N-Alkyl Interstitial Spacers and Terminal Pendants Influence the Alkaline Stability of Tetraalkylammonium Cations for Anion Exchange Membrane Fuel Cells

Article abstract of DOI:10.1021/acs.chemmater.5b04767

Current performance targets for anion exchange membrane (AEM) fuel cells call for greater than 95% alkaline stability for 5000 h at temperatures of up to 120 °C. Using this target temperature of 120 °C, we provide an incisive ¹H nuclear magnetic resonance-based alkaline degradation method to identify the degradation products of n-alkyl spacer tetraalkylammonium cations in various AEM polymers and small molecule analogues. The operative alkaline degradation mechanisms and rates on benzyltrimethylammonium-, n-alkyl interstitial spacer-, and n-alkyl terminal chain-cations are compared in several architectures. Our findings indicate that benzyltrimethylammonium and n-alkyl terminal pendant cations are significantly more labile than an n-alkyl interstitial spacer cation. Additionally, we found that the alkaline stability of an n-alkyl interstitial spacer cation is enhanced when it is combined with an n-alkyl terminal pendant. At 120 °C, an inverse trend was observed in the overall stability of AEM poly(styrene) and AEM poly(phenylene oxide) samples compared to what has been shown at 80 °C. Follow-up small molecule studies suggest that at 120 °C, a 1,4-elimination degradation mechanism may be activated on styrenic AEM polymers capable of forming hyperconjugated resonance hybrids.

Full text of DOI:10.1021/acs.chemmater.5b04767

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Article
N-Alkyl Interstitial Spacers and Terminal Pendants
Influence the Alkaline Stability of Tetraalkylammonium
Cations for Anion Exchange Membrane Fuel Cells
Sean A. Nunez, Clara Capparelli, and Michael A. Hickner
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04767 • Publication Date (Web): 03 Mar 2016
Downloaded from http://pubs.acs.org on March 18, 2016
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Chemistry of Materials
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N-Alkyl Interstitial Spacers and Terminal Pendants Influence the
Alkaline Stability of Tetraalkylammonium Cations for Anion Ex-
change Membrane Fuel Cells
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Sean A. Nuñez, Clara Capparelli, and Michael A. Hickner*
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802,
United States
ABSTRACT: Current performance targets for anionꢀexchange membrane (AEM) fuel cells call for greater than 95% alkaline staꢀ
bility for 5000 hours at temperatures up to 120 °C. Using this target temperature of 120 °C, we provide an incisive 1H NMRꢀbased
alkaline degradation method to identify the degradation products of nꢀalkyl spacer tetraalkylammonium cations in various AEM
polymers and small molecule analogs. The operative alkaline degradation mechanisms and rates on benzyltrimethylammoniumꢀ, nꢀ
alkyl interstitial spacerꢀ and nꢀalkyl terminal pendantꢀcations are compared in several architectures. Our findings indicate that benꢀ
zyltrimethylammoniumꢀ and nꢀalkyl terminal pendant cations are significantly more labile than an nꢀalkyl interstitial spacerꢀcation.
Additionally, we found that the alkaline stability of an nꢀalkyl interstitial spacer cation is enhanced when combined with an nꢀalkyl
terminal pendant. At 120 °C, an inverse trend was observed in the overall stability of AEM poly(styrene) and AEM poly(phenylene
oxide) samples compared to what has been shown at 80 °C. Followꢀup small molecule studies suggest that at 120 °C, a 1,4ꢀ
elimination degradation mechanism may be activated on styrenic AEM polymers capable of forming hyperconjugated resonance
hybrids.
INTRODUCTION
The global demand for energy is projected to rise
alkaline stability of AEM polymer backbones and cations posꢀ
es a significant challenge for developing AEM fuel cell devicꢀ
es capable of achieving the Department of Energy’s 2015 perꢀ
formance targets calling for greater than 95% alkaline stability
for 5000 hours at temperature up to 120 °C.15
1.8% YOY until 2020 with an expected fossil fuel consumpꢀ
tion growth of only 0.8% YOY.1 This discrepancy between
growing demand and fossil fuel supply increase has driven the
development of alternative energy technologies that utilize
renewable materials and employ renewableꢀ and regenerationꢀ
schemes bearing low environment impact. Fuel cells rely on
renewable feedstocks for the electrochemical conversion of
renewable fuels to electrical energy for primary power applicaꢀ
tions and energy storage systems. Anion exchange membrane
(AEM) fuel cells have been proposed as viable alternatives to
proton exchange membrane fuel cells (PEMFCs)2–4 arising
from their use of platinumꢀfree electrodes.5ꢀ7 Additionally,
AEMs may be useful in electrolyzers7 that feature precious
metalꢀfree catalysts.8ꢀ10 The low cost of nonꢀprecious metal
catalysts is the clear motivator for AEMꢀbased devices, and
these types of cells also have increased tolerance to CO11 or
Polymerꢀbound cations for use in AEMs have includꢀ
ed benzimidazolium,16 imidazolium,14,17,18 amino phosphoniꢀ
um19 and sulfonium,20 but the most commonly studied cations
are tetraalkylammonium (TAA) salts due to their synthetic
accessibility, inexpensive starting materials, and verified pracꢀ
tical membrane properties. TAA cations have demonstrated
appreciable alkaline stability, but remain susceptible to a varieꢀ
ty of chemical degradation pathways. These degradation
mechanisms include substitutions (SN2),21,22 eliminations
(E2),22,23 Stevens or Sommelet—Hauser rearrangements23 and
potentially, anionꢀinduced 1,4ꢀeliminations14 (Figure 1).
Several studies24,25 on polymerꢀbound TAA cations
have substantiated the occurrence of substitution reactions,26,27
but the contribution of Hofmann eliminations remains a matter
of discussion with little experimental evidence in AEM polyꢀ
mers. Stevens and Sommelet—Hauser rearrangements on benꢀ
zyltrimethylammonium (BTMA) cations have been demonꢀ
strated under alkaline aqueous conditions28 and in silico,29,30
but have not been experimentally observed in AEM materiꢀ
als.24 A possible explanation for this result may involve side
reactions in the presence of nucleophilic counterions.28
12
NH3 impurities in the fuel which can extend their applicabilꢀ
ity significantly over PEMꢀbased devices. In early work,
AEM devices showed low polarization performance, but new
AEMs have conductivities that rival PEMs.13,14 However, the
use of AEMs in a wide range of devices has been limited due
to the polymers’ poor stability in the highly alkaline electroꢀ
chemical environment within fuel cells that require the generaꢀ
tion of hydroxide anions and transport through the solid polyꢀ
mer membrane. Consequently, at higher temperatures, the
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a
Figure 1. Chemical degradation reaction mechanisms for a TAA small molecule under (i) alkaline aqueous conditions. A minor product
of this substitution reaction results in 1ꢀhexanol (not shown) as the dealkylation product.31 bA minor product of the Steven’s rearrangement
results in the [2,3]ꢀshift product (not shown).30 Anionꢀinduced 1,4ꢀeliminations (f) arise from the presence of an acidic hydrogen paired
with a good leaving group (e.g., TAA cation) in the highlighted configuration to generate a transient 1,4ꢀquinodimethane intermediate that
hydrolyses under aqueous conditions.
Finally, anionꢀinduced 1,4ꢀeliminations, a degradaꢀ
tion mechanism (seldom discussed in the context of AEM degꢀ
radation) is fundamentally allowed when BTMA analogues
(e.g., a TAA functionalized polystyrene) contain an acidic
hydrogen in ringꢀactivated positions that is electronically
paired with a leaving group (Figure 1f).32 Unfortunately, the
contribution of 1,4ꢀeliminations cannot easily be discerned
from benzyl substitutions under aqueous alkaline degradation
conditions without sophisticated isotopic labeling studies and
full analysis of the degradation products.33
interstitial nꢀalkyl spacers, most attempts at increased stability
of TAAꢀbased AEM polymers feature diamine crosslinkers or
terminal alkyl chains. Diamine crosslinkers and terminal spacꢀ
ers, while sterically shielding, generally contain βꢀhydrogen(s)
that may introduce the potential for Hofmann eliminations.
Interestingly, the specific contribution of Hofmann eliminaꢀ
tions to polymerꢀbound TAA cations, contrary to many claims,
has not been directly substantiated or compared to the relative
contributions of other possible degradation mechanisms.
Therefore, incisive chemical methods that provide robust idenꢀ
tification of reaction products bound to the polymer chain and
in the degradation solutions are required for measuring the
degradation products, rates, and resulting changes to the polyꢀ
mer structure with the aim of designing robust AEM polymers.
Our prior work26 has: (i) highlighted the utility of
small molecule analogs for the study of AEM polymer degraꢀ
dation, (ii) demonstrated a spectroscopic method to systematiꢀ
cally measure AEM alkaline stability under controlled and
reproducible test conditions, and (iii) substantiated the contriꢀ
bution of polymer backbones on alkaline stability. Furtherꢀ
more, an emphasis was placed on the importance of ranking
alkaline stability under harsh conditions where degradation is
clearly observed. The given suggestion was that degradation
could not be readily assessed unless there is significant reacꢀ
tion or conversion of the polymer or small molecule.
A recent review by Kreuer argued that claims about
base stable [TAA cations] are abundant, but so far, many reꢀ
sults have not been verified by independent studies.34 In addiꢀ
tion, Kreuer surmised that the steric shielding of cations with
(alkyl) spacers offers the most promising approach to increase
alkaline stability, which is likely the result of increasing hyꢀ
drophobicity or steric hindrance around the cationic center.35
Indeed these claims of enhanced alkaline stability have been
corroborated in AEM polymers with the implementation of: (i)
interstitial nꢀalkyl spacers between the polymer backbone and
nitrogen of the TAA,36,37 (ii) terminal nꢀalkyl pendants from
the nitrogen of the TAA,38 and (iii) crosslinking spacers adꢀ
joining TAA groups from two congruent polymer backꢀ
bones.39,40
Given the synthetic challenges of simple and effecꢀ
tive postꢀpolymerization chemical modifications to incorporate
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noted. Poly(2,6ꢀdimethylꢀ1,4ꢀphenylene oxide) (Aldrich),
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poly(styreneꢀcoꢀchloromethylstyrene) (Aldrich), benzyltrimeꢀ
thylammonium chloride (≥98.0%,), CD3OD (D, 99.8%) and
D2O (D, 99.9%) (Cambridge Isotope Laboratories), sodium
deuteroxide (40 wt. % in D2O, D, 99.5%), 1,4ꢀdioxane (anhyꢀ
drous, 99.8%, Aldrich), 1ꢀhexene (97%, Aldrich), trimethylaꢀ
mine (~45 wt. % in H2O, Aldrich), 4ꢀmethylbenzyl alcohol
(98%, Aldrich), 1ꢀhexanol (98%, Aldrich), benzyl alcohol
(99%, Alfa Aesar), 3ꢀphenylꢀ1ꢀpropanol (98%, TCI), N,Nꢀ
dimethylbenzylamine (≥99%, Aldrich), and all other reagents
were purchased commercially and were used as received unꢀ
less otherwise noted. Thinꢀlayer chromatography was carried
out on Dynamic Adsorbents silica gel TLC (20 × 20 w/h, Fꢀ
254, 250 µm). Deionized water was purified with a Millipore
purification system (Milli–Q UV Plus). High temperature
NMR samples were prepared in Wilmad quick pressure valve
tubes (5 mm, heavy wall, 300 MHz or better).
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Instrumentation
Proton nuclear magnetic resonance (1H NMR) spectra were
obtained on a Bruker DPXꢀ300. 1H NMR spectra of nonꢀ
degradation samples were recorded at 25 °C unless otherwise
specified. Proton chemical shifts are expressed in parts per
million (ppm, δ scale), and are referenced to tetramethylsilane
((CH3)4Si, 0.00 ppm) or to residual protium in the solvent
(CHCl3, 7.27 ppm; CD2HOD, 3.31 ppm; CD3SOCD2H, 2.50
ppm) or to an internal standard (1,4ꢀdioxane, 3.70 ppm). Data
are represented as follows: chemical shift, multiplicity (s =
singlet, d = doublet, t = triplet, q = quartet, m = multiplet
and/or multiple resonances), integration, and coupling constant
(J) in hertz.
Figure 2. Chemical structures of TAA small molecule AEM anaꢀ
logs and polymers studied.
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The AEM literature often claims good stability at low
temperatures (e.g., 60 °C) while often reporting poor device
stability. This contradiction identifies a clear disconnect beꢀ
tween membrane and device studies. To consolidate these isꢀ
sues, followꢀup studies have focused on methods that are speꢀ
cifically designed to cause the degradation of both the polymer
backbone and TAA cation architectures to identify leads for
advanced synthetic designs. Raising the temperatures in exꢀ
cess of 80 °C is needed to assess the intrinsic stability of AEM
structures. Additionally, lowering the dielectric constant of
the degradation medium appears to be critical in observing
functional group degradation in a reasonable time period.38,39
Processing
Acquisition data was processed using Bruker Topspin 3.2
software. Sample peak areas were assigned based on standards
and further normalized to the peak area of the internal standꢀ
ard. Stacked overlay visualizations are scaled to their normalꢀ
ized peak area values.
Herein, a comparison between the relative alkaline
stability and operative degradation mechanisms in BTMAꢀ and
various TAA samples are determined by 1H NMR at 120 °C in
the presence of excess NaOD in a 3:1 solution of CD3OD–
D2O. Three BTMAꢀfunctionalized compounds consisting of
pMeBTMA, BTMA-PS, and BTMA-PPO (Figure 2) were
evaluated. Additionally we examined compounds containing
nꢀalkyl terminal spacers of variable length: pMeBQA6, QA6-
PS, and QA6-PPO. Finally, we studied the degradation of
ArPrTMA as a small molecule analog containing an interstiꢀ
tial spacer and ArPrHxDMA that contains both an interstitial
and terminal spacer. The approach presented offers mechanisꢀ
tic insight into the contribution of the polymer backbones to
the stability of these materials and the effect of nꢀalkyl spacers
on the degradation of TAAꢀbased cations.
General Procedure for Sample Preparation and Data Ac-
quisition.
To a BTMA–PS sample (15.8 mg) was added CD3OD (1875
ꢁL), D2O (429.5 ꢁL) and a 1.8 M solution of 1,4ꢀdioxane (inꢀ
ternal standard) in D2O (41.5 ꢁL). The vial was capped and
vortexed until the polymer was completely dissolved ( 60 s).
To the resulting solution was added a 40% wt/v solution of
NaOD in D2O (140 ꢁL), and the vial was recapped and briefly
vortexed ( 5 s). The solution was evenly divided into two
heavyꢀwall NMR tubes (300 MHz, 5 mm, 7 in., max. pressure
205 psi) and fitted with a PTFE screw cap. The sample placed
in a ceramic collar and inserted into a 300 MHz NMR specꢀ
trometer equipped with a variableꢀtemperature probe preheated
to 60 °C. The sample was allowed to equilibrate (~5 min) and
the spectrometer programed to acquire an initial spectrum with
48 scans and a relaxation time of 5 s. Upon completion of the
acquisition, the sample NMR tube was removed from the specꢀ
trometer and inserted into a solidꢀstate aluminum heating
block preheated to 120 °C (see SI for specifications). After
each consecutive hour, the sample tube was removed from the
heating block, agitated by inversion (2 ×) and inserted into the
EXPERIMENTAL
Materials
All reactions were performed in flameꢀdried glassware under
positive pressure of nitrogen, unless otherwise noted. Airꢀ and
moistureꢀsensitive liquids were transferred by syringe or canꢀ
nula. Organic solutions were concentrated by rotary evaporaꢀ
tion (1–20 mmHg) at ambient temperature, unless otherwise
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spectrometer for additional acquisitions using identical paramꢀ %, 10 mL). The flask was sealed and was stirred for 24 h at 25
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eters to the initial spectrum.
°C. Excess trimethylamine was removed by bubbling nitrogen
gas through the product mixture for 3 h at 25 °C. The product
mixture was poured into a recrystillization dish and the solvent
removed in a vacuum (~1 torr) oven at 35 °C. The resulting
brittle film was removed to afford BTMA-PPO as yellow
Synthesis
pMeBTMA. To
a roundꢀbottom flask was added 4ꢀ
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methylbenzylbromide (1.0 g, 5.4 mmol, 1 equiv.) and tetrahyꢀ
drofuran (5 mL). The flask was cooled to 0 °C and an aqueous
solution of trimethylamine (~45 wt. %, 5 mL) was added. The
flask was warmed to 25 °C and was stirred for 12 h. The exꢀ
cess trimethylamine was removed by bubbling nitrogen gas
through the product mixture for 3 h at 25 °C. The product mixꢀ
ture was then concentrated under reduced pressure (0.1 torr) at
25 °C for 24 h to afford pMeBTMA as a white powder. H
NMR (CD3OD): δ 7.53 (d, 2H, J = 8.1 Hz), 7.34 (d, 2H, J =
7.7 Hz), 4.64 (s, 2H), 3.14–3.17 (s, 9H), 2.39 (s, 3H).
powder. H NMR (CDCl3): δ 6.63–6.70 (m), 6.46–6.51 (m),
4.33 (s), 2.08 (s).
9
QA6-PPO. To a round bottom flask equipped with a stir bar
was added poly(styreneꢀcoꢀchloromethylstyrene) (1.0 equiv.,
14.2 mmol, 2.0 g), dimethylacetamide (20 mL), deionized
water (4 mL), and N,Nꢀdimethylhexylamine (2.5 equiv., 35.5
mmol, 6.1 mL). The flask was sealed and stirred for 24 h at 25
°C. To the product mixture was added diethyl ether (40 mL)
and the organics washed with 0.1 M HCl (3 × 40 mL). The
organics were then washed with deionized water (2 × 20 mL)
and the organics collected and dried over sodium sulfate and
evaporated under reduced. The product residue was then furꢀ
ther dried under reduced pressure (0.1 torr) at 25 °C for 24 h to
afford the chloride salt of QA6-PPO as a tacky yellow crystalꢀ
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pMeBQA6. To a roundꢀbottom flask equipped with a stir bar
was added 4ꢀmethylbenzylbromide (1.0 equiv., 1.35 mmol,
250 mg), N,Nꢀdimethylhexylamine (1.0 equiv., 1.35 mmol,
0.45 mL) and THF (1.35 mL). The flask was sealed and stirred
for 24 h at 25 °C. The residual solvent was evaporated from
the crude product mixture under reduced pressure followed by
the addition of ethyl acetate (10 mL) and the organics were
washed with 0.1 M HCl (2 × 10 mL). The organics were
washed with deionized water (2 × 10 mL) and the organics
collected and dried over sodium sulfate and evaporated under
reduced. The product residue was then further dried under
reduced pressure (0.1 torr) at 25 °C for 24 h to afford the chloꢀ
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line solid. H NMR (DMSOꢀd6): δ 6.50–6.97 (bm, 2H), 4.36
(bs, 2H), 3.17 (bm, 2H), 2.96 (bs, 6H), 2.02 (bm, 2H), 1.62
(bm, 2H), 1.21 (bm, 6H), 0.80 (bs, 3H).
ArPrHxDMA. To a round bottom flask equipped with a magꢀ
netic stir bar was added 1ꢀbromoꢀ3ꢀphenylpropane (1.0 equiv.,
13.2 mmol, 2.0 mL), tetrahydrofuran (10 mL), and N,Nꢀ
dimethylhexylamine (3.0 equiv., 39.4 mmol, 6.85 mL). The
flask was sealed and stirred for for 24 h at 25 °C. The residual
solvent was evaporated from the crude product mixture under
reduced pressure followed by the addition of ethyl acetate (10
mL) and the organics were washed with 0.1 M HCl (2 × 10
mL). The organics were washed with deionized water (2 × 10
mL) and the organics collected and dried over sodium sulfate
and evaporated under reduced. The product residue was then
further dried under reduced pressure (0.1 torr) at 25 °C for 24
h to afford the chloride salt of ArPrHxDMA as white foam.
1H NMR (CD3OD:D2O (3:1)): δ 7.27–7.31 (m, 5H), 3.05 (s,
6H), 2.71 (m, 2H), 2.06 (m, 2H), 1.64 (m, 2H), 1.34 (m, 6H),
0.93 (t, 3H).
1
ride salt of pMeBQA6 as a clear viscous liquid. H NMR
(CD3OD): δ 7.43 (d, 2H, J = 10.8), 7.34 (d, 2H, J = 7.2), 4.50
(s, 2H), 3.28 (m, 2H), 3.02 (s, 6H), 2.41 (s, 3H), 1.86 (m, 2H),
1.40 (m, 6H), 0.93 (t, 3H, J = 7.2).
BTMA-PS. To a roundꢀbottom flask was added poly(styreneꢀ
coꢀchloromethylstyrene) (2.0 g), tetrahydrofuran (10 mL), and
an aqueous solution of trimethylamine (~45 wt. %, 10 mL).
The flask was sealed and was stirred for 24 h at 25 °C. The
excess trimethylamine was removed by bubbling nitrogen gas
through the product mixture for 3 h at 25 °C. The product mixꢀ
ture was then concentrated under reduced pressure (0.1 torr) at
25 °C for 24 h to afford the chloride salt of BTMA-PS as a
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white translucent film. H NMR (CD3OD): δ 6.56–7.20 (m),
QA6-PS. To a roundꢀbottom flask was added poly(styreneꢀcoꢀ
chloromethylstyrene) (2.0 g), dimethylacetamide (20 mL),
deionized water (4 mL), and N,Nꢀdimethylhexylamine (5.0
equiv., 71 mmol, 12.3 mL). The flask was sealed and was
stirred for 24 h at 25 °C. To the product mixture was added
diethyl ether (40 mL) and the organics washed with 0.1 M HCl
(3 × 40 mL). The organics were then washed with deionized
water (2 × 20 mL) and the organics collected and dried over
sodium sulfate and evaporated under reduced pressure. The
product residue was then further dried under reduced pressure
(0.1 torr) at 25 °C for 24 h to afford the chloride salt of QA6ꢀ
PS as a white foam. 1H NMR (CD3OD:D2O (3:1)): δ 6.47–7.22
(bm, 4H), 4.53 (bs, 2H), 3.26 (bm, 2H), 2.86 (bs, 6H), 1.76
(bm, 2H), 1.27 (bm, 6H), 0.85 (bm, 3H).
4.56 (br s), 3.05 (br s), 1.0–2.5 (m).
BTMA-PPO. To a roundꢀbottom flask was added poly(2,6ꢀ
dimethylꢀ1,4ꢀphenylene oxide) (15 g, 125 mmol) and chloroꢀ
benzene (120 mL, 1.04 M) until the polymer was completely
dissolved. To the flask was added Nꢀbromosuccinimide (11.1
g, 62.5 mmol) and 2,2’ꢀazobisꢀisobutyronitrile (0.63 g, 3.75
mmol). The flask was fitted with a reflux condenser and heated
to 125 °C for 3 h. The product mixture was cooled to 25 °C
and slowly poured into a flask of stirring ethanol (~1 L) at
room temperature to precipitate the product residue. The prodꢀ
uct residue was filtered and washed with ethanol (2 × 200
mL). The product residue was collected and dissolved in chloꢀ
roform (150 mL) and then reprecipitated in ethanol (~1 L).
The product was collected and dried under reduced pressure
(0.1 torr) for 12 h to afford a yellow powder. The aforemenꢀ
tioned yellow powder precursor (2.0 g) was added to a roundꢀ
bottom flask to the flask was added N,Nꢀdimethylacetamide
(10 mL) and an aqueous solution of trimethylamine (~45 wt.
ArPrTMA. To a round bottom flask equipped with a magnetic
stir bar was added 1ꢀbromoꢀ3ꢀphenylpropane (1.0 equiv., 13.2
mmol, 2.0 mL), tetrahydrofuran (10 mL).
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Figure 3. Rates of degradation and offset H NMR spectral overlays of pMeBTMA and pMeBQA6 as (i) 30 mM solutions in CD3OD–
D2O (3:1), 1,4–dioxane (int. std.), and NaOD (20 equiv., 0.6 M) from 0–3 h shown in hourly intervals at 120 °C. QAꢀPPO displayed turꢀ
bidity at 3 h hours and was not included in our analyses. Mechanism abbreviations are benzyl substitution (SN2Bn), dealkylation (SN2NMe),
Hofmann elimination (E2), nonꢀspecific (N–S); the difference in decay between the TAA cation and the sum of observed byproducts.
The flask was cooled to 0 °C and an aqueous solution of trimeꢀ
thylamine (~45 wt. %, 5 mL) was added. The flask was
warmed to 25 °C and was stirred for 24 h. The excess trimeꢀ
thylamine was removed by bubbling nitrogen gas through the
product mixture for 3 h at 25 °C. The product mixture was
then concentrated under reduced pressure (0.1 torr) at 25 °C
for 24 h to afford ArPrTMA as a white foam. 1H NMR
(CD3OD:D2O (3:1)): δ 7.24–7.35 (m, 5H), 3.36 (m, 2H), 3.15
(s, 9H), 2.78 (m, 2H), 2.18 (m, 2H).
RESULTS AND DISCUSSION
The products resulting from the degradation of TAA
cations are extensive but were simplified by considering only
the degradation products that were most likely to occur. For
instance, the absence of βꢀhydrogens for benzyltrimeꢀ
thylammonium samples analyzed precludes Hofmann eliminaꢀ
tions (E2). Likewise, the presence of an interstitial spacer limꢀ
its the possibility of Sommelet–Hauser and Stevens rearꢀ
rangements in ArPrHxDMA. Equally important is the inabilꢀ
ity to distinguish between anionꢀinduced 1,4ꢀeliminations (in
all samples excluding PPO where this mechanism is not possiꢀ
ble)
and
benzyl
substitution.
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Figure 4. (a) Degradation rates and (b) degradation kinetics for BTMA-PS, QA6-PS, BTMA-PPO, and QA6-PPO as 30 mM solutions in
CD3OD–D2O (3:1), 1,4–dioxane (int. std.), and NaOD (20 equiv., 0.6 M) from 0–3 h shown in hourly intervals at 120 °C. Mechanisms
depicted are benzyl substitution (SN2Bn), dealkylation (SN2NMe), Hofmann elimination (E2), nonꢀspecific (N–S).
For this reason, the shared products of these degradation pathꢀ
ways were strictly treated as substitution (SN2Bn or SN2NMe) at
the benzyl position.
the operative mechanism in both samples followed by dealkylꢀ
ation (SN2NMe) to a much lesser extent. Interestingly, the presꢀ
ence of Hofmann elimination (E2) in pMeBQA6 is nearly
insignificant (t = 3 h, 4.0%). Degradation was assessed using
the aliphatic 1H NMR regions displaying the characteristic
products of degradation consisting of trimethylamine, pꢀ
methylꢀN,Nꢀdimethylbenzylamine, and pꢀ methylbenzyl alcoꢀ
hol for pMeBTMA (Figure 3b) and N,Nꢀdimethylhexylamine,
and pꢀmethylꢀNꢀmethylꢀNꢀhexylbenzylamine for pMeBQA6
(Figure 3c). The occurrence of Hofmann elimination was deꢀ
termined by the unsaturated protium of 1ꢀhexene (not shown).
Any decay of starting TAA spectral peaks without
corresponding products of degradation were categorized as
nonꢀspecific (NꢀS) degradation. Additionally, in selected exꢀ
periments precipitation and/or turbidity resulted during the
heating process (e.g., QA6ꢀPPO). In such instances, an analyꢀ
sis was conducted only on spectra wherein the sample reꢀ
mained soluble.
To aid in the interpretation of our results, we conꢀ
ducted small molecule 1H NMR reference experiments on
standards under analogous alkaline solution conditions. These
standards consisted of commerciallyꢀavailable or synthesized
small molecule analogs of the major degradation products.
The comparison between pMeBTMA and
pMeBQA6 small molecules was designed to approximate the
degradation kinetics and product distribution of their respecꢀ
tive poly(styrene) macromolecules. pMeBTMA displayed a
reduced loss of the TAA concentration (38.6% loss) compared
to 55.7% loss of TAA for pMeBQA6 at 3 h (Figure 3a) with
both samples experiencing similar degradation from benzyl
substitution (SN2Bn) and dealkylation (SN2NMe). Furthermore,
the degradation product distributions for pMeBTMA indicated
by Hf (2.25–2.33 ppm) and Hg (2.11–2.25 ppm) (Figure 3b),
suggests the increase in degradation of pMeBQA6 does not
result from Hofmann elimination. Figure 3d shows the kinetic
profiles of pMeBTMA and pMeBQA6 with half lives of 267
min and 151 min, respectively, a significantly reduction over
studies conducted at 80 °C.
1
However in some cases, where H NMR spectral analyses disꢀ
cerned only one byproduct of a specific degradation pathway
due to overlapping peaks, the discernable mechanismꢀspecific
product was used for the determination of the operative degraꢀ
dation mechanism.
Influence of Terminal N-Alkyl Pendants on Benzyl-Linked
Cations
The incorporation of terminal nꢀalkyl pendants in
TAAs has been identified as a facile route to achieve alkaline
stability in cast AEMs.38 In solution studies on small moleꢀ
cules and polymers, we observed that pendant nꢀalkyl chains
decreased the stability of benzyldimethyl nꢀalkyl ammonium
groups compared to BTMA analogs. Shown in Figure 3a is the
contribution of each degradation mechanism to the overall
degradation of the TAA group in pMeBTMA and pMeBQA6
in a solution of 3:1 CD3OD–D2O with 20 equivalents of NaOD
at 120 °C.
The comparative degradation between our dissolved
pMeBTMA and pMeBQA6 samples were contrary to the
increased stability of immersed solid membrane samples reꢀ
ported reported by Li, et al. in poly(phenylene oxides) (PPO)
with terminal nꢀalkyl pendants,43 and Pan, et al. with backꢀ
boneꢀappended alkyl chains.44 In those membrane studies, reꢀ
duced solvation and tertiary structural effects (e.g., phase sepaꢀ
ration) limit interactions of the tetraalkylammonium cation
with the surrounding alkaline media thus promoting stability in
nꢀalkylated pendant functionalized AEMs in the solid state.
The progressive summation of the degradation of the
initial compound indicates that benzyl substitution (SN2Bn) is
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Figure 5. (a) Degradation rates for ArPrTMA and ArPrHxDMA as 30 mM solutions in either CD3OD–D2O (3:1) (i) or D2O (ii), 1,4–
dioxane (int. std.), and NaOD (20 equiv., 0.6 M) from 0–3 h shown in hourly intervals at 120 °C. (b) Degradation kinetics and half lives for
ArPrTMA and ArPrHxDMA. Mechanisms depicted are dealkylation (SN2NMe), aliphatic methylene substitution (SN2Aliph).
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In the presented solution studies whereby our samꢀ
ples remain soluble in a medium of reduced dielectric polarity,
a more fundamental analysis of the reaction mechanism is
fitting where tertiary structural effects such as phase separaꢀ
tion are nonexistent. In the simplest definition, benzyl substiꢀ
tution and dealkylation are reactions that proceed via an elecꢀ
tronꢀdeficient carbon intermediate. In the ground state, carbon
atoms will have a partial positive charge due to the ionic charꢀ
acter in the Cα–N+R3 bond.45
ty and stabilization of the electronꢀdeficient benzyl intermediꢀ
ate leading to an increase in the observed degradation over
that of pMeBTMA.
Consistent with the results of pMeBTMA and
pMeBQA6 small molecule analogs, our poly(styrene) sample,
QA6-PS, experienced a higher loss of its TAA functionality
over BTMA-PS, albeit at a much higher rate of 92% loss after
3 h at 120 °C, respectively (Figure 4a). Poly(phenylene oxꢀ
ide)ꢀbased samples were surprisingly more resistant to alkaline
degradation; BTMA-PPO lost 66.9% of its TAA groups after
3 hours while QA6-PPO was observed to precipitate from
solution at 3 h, but yielded a 2 h degradation of 62.9%. Interꢀ
estingly the increased stability of BTMA-PPO over BTMA-
PS with respective half lives of 120 min and 56.8 min is in
distinct contrast with our degradation observations at 80 °C
wherein the poly(styrene) derivative (QAꢀPS) was found to
degrade less than its poly(phenylene oxide) analog (QAꢀPPO)
(Figure 4b).28
If the amount of positive charge on the αꢀcarbon of
the TAA (i.e., benzyl or methyl, or methylene in nꢀalkyl
chains) in the transition state is greater than in the ground
state, then substituents which can stabilize the positive charge
with throughꢀspace fieldꢀ (ionꢀdipole interaction energy and
formal charge), inductiveꢀ (hyperconjugative electron withꢀ
drawal by tetraalkylammonium cation), and/or resonance (σ–π
bond overlap with benzene ring) effects will cause a rate acꢀ
celeration.31 However, pMeBTMA and pMeBQA6 are equalꢀ
ly affected by these three factors, yet the latter experiences
greater degradation.
In comparison to our previous work that employed
KOD at 80 °C,26 this study utilizes NaOD at 120 °C and eleꢀ
vated pressures (>13 bar) within our sealed NMR vessel.
Temperature and pressure differences can often lead to small
but consistent changes in relative kinetic rates. Our observaꢀ
tions indicate that structural variation(s) (e.g., activation volꢀ
umes, transition states, sterics, etc.,)51 are introduced under our
conditions at 120 °C that result in the observed differences in
relative rates between PSꢀ and PPOꢀbased samples from those
conducted at 80 °C.
A study on benzyl leaving groups such as benzyltriꢀ
alkylammonium salts, in aqueous and nonꢀaqueous solvents,
found that hydration numbers significantly increase with longꢀ
er nꢀalkyl chain lengths.46 Hydration is reported to lead to
greater stabilization of the associative/dissociative character of
the degradation pathway’s transition state.47,48 Additionally,
the introduction of a βꢀcarbon capable of extended hyperconꢀ
jugative donation (i.e., between Cβ–CαN+R3) as the case with
pMeBQA6, results in a longer CBn–N+R3 bond that more
closely approximates the transitionꢀstate structure and faciliꢀ
tates faster solvolytic degradation kinetics.48–50
Influence of N-Alkyl Interstitial Spacers and Dielectric
Effects on Cation Degradation
We thus postulate that the observed increase in degꢀ
radation of pMeBQA6 compared to pMeBTMA results from
these solvent effects and a greater polarizable CBn–N+R3 bond.
That is, the larger and more structured hydration of the longer
nꢀalkyl chain of the pMeBQA6 cation likely aids the solubiliꢀ
In both small molecules and polymers, substitution at
the benzyl position was the obvious major degradation route.
Therefore the effect of interstitial nꢀalkyl spacers between the
aryl ring and the trimethylammonium cation was investigated.
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This strategy has shown promise in the literature36,37 and conꢀ
firmation of these previous membrane studies with an incisive
small molecule measurement was justified.
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2
Pivovar, et al.29 showed that the background dielecꢀ
tric constant for the benzyl substitution of BTMA has a large
effect on the reaction barrier. We observe this dielectric conꢀ
stant effect in our experiments by changing the solvent form
pure D2O to a 3:1 CD3OD:D2O mixture. The varying rates of
degradation in D2O and CD3OD:D2O can be explained by an
analysis of the respective dielectric constants of the solutions.
The dielecric constant of the D2O mixture with NaOD is ~72
at 25 °C and ~42 at 120 °C. This type of solution study is
reasonable because the total dielectric constant of a hydrated
polymer has been shown to be between 10–30 depending on
the water content of the system.52
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In Figure 5, the alkaline degradation of ArPrTMA
in pure D2O and in a 3:1 CD3OD:D2O mixture is shown. With
the inclusion of a three carbon interstitial spacer, ArPrTMA,
loses 11.6% of its TAA after 3 h in CD3OD:D2O to substituꢀ
tional displacement (SN2Aliph) of the trimethylamine and
dealkylation (SN2NMe). In D2O, ArPrTMA degrades 3.3%
after 3 h with substitutional displacement of the trimethylaꢀ
mine alone.
Figure 6. Alkaline degradation of pMeBTMA and ArBTMA as
30 mM solutions in CD3OD–D2O (3:1), 1,4–dioxane (int. std.),
and NaOD (20 equiv., 0.6 M) from 0–3 h shown in hourly interꢀ
vals at 120 °C. Mechanism abbreviations are benzyl substitution
(SN2Bn), dealkylation (SN2NMe), Hofmann elimination (E2), nonꢀ
specific (N–S); the difference in decay between the TAA cation
and the sum of observed products.
When both a three carbon interstitial spacer and a
terminal chain are added in the case of ArPrHxDMA, an adꢀ
ditive effect on the stability is observed as the TAA loss totals
just 6.6% from dealkylation alone. In D2O with NaOD, the
degradation of ArPrHxDMA is expectedly less yielding a
combined 2.7% degradation to substitutional displacement and
dealkylation.
possibility of a parallel mechanism concerning the elimination
of trimethylamine in an anionꢀinduced 1,4ꢀelimination on a
styrenic small molecule AEM analog. Specifically, we chose
to compare pMeBTMA that contains a pꢀmethyl group that
operates as an extension of the πꢀbond system of the benzene
ring in the form of a 1,4ꢀquinodimethane or similarly, an exꢀ
tended hyperconjugated resonance hybrid, to a small molecule
without this effect, ArBTMA.
The addition of an interstitial nꢀalkyl spacer has a
pronounced effect on the stability of both ArPrHxDMA and
ArPrTMA compounds. For both samples, the removal of an
αꢀCBn to both a benzene ring and TAA cation, eliminated the
pronounced σ–π bond resonance effect that stabilizes the elecꢀ
tronꢀdeficient carbon intermediate of the SN2 reaction as disꢀ
cussed with pMeBTMA and pMeBQA6. The terminal nꢀalkyl
pendant in ArPrHxDMA yielded marginal gains in stability
compared to the nꢀinterstitial spacer of ArPrTMA alone. The
greater effective steric bulk of the nꢀalkyl pendant of
ArPrHxDMA likely opposes the close approach of polar
solvated nucleophiles leading to marginal gains in stability,
but overall the interstitial spacer was observed to have the
greatest impact on alkaline stability.
In Figure 6, the alkaline degradation of ArBTMA
resulted in a TAA loss of 20.3% after 3 h compared to a sigꢀ
nificantly higher degradation rate for pMeBTMA of 38.6%.
This result is intriguing if only the electronꢀdonating properꢀ
ties of a paraꢀmethyl group on an aromatic ring are considered.
Inductive donation should yield a greater partial negative
charge on the benzyl carbon and therefore result in a reduction
of electrophilicity. However, when considering the partial
positive charge on the benzyl carbon (CBn) that results from
the withdrawing effect of the ammonium, then any substituent
that can stabilize the positive charge by resonance or inductive
effects will cause a rate acceleration.45 Distinguishing between
the relative contributions of hyperconjugated resonance strucꢀ
tures or the formation of intermediates such as the 1,4ꢀ
quinododimethane that results from anionꢀinduced 1,4ꢀ
eliminations remains difficult considering that both events
result in the products of SN2Bn.
Although not experimentally evaluated in this manuꢀ
script, kinetic isotope studies on the elimination reaction (E2)
of 2ꢀphenylethyltrimethylammonium ions (i.e., a small moleꢀ
cule containing a two carbon interstitial spacer) was found to
lay so far on the side of the products, styrene and trimethylaꢀ
mine, that measuring equilibrium constants at 40 °C in EtO–
/EtOH presented a challenge.53 This propensity for degradaꢀ
tion is not expected in structures containing ≥ 3 carbon interꢀ
stitial spacers, and thus we recommend this approach for new
AEM synthetic designs.
Studies conducted at 80 °C concluded that QA-PS is
less susceptible to alkaline degradation than QA-PPO,26 but
the opposite trend was observed for BTMA-PS and BTMA-
PPO at 120 °C. Based on these results, we postulate the inꢀ
crease in thermal energy at 120 °C may be sufficient to overꢀ
come the energy barrier necessary to activate the 1,4ꢀ
Evidence for Contribution of 1,4-eliminations to
Poly(styrene) Analogs
Prior work conducted on selfꢀimmolative systems
that generate 1,4ꢀquinodimethane analogs33 invoked the
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(2) Robertson, N. J.; Kostalik, H. A., IV; Clark, T. J.; Mutolo, P. F.;
elimination mechanism or increase the effects of extended
hyperconjugated structures in accelerating the degradation of
the corresponding BTMA-PS macromolecule.
Abruña, H. D.; Coates, G. W. Tunable High Performance Crossꢀ
Linked Alkaline Anion Exchange Membranes for Fuel Cell Applicaꢀ
tions. J. Am. Chem. Soc. 2010, 132, 3400–3404.
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Exchange Membranes in Low Temperature Fuel Cells. Fuel Cells
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CONCLUSIONS
In summary, a high temperature 1H NMRꢀbased
method to study AEM polymers and analogs under high temꢀ
perature conditions was introduced that ensures their degradaꢀ
tion for meaningful comparisons. The findings indicate that
BTMA cations and benzyldimethylammonium analogs with nꢀ
alkyl terminal pendants are significantly more susceptible to
degradation than TAA cations containing an nꢀinterstitial
spacer. Further but minor enhancements in alkaline stability
were observed when nꢀalkyl interstitial and nꢀalkyl pendant
functionalities were combined on a single cation.
(4) Kruusenberg, I.; Matisen, L.; Shah, Q.; Kannan, A. M.; Tamꢀ
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The study presented suggests that an AEM
poly(phenylene oxide) is more resistant to alkaline degradaꢀ
tion at 120 °C than an AEM poly(styrene) polymer; a reversal
of stability observed at 80 °C in a previous study. A follow up
study with AEM poly(styrene) small molecule analogs sugꢀ
gests that an anionꢀinduced 1,4ꢀelimination degradation mechꢀ
anism is activated at 120 °C.
These conclusions have potential consequences for
BTMA containing poly(styrene)ꢀbased polymers in AEMFC
highꢀtemperature applications. Although more studies are
needed to confirm the findings and given the added stability
shown by the addition of the addition of nꢀalkyl interstitial
spacers, it is suggested BTMAꢀpoly(styrene) polymers are
replaced with more robust polymer backbones and chemistries
are developed for synthesizing AEMs with nꢀinterstitial spacꢀ
ers.
Supporting Information
The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org.
Heating block design and 1H NMR degradation specꢀ
tra of pMeBTMA, QAꢀPPO, pMeBQA6, QA6ꢀPS,
QAꢀPS, ArBTMA, ArPrTMA, ArPrHxDMA, and
QA6ꢀPPO. H NMR characterization of pMeBQA6,
pMeBTMA, ArPrHxDMA, BTMAꢀPS, QA6ꢀPS,
ArPrTMA, ArBTMA, BTMAꢀPPO, and QA6ꢀPPO.
1
Corresponding Author
*Eꢀmail: hickner@matse.psu.edu
Acknowledgments
This work was funded by the Advanced Research Projects
Agency – Energy (ARPAꢀE), U.S. Department of Energy,
under award number: DEꢀAR0000121, the United Statesꢀ
Israel Binational Science Foundation (BSF) through Energy
Project no.2011521, and NSF DMREF program under award
number CHEꢀ1534326.
References
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http://energy.gov/eere/fuelcells/fuelꢀcellꢀtechnologiesꢀoffice (acꢀ
cessed May 22, 2014).
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“for Table of Contents use only”
NꢀAlkyl Interstitial Spacers and Terminal Pendants Influence
the Alkaline Stability of Tetraalkylammonium Cations for
Anion Exchange Membrane Fuel Cells
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Chemistry of Materials
Sean A. Nuñez, Clara Capparelli, and Michael A. Hickner
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