Cationic Metallo-Polyelectrolytes for Robust Alkaline Anion-Exchange Membranes

Article abstract of DOI:10.1002/anie.201712387

Chemically inert, mechanically tough, cationic metallo-polyelectrolytes were conceptualized and designed as durable anion-exchange membranes (AEMs). Ring-opening metathesis polymerization (ROMP) of cobaltocenium-containing cyclooctene with triazole as the only linker group, followed by backbone hydrogenation, led to a new class of AEMs with a polyethylene-like framework and alkaline-stable cobaltocenium cation for ion transport. These AEMs exhibited excellent thermal, chemical and mechanical stability, as well as high ion conductivity.

Full text of DOI:10.1002/anie.201712387

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Cationic Metallo-Polyelectrolytes for Robust Alkaline Anion-
Exchange Membranes
Authors: Chuanbing Tang, Tianyu Zhu, Shichao Xu, Md. Anisur
Rahman, Emir Dogdibegovic, Peng Yang, Parasmani Pageni,
Mohammad Pabel Kabir, and Xiaodong Zhou
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201712387
Angew. Chem. 10.1002/ange.201712387
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10.1002/anie.201712387
Angewandte Chemie International Edition
COMMUNICATION
Cationic Metallo-Polyelectrolytes for Robust Alkaline Anion-
Exchange Membranes
Tianyu Zhua‡, Shichao Xua‡, Md Anisur Rahmana, Emir Dogdibegovicb, Peng Yanga, Parasmani
Pagenia, Mohammad Pabel Kabira, Xiaodong Zhoub and Chuanbing Tanga*
Abstract: Chemically inert, mechanically tough, cationic metallo-
sacrificed ion conductivity or still faced challenges on achieving
long-term stability.
polyelectrolytes were conceptualized and designed as durable anion-
exchange
membranes
(AEMs).
Ring-opening
metathesis
Of equal importance, other components of polyelectrolytes as
AEMs are also required to be stable for withstanding harsh
polymerization (ROMP) of cobaltocenium-containing cyclooctene with
triazole as the only linker group, followed by backbone hydrogenation,
led to a new class of AEMs with a polyethylene-like framework and
alkaline-stable cobaltocenium cation for ion transport. These AEMs
exhibited excellent thermal, chemical and mechanical stability, as well
as high ion conductivity.
chemical environments.
A degradation or deterioration of
chemical compositions of polyelectrolytes would have
a
catastrophic impact on membrane performance. It is essential to
design novel cationic polyelectrolytes with high cation stability and
superior chemical durability of the polymer framework.
Herein we report a new class of robust AEMs based on
cobaltocenium-containing polyelectrolytes that bear promising
hydroxide conductivity as well as mechanical, thermal and
chemical stability. These novel membranes exhibited long-term
stability that is unprecedented over many reported polymeric
systems. Specifically, we developed side-chain cobaltocenium-
containing vinyl polymers, which have non-hydrolysable
hydrocarbon backbones and extraordinarily stable side chains,
yet with high modulus and toughness.
We along with others have discovered that cationic
cobaltocenium exhibits good stability toward harsh chemical
environments in both strong acidic and basic conditions.[8] As
shown in Figure S1, there is negligible change in characteristic
UV absorption corresponding to cobaltocenium after it was
dissolved in a solution of NaOH (pH = 14) or HCl (pH = 1.5) for
two weeks. Moreover, published work by Yan[9] and Zhu[10] both
demonstrated that cobaltocenium cations with substituent group
in Cp ring had superior thermal and chemical stability over other
reported cations. Such extraordinary stability of cationic
Polyelectrolytes have vast applications ranging from coating to
biomedical materials.[1] Among them, cationic polyelectrolytes are
widely pursued as anion-exchange membranes (AEMs) in
alkaline fuel cells for converting fuels into electricity,[2] which can
be operated below 100 ˚C and thus are more attractive as energy
conversion devices for electric vehicles and powering portable
devices.
Commonly, AEMs are copolymers containing cations that are
covalently linked to hydrophobic polymer backbones.[2a, b, m, 3] The
most popular approach is the use of quaternary ammonium cation
(QAC)-containing polymers. However, QACs are prone to
degradation through Hofmann elimination (E2) or nucleophilic
substitution (SN2) under highly basic conditions. Over the past few
years, several strategies have been adopted to improve the
alkaline stability of AEM backbones: (i) protect ammonium cations
using steric protection, conformational restrictions or inductive
effects;[4] (ii) explore alternative stable cations such as benzyl
quaternary guanidinium,[5] imidazolium,[6] and phosphonium[7] to
slow down degradation. However, these methods have either
cobaltocenium
motivated
us
to
design
ultra-stable
polyelectrolytes in both cations and other compositions of
frameworks for AEMs. To construct the new polymeric systems,
we need to avoid the integration of functional groups (e.g. ester,
amide, anhydride, carbonate, etc.) and linkers that are
susceptible to degradation under strong basic conditions for a
prolonged period of time. Due to the instabilities of ester or imide
group, most cobaltocenium polymers reported in literature could
not be used as stable AEMs directly.[11]
To place our work in context, metal cations have been used for
AEMs. Hickner and Tew reported the synthesis of AEMs
functionalized with bis(terpyridine)-ruthenium(II) complexes.[2l]
These membranes exhibited good ion conductivity and
mechanical properties. However, the decline of conductivity in
alkaline condition over a long term was a concern. Yan and
[a]
T. Zhu, Dr. S. Xu, Md. A. Rahman, Dr. P. Yang, P. Pageni, Md. P.
Kabir, Prof. C. Tang*
Department of Chemistry and Biochemistry
University of South Carolina
Columbia, South Carolina 29208, United States
E-mail: tang4@mailbox.sc.edu
[b]
[‡]
Dr. E. Dogdibegovic, Prof. X. Zhou
Department of Chemical Engineering
University of South Carolina
Columbia, South Carolina 29208, United States
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end
of the document.
coworkers
reported
permethyl
cobaltocenium-containing
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10.1002/anie.201712387
Angewandte Chemie International Edition
COMMUNICATION
polysulfone as AEMs that exhibited extraordinary stability of
cations and chemical stability of polymers.[9] The ion conductivity
was mediocre, partially due to the steric hindrance for the
pentamethyl group. These polymers were quite brittle with
elongation at break only about 10%. Later Zhu and coworkers
synthesized polybenzimidazole polymers with cobaltocenium
cations in the main-chain.[10] These membranes showed improved
thermal stability and ion-exchange capacity, but the mechanical
and alkaline stability of these AEMs were inferior because of the
poor stability of linkage groups and the rigid polymer backbones.
Both approaches adopted condensation polymerization to
prepare rigid polymers with relatively limited molecular weight. In
addition, the synthesis was particularly laborious with low yields.
In this work, we followed three design principles: (1) eliminate any
hydrolysable functionalities; (2) maximize the content of
hydrocarbon in the compositions; (3) enable polymers with
controlled molecular weight. Specifically, we used highly efficient
click chemistry to attach cobaltocenium onto a cyclic vinyl
monomer, which was executed for ring-opening metathesis
polymerization (ROMP).[12] The resultant polymer has only one
triazole group as a linker between cobaltocenium and the polymer
backbone. This powerful synthetic tool allows the preparation of
high molecular weight toward mechanically flexible and tough
polymers. In addition, these polymers have a polyethylene-like
backbone to warrant mechanical integrity and chemical stability
under harsh basic conditions.
between
1
and ethynylcobaltocenium hexafluorophosphate
yielded cobaltocenium cyclooctene with a triazole as the linker (2),
which was subsequently carried out for ROMP with cyclooctene
as a co-monomer with the aid of Grubbs II catalyst to get a
copolymer 3. The molecular weight was well controlled by
adjusting the molar ratio of monomers to catalysts. In this study,
we chose a polymer with molecular weight at 80,000 g/mol (Table
S1), which is sufficiently high to warrant the formation of flexible
membranes. The cobaltocenium monomer and cationic
copolymers in hexafluorophosphate form were characterized by
1H NMR spectroscopy (Figure 1), which unambiguously
confirmed their structures with specific assignments to each
proton.
Scheme 1. (a) Synthesis of cobaltocenium monomer 2, copolymer 3 and
hydrogenated polymer 4 as AEMs; (b) Images of transparent and flexible
cobaltocenium AEMs; (c) Proposed structures of cobaltocenium-containing
AEMs in a hydroxide solution.
Figure 1. 1H NMR spectra of cobaltocenium monomer 2 (top), copolymer 3
(middle) and hydrogenated polymer 4 (bottom).
(a)
The presence of unsaturated bonds in the backbone of polymer 3
could be a concern on the long-term stability in alkaline conditions.
The alkaline stability of this copolymer was improved by
hydrogenation of the backbone. The reduction of double bonds
could be confirmed with 1H NMR (Figure 1) and disappearance of
characteristic absorption in the range of 1686~1796 cm-1 in FTIR
spectrum (Figure S2). These hydrogenated copolymers were
x
y
N
N
THF/H
2O, CuSO4
N
N
N3
1
N
N
+
L-ascorbate sodium
Grubbs II, 60 °C
PF6
Co PF6
Co
PF6
Co
3
2
x
x
y
y
N
N
subsequently
carried
out
ion-exchange
from
N
N
N
4Cl, 60 °C
1) 2M NH
Tosyl hydrazide
N
hexafluorophosphate (PF6¯) to chloride (Cl¯) and then to
hydroxide (OH¯) ion, and further fabricated to form anion
exchange membranes H-AEMx-OH (5) (x represents the molar
fraction of cobaltocenium units in a polymer composition).
DMF/xylene,
Et3N, 100 °C
2) 1M KOH, r.t.
PF6
Co
Co OH
4
5
Cobaltocenium monomer 2 with Cl¯ anion was evaluated for its
alkaline stability by UV-vis spectroscopy, as cobaltocenium has a
characteristic UV absorption at ~ 282 nm (Figure S3c).[8a, b] The
monomer was dissolved in 1 M NaOH at 80 ˚C and then collected
for time-dependent UV-vis spectra. Two representative
absorption peaks appeared at 282 nm and 346 nm,
characteristically originated from the cobaltocenium and triazole
groups respectively. The absorption spectrum obtained after 10
days perfectly overlapped with the initial one. Both peaks
maintained nearly 100% of initial intensity even after 10 d,
unambiguously indicating excellent alkaline stability of
cobaltocenium and triazole at elevated pH and temperatures.
Furthermore, the stability of cobaltocenium monomer 2 was
We prepared cobaltocenium-containing cyclooctene that was
subject to ROMP, as illustrated in Scheme 1. 5-Bromocyclooct-1-
ene was derived from cyclooctadiene,[7d] and then converted to 5-
1
confirmed by H NMR (Figure S4a) and mass spectrum (Figure
S4b) after exposure in alkaline conditions. We noticed that proton-
deuterium exchange occurred on protons from both aromatic
azidocyclooct-1-ene (1).
A copper-catalyzed click reaction
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10.1002/anie.201712387
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COMMUNICATION
rings and alkenes in the monomer structure, while no signals
indicated any degraded products. As investigated recently by the
Patric group,[13] this kind of proton-deuterium exchange is
reversible and does not lead to any structural degradation. All
above stability tests suggested cobaltocenium is a stable cation
suitable for AEM applications.
Table 1. Properties of cobaltocenium AEMs
Sample
x[a]
y[a]
IECtheo
IECtitr
(mmol/g)
λ
(mmol/g)
H-AEM40-OH
H-AEM50-OH
40
50
60
50
1.81
2.02
1.53
1.86
11.2
11.6
For cobaltocenium-containing polymers, direct anion exchange
―
from hydrophobic PF6 to hydrophilic OH― was challenging.[2l, 9]
We resolved this problem by first exchange to Cl― in 2 M aqueous
solution of NH4Cl at 60 ˚C. Then, exchange from Cl― to OH― was
carried out by immersing polymer membranes into 1 M NaOH
solution. Complete ion-exchange was confirmed by ion exchange
capacity (IEC) titration (Table 1) and Energy Dispersive X-ray
(EDX) characterization (Figure S5). These membranes were in
yellow color, flexible and transparent (Scheme 1b). All
membranes after anion exchange were not soluble in water or
other polar aprotic solvents like dimethylacetamide and
acetonitrile.
The thermal stability of cobaltocenium-containing polymers was
studied by thermogravimetric analysis (TGA) (Figure 2a). The
decomposition of hydrogenated H-AEMx-OH membranes under
N2 atmosphere consisted of two different stages and occurred
only well above 300 ˚C. The 5% weight-loss decomposition
temperature (Td) of H-AEM40-OH and H-AEM50-OH were 347 ˚C
and 328 ˚C, respectively. In comparison, many reported AEMs
with ammonium or imidazole cations have much lower
decomposition temperature for their cations (< 200 ˚C).[14] Our
results suggested both cobaltocenium cations and the
hydrocarbon backbone have excellent thermal stability.
Mechanical properties of AEMs are largely dependent on the
choice of polymer backbones. For well-studied poly(p-phenylene
oxide), polybenzimidazole, and polystyrene based AEMs, the
tensile strain at break is usually lower than 80% due to their rigid
aromatic backbones. While for flexible poly(4-methyl-1-pentene)
based films,[15] the tensile strength is relatively low. We intended
cobaltocenium polymers to be tough: flexible and strong.
Mechanical properties of cobaltocenium AEMs were
characterized at 80% RH and under ambient temperature (Figure
S6). Both H-AEM40-OH and H-AEM50-OH retained tensile strain
over 120% attributed to the existence of a highly flexible backbone.
These cobaltocenium membranes were expected to have phase
separation due to immiscibility of the hydrophobic backbone and
hydrophilic side chain in the copolymers. Small-angle X-ray
scattering (SAXS) was used to investigate the morphology of
cobaltocenium membranes (Figure 2b). Both of the membranes
exhibited a broad primary scattering peak around q* = 1.28 and
1.54 nm-1 with a d spacing (2π/q*) of 4.9 and 4.1 nm. The lack of
higher orders of scattering peaks implied that the phase
separation is probably short-range correlated,[16] which is
reasonable given the membranes are not block copolymers.
Atomic force microscopy (AFM) images also confirmed the
microphase separation between hydrophilic and hydrophobic
domains under spin casting conditions (Figure S7). The dark
areas represent the soft matrix from polyethylene backbone,
whereas the brighter ones represent hard domains from
cobaltocenium side chains. Such phase-separated morphology is
essential for anion exchange materials with desirable properties.
The hydrophobic backbone provided mechanically flexible and
chemically stable scaffold, whereas the hydrophilic side-chains
facilitated ion transport through connected ionic channels.
[a] x and y represent molar fractions of cobaltocenium and
cyclooctene units respectively (Scheme 1a).
Figures S8 show the water uptake and swelling ratio of
hydrogenated membranes in DI water at different temperature.
Even under high cation loading, side-chain cobaltocenium AEMs
still exhibited relatively low water uptake and swelling ratio
especially at room temperature, which can be explained by the
hydrophobic nature of polyethylene-like backbone. Furthermore,
the number of absorbed water molecules per cobaltocenium
cation was calculated to be in the range of 11.2 to 11.6, and
showed a tendency of slight increase with higher IEC (Table S1).
Hence, both cobaltocenium copolymers showed good resistance
to water uptake at various temperature. Such water management
property could play a critical role in forming ion conducting
pathways (Scheme 1c) and ensure the membranes to be
dimensionally stable over a long term. While for many quaternary
ammonium or phosphonium polymers, high cation loading for
better electrical property makes them much more hydrophilic and
absorbs excess water, which in turn sacrifices the overall
stability.[7d, 14a, 17] Our study on cobaltocenium AEMs indicated the
possibility of simultaneously making high IEC and having good
water management property.
Figure 2. Cobaltocenium AEMs: (a) TGA and DTG curves; (b) SAXS profiles;
(c) Hydroxide conductivity of membranes as a function of temperature in fully
hydrated condition; (d) Room temperature conductivity as
immersion time of membranes in 1 M NaOH solution at 80˚C.
a function of
The hydroxide conductivity of membranes with different loading of
cobaltocenium cation as a function of temperature is shown in
Figure 2c. Both membranes showed steadily increased
conductivity at elevated temperature likely due to higher water
uptake and faster ion migration. The hydroxide conductivity of H-
AEM40-OH and H-AEM50-OH reached pretty high value at 90 ˚C:
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72 and 90 mS/cm, respectively. Considering the processability
and durability of membranes, we only incorporated up to 50 mol%
cobaltocenium in copolymers at this stage. It could be possible to
achieve even higher hydroxide conductivity by increasing the IEC
and optimizing the compositions of copolymers. Moreover, the
hydroxide conductivity at different temperature generally followed
an Arrhenius relationship (Figure S8). The activation energy for
ion transport was calculated to be 19.2 to 20.2 kJ/mol.
The alkaline stability at high operating temperature has always
been a key concern for AEMs. For polymers prepared via ROMP,
the unsaturated double bond in repeating units is considered to
not only limit the chain flexibility, but also make membranes less
stable in harsh basic conditions. We employed direct
hydrogenation to reduce the double bonds in the polymer
backbone. In addition, polymers with polyethylene-like backbone
exhibited improved durability and ductility. H-AEM50-OH was
chosen for the long-term stability test because of its higher ion
capacity and hydroxide conductivity. FTIR spectra (Figure S9)
showed that all chemical structures of membranes remained
almost unchanged before and after the test. Moreover, initial
hydroxide conductivity of hydrated membranes was maintained
over 95% after soaking in 1 M NaOH at 80 ˚C for one month
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This work is partially supported by the National Science
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Keywords: cobaltocenium polyelectrolytes • anion-exchange
membranes • ion conductivity • ROMP • microphase separation
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10.1002/anie.201712387
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
T. Zhu, S. Xu, Md A.
Rahman, E. Dogdibegovic, P.
Yang, P. Pageni, Md P. Kabir,
X. Zhou, C. Tang*
Page No. – Page No.
Cationic Metallo-
Polyelectrolytes for Robust
Alkaline Anion-Exchange
Membranes
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