
generally much too low for fuel cell applications.33 In fact,
imidazolium cations with higher stability would be beneficial in
many applications, such as organocatalysis,34 solar cell electro-
lytes,35 phase transfer catalysis,36 and as carbon material
precursors,37 in addition to AAEMs. Imidazoliums degrade
under alkaline conditions via four distinct mechanisms, and the
identities of the substituents direct the degradation pathways
(Scheme 1).38 By selecting the appropriate substituents, several
degradation routes are inhibited, thus deterring reactions
Model compound studies, wherein the degradation rates of
small molecules are assessed under alkaline conditions, are
effective at determining the relative stabilities of a series of
compounds. Once promising cations are identified, they must
be incorporated into polymers where the collective stability of
the AAEM can be determined under real-world operating
conditions. In a recent report from Mohanty et al.,42 a variety of
quaternary ammonium hydroxide complexes were prepared and
studied under relevant fuel cell operating temperatures. Several
other protocols have been reported for determining model
compound stabilities; however, the conditions vary widely,
making productive comparisons between individual accounts
difficult.40 To rigorously assess the performance of new cations,
we have developed an NMR spectroscopy method that
unambiguously ranks the stability of cations.43 Solutions of
the cation are prepared in basified methanol-d3 (KOH/
CD3OH) and stored in flame-sealed NMR tubes at 80 °C. At
Scheme 1. Degradation Pathways of Imidazolium Cations
under Alkaline Conditions
1
uniform time intervals, the solutions are analyzed by H NMR
spectroscopy for the amount of cation remaining relative to an
internal standard.44 The use of CD3OH precludes a hydrogen/
deuterium exchange process that causes a reduction in the
cation signals (not related to degradation) and obscures new
product signals. Methanol is a more universal solvent and
conveniently dissolves organic cations and potential degrada-
tion products, a consideration of paramount importance for
NMR spectroscopy studies. Key aspects of the degradation
routes were revealed with this new protocol, which facilitates
the design of new imidazoliums with strategically placed
substituents to prevent decomposition.
B
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Scheme 2. Summary of Model Compounds Investigated, Including the Synthesis of Imidazoliums with Varied Substitution
Patterns
To date, investigations of imidazolium substitution patterns
have been typically restricted to commercially available
imidazoles. Fortunately, tetrasubstituted imidazoles, the neutral
precursors to imidazoliums, can be prepared using simple
multicomponent reactions.45 Herein, we report the synthesis of
a variety of imidazoliums (Scheme 2), systematically altering
the structures to examine the precise influence of substitution
patterns on the alkaline stability. Ultimately, these experiments
led to the synthesis of imidazolium cations with higher
resistance to reaction with bases and nucleophiles than any
previously reported model compound studies. Furthermore, the
synthetic accessibility of imidazoliums simplifies their incorpo-
ration into polymer architectures to achieve AAEMs with high
conductivities and stabilities.
liums with N1-benzyl and N3-methyl groups, as the majority of
imidazolium-based AAEMs reported in the literature have these
functionalities. This series of cations was compared to benzyl
trimethylammonium (1) (BTMA) and the C2 unsubstituted
imidazolium, 1-benzyl-3-methylimidazolium bromide (2a)
(Scheme 2).
As previously reported, 2a degrades rapidly under mildly
basic conditions, with less than 2% cation remaining after 5
days.43 In contrast, the imidazolium with a C2-methyl
substituent (2b) reacts more slowly, leaving 36% remaining
after 30 days. This simple imidazolium 2b is already an
improvement over BTMA (1), which degraded to 11%
remaining in the same time. BTMA degrades by nucleophilic
attack at the benzylic and methyl positions, producing N,N-
dimethylbenzylamine and benzyl methyl ether, as evident by 1H
NMR spectroscopy.30 The resonances for similar nucleophilic
displacement products (benzyl methyl ether, dimethyl ether,
and the corresponding N1-benzyl or N3-methyl imidazoles) are
not observed for 2a or 2b indicating that SN2 reactions did not
occur (Scheme 1c). Nucleophilic addition of hydroxide to the
C2 position and concomitant ring-opening is reported for
imidazolium degradation (Scheme 1a); however, the amide and
imine signals corresponding to this degradation pathway are
not observed for either compound. In fact, analysis by 1H NMR
spectroscopy indicates that imidazolium decomposition is more
complicated than suggested in Scheme 1. We propose that the
initial product(s) formed are not stable under the protocol
conditions and undergo further degradation or rearrangement
reactions. Additional work is needed to confirm the identity of
the products and elucidate the secondary degradation
mechanism(s). Introducing substituents to the C4,5 positions
results in a substantial increase in stability. The imidazolium
with C4,5-phenyl substituents (4d) degrades moderately faster
than the C4,5-methyl version (3d), yielding 80% and 87%
cation after 30 days, respectively. The degradation products for
3d are not yet identified, although 3d does not appear to
degrade by an SN2 mechanism. A small amount of nucleophilic
displacement is observed for 4d; however, other degradation
pathways are more prominent. Deprotonation of substituent
hydrogens (i.e., C2-methyl or benzylic protons) followed by
rearrangement is a plausible mode of degradation for both 3d
and 4d (Scheme 1d). These examples strongly indicate that
imidazolium stability is enhanced by C4,5 substitution, which
agrees with the results obtained by Yan,40e Zhang,38a and co-
workers. Therefore, this work focuses on compounds with
RESULTS AND DISCUSSION
■
Initially, we explored the impact of C4,5 substitution on cation
stability by evaluating imidazoliums with hydrogen, methyl, or
phenyl groups in the C4,5 positions (Figure 1). Methyl groups
were installed at the C2 position because these substituents
produce cations that are more stable than their C2-
unsubstituted counterparts. To start, we investigated imidazo-
Figure 1. Stability of C4,5-substituted imidazolium cations (0.05 M) in
1 M KOH/CD3OH at 80 °C.
C
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C4,5-methyl or phenyl substituents, but other alkyl and aryl
substituents should behave similarly.
Next, we investigated the effects of C2 substitution on
imidazolium stability (Scheme 3). Imidazoliums generated from
imidazoliums with C2-aryls (4a and 4b) clearly degrade by SN2
attack at the nitrogen substituents, and the resonances for
benzyl methyl ether, dimethyl ether, and both imidazole
products are observed (Figure 2, diagnostic signals are
Scheme 3. Influence of C2 Substituents on Imidazolium
Stability: Percent Cation Remaining after 30 days at 80 °C
a
Figure 2. Degradation of 4a after 3 months in 1 M KOH/CD3OH at
80 °C.
a
1
Determined by H NMR spectroscopy.
highlighted for 4a). Imidazoliums 4a and 4b react via SN2
pathways primarily at the benzylic position followed by the N3-
methyl position as evidenced by the distribution of products in
the H NMR spectra. Overall, C2-aryl groups improve the
resistance of the imidazolium cation to base and were selected
for continued examination.
We evaluated the impact of changing the nitrogen
substituents by increasing the steric bulk of the N3-alkyl
group from an N3-methyl to an N3-ethyl or N3-butyl.
Synthesizing imidazoliums with both N1- and N3-butyl groups
eliminated the reactive benzylic position altogether. We
assessed imidazoliums with C4,5-phenyl groups because
stability trends were more apparent with faster degradation
rates (Scheme 4); although, after determining the best N1 and
N3 substituents, we reinvestigated imidazoliums with C4,5-
commercially available imidazoles (C2-methyl, isopropyl or
phenyl groups, and C4,5-hydrogens) were previously studied
by Lin et al.40d Holdcroft and co-workers investigated bulky
C2-aryl groups and found that 2,6-dimethylphenyl substituents
improved the stability of benzimidazoliums, as compared to
phenyl groups alone.20e For comparison, we combined these
four C2 substituents with our substitution patterns and
evaluated their impact on imidazolium stability by reporting
the percent cation remaining after 30 days (approximately 720
h).
Imidazoliums with C4,5-methyl substituents (Scheme 3, 3a−
3d) are more stable than the analogous compounds with
phenyl groups (Scheme 3, 4a−4d). In fact, little decomposition
is observed over 30 days for the C4,5-methyl-substituted
compounds, with the exception of 3d, as discussed previously.
For this reason, the C4,5-phenyl-substituted series, which
experienced moderate degradation, is used to analyze trends in
the impact of C2 substitution (vide infra).
1
Scheme 4. Influence of N1 and N3 Substituents on
Imidazolium Stability: Percent Cation Remaining after 30
a
days at 80 °C
Imidazoliums with aryl groups at the C2 position (Scheme 3,
4a and 4b) are more base-stable than those with alkyl groups
(Scheme 3, 4c and 4d). This observation contrasts with C2
trends observed by Yan and co-workers where alkyl
substituents improved stability as compared to phenyl
groups.40d The degradation rates for C2-aryl-substituted
compounds (Scheme 3, 4a and 4b) are very similar (87%
and 91% cation remaining, respectively). Likewise, varying the
steric bulk of the C2-alkyl substituent does not strongly
influence the base stability (Scheme 3, 4c and 4d). These
results indicate that nucleophilic addition to the C2 position is
not a major degradation pathway for 4a−4d. Amide and
1
enamine resonances are not observed by H NMR spectros-
copy, suggesting that ring-opening decomposition is not
occurring (Scheme 1a). The lack of ether and imidazole
degradation products suggests that imidazoliums with C2-alkyls
(4c and 4d) do not degrade by SN2 attack, leaving substituent
deprotonation and subsequent rearrangement reactions as
potential decomposition routes (Scheme 1d). In contrast, the
a
1
Determined by H NMR spectroscopy.
D
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methyl groups. To delineate trends in stability in a more
convenient time frame, we raised the base concentration from 1
to 2 or 5 M KOH, increasing the rate of the degradation
reactions.
from interaction with the neighboring nucleophile. The results
in Scheme 4 establish that imidazoliums with N1-butyl groups
are less prone to nucleophilic attack as compared to the N1-
benzyl counterparts and addition reactions at the C2 position
are least likely with 2,6-dimethylphenyl substituents. We
prepared optimized imidazoliums, which incorporated the
best substituents at each of the ring positions, and assessed
their alkaline stability (Scheme 5).
In a series of benzyl imidazoliums with either C2-phenyls
(Scheme 4, 4b, 5b, and 6b) or C2-aryls (Scheme 4, 4a, 5a, and
6a), the stability improved by switching from N3-methyl to N3-
ethyl or N3-butyl substituents. A similar result was reported by
Gu et al.,40b which showed that n-butyl groups improved
imidazolium stability as compared to methyl groups. For
example, in 1 M KOH, 87% of 4a remains after 30 days,
whereas 91% and 95% of 5a and 6a remain, respectively. The
differences became larger as the concentration of base increases
to 2 M KOH; 66% of 4a remains, while 84% and 86% of 5a and
6a remain, respectively. When comparing 1 to 2 M KOH
conditions (Scheme 4, 4a−6a and 4b−6b), the reaction rates
increase consistently with the increase in base concentration
and degradation continues to occur by an SN2 mechanism. A
decrease in nucleophilic attack at the α-carbon of the N3-
substituent is observed as the length of the N3-alkyl group
increases, which explains the prior observation that longer alkyl
chains improve cation stability. 1H NMR signals corresponding
to ethylene or 1-butene are not readily detected, suggesting that
Hofmann elimination (deprotonation and elimination of alkyl
groups that contain β-hydrogens) is not a major degradation
pathway for alkyl imidazoliums. An increase in degradation rate
larger than predicted is observed for 4b, 5b, and 6b in 5 M
KOH, and 1H NMR signals related to new degradation
products are observed. At very high concentrations of base,
these imidazoliums degrade by a mechanism other than SN2
attack at the substituents bound to the nitrogens. The trend is
only observed for the C2-phenyl imidazoliums (Scheme 4, 4b,
5b, and 6b), which suggests that the new degradation pathway
involves addition of base to the C2 position. One possible
explanation involves the participation of multiple species of
base (−OH or −OMe) in the rate-determining step of the
reaction, resulting in a higher order dependence of base under
very high concentrations of base. It is important to emphasize
that the goal of increasing the base concentration is to
accelerate reactions that occur at 1 M KOH, not to introduce
new degradation pathways. Stability studies that are conducted
in excess base are not necessarily representative of fuel cell
operation, and extrapolating degradation rates must be
judiciously considered. Nevertheless, a cation that is stable
under such caustic conditions will likely demonstrate excellent
stability at lower base concentrations. Importantly, this
facilitates membrane electrode assembly (MEA) fabrication
due to the improved cation stability at elevated temperatures in
the absence of water.
Scheme 5. Optimization of Base Stable Imidazoliums:
a
Percent Cation Remaining after 30 days at 80 °C
a
1
Determined by H NMR spectroscopy.
As predicted, imidazoliums with methyl groups at the C4,5
positions (Scheme 5, 7a and 7b) are quite stable at 2 M KOH
concentrations. At 5 M KOH concentrations, the stability of 7b
drops off; however, the stability remains higher than the
analogous cation 8b with C4,5 phenyl groups. Significant
changes in the signal integrations for 7a are not observed over
30 days, even at 5 M KOH and 80 °C (Figure 3). By
systematically screening substituent effects on the overall
imidazolium stability, we developed cations with exceptionally
high resistance to reaction with base.
We compared the stability of 7a and 8a to imidazoliums that
have been previously reported, and some important trends are
highlighted (Figure 4). Imidazoliums with substitution at the
C2 position demonstrate greatly improved stability over the
unsubstituted version and better resistance to degradation than
BTMA. Introducing alkyl substituents to the nitrogens
consistently enhances the stability as compared to the benzylic
counterparts. The best stability is observed when both N1- and
N3-alkyls are larger than methyls and substituents are present
at the C4,5 positions. The use of 2 and 5 M test conditions
permits quantitative comparison between highly stable systems.
CONCLUSIONS
The effect of imidazolium substituents on base stability was
systematically studied. The model compounds were assessed
for stability under alkaline conditions by H NMR spectrosco-
■
Replacing the N1-benzyl with an N1-butyl group and
retaining the N3-butyl group led to the most stable
imidazoliums in the series (8a and 8b) for which signal
integrations are essentially unchanged at 2 M KOH after 30
days and very little degradation is observed even at 5 M KOH
(Scheme 4, 8a and 8b). Interestingly, the stability of 8b with a
C2-phenyl group is only slightly altered at 5 M KOH, unlike
4b, 5b, and 6b, which appear to react with methoxide at the C2
position at high base concentrations. This may indicate that an
N1-butyl group is more effective than an N1-benzyl group at
blocking nucleophilic addition to the C2 position of the
imidazolium ring. Alternatively, a more soluble organic cation
with aliphatic groups may remain better solvated, which can
reduce the effective strength of the base, shielding the cation
1
py, and the degradation modes were analyzed. On the basis of
these structural trends, we rationally modified the imidazolium
ring to install substituents that would impede reaction with
anions. Ultimately, we arrived at cations that were stable under
the harsh alkaline conditions we assessed, in excess of operating
fuel cell hydroxide concentrations. We found that C4 and C5
substitution was very important to the alkaline stability of the
imidazolium cations, with methyls groups slightly improving
the stability relative to phenyl groups. Moreover, methyl groups
offer an advantage over phenyl groups when used in AAEMs
because they increase the ion exchange capacity (IEC) of the
membrane. Substitution at the C2 position inhibited degrada-
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1
Figure 3. Analysis of 7a under alkaline conditions using H NMR spectroscopy in CD3OH. Residual signals between 5.5−7.0 ppm are due to
solvent; see the Supporting Information for discussion on solvent suppression.44
Figure 4. Comparison of model compound stabilities, percent remaining after 30 days at 80 °C (determined by 1H NMR spectroscopy relative to an
internal standard).44
tion, and 2,6-dimethylphenyl substituents were the most
effective. The use of alkyl substituents on the nitrogens,
particularly n-butyl groups, prevented degradation better than
benzyl or methyl groups. Because the majority of polymer-
ization techniques applied to synthesize AAEMs append cations
to benzylic positions, it will be necessary to develop new
synthetic routes to attach the base-stable imidazoliums to
polymers. Furthermore, several of the commonly employed
polymer architectures have been discovered to be unstable
under fuel cell operating conditions.46 Developing AAEMs
based on inert polymer backbones, such as the recent work by
Coughlin and co-workers,47 which describes the synthesis of a
copolymer of isoprene and ammonium-functionalized styrene,
may bypass these issues. Future work will focus on appending
these cations to highly stable polymer architectures and
characterizing the membranes for hydroxide conductivity and
alkaline stability.
AUTHOR INFORMATION
■
Corresponding Author
*coates@cornell.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon work at the Energy Materials
Center at Cornell (emc2) supported by the New York State
Energy Research and Development Authority (NYSERDA)
under contract number 38304 and the U.S. Department of
Energy, Office of Science, and Office of Basic Energy Sciences,
under Award DE-SC0001086. We thank Ford Motor Co. for a
University Research Program Award, and Dr. I. Keresztes for
assistance with NMR spectrum interpretation and for helpful
discussions.
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* Supporting Information
Detailed experimental procedures, full characterization data,
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