Enhancement of anhydrous proton transport by supramolecular nanochannels in comb polymers

Article abstract of DOI:10.1038/nchem.629

Transporting protons is essential in several biological processes as well as in renewable energy devices, such as fuel cells. Although biological systems exhibit precise supramolecular organization of chemical functionalities on the nanoscale to effect hi

Full text of DOI:10.1038/nchem.629

ARTICLES
PUBLISHED ONLINE: 25 APRIL 2010 | DOI: 10.1038/NCHEM.629
Enhancement of anhydrous proton transport by
supramolecular nanochannels in comb polymers
Yangbin Chen1, Michael Thorn2, Scott Christensen3, Craig Versek2, Ambata Poe1, Ryan C. Hayward3 ,
*
Mark T. Tuominen2 and S. Thayumanavan1
*
*
Transporting protons is essential in several biological processes as well as in renewable energy devices, such as fuel cells.
Although biological systems exhibit precise supramolecular organization of chemical functionalities on the nanoscale to
effect highly efficient proton conduction, to achieve similar organization in artificial systems remains a daunting challenge.
Here, we are concerned with transporting protons on a micron scale under anhydrous conditions, that is proton transfer
unassisted by any solvent, especially water. We report that proton-conducting systems derived from facially amphiphilic
polymers that exhibit organized supramolecular assemblies show a dramatic enhancement in anhydrous conductivity
relative to analogous materials that lack the capacity for self-organization. We describe the design, synthesis and
characterization of these macromolecules, and suggest that nanoscale organization of proton-conducting functionalities is
a key consideration in obtaining efficient anhydrous proton transport.
fficient and selective transport of protons is critical both in identified, one avenue that was not explored in these anhydrous
biological contexts1 and in materials for renewable energy2. proton-conducting systems is the role of supramolecular organiz-
E
In biological systems, nature has optimized proton conduction ation in nanoscale ion-conducting channels. This is surprising
on a nanometre scale by using secondary and tertiary structures of because, in the context of hydrated proton-conducting systems,
proteins to arrange precisely the appropriate side chains of amino such as Nafion9–11 and sulfonated block copolymers12,23–26, as well
,
acids, for example in the membrane protein M2 (refs 3–5). as lithium-ion conducting supramolecular assemblies27–29, it is
Although control of proton transfer on this scale is adequate for well-established that the formation of nanoscale domains enriched
most biological processes, it is essential that efficient proton conduc- in the ion-conducting materials is critical to the resulting macro-
tion be obtained on a micron scale for clean-energy applications6,7. scopic ionic conductivity.
In hydrogen fuel cells, for example, after oxidation of molecular
In this paper, we describe the molecular design and synthesis of a
hydrogen at the anode, the resulting protons must be transported novel class of comb polymers with amphoteric proton-transfer
across a selective membrane to reach the cathode and complete functionalities that can self-assemble into organized supramolecular
the conversion of chemical energy into electrical energy. The structures. We also show that very subtle changes in the monomer
proton conductivity of this membrane, often called the proton- and analogous polymer provide solid-state structures that lack
exchange membrane or the polymer electrolyte membrane (PEM), such nanoscale organization. By comparing these polymers, we
has been one of the bottlenecks to achieving affordable fuel-cell show that the self-assembled structures yield a dramatic increase
technology. Nafion, a poly(tetrafluoroethylene)-based polymer in proton conductivity (by as much as three orders of magnitude),
with sulfonic acid groups arranged at intervals along the backbone, presumably because of the locally increased concentration of
is one of the most widely used materials for this membrane8. The proton-transport functionalities within the nanophase-separated
key to proton transport in Nafion is thought to be nanochannels domains. These results suggest that a careful consideration of
of sulfonic acid groups, through which ‘hydrated’ protons can macromolecular architecture and nanoscale assembly is critical to
pass efficiently9–11. Although a good proton conductor for hydrated optimizing anhydrous proton transport in new materials for PEMs.
protons, Nafion suffers from poor conductivity in unassisted proton
transfer, that is Grotthuss or anhydrous proton transfer12,13, which Results
results in low conductivities at temperatures above the boiling To prepare polymers that form supramolecular assemblies with
point of water. PEMs with high proton conductivities at tempera- proton-transporting functionalities concentrated within nanoscale
tures of 120–200 8C are desirable, because operation at higher temp- domains, we made use of comb polymer architectures (Fig. 1).
eratures can increase fuel-cell efficiency, reduce cost, simplify heat One of our groups recently used this architecture to prepare amphi-
management and provide better tolerance of the catalysts against philic comb polymers by attaching lipophilic and hydrophilic func-
poisoning14.
tionalities at the meta-positions of the benzene ring of each styrenic
One approach to address this issue is to use amphoteric func- repeat unit30,31. Such polymers were shown to form assemblies of a
tional groups that allow anhydrous proton transport15,16, for micelle type in aqueous milieu and of an inverse-micelle type in
example imidazole, which is a common motif in biological proton apolar organic solvents. Thus, we hypothesized that similar poly-
transport in the form of the amino acid histidine. Several groups mers would also form nanoscale assemblies in the melt state. For
have studied synthetic polymers that contain such amphoteric func- this purpose, we designed a series of styrenic comb polymers in
tional groups as candidates for high-temperature proton transfer17–22
.
which one of the meta-positions contained a polar N-heterocyclic
Although a number of interesting candidate materials were functionality capable of proton transport, and the other contained
1Department of Chemistry, 2Department of Physics, 3Department of Polymer Science and Engineering, University of Massachusetts, Amherst,
Massachusetts 01003, USA. e-mail: thai@chem.umass.edu; tuominen@physics.umass.edu; rhayward@mail.pse.umass.edu
*
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.629
ARTICLES
H
N
H
a
N
O
O
N
N
O
O
O
N
N
n-C10H21
N
N
H
H
n
n
1
2
b
O
O
HO
OH
O
O
O
O
n-C10H21
O
n-C10H21
O
i. K2CO3, 18-crown-6
NaI, n-C10H21Br
i. PCC
ii. K2CO3, 18-crown-6
NaI, Br(CH2)6CO2Et
ii. KO-t-Bu
CH3PPh3Br
OH
OH
5
3
4
O
125 °C
N
H
N
O
O
i. Ester hydrolysis
N
O
O
O
O
n-C10H21
O
N
n-C10H21
N
H
O
ii. Et3N
Cl
O
H
N
n
then
n
N
N
1
6
H2N
Figure 1 | Structures and synthesis of benzotriazole-based polymers. a, Benzotriazole-based polymers that only differ by the inclusion of a decyl chain.
b, Synthetic scheme for benzotriazole-based polymers. This synthesis was modified slightly to prepare proton-conducting polymers without decyl chains and
those that contain imidazole in place of benzotriazole. Details are given in the Supplementary Information. PCC ¼ pyridinium chlorochromate.
a non-polar alkyl chain to drive phase segregation. For example, from 6 × 1026 S cm21 at ambient temperature to 1.3 × 1023 S cm21
polymer 1 (Fig. 1a) contains non-conducting decyl chains and the at 200 8C, at least two orders of magnitude larger than the conduc-
proton-conducting heterocyclic functionality benzotriazole. tivity of 2 across the same temperature range, which varies from
Although Liu et al.18 reported that triazole appreciably enhances 4 × 1029 S cm21 at ambient temperature to 1.2 × 1025 S cm21 at
proton conductivities with respect to imidazole (also benzimidazole 200 8C. As a benchmark, Nafion membranes show room-temperature
has been studied20 as a proton-transporting functionality), we are conductivities of 1022 to 1021 S cm21 when fully hydrated32, but at
not aware of any reports of benzotriazole used as a proton low humidity (below 5%) their conductivities were reported as 1027
conductor. To test our premise that nanoscale phase segregation to 1025 S cm21 (ref. 33). (Under our experimental conditions,
of 1 would facilitate proton transport, we synthesized the measured conductivities of Nafion were below the noise floor of the
analogous polymer 2 that has no alkyl chains and therefore did measurements of ꢀ1029 S cm21.) Thus, although the conductivities
not undergo nanoscale assembly. Syntheses of these polymers are of our materials remain significantly below those of Nafion under
exemplified with polymer 1 in Fig. 1b.
ideal conditions, the dramatic increase in conductivity from 2 to 1
To determine proton-conductivity values, polymer films were suggests an important design principle for optimizing proton transport
drop-cast from solution onto a hole in a piece of Kapton tape and under anhydrous conditions.
subsequently sandwiched between two electrodes to allow character-
At first, it may seem surprising that the addition of a decyl chain
ization by impedance spectroscopy, as described previously19. The to each repeat unit of a polymer could boost proton conductivity by
Kapton tape determined the thickness of the polymer film, which two orders of magnitude. After all, the average density of proton-
was therefore constant at 125 mm for all impedance measurements. transporting groups is lowered by the presence of the decyl chain;
Separate thermogravimetric analyses (TGAs) were conducted to the benzotriazole unit makes up only 23 weight per cent (wt%) of
verify that all polymers reported in this study were thermally 1 as compared to 34 wt% of 2. However, we hypothesized that the
stable up to at least 200 8C, which was the highest temperature decyl chain renders the mixing of 1 with the amphoteric hetero-
investigated in the impedance measurements. Conductivities of cycles incompatible, and so drives 1 to self-assemble and form
the polymer samples were measured through several heating– nanoscale domains that contain enhanced local concentrations of
cooling cycles (40–200 8C) under high vacuum and were found to benzotriazole, and thereby facilitates proton transport.
be consistent from cycle to cycle, which eliminates any possible
effects of residual solvent on the performance of these polymers.
To test this hypothesis, we characterized the structures of
these polymers using small-angle X-ray scattering (SAXS). As
Proton conductivities for 1 and 2 measured as a function of shown in Fig. 2b, polymer 1 gave rise to scattering peaks that
temperature between 40 and 200 8C are shown in Fig. 2a. Both indicate self-assembled nanostructures, but the control polymer
polymers show qualitatively similar non-Arrhenius increases in 2 yielded a completely featureless pattern that indicates a homo-
conductivity with temperature that are typical for anhydrous geneous phase-mixed structure. The first-order scattering peak
proton-conducting polymers. However, the conductivity of 1 ranges from 1 falls at q* ¼ 1.47 nm21, which corresponds to a real-space
504
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.629
ARTICLES
a
Temperature (°C)
b
227
10–2
181
144
111
84
60
39
q*=1.47nm–1
1
2
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
10–3
10–4
10–5
10–6
10–7
10–8
10–9
4q*
1
2
2.0
2.2
2.4
2.6
1,000/T (K–1
2.8
3.0
3.2
1
2
3
4
5
q (nm–1
)
)
c
Polymer backbone
O
H
4
N
O
5
N
O
N
H
N
O
O
4
N
O
N
5
Decyl chain
Spacer
Benzotriazole
Figure 2 | Conductivity and SAXS results for benzotriazole polymers. a, Proton conductivity over a wide temperature range, in which the polymer that
contains the decyl chain exhibits a much higher conductivity than that of its counterpart. b, SAXS profiles of benzotriazole polymers that indicate ordering of
the alkylated polymer. Curves are shifted vertically for clarity. I(q) is the azimuthally-averaged scattering intensity as a function of scattering wave-vector (q).
c, An illustration of the proposed structure of 1, with two units arranged with the benzotriazoles head-to-head, which thus allows for hydrogen bonding.
a.u. ¼ arbitrary units.
distance of d ¼ 4.3 nm, and a faint, although clearly resolvable, at half-maximum, Dq) of 0.6 nm21 for polymer 1, which yielded a
p
second-order peak at 4q*. Although the structure cannot be deter- correlation length of 2p/Dq ≈ 10 nm, indicating that the size of
mined unambiguously from these data, the presence of only a ordered domains is small, with positional correlations that extend
second-order peak suggests a lamellar structure with a repeat only over several repeat units.
spacing of 4.3 nm. We estimate the fully stretched length of a
As demonstrated by Ikkala and co-workers34, self-assembly into
monomer, from the tip of the decyl chain to the benzotriazole anisotropic nanostructures yields orientation-dependent conduc-
group, as ꢀ3 nm, and thus the observed repeat spacing is consistent tivity, and McGrath and co-workers have shown that continuity of
with ‘back-to-back’ stacking of polymer chains with some interdigi- nanoscale domains is critical to efficient proton transport in sulfo-
tation of the decyl chains and/or benzotriazole group and spacer. nated polymers35. For our polymers, although the limited length of
A schematic of this proposed structure is shown in Fig. 2c, in which ordering precludes any considerations of the effects of orientation or
two repeating units arranged with the benzotriazoles head-to-head dimensionality of nanoscale domains on conductivity, the nanoscale
allow hydrogen bonding, with the alkyl groups pointed away from organization provided by supramolecular assembly clearly enhances
each other. Analysis of the first-order peakrevealedawidth (full-width anhydrous proton conductivity by at least two orders of magnitude
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505

NATURE CHEMISTRY DOI: 10.1038/NCHEM.629
ARTICLES
first-order scattering peak decreased continuously with increasing
temperature, its position and width remained nearly constant,
which indicates that a similar level of nanoscale organization was
present in these materials over the entire temperature range of inter-
est (see Supplementary Information for details).
An additional factor to consider when the proton conductivities
of two polymer chains that bear the same functional group are com-
pared is the glass-transition temperature (Tg), because the mobility
of the polymer chain is well-known to influence the rate of proton
transport36. To test whether the difference in conductivity observed
between 1 and 2 simply reflects a decrease in Tg because the decyl
chain is present, we carried out differential scanning calorimetry
Table 1 | Properties of polymers 1, 2, 7 and 8.
Polymer Decomposition
onset (8C) (5%
Tg
N-heterocycle
Molecular
(8C) weight fraction weight (Mn)
weight loss)
(%)
1
218
233
225
221
55
67
61
71
23
34
14
25,000
25,000
23,000
24,000
2
7
8
20
Glass-transition temperatures, decomposition onset temperatures, N-heterocycle weight fractions
and molecular weight of polymers 1, 2, 7 and 8. Mn ¼ number-average molecular weight.
compared with that of the phase-mixed control polymer. The exact experiments. As summarized in Table 1, the Tg values of 1 and 2
mechanism of enhancement is currently unclear, although we were 55 8C and 67 8C, respectively. The modest difference in Tg
speculate that self-organization leads to interconnected channels between these polymers suggests that the mobility of the polymer
of locally enriched benzotriazole concentrations that facilitate backbone is not a major contributor to the difference in proton con-
proton hopping through the membrane. To test for the possibility ductivities observed. As relatively high conductivity values of 1 were
of disordering or nanoscale structural transitions at high tempera- achieved at temperatures well above Tg, the mechanical properties of
ture (which, in some cases, have yielded dramatic changes in ionic this polymer at such temperatures are not well-suited for application
conductivity27), we carried out variable-temperature SAXS measure- as PEMs. Although components with higher Tg values or semicrys-
ments over the range 40–200 8C. Although the intensity of the talline components need to be incorporated to provide materials
a
O
N
Temperature (°C)
b
NH
O
O
227
181
144
111
84
60
39
n-C10H21
N
H
10–2
10–3
10–4
10–5
10–6
10–7
10–8
10–9
10–10
7
8
n
7
O
N
NH
O
N
H
n
8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
1,000/T (K–1
)
c
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
d
q*=1.64nm–1
7
8
Decyl chain
Spacer
3q*
Imidazole
1
2
3
4
5
q (nm–1
)
Figure 3 | Results for imidazole-based polymers a, Structures of imidazole polymers that only differ by the inclusion of a decyl chain. b, Proton conductivity
over a wide temperature range, which again demonstrates that the polymer with an added decyl chain has a much higher conductivity. c, SAXS profiles of
imidazole polymers, which show order in polymer 7 and a disordered polymer 8. Curves are shifted vertically for clarity. d, An illustration of the proposed
structure of 7, which self-assembles into cylindrical domains.
506
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.629
ARTICLES
with adequate mechanical properties, we emphasize that the poly-
mers described here provide proof-of-concept results and demon-
strate the importance of nanoscale organization in anhydrous
proton conductivity.
Methods
TGA was carried out using a TA Instruments TGA 2950 thermogravimetric analyser
with a heating rate of 10 8C min21 from room temperature to 500 8C under nitrogen.
Glass-transition temperatures were obtained by differential scanning calorimetry
using a TA Instruments Dupont DSC 2910. Samples were analysed at a heating rate
of 10 8C min21 from 0 8C to 150 8C under a flow of nitrogen (50 ml min21).
Electrochemical impedance data were obtained using a Solartron 1287
potentiostat and 1252A frequency response analyser in the range 0.1 Hz to 300 kHz.
Measurements were conducted under vacuum at temperatures between 40 8C and
200 8C with a sinusoidal excitation root-mean-square voltage of 0.1 V. The sample
thickness and contact surface area were controlled by a 125 mm thick Kapton tape
with a 0.3175 cm diameter hole.
If our hypothesis and conclusions are correct, this molecular
design strategy should also work for other amphoteric heterocycles.
To test the generality of our molecular design, we prepared polymers
that contained imidazole as a proton-transporter functionality,
which was also shown to be capable of anhydrous proton trans-
fer16–19 and has a very different dissociation constant (pKa) value
in the protonated form as compared to that of benzotriazole. In
analogy to 1 and 2, we synthesized polymers 7 and 8 with imidazole
moieties (Fig. 3a). Alternating current impedance measurements
revealed that polymer 7, with its decyl chain, exhibits dramatically
higher conductivity than that of the corresponding control
polymer 8, in this case by more than three orders of magnitude
(Fig. 3b). The morphologies of these polymers were investigated
using SAXS and revealed that 7 gave two well-defined scattering
peaks, which indicates the presence of self-assembled nanostruc-
SAXS measurements were carried out on an in-house beamline using a Rigaku
rotating anode source to generate Cu Ka radiation (wavelength l ¼ 0.154 nm).
Scattering patterns were collected on an image plate positioned a distance of 500 mm
from the sample. All samples yielded isotropic patterns, and thus data were
integrated to yield plots of intensity as a function of the magnitude of the scattering
vector, q ¼ (4p/l)sin(u), where 2u is the total scattering angle. The actual scattering
angles were calibrated using the known reflection from silver behenate.
Received 16 November 2009; accepted 16 March 2010;
published online 25 April 2010
tures, but 8 showed no signs of structure (Fig. 3c). The second scat-
p
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(Fig. 3d). Tg values determined for these polymers (Table 1) are very
similar, which once again reveals that mobility of the polymer back-
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Acknowledgements
This work was supported by the National Science Foundation through the Fueling the
Future Center for Chemical Innovation at the University of Massachusetts Amherst
(CHE-0739227). We thank W. de Jeu for discussions on the X-ray scattering results.
Author contributions
S.T. and Y.C. conceived the molecular design. S.T., R.H. and Mark T. planned the project.
Y.C, Michael T., S.C. and C.V. carried out the experiments and analysed the data. Y.C. and
A.P. synthesized the discussed compounds, Michael T. and C.V. measured ionic
conductivities, and S.C. performed SAXS. Results were discussed by R.H., Mark T. and S.T.
All authors contributed to writing the manuscript.
Additional information
33. Sanders, E. H. et al. Characterization of electrosprayed Nafion films. J. Power
Sources 129, 55–61 (2004).
34. Ruotsalainen, T. et al. Structural hierarchy in flow-aligned hexagonally self-
organized microphases with parallel polyelectrolytic structures. Macromolecules
36, 9437–9442 (2003).
The authors declare no competing financial interests. Supplementary information and
chemical compound information accompany this paper at www.nature.com/
naturechemistry. Reprints and permission information is available online at http://npg.nature.
com/reprintsandpermissions/. Correspondence and requests for materials should be addressed
to R.C.H., M.T.T. and S.T.
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