Showcasing a study on proton-conductive polymer membranes
exhibiting anhydrous conductivity higher than 100 mS cm-1 ataround 100 °C, by Prof. Atsushi Noro at Nagoya University and
Dr Naoki Nakamura at Toyota Motor Corporation.
Acidic liquid-swollen polymer membranes exhibiting anhydrous
proton conductivity higher than 100 mS cm−1 at around 100 °C
Polymer membranes exhibiting anhydrous proton conductivity
higher than 100 mS cm-1 at around 100 °C are prepared byswelling a cross-linked ﬂexible basic polymer with a nearlynon-volatile strong acid via ionic interaction. The incorporated
acidic liquid does not leach out from the membranes, even whenthe membranes contain a large amount of acidic liquid.

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Materials Chemistry A
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Cite this: J. Mater. Chem. A, 2019, 7,
15585
Received 19th February 2019
Accepted 23rd April 2019

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Acidic liquid-swollen polymer membranes
exhibiting anhydrous proton conductivity higher
than 100 mS cm1 at around 100  C†
Takato Kajita, ‡a Haruka Tanaka,
and Naoki Nakamura ‡b‡a Atsushi Noro,‡*a Yushu Matsushita‡a

DOI: 10.1039/c9ta01890ersc.li/materials-a

Anhydrous proton-conductive membranes are prepared by incorporating sulfuric acid (H2SO4) into cross-linked poly(4-vinylpyridine) (CLP), where H2SO4 mixes CL-P homogeneously at the molecular level,
and therefore incorporated H2SO4 does not leach out from CL-P, evenwhen the weight ratio of H2SO4 to CL-P is about four. The membranes
are soft but self-standing and exhibit anhydrous conductivity higher
than 100 mS cm1 at around 100  C. Furthermore, a membranecomposed of polystyrene-b-poly(4-vinylpyridine)-b-polystyrene triblock copolymers and H2SO4 achieves not only high anhydrous
conductivity but also good mechanical properties as a membrane.

Introduction
High-performance polymer membranes, such as lithium ionconductive
membranes1–7
andproton-conductive
8–15
membranes,
have been receiving attention in recent years.
Proton conductive-membranes are useful in polymer electrolyte
fuel cells (PEFCs) that generate electrical energy througha reaction of hydrogen and oxygen gases, producing water only
as a byproduct. Other fuel cells, such as solid oxide fuel cells(SOFCs),16–18 molten carbonate fuel cells (MCFCs),19,20 etc., must
be typically operated at a higher temperature than at least
200  C to attain high ionic conductivity of 100 mS cm1 and theyalso require long time to start up.21 On the other hand, theproton-conductive membranes in PEFCs exhibit relatively highaDepartment of Molecular and Macromolecular Chemistry, Graduate School ofEngineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan.
E-mail: noro@nagoya-u.jpbAdvanced Material Engineering Division, Advanced R&D and Engineering Company,
Higashifuji Technical Center, TOYOTA Motor Corporation, 1200 Mishuku, Susono,
Shizuoka, 410-1193, Japan† Electronic supplementary information (ESI) available: Experimental details,
supplementary gures and supplementary tables. See DOI: 10.1039/c9ta01890e‡ Author contributions: A. N. conceived the original idea and designed the study
with T. K. and N. N. T. K. and H. T. carried out the experiments, and A. N., T. K.,
Y. M., and N. N. analyzed data. A. N., T. K., and Y. M. co-wrote the paper with input
from all the authors.

This journal is © The Royal Society of Chemistry 2019

proton conductivity of 100 mS cm1 even at 80  C; hence, PEFCs
are promising systems for use in clean power generation forvehicles. Naon®, a peruorosulfonic acid polymer membranedeveloped by DuPont, is the most well-known protonconductive membrane since it exhibits a relatively highproton conductivity of 100 mS cm1 or higher at 70–90  C underhydrous conditions.8,22–29 However, the conductivity of Naon isextremely low under dry conditions, especially over the boilingpoint of water (100  C), where almost no water molecules
exist.22–24,29 Hence, PEFCs with Naon membranes cannot be
operated without having humidication systems, especiallyover 100  C. Note that a glass transition temperature (Tg) ofNaon is about 100  C.23,26,29 Thus, Naon cannot be used asa membrane over the Tg, i.e., about 100  C.
To overcome this drawback, membranes with moderate
proton conductivity even under anhydrous conditions havebeen developed.30–40 One of the most well-known examples ispolybenzimidazole (PBI; Fig. S1(a), ESI†) membranes dopedwith phosphoric acid (H3PO4) or sulfuric acid (H2SO4; Fig. S1(b),ESI†).30–32 These membranes exhibit a conductivity of a few tensof mS cm1 at relatively high temperatures of about 200  C;
however, they exhibit the low conductivity of 5.6 mS cm1 at
a moderately high temperature of 130  C. Moreover, theconductivity of the membranes spontaneously decreases overtime because H3PO4 or H2SO4 in the membranes naturally
leaches out. Thus, acid-doped PBI membranes are still insuﬃcient for use in fuel cells, though various improvements havebeen made to the membranes to date.35,36,38,40 As anotherexample, anhydrous proton-conductive metal organic framework (MOF) has also been developed,41–45 but the anhydrous
conductivity was at most 10 mS cm1 even at 150  C,46 which isalso inferior to the conductivity of humidied Naon. Spontaneous leaching out of the acid from acid-doped PBI is probably
attributed to the rigidness of PBI, which shows insuﬃcientcompatibility with even low-molecular-weight substances, suchas H3PO4 or H2SO4. It should be noted that PBI has rigid
aromatic rings on the backbone and hence it shows long Kuhn
segment length of 3.0 nm,47 while a typical exible vinyl polymer

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Journal of Materials Chemistry A(e.g., polystyrene) has a Kuhn segment length around 0.70 nm.48
Therefore, if a exible basic polymer without aromatic rings on
the backbone chain is mixed with an acidic low-molecularweight liquid medium that can be attached to a basic groupon the side chain of exible polymers via an attractive interaction, then spontaneous leaching out of acidic liquid would notoccur because a large amount of acidic liquid can inltrate theexible polymer homogeneously at the molecular level.
However, the exible basic polymer mixed with a large amountof acidic liquid usually gives a homogeneous molten mixture
that cannot be used as a membrane.
In order to prepare a so-solid self-standing membrane,cross-linking of the polymer is typically required. In otherwords, there must be excellent utility in applying the molecular
design concept of polymer gels49,50 that typically contain a large
amount of solvents. Although the Tg of polymer gels is lower
than ambient temperature due to the large amount of solvent,they do not ow and maintain their shape because the chainsare cross-linked. If such polymer gels have adequate mechanicalproperties enabling self-standing abilities, then they can beused as so membranes.
Based on the above consideration, we prepare an anhydrous
proton-conductive polymer membrane that can be used even
without humidication, while providing high conductivity ofmore than 100 mS cm1 that is comparable to the performanceof hydrous Naon, by using both cross-linking of a exible basic
polymer and swelling with a large amount of nonvolatile strong
acid, H2SO4, that is useful for attaining high proton conduction.51 First, we synthesize a cross-linked exible basic polymer(CL-P; Fig. 1(a)) of poly(4-vinylpyridine) (Kuhn segment length:0.797 nm)52 which can form acid–base complexes53 with nearly
nonvolatile H2SO4 via ionic interactions.54 Then, we prepare ananhydrous proton-conductive membrane by swelling the crosslinked polymer with a nearly nonvolatile H2SO4. Sulfuric acidmolecules spontaneously and homogeneously inltrate thecross-linked basic polymer via ionic interactions among theCommunication

proton, sulfonate anion, and pyridinium cation. We alsomeasure the anhydrous conductivity of the membranes.

Results and discussion
Preparation of mixtures and membranes
Before attempting our molecular design to preparing anhydrous
proton-conductive membranes, we rst prepared an H2SO4doped PBI membrane (weight ratio H2SO4/PBI: 76/24) by inltrating H2SO4 into a neat PBI membrane; which is a previously
reported anhydrous proton-conductive membrane.30,31 Unfortunately, leaching out of H2SO4 was seen over several days aer
the preparation (Fig. S1(c) and (d), ESI†). This probably
occurred due to poor solubility of PBI in H2SO4.
To achieve high solubility of polymers in H2SO4, we used
exible poly(4-vinylpyridine) instead of rigid PBI. A membrane
containing 20 wt% H2SO4 and 80 wt% uncross-linked poly(4vinylpyridine) (P; Fig. 1(b)) was prepared (see also Table S1,
ESI†) and resulted in a self-standing, homogeneous, hard
membrane (Fig. 2(a)). However, this membrane exhibited almost
no anhydrous conductivity. Narayanan and coworkers34 investigated a sample of uncross-linked poly(4-vinylpyridine) mixed
with an equimolar amount of H2SO4 to the pyridyl groups on thepolymer, and the mixture became a self-standing membrane andexhibited anhydrous conductivity; however, the conductivity was
merely on the order of 0.1 mS cm1, even at 140  C, which was far
below the 100 mS cm1. Although the conductivity is insuﬃcient,
they did not further investigate a sample containing a larger
amount of H2SO4. Therefore, to aim for a higher conductivity, we
prepared an uncross-linked P/H2SO4 mixture containing much
more H2SO4 (80 wt%) (see also Table S1, ESI†). Although the
mixture had a homogeneous appearance as shown in Fig. 2(b)because H2SO4 in the mixture formed acid–base complexes withpyridyl groups on polymer P (see the Fourier transform infrared(FT-IR) spectra in Fig. S3, ESI†), the mixture showed uidity.
Thus, even if the mixture shows anhydrous high proton

Fig. 1 Synthetic schemes and schematic illustration. (a) Chemically cross-linked poly(4-vinylpyridine) (CL-P). (b) Uncross-linked poly(4-vinylpyridine) (P).

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Journal of Materials Chemistry A

Fig. 2 Schematic illustrations and photos of P/H2SO4 and CL-P/H2SO4. (a) A membrane containing 20 wt% H2SO4 and 80 wt% uncross-linked
polymer P. The membrane was self-standing, homogeneous, and hard; however, its conductivity was very low under anhydrous conditions. (b) A
mixture containing 20 wt% P and 80 wt% H2SO4. The mixture was homogeneous but could not be used as a membrane due to its ﬂuidity. (c) A
membrane prepared by swelling a cross-linked polymer CL-P membrane with the same weight of H2SO4. The membrane was self-standing,homogeneous, and hard, but its conductivity was very low (see Table 1). (d) A membrane containing 18 wt% CL-P and 82 wt% H2SO4. Since thepolymer was cross-linked, the membrane was self-standing, homogeneous, and soft. Furthermore, it exhibited high conductivity even underanhydrous conditions (see also Fig. 3 and Table 1). In schematic, gray lines represent polymer chains with a higher Tg than ambient temperature,and pink lines represent chains with a lower Tg than ambient temperature. As shown in FT-IR spectra of Fig. S3,† a pyridyl group and H2SO4interact with each other, but broken lines and dotted lines indicating the interaction are not shown here for simplicity.

conductivity, it could not be used as a membrane.55–57 The reason
for the liquidity was quite natural, namely the uncross-linked
polymer was actually dissolved in a large quantity of a lowmolecular-weight liquid.
To overcome these diﬃculties, we prepared a cross-linkedpoly(4-vinylpyridine) (CL-P)/H2SO4 membranes by immersing
the CL-P membrane in a solution of nearly nonvolatile H2SO4
dissolved in methanol, and then the volatile solvent wascompletely removed by heating at 50  C. With this method,membranes can be prepared with good reproducibility (see also
Fig. S4, ESI†). The sample containing 50 wt% H2SO4 was a selfstanding, glassy, hard membrane and had a macroscopicallyhomogeneous appearance (Fig. 2(c)). The sample containing
82 wt% H2SO4 was also self-standing but it formed a so
membrane (Fig. 2(d)). It should be noted that, unlike the H2SO4doped PBI, the CL-P/H2SO4 membrane was macroscopicallyhomogeneous and H2SO4 has never leached out, even it has
been le for a long time (>100 days). This is because exible CLP is much more easily miscible with H2SO4 compared with rigid
PBI. In fact, only the surface layer of PBI is likely to dissolve with
H2SO4 in H2SO4-doped PBI. This also means that the number ofcontacts between the pyridyl groups on exible CL-P and H2SO4at the molecular level is much larger than that between

This journal is © The Royal Society of Chemistry 2019

imidazole groups on PBI and H2SO4. Thus, the Gibbs energy of
mixing for the case, DmixG, represented by DmixH  TDmixS(where DmixH, DmixS, and T are enthalpy of mixing, entropy of
mixing, and absolute temperature, respectively), is mainly
dominated by negative DmixH with a large absolute value. This
prevents spontaneous leaching out of H2SO4 from themembrane. The above results show that it is eﬀective to swella cross-linked exible base polymer with an acidic liquid in the
preparation of an anhydrous proton-conductive membrane. It
should be also noted that CL-P/H2SO4 containing 85 wt% H2SO4
was also prepared, but the sample was so so and it was diﬃcult
to handle as a membrane. Therefore, swelling with 82 wt%
acidic liquid could be the best result for the membrane
preparation.
Anhydrous conductivity of CL-P/H2SO4
The conductivity of the CL-P/H2SO4 membranes in the temperature range from 50  C to 120  C without humidication wasdetermined by the AC impedance method, where the overall ionic
conductivity contributed by all ion species including protons wasmeasured. Contributions of proton conductivity to the overall
ionic conductivity can be expressed by the proton transference
number, and the proton transference number of concentrated

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sulfuric acid is probably very close to unity as phosphoric acid,58
since the proton is the smallest ion in sulfuric acid and move
much more easily than other ions such as a sulfate ion. Therefore, the overall ionic conductivity measured by AC impedance
method is considered to be equivalent to the proton conductivity
in this study. Table 1 summarizes the anhydrous conductivity
and Fig. 3 shows a plot of the anhydrous conductivity of the CL-P/
H2SO4 membranes against the reciprocal of the absolutetemperature. In the membrane with an H2SO4 content of 50 wt%(Fig. 2(c)), we could not obtain an impedance spectrum because
the resistance was extremely large; i.e., the anhydrous conductivity was extremely low. However, for the membrane in which thecontent of H2SO4 increased by 5 wt% from 50 wt%, i.e., H2SO4
content was set to be 55 wt% (Fig. S4(a), ESI†), the membrane
exhibited a conductivity of 0.077 mS cm1 at 80  C. Moreover, theanhydrous conductivity was 13 mS cm1 at 80  C for themembrane with an H2SO4 content of 60 wt% (Fig. S4(b), ESI†);the conductivity increased drastically by 2 to 3 orders of magnitude due to a mere 5 wt% increase in the H2SO4 content.
Furthermore, the conductivity increased as the H2SO4 content
increased; notably, the membrane with an H2SO4 content of
82 wt% (Fig. 2(d)) exhibited a very high conductivity of more than
100 mS cm1 (120 mS cm1 at 80  C), even under anhydrous
conditions. At temperatures above 100  C, this membrane
showed even higher conductivity (e.g., 160 mS cm1 at 120  C).
The present molecular design which enables to maintain themembrane shape by homogeneous and spontaneous swellingwith a large amount of H2SO4, probably resulted in much higher
conductivity than that in previously reported H2SO4-doped PBI
membranes (the H2SO4 content: 44 wt%), i.e., ca. 0.35 mS cm1 at
85  C.31 The molecular design for membrane preparation canalso be applied to another cross-linked exible basic polymer,
cross-linked poly(1-vinylimidazole) (CL-I, see Scheme S1, Table S2and Fig. S6, ESI†). Note that the leaching out of the H2SO4 fromthe membranes did not occur, either.
To further investigate the dependence of the anhydrous
conductivity on the H2SO4 content in CL-P/H2SO4 membranes,
Tgs of uncross-linked P/H2SO4 mixtures were measured by
diﬀerential scanning calorimetry (DSC). DSC thermograms areshown in Fig. S5 (ESI†). Fig. 4 shows a plot of Tg (the le side ofTable 1

Fig. 3 Anhydrous conductivity versus reciprocal absolute temperaturefor several CL-P/H2SO4 membranes. Solid lines are drawn as guide foreyes.

vertical axis) against the molar ratio of H2SO4 to the pyridylgroup on the polymer P, nAcid/nBase (see also Table S1, ESI†).
Here we used uncross-linked P because it can be easily
synthesized and its molecular characterization is also easy (seealso ESI†). Note that the Tg dependence of uncross-linked P/
H2SO4 and CL-P/H2SO4 should be almost the same, since themolar amount of cross-links in CL-P is very small. When theweight contents of H2SO4 were 10 wt% (nAcid/nBase ¼ 0.12),
20 wt% (nAcid/nBase ¼ 0.27), 30 wt% (nAcid/nBase ¼ 0.46), and40 wt% (nAcid/nBase ¼ 0.71), where the molar ratio of H2SO4 to
the pyridyl group is below unity, the Tg values were high: 141  C,
158  C, 170  C, and 171  C, respectively. This is presumably
because most of H2SO4 in the mixtures formed acid–basecomplexes with pyridyl groups on polymer P (see the FT-IR
spectra in Fig. S3, ESI†), and the segmental motions of thepolymer were strongly restricted. On the other hand, when theweight contents of H2SO4 were 50 wt% (nAcid/nBase ¼ 1.1),
55 wt% (nAcid/nBase ¼ 1.3), and 60 wt% (nAcid/nBase ¼ 1.6), where
the molar ratio of H2SO4 to the pyridyl group on the polymer P
exceeds unity, the Tg values suddenly dropped and became
141  C, 43  C, and 52  C, respectively. This is probably because
an excessive molar amount of H2SO4 to the pyridyl groups

Anhydrous conductivity for a series of CL-P/H2SO4, Post-CL-P/H2SO4 and S–P–S/H2SO4
sDCd [mS cm1]

Polymer

H2SO4 weight contenta [wt%]nAcid/nBaseb [mol mol1]

H2SO4 mole contentc [mol%]

50  C80  C95  C110  C

120  C

CL-P

50
55
60
70
82
80
80

1.1
1.3
1.6
2.6
5.0
4.3
4.5

52
57
62
72
83
81
82

—e
0.022
7.4
25
87
60
63

—e
0.077
13
36
120
94
100

—e
0.11
15
40
140
120
130

—e
0.14
18
49
150
140
—f

—e
0.15
19
59
160
150
—f

Post-CL-P
S–P–Sa
The weight content of H2SO4 in polymer membranes. b The molar ratio of H2SO4 to the pyridyl group on the polymer. c The mole content of H2SO4,
which can be calculated according to (1 + nBase/nAcid)1  100. d The DC conductivity estimated from the |Z| value of the plateau in the AC
impedance spectrum, i.e., the bulk resistance. Here, the proton transference number is assumed to be unity. Therefore, the overall ionic
conductivity is regarded as the proton conductivity. e The conductivity cannot be determined because it is very low. f The conductivity was notmeasured because the temperature is higher than the Tg of polystyrene.

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Fig. 4 Tg of P/H2SO4 mixtures (left axis) and anhydrous conductivity at
120  C of CL-P/H2SO4 membranes (right axis) against H2SO4/pyridylgroup molar ratio (nAcid/nBase) and the content of H2SO4 in mixtures.
Solid and broken lines are drawn as guide for eyes.

served as a plasticizer, causing activated segmental motions of
the polymer chain. When the H2SO4 weight content was further
increased, the Tg gradually decreased; the Tgs at 70 wt% (nAcid/
nBase ¼ 2.5) and 80 wt% (nAcid/nBase ¼ 4.3) are very low at 86  C
and about 95  C, respectively.
In the region where nAcid/nBase is larger than unity, theconductivity also varied remarkably. Fig. 4 also shows thedependence of the conductivity of CL-P/H2SO4 at 120  C (theright side of vertical axis) against nAcid/nBase. When nAcid/nBase
was 1.1 or less, i.e., the H2SO4 content was 50 wt% or less, theconductivity was not able to be estimated by impedance
measurements because of high resistance. However, when nAcid/
nBase was varied slightly from 1.3 (H2SO4 content: 55 wt%) to 1.6(H2SO4 content: 60 wt%), the conductivity increased drasticallyby 2 to 3 orders of magnitude. This drastic increase in the
conductivity was attributed to both an increase in the number offree protons derived from an excessive molar amount of H2SO4
that is not used for acid–base complexation and a drastic
decrease in Tg of the membrane. Furthermore, the membraneof CL-P/H2SO4 containing even 82 wt% H2SO4 maintained its
shape due to cross-linking, despite of the low Tg, thereby
attaining the very high conductivity of more than 100 mS cm1(160 mS cm1 at 120  C) even under anhydrous conditions.
Properties of post-cross-linked polymer/H2SO4 and triblock
copolymer/H2SO4
Although CL-P described above was prepared by cross-linking
a polymer during polymerization, a post-cross-linked basepolymer (Post-CL-P) can also be prepared by mixing 1,4-dibromobutane (BrC4Br) as a cross-linking agent with the uncrosslinked homopolymer P (Fig. 5(a); see also Scheme S2, ESI†),so that a quaternization reaction of the pyridyl group occurs (seeFT-IR spectra in Fig. S7, ESI†).59 Thus, anhydrous protonconductive membranes can also be easily prepared by inltrating H2SO4 into the Post-CL-P membranes. Fig. 5(b) and (c)show a schematic illustration and a photograph of Post-CL-P/
H2SO4 containing 80 wt% H2SO4. The Post-CL-P/H2SO4This journal is © The Royal Society of Chemistry 2019

Journal of Materials Chemistry A

membrane was so and homogeneous. As shown in Fig. 5(d),the conductivity of Post-CL-P/H2SO4 was also very high, forexample, 94 mS cm1 at 80  C, 150 mS cm1 at 120  C, and 220
mS cm1 at 160  C (see also Tables 1 and S3, ESI†) underanhydrous conditions. It should be noted that the conductivityof Post-CL-P/H2SO4 at 95  C decreased slightly over time, but the
conductivity in approximately 300 hours aer keeping at 95  C
was still around 94% of the initial conductivity, 120 mS cm1
(Fig. S8, ESI†). Leaching out of the acid did not occur, either,even aer heating at 95  C for approximately 300 hours. This
also means fatal sulfate poisoning to catalyst does not occur at
this temperature.
Proton-conductive membranes can also be prepared not
only by chemical cross-linking but also by physical crosslinking.60,61 One example is self-assembly of an ABA triblock
copolymer62 that forms a nanophase-separated structure63
consisting of incompatible polymer blocks, where the middle
block B is bridged between the glassy domains of the end
blocks A. As a pioneering study of anhydrous protonconductive membranes using a block copolymer, Park andcoworkers prepared a membrane exhibiting the anhydrous
conductivities of 20 mS cm1 at 115  C and 45 mS cm1 at
165  C by inltrating a neutral aprotic ionic liquid into a poly(styrenesulfonate)-b-polymethylbutylene diblock copolymer,
which can form a nanophase-separated structure, but cannot
adopt a bridging conformation because of the nature of
diblock copolymer.64–66 Segalman and coworkers also prepareda membrane exhibiting the anhydrous conductivities of 3
mS cm1 at 65  C and 10 mS cm1 at 140  C by inltratinga protic ionic liquid into a poly(styrene)-b-poly(2-vinylpyridine)
diblock copolymer.67,68 More recently, Lodge, Hillmyer, andcoworkers prepared an anhydrous proton-conductive
membrane (4 mS cm1 at 120  C and 14 mS cm1 at 180  C)
by incorporating a protic ionic liquid of 1-ethylimidazolium
N,N-bis(triuoromethylsulfonyl)imidide into a poly(ethyleneoxide)-b-polystyrene diblock copolymer where the polystyrene
domain was chemically cross-linked.69 In our study, we
prepared an anhydrous proton-conductive membrane bysimply inltrating H2SO4 into the polystyrene-b-poly(4-vinylpyridine)-b-polystyrene (S–P–S) triblock copolymer (Fig. 5(e);see also Scheme S3, ESI†) membrane via acid–base complexation and ionic interaction. Note that H2SO4 can selectively
dissolve the middle block P only in S–P–S. As expected, even
the S–P–S/H2SO4 containing 80 wt% H2SO4 was a self-standingso membrane (Fig. 5(g)), because the middle chains P arebridging the glassy S domains, probably forming thenanophase-separated structure (Fig. 5(f)).60 The S–P–S/H2SO4
membrane also exhibited very high conductivity (for example,
100 mS cm1 at 80  C and 130 mS cm1 at 95  C; see also Table1), whose value is comparable to that of the humidied Naonmembrane. To the best of our knowledge, this is the rst
report of a proton-conductive membrane comprising a selfassembled block copolymer that exhibits a quite highconductivity of higher than 100 mS cm1 at approximately
100  C under almost no humidity; furthermore, it can befavorably processed with very little thickness and a large area

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Fig. 5 (a) Chemical structure of a post-cross-linked P (Post-CL-P) with base groups on the P chain. (b) Schematic illustration and (c) a photo ofa proton-conductive membrane of Post-CL-P/H2SO4 containing 80 wt% H2SO4. (d) Plots of anhydrous conductivity against reciprocal absolutetemperature for Post-CL-P/H2SO4 and S–P–S/H2SO4, where the weight content of H2SO4 is 0.8. Solid lines are drawn just as guide for eyes. (e)Chemical structure of an ABA triblock copolymer (S–P–S) comprising glassy polystyrene (S) and glassy poly(4-vinylpyridine) (P) at ambient
temperature. (f) Schematic illustration and (g) a photo of a proton-conductive membrane of S–P–S/H2SO4 containing 80 wt% H2SO4. (h) Stress–
strain curves for Post-CL-P/H2SO4 and S–P–S/H2SO4, where the weight content of H2SO4 is 0.8. In schematic, as shown in the FT-IR spectra ofFig. S3,† a pyridyl group and H2SO4 interact with each other, but broken lines and dotted lines indicating the interaction are not shown here forsimplicity.

since these membranes show thermoplastic property, which isa very useful feature for practical applications.
The mechanical properties were also evaluated by tensile
tests of the above proton-conductive membranes. Fig. S9(a) and(b) (ESI†) show photographs of the samples before and aer/during elongation for Post-CL-P/H2SO4 and S–P–S/H2SO4,
respectively. Fig. 5(h) exhibits stress–strain curves acquired, andTable S4 (ESI†) summarizes the following mechanical properties: Young's modulus (E), elongation at break (3b), maximum
stress (smax), and inner area of the stress–strain curve (S). E is theinitial slope of the stress–strain curve estimated from the low
strain region between 0% and 10% by the least squares method.
S corresponds to the energy used until break, which is an index
representing the robustness of the material. The E, 3b, smax, andS of the Post-CL-P/H2SO4 membrane are 0.10 MPa, 60%,
0.054 MPa, and 0.021 MJ m3, respectively. From these results,
Post-CL-P/H2SO4 is found to be a so solid membrane, although
it cannot be largely deformed. In contrast, the E, 3b, smax, and S
of the S–P–S/H2SO4 membrane are 0.10 MPa, 880%, 0.56 MPa,

15590 | J. Mater. Chem. A, 2019, 7, 15585–15592

and 2.1 MJ m3, respectively. Here, E is the same value as that of
the Post-CL-P/H2SO4 membrane, while both 3b, and smax have 10
times larger values. Therefore, S is 100 times larger than that of
the Post-CL-P/H2SO4 membrane, revealing that the S–P–S/H2SO4
membrane has far superior mechanical properties than Post-CLP/H2SO4 membrane. Since Post-CL-P is chemically cross-linked
and the cross-linking points are generated nonuniformly(Fig. 5(b)), the stress concentration at certain cross-linking
points tends to occur during elongation, which probably
induces easy fracture of the membrane. On the other hand, the S
hard domains formed by the self-assembly of the S–P–S block
copolymer are located uniformly (Fig. 5(f)), which reduces thestress concentration on certain domains that behave as physicalcross-links, resulting in the large 3b of S–P–S/H2SO4.

Conclusions
In conclusion, we report the preparation of anhydrous protonconductive polymer membranes exhibiting high conductivity

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Communicationof higher than 100 mS cm1, comparable to that of humidied
Naon. The preparation procedure adopted in this study
includes the spontaneous and homogeneous inltration ofa strong acidic liquid into the cross-linked exible basic polymer. Even if the membranes contain a large amount of H2SO4 (4times the weight of the polymer), no spontaneous leaching out
was observed. In addition, even if Tg is far below the ambient
temperature due to the incorporation of a large amount of
H2SO4, the membranes are self-standing because the polymers
are cross-linked. When the molar ratio of the acid to the basegroup (nAcid/nBase) exceeds unity, the excess molar amount of
H2SO4 acts as a plasticizer and the concentration of free protons
increases, resulting in a drastic decrease in Tg and a remarkableincrease in anhydrous conductivity. Moreover, by incorporating
about 4 times the weight of H2SO4 in CL-P, the membrane,
despite anhydrous conditions, exhibits the very high conductivity of more than 100 mS cm1, comparable to humidied
Naon. Furthermore, by swelling an S–P–S triblock copolymer
membrane with H2SO4, it exhibits not only anhydrous highconductivity, but also excellent robustness, especially to a large
breaking elongation of 800% or more, since the hard domains
serving as physical cross-links are located uniformly. In the
future, we will investigate eﬀects of humidity on the conductivity of membranes and the performance of assembled fuelcells under anhydrous conditions (see also Fig. S10 (ESI†) of thepreliminary result on performance of a fuel cell assembled withthe anhydrous proton-conductive membranes under anhydrous
conditions, though the performance was not good; this isprobably because preparation and measurements of theassembled fuel cell have not been optimized yet. If catalyst layer
preparation by inltrating ion species is optimized, the muchbetter fuel cell performance will be attained even under anhydrous conditions). Further development of higher-performance
anhydrous proton-conductive polymer membranes based ona similar molecular design are desired to achieve practicalapplications of next generation fuel cells.

Conﬂicts of interest
Two patent applications covering this work have been led by
Nagoya University and Toyota Motor Corporation,naming N. N., A. N., T. K., H. T., and Y. M. as inventors.

Acknowledgements
The authors thank Mr Masashi Suzuki and Mr Naoki Inukai at
the Aichi Center for Industry and Science Technology, Japan, fortheir kind help for AC impedance measurements. This work waspartially supported through KAKENHI grant number 15K13785(A. N.) from JSPS, Japan. A. N. is also grateful for Toyota Most
Advanced Collaborative Research Grant.

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