Phosphonium-functionalized polyethylene: A new class of base-stable alkaline anion exchange membranes

Article abstract of DOI:10.1021/ja307466s

A tetrakis(dialkylamino)phosphonium cation was evaluated as a functional group for alkaline anion exchange membranes (AAEMs). The base stability of [P(N(Me)Cy)₄]⁺ was directly compared to that of [BnNMe₃]⁺ in 1 M NaOD/CD₃OD. The high base stability of [P(N(Me)Cy)₄]⁺ relative to [BnNMe ₃]⁺ inspired the preparation of AAEM materials composed of phosphonium units attached to polyethylene. The AAEMs (hydroxide conductivity of 22 ± 1 mS cm^-1 at 22 °C) were prepared using ring-opening metathesis polymerization, and their stabilities were evaluated in 15 M KOH at 22 °C and in 1 M KOH at 80 °C.

Full text of DOI:10.1021/ja307466s

Communication
pubs.acs.org/JACS
Phosphonium-Functionalized Polyethylene: A New Class of Base-
Stable Alkaline Anion Exchange Membranes
Kevin J. T. Noonan, Kristina M. Hugar, Henry A. Kostalik, IV, Emil B. Lobkovsky, Hector D. Abruna,
́
̃
and Geoffrey W. Coates*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States
S
* Supporting Information
cell operating conditions remains a concern.11 This has sparked
ABSTRACT: A tetrakis(dialkylamino)phosphonium cati-
on was evaluated as a functional group for alkaline anion
exchange membranes (AAEMs). The base stability of
[P(N(Me)Cy)4]+ was directly compared to that of
[BnNMe3]+ in 1 M NaOD/CD3OD. The high base
stability of [P(N(Me)Cy)4]+ relative to [BnNMe3]+
inspired the preparation of AAEM materials composed
of phosphonium units attached to polyethylene. The
AAEMs (hydroxide conductivity of 22 1 mS cm−1 at 22
°C) were prepared using ring-opening metathesis polymer-
ization, and their stabilities were evaluated in 15 M KOH
at 22 °C and in 1 M KOH at 80 °C.
investigations of other cationic species appended to polymeric
supports for use in alkaline membrane fuel cells (AMFCs),
including guanidinum12 and imidazolium13 cations. Holdcroft
and co-workers recently reported a sterically crowded
benzimidazolium polymer that exhibits excellent base stability
when heated to 60 °C in 2 M KOH over a 13 day period.14
Tetraalkylphosphonium cations represent another exciting class
of functional groups for AAEMs, as the synthesis of
phosphonium ionomers has already been established.15 Both
the benzyltrimethylphosphonium ([BnPMe3])16 and benzyltris-
(2,4,6-trimethoxyphenyl)phosphonium17 cations are being
evaluated in AAEM materials. While the tetraalkylphosphonium
cation ([PR4]+) is attracting attention in AAEMs, the
tetrakis(dialkylamino)phosphonium cation ([P(NR2)4]+) is
also of interest.18,19 Recent reports have shown the exceptional
uel cell devices are currently being investigated for a variety
Fof applications, as they efficiently convert the chemical stability of [P(NR2)4]OR′ compounds (R′ = H, alkyl), and
their applications in phase transfer,19 transesterification,20 and
energy stored in a fuel (e.g., H2 or CH3OH) directly into
polymerization21 reactions have been demonstrated. Herein we
report a new class of hydroxide ion exchange membranes that
consist of a tetrakis(dialkylamino)phosphonium cation ap-
pended to polyethylene. These materials exhibit excellent
stability in strongly basic solution.
electrical energy.1 One type of fuel cell that has been
investigated extensively is the so-called proton exchange
membrane fuel cell (PEMFC). PEMFCs operate at relatively
low temperatures (50 to 100 °C) with high efficiencies and
consist of an ionically conducting polymeric material pressed
between the cathode and anode to provide ion conduction and
electrical insulation. PEMFCs operating under acidic conditions
commonly employ Nafion, a perfluorinated polymer with
pendant sulfonic acid groups that facilitate the flow of protons.2
Unfortunately, the oxygen reduction reaction is rate-limiting for
acidic fuel cells, and the most commonly employed cathode
materials are based on platinum and its alloys.1 The high cost of
platinum has led to investigations into fuel cells that operate
under alkaline conditions, since less expensive metals, such as
silver, can be used as the cathode catalyst.3 Consequently,
researchers have attempted to prepare alkaline anion exchange
membranes (AAEMs) with stability and conductivity similar to
those of Nafion.4
The [BnNMe3]+ cation has been commonly employed in
AAEMs. As a result, we chose to compare its base stability to
that of the bulky [P(N(Me)Cy)4]+ cation. Since [BnNMe3]OH
exhibits negligible degradation over 29 days in 1 M NaOH in
water at 80 °C,10f the stability in methanol was explored
because this solvent (1) accelerates cation degradation, (2)
often dissolves polyatomic cations better than water, and (3) is
applicable to direct methanol fuel cells (DMFCs).22
In a sealed fluoropolymer-lined vessel, [BnNMe3]Br (0.1 M)
and NaOD (1 M) were combined in CD3OD at 80 °C. This
resulted in 66% degradation of the BnNMe3 cation after 20
days (Figure 1).22 In solution, [BnNMe3]+ seems to degrade
primarily by nucleophilic attack at either the benzylic or methyl
1
positions (as confirmed by H NMR and GC−MS analyses).
Ideal AAEMs would be mechanically and chemically robust,
exhibit good hydroxide ion conductivity, and display limited
swelling. Various polymer backbones have been investigated to
date, including hydrocarbon polymers prepared via ring-
opening metathesis polymerization (ROMP),5 fluoropolymers,6
polysulfones,7 polyarylenes,8 and poly(ether−imide)s.9 These
polymeric supports contain pendant ammonium cations to
facilitate hydroxide ion conduction from the cathode to the
anode. However, the degradation pathways for ammonium
cations under alkaline conditions have been well-documented,10
and the long-term stability of the ammonium cation under fuel
Deuterium scrambling was also observed at the benzylic and N-
methyl positions in the degradation products, suggesting
reversible formation of a nitrogen ylide species in solution. In
stark contrast, when [P(N(Me)Cy)4]BF4 (0.1 M) and NaOD
(1 M) were dissolved in CD3OD, no degradation of
[P(N(Me)Cy)4]+ was observed over a 20 day period at 80
1
°C (as measured by H and 31P NMR spectroscopy). This
Received: August 2, 2012
Published: September 27, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
[1.636(1) Å].19 The anionic methoxide is hydrogen-bonded
to three methanol molecules.25 The short O−O separations
between the neighboring methanol molecules and the
methoxide anion of 1 (2.576−2.636 Å) suggest relatively
strong hydrogen bonding; a typical O−O distance for a
hydrogen-bonded system is 2.75 Å. These strong O−H
contacts provide significant energetic stabilization to the
reactive methoxide anion.26
The stability of [P(N(Me)Cy)4]BF4 in 1 M NaOD/CD3OD
and the preparation of crystalline 1 suggested that these
delocalized phosphonium cations might be suitable for AAEM
applications. Although there have been previous reports
describing polymers with appended dialkylaminophospho-
niums,18,27 none have been examined under fuel-cell-relevant
conditions. This prompted us to target a polyethylene-based
phosphonium ionomer.
Figure 1. Stability of [BnNMe3]+ and [P(N(Me)Cy)4]+ in 1 M
NaOD/CD3OD at 80 °C.
stability under basic conditions in methanol is noteworthy and
suggests that the tetrakis(dialkylamino)phosphonium cation
may be a promising functional group choice for AAEM fuel
cells, especially DMFCs.
To investigate further the stability of the tetrakis-
(dialkylamino)phosphonium cation, we sought to synthesize
and characterize a discrete alkoxide species. [P(N(Me)Cy)4]-
BF4 was combined with KOH in methanol; precipitation and
filtration of KBF4 followed by removal of the solvent afforded
crystals suitable for X-ray diffraction. The solid-state molecular
structure of [P(N(Me)Cy)4][OMe]·3MeOH (1) is displayed
in Figure 2. Our ability to obtain this crystal structure further
Our group has previously reported two separate cross-linked
polymer networks bearing pendant ammonium groups as
potential AAEMs.5c,d The ammonium groups were appended to
norbornyl and cyclooctenyl derivatives, and the functionalized
monomers were polymerized via ROMP. The commercially
available Grubbs second-generation catalyst (Ru-cat) was
employed since it is tolerant of quaternary ammonium
halides.5,28 The synthesis of non-cross-linked AAEMs has also
been achieved using ROMP.5b A cyclooctenyltrimethylammo-
nium monomer was copolymerized with cyclooctene in the
presence of Ru-cat to afford an unsaturated copolymer.
Hydrogenation of this material yielded a solution-processable
and mechanically strong ammonium-functionalized polyethy-
lene [hydroxide conductivity at 50 °C (OH− σ50) = 65 mS
cm−1]. Using a similar strategy, a tetrakis(dialkylamino)-
phosphonium-functionalized cyclooctene was targeted and
prepared in six steps from 5-hydroxy-1-cyclooctene (2)
(Scheme 1).29 Mesylation of 2 followed by reaction with
a
Scheme 1. Phosphonium Monomer Synthesis
a
Reagents and conditions: (a) 1.2 equiv of MsCl, pyridine, 0 °C; (b)
1.5 equiv of NaN3, DMSO, 50 °C; (c) 5 equiv of PCl3, toluene, 70 °C;
(d) CH2Cl2, 0 °C, followed by workup with 1 M NaBF4; (e) 5 equiv of
Me2SO4, 40 equiv of NaOH, H2O, chlorobenzene, 70 °C; (f) chloride
anion exchange resin.
Figure 2. Molecular structure of 1, with thermal ellipsoids set at 40%
probability.
documents the unique stability of the phosphonium cation,
since very few other organic cations are known to be stable
enough to crystallize with a methoxide anion. Two other
reports describe salts of organic cations with methoxide. Love
and co-workers reported a tetraprotonated polypyrrole macro-
cycle crystallized with three tosylate counterions and one
methoxide counterion,23 and Ou and co-workers described an
ion pair in which a hydrogen bond exists between an NH3
moiety and the anionic methoxide.24
NaN3 afforded 5-azido-1-cyclooctene (4). A Staudinger
reaction yielded iminophosphorane 5, whose immediate
reaction with cyclohexylamine produced phosphonium salt 6.
Methylation conducted using standard phase-transfer protocols
yielded the tetrakis(dialkylamino)phosphonium cation (7)
(Scheme 1). Monomer 7 was copolymerized with cyclooctene
(COE) in the presence of Ru-cat in chloroform for 18 h
1
(Scheme 2). H NMR spectroscopy of the reaction mixture
indicated that COE was incorporated into the growing polymer
chain more readily than 7. Thus, this polymerization strategy
could potentially be modified to yield block copolymer
The P−N bond lengths in 1 [1.639(1)−1.655(1) Å] are
quite similar to those reported for [P(N(Me)Cy)4]PF6
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Journal of the American Chemical Society
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Scheme 2. Synthesis of Phosphonium-Functionalized
Polyethylene
Figure 3. AAEM-17 hydroxide conductivity as a function of time after
immersion in 15 M KOH(aq) at 22 °C. Inset: AAEM-17 hydroxide
conductivity as a function of time after immersion in 1 M KOH(aq) at
80 °C.
architectures, an area currently drawing interest with respect to
AAEMs.7a,30 We are currently investigating the synthesis of
random and block copolymers bearing these phosphonium
moieties.
In conclusion, the alkaline stability of a tetrakis-
(dialkylamino)phosphonium cation was evaluated and directly
compared with that of benzyltrimethylammonium cation. In
model compound investigations, the [P(N(Me)Cy)4]+ out-
performed [BnNMe3]+. Consequently, a new methodology for
appending these delocalized phosphonium cations to poly-
ethylene was developed. The membrane stability of AAEM-17
in 15 M KOH at 22 °C and 1 M KOH at 80 °C confirmed that
tetrakis(dialkylamino)phosphonium materials are promising
candidates for testing in AMFCs. Our future work will focus
on controlling the polymer morphology in an effort to increase
the conductivity of these materials while retaining their
excellent mechanical properties.
The unsaturated ROMP polymer was hydrogenated
([(COD)Ir(py)(PCy3)]PF6, CHCl3/MeOH, 600 psig H2, 17
1
h), and H NMR spectroscopy revealed nearly quantitative
hydrogenation. The reduced copolymers (chloride form) were
dissolved in a 1,2-dichloroethane/ethanol cosolvent mixture
(1:1) and cast onto a glass dish preheated to 45 °C, from which
the volatiles were slowly evaporated to yield a film. The
polymers were converted into their hydroxide form by soaking
them in 1 M KOH for 2 h and washing with deionized water
for 1 h. Samples containing a range of comonomer ratios were
tested, but polymers with higher percentages of 7 exhibited
excessive swelling at higher temperatures. The optimized
AAEM used for conductivity and base-stability studies,
AAEM-17, contained 17 mol % 7. Conductivity, ion exchange
capacity and water uptake data for AAEM-17 are presented in
Scheme 2.31 We suspect that the size of the tetrakis-
(dialkylamino)phosphonium cation inhibits packing of the
polymer chains, resulting in a material with a high water uptake
in view of the relatively low ion exchange capacity.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization data. This
material is available free of charge via the Internet at http://
pubs.acs.org. Crystallographic data are also available from the
Cambridge Crystallographic Data Centre (http://www.ccdc.
cam.ac.uk/) under deposition number CCDC 889152.
To investigate the stability of the phosphonium-function-
alized membrane materials under highly alkaline conditions, we
exposed AAEM-17 (OH− σ22 = 22 1 mS cm−1) to a solution
of 15 M KOH in water. The chloride form of the hydrogenated
copolymer was immersed in 1 M KOH(aq) following the
standard exchange procedure31 to obtain AAEM-17, which was
immediately exposed to 15 M KOH(aq) at 22 °C. Over a 138
day period, AAEM-17 was periodically removed from the 15 M
solution and soaked in deionized water for 18 h to ensure
complete rehydration. The membrane was re-exchanged with 1
M KOH(aq) and washed with water to remove any residual
base, and the in-plane hydroxide conductivity was measured at
22 °C. The data obtained are presented in Figure 3.
Interestingly, no significant loss of conductivity was observed
for AAEM-17 after exposure to 15 M KOH(aq) over the 20
week period. To investigate the stability of the membrane at
elevated temperatures, AAEM-17 was exposed to 1 M
KOH(aq) at 80 °C. A small initial loss of conductivity (from
22 to 18 mS cm−1) was observed after 3 days, but no further
loss in conductivity was evident up to 22 days. The alkaline
stability of the phosphonium-functionalized AAEM in 1 M
KOH(aq) at 80 °C suggests that these may be excellent
candidates for higher-temperature AMFC devices.
AUTHOR INFORMATION
Corresponding Author
gc39@cornell.edu
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Energy Materials Center at
Cornell (EMC2), an Energy Frontier Research Center funded
by the U.S. Department of Energy, Office of Science, Office of
Basic Energy Sciences under Award DE-SC0001086. K.J.T.N.
gratefully acknowledges an NSERC Postdoctoral Fellowship.
We thank E. Rus, J. John, I. Keresztes, and P. Mutolo for helpful
discussions.
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(14) Thomas, O. D.; Soo, K. J. W. Y.; Peckham, T. J.; Kulkarni, M. P.;
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