High anhydrous proton conductivity of imidazole-loaded mesoporous polyimides over a wide range from subzero to moderate temperature

Article abstract of DOI:10.1021/ja511389q

On-board fuel cell technology requires proton conducting materials with high conductivity not only at intermediate temperatures for work but also at room temperature and even at subzero temperature for startup when exposed to the colder climate. To develop such materials is still challenging because many promising candidates for the proton transport on the basis of extended microstructures of water molecules suffer from significant damage by heat at temperatures above 80 °C or by freeze below -5 °C. Here we show imidazole loaded tetrahedral polyimides with mesopores and good stability (Im@Td-PNDI 1 and Im@Td-PPI 2) exhibiting a high anhydrous proton conductivity over a wide temperature range from -40 to 90 °C. Among all anhydrous proton conductors, the conductivity of 2 is the highest at temperatures below 40 °C and comparable with the best materials, His@[Al(OH)(1,4-ndc)]_n and [Zn₃(H₂PO₄)₆(H₂O)₃](Hbim), above 40 °C.

Full text of DOI:10.1021/ja511389q

Article
pubs.acs.org/JACS
High Anhydrous Proton Conductivity of Imidazole-Loaded
Mesoporous Polyimides over a Wide Range from Subzero to
Moderate Temperature
Yingxiang Ye,† Liuqin Zhang,† Qinfang Peng,† Guan-E Wang,‡ Yangcan Shen,† Ziyin Li,† Lihua Wang,†
Xiuling Ma,† Qian-Huo Chen,† Zhangjing Zhang,*,†,‡ and Shengchang Xiang*,†,§
†College of Chemistry and Chemical Engineering, Fujian Provincial Key Laboratory of Polymer Materials, Fujian Normal University,
32 Shangsan Road, Fuzhou 350007, China
‡State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, PR China
§College of Life and Environmental Sciences, Minzu University of China, 27 South Zhongguancun Boulevard, Haidian District,
Beijing 100081, China
S
* Supporting Information
ABSTRACT: On-board fuel cell technology requires proton
conducting materials with high conductivity not only at
intermediate temperatures for work but also at room
temperature and even at subzero temperature for startup
when exposed to the colder climate. To develop such materials
is still challenging because many promising candidates for the
proton transport on the basis of extended microstructures of
water molecules suffer from significant damage by heat at
temperatures above 80 °C or by freeze below −5 °C. Here we
show imidazole loaded tetrahedral polyimides with mesopores
and good stability (Im@Td-PNDI 1 and Im@Td-PPI 2)
exhibiting a high anhydrous proton conductivity over a wide temperature range from −40 to 90 °C. Among all anhydrous proton
conductors, the conductivity of 2 is the highest at temperatures below 40 °C and comparable with the best materials,
His@[Al(OH)(1,4-ndc)]n and [Zn3(H2PO4)6(H2O)3](Hbim), above 40 °C.
Recently, crystalline porous materials,9,10 such as metal−
INTRODUCTION
■
organic frameworks (MOFs)/porous coordination polymers
Proton exchange membrane fuel cells (PEMFCs) are very
promising as a replacement for traditional engines due to their
high power density and ultralow emission features. The state-
(PCPs) and covalent organic frameworks (COFs), show
tremendous possibilities to be candidates for electrolytes and
for better understanding of proton conduction in diverse
of-the-art PEMFCs with Nafion as electrolytes can reach a
structures.11 Their designable framework structures and high
conductivity on the order of 10−1−10−2 S cm−1 under a limited
surface areas provide great opportunities to orderly accom-
modate proton carriers as well as to systemically modify the
condition with moderate temperatures (60−80 °C) and high
relative humidity (98% RH).1 Platinum catalysts are necessary
concentration and mobility of proton carriers in pores. Many
for proton exchange membranes (PEMs) under this operating
porous MOFs with hydrated water could possess interesting
temperature, which will lead to high cost and potential for CO
poisoning.2 In addition, its conductivity will drop significantly
above 80 °C due to the loss of inner water, while significant
damage to the hydrated PEMs will be induced by freeze/thaw
cycles after operation below −5 °C.3 Much attention has been
paid to develop new electrolytes for PEMs able to operate at
the temperatures above 80 °C4−8 but little to develop the PEM
electrolytes at subzero temperatures. Therefore, it is desirable
but challenging to develop high proton-conductive electrolytes
operating over a wide performing temperature range, especially
for fuel cells in on-board automotive application: working at
moderate temperature and startup at room temperature, even
subzero temperature when exposed in the colder climate.
proton conducting properties at moderate temperature and
high humidification.12−15 For the purpose of a higher
temperature (>80 °C) operation system without dependency
on humidity variations, non-water-media molecules, including
nonvolatile acids16 (such as H2SO4 and H3PO4) and organic
aryl molecules4−7 (such as triazole and imidazole), have been
introduced into pores of MOFs to form anhydrous proton
carrier pathways. Among them, H2SO4@MIL-101 achieved the
highest σ value of 1 × 10−2 S·cm−1 at 150 °C under very low
humidity (0.13% RH).16 [Zn3(H2PO4)6(H2O)3](Hbim) and
Received: November 5, 2014
Published: December 30, 2014
© 2014 American Chemical Society
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Article
Scheme 1. Synthesis Routes to Im@Td-PNDI and Im@Td-PPI
Figure 1. N2 sorption isotherms of (a) Td-PNDI and (b) Td-PPI at 77 K. Solid and open symbols represent adsorption and desorption, respectively.
(Inset) Pore size distributions of (a) Td-PNDI and (b) Td-PPI calculated by the NLDFT method.
His@[Al(OH)(1,4-ndc)]n presented intrinsic anhydrous pro-
ton conductivity as high as 1.3 × 10−3 S cm−1 at 120 °C and 1.7
× 10−3 S cm−1 at 150 °C,4a,6a respectively. However, poor
hydrolytic stability of the coordination bonds and the
framework backbone of MOFs limit their industrial applicability
under fuel cell operating conditions. Covalent organic frame-
works (COFs),17 being one type of porous polymers, resemble
perfect crystalline organic analogues to the MOFs.18 Research
work on proton conducting COFs is still in its infancy because
they suffer from chemical and hydrothermal instability, and
only specially designed monomers can enhance their stability.19
To date, only one example, H3PO4@Tp-Azo, has been tested
and shows proton conducting behaviors under both hydrous
and anhydrous conditions (9.9 × 10−4 and 6.7 × 10−5 S cm−1,
respectively).20 Its anhydrous proton conductivity reaches a
maximum around 59−67 °C and then decreases gradually upon
increasing temperature. Thus, it is crucial to explore new
porous materials with good stability and proton-conducting
performance under different working conditions.
In this paper, the strategy through loading imidazole carriers
into mesoporous polyimides is proposed to realize the first
examples of porous organic polymers (POPs based proton
conductors (Im@Td-PNDI 1 and Im@Td-PPI 2) operating
over a wide temperature range from subzero to moderate
temperatures above 80 °C, under the following considerations:
(1) Imidazole (Im) is chosen as the proton transfer agent
because it shows fast proton transfer behavior and an
amphiprotic nature, analogous to that of water.5 Superior to
water media, imidazole with high melting and boiling points
will support a more stable proton-transport pathway in a wide
temperature range. (2) The tetrahedral polyimides (Td-PNDI
and Td-PPI) show robust frameworks, good thermal and
hydrolytic stability even when exposed to aqueous and acidic
conditions,21,22 and designable large mesopores favoring an
increase in the moving freedom of proton carriers to achieve
good proton conducting characteristics, while all hosts reported
for the proton conductor with imdazole carriers are micro-
porous coordination polymers.5 (3) These POPs without metal
ions may be more electrochemically stable in comparison with
MOFs.
RESULTS AND DISCUSSION
■
The imidazole-loaded proton-conducting POP materials 1 and
2 were prepared by vaporizing imidazoles into the solvent-free
polyimides Td-PNDI and Td-PPI at 120 °C for 36 h. As shown
in Scheme 1, Td-PNDI and Td-PPI are both constructed from
the condensation of tetrahedral building blocks tetrakis(4-
aminophenyl)-methane (TAPM) and rigid dianhydride linkers,
perylene dianhydride (PTDA) linker in Td-PPI, instead of
naphthalene dianhydride (NTDA) in Td-PNDI. Solvent-free
Td-PNDI has been confirmed to be permanently porous and
robust.21a Computational simulations have been used pre-
viously to predict or rationalize the amorphous structures of
Td-PNDI and Td-PPI with a diamond topology.23 The
permanent porosity of solvent-free polyimide networks in Td-
PNDI and Td-PPI was evaluated by the N2 adsorption
isotherm at 77 K. As illustrated in Figure 1, Td-PNDI shows
a type-I sorption behavior typical for microporous materials
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Figure 2. Solid-state UV−vis absorption spectra (a, b) and TGA curves (c, d) of polyimides (black) and imidazole loaded polyimides (red).
with a Brunauer−Emmett−Teller (BET) surface area of 465 m2
g−1, while the isotherm of Td-PPI is typical of Henry linear
adsorption for mesoporous structures with a BET surface area
of 325 m2/g. The single point pore volume at P/Po of 0.97 for
Td-PNDI and Td-PPI are 0.245 and 0.319 cm3/g, respectively.
The differential pore volume distributions for both polymers
are calculated using the nonlocal density functional theory
(NLDFT) method. The major pores in Td-PNDI locate at
around 0.8 and 1.2 nm, while Td-PPI has a wide pore-size
distribution in the mesopore region ranging from 3 to 13 nm,
indicating that Td-PPI has a larger space than Td-PNDI to hold
up imidazole molecules (4.3 × 3.7 Å2).
bands around 1053 cm−1 agree well with the C−N stretching
vibrations of imidazole in the compounds. The FT-IR spectral
information indicated the imidazole molecules were successfully
loaded in the polyimide frameworks. After imidazole was
vaporized into guest-free Td-PNDI and Td-PPI at 120 °C for
36 h, obvious color changes were observed from tan to red-
brown for 1 and from dark green to dark red for 2, respectively.
Correspondingly, two new peaks appearing at around 475 and
520 nm, respectively, in the UV−vis absorption spectra of 1
and 2 are not observed in the spectra of imidazole24 or
polyimides (Td-PNDI and Td-PPI), which can be attributed to
the host−guest interactions between the host POP frameworks
and the guest imidazole molecules (Figure 2a,b).
It is essential to examine the existence of imidazole molecules
in the pores of polyimide, rather than aggregation on the outer
surface, which has been confirmed by solid-state FT-IR spectra,
UV−vis absorption spectra, powder X-ray diffraction (PXRD),
scanning electron microscope (SEM), and TGA and DSC
profiles in Figure 2 and Figures S2−S5 (Supporting
Information). SEM images show no obvious change in the
surfaces of 1 and 2 in comparison with their own parent POP
materials, indicating imidazole molecules entered into pores of
polyimide frameworks without being aggregated on the outer
surface of polyimides (Figure S2, Supporting Information).
PXRD measurements of polyimides reveal broad peaks,
implying that both polyimides are amorphous, and the presence
of several peak maxima in the diffractogram indicates the locally
ordered nature of these networks (Figure S3, Supporting
Information).21,22 After imidazole was loaded, 1 and 2
presented the same angle peaks with polyimide powders.
Solid-state FT-IR spectra of the initial monomers and
polyimides are presented in Figure S4 (Supporting Informa-
tion). In the imidazole loaded polyimides, the weak absorption
at around 3140 cm−1 could be attributed to the unsaturated C−
H stretching vibrations of imidazole molecules. The absorption
TGA measurements were carried out to examine the thermal
stability of the porous networks (Figure 2c,d). Both parent
polyimide frameworks are stable up to 500 °C. The
thermogravimetric profiles show 28.4% weight imidazole
loading or 4.9 imidazole/1 TAPM for 1, and 30% weight or
6.9 imidazole/1 TAPM for 2. The release of accommodated
imidazole in 1 starts at 130 °C and finishes at 238 °C, and the
release in 2 begins at 120 °C and finishes at 240 °C. The lower
starting temperature in 2 implies that the imidazole guests have
less interaction with Td-PPI hosts, and thus may have higher
mobility in 2, in contrast with those in 1. Considering that the
dispersion of imidazole is uniform in the pores, the density of
imidazole loaded in the pores could be calculated from the
single point pore volumes of the hosts. The values are 1.15 and
0.94 g/cm3 for 1 and 2, respectively, lower than 1.23 g/cm3 for
bulk imidazole solid at ambient temperature,25 indicating that
the loaded imidazole molecules in polyimide frameworks have a
different packing arrangement with bulk imidazole. It is further
confirmed by the DSC profiles in Figure S5 (Supporting
Information). For the bulk imidazole, it shows a sharp
exothermic peak at the melting temperature (93 °C), while
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Figure 3. Arrhenius plots for 1 (a) and 2 (b) under anhydrous conditions. Least-squares fitting is shown as a solid line. (Inset) Typical Nyquist plots
of 1 and 2 under anhydrous conditions at different temperatures.
the imidazole loaded 1 and 2 do not present a clear peak from
room temperature to 120 °C, suggesting that the accom-
modated imidazole does not undergo phase transition in the
temperature range measured.
S cm−1, which has never been shown in crystalline porous
materials. It can be contributed from imidazole molecules with
high melting points. Upon heating, their conductivity values
gradually increase to 2.26 × 10−5 S cm−1 for 1 and 7.64 × 10−5
S cm−1 for 2 at ambient temperature (30 °C), much higher
than the values for solid bulk imidazole (10−8 S cm−1)5a as well
as for other imidazole loaded microporous materials (3.3 ×
10−8 and 5.5 × 10−8 S cm−1 for [Zn(HPO4)(H2PO4)2] and
[Al(μ2-OH)(1,4-ndc)], respectively).5a,b The value of 2 is the
highest among all reported anhydrous proton conductors under
these working conditions (Figure 4 and Figure S8, Supporting
Information).4−8,13f,20 Until a temperature of 90 °C, their
conductivity values reach a maximum of 9.04 × 10−5 S cm−1 for
1 and 3.49 × 10−4 S cm−1 for 2, respectively, which is
Temperature-dependent proton conductivities of solvent-free
polyimides (Td-PNDI and Td-PPI) and imidazole loaded
polymers (1 and 2) were measured by using AC impedance
spectroscopy under anhydrous conditions. No semicircle was
observed in Td-PNDI and Td-PPI, which is indicative of
negligible proton conductivity for these parent polyimide
frameworks. The existence of imidazoles leads to typical proton
conductivities of 1 and 2, determined from the semicircles in
the Nyquist plots, as shown in Figure 3 and Figures S6 and S7
(Supporting Information). The proton conductivity of 2 is
higher than that of 1 over the temperature range from −40 to
90 °C, which can be attributed to the lower density and higher
mobility of imidazole guests accommodated in the larger
mesopore of Td-PPI. The comparison of the conductivities of 1
and 2 with some representative MOF and COF materials under
anhydrous conditions is shown in Figure 4, Figure S8
comparable to [ImH2][Cu(H2PO4)2Cl]·H2O (5.12 × 10−4
S
cm−1)5c and higher than other imidazole-impregnated MOFs.
In particular, the value of 2 at 90 °C is comparable with the
highest values (7.02 × 10−4 S cm−1 for His@[Al(OH)(1,4-
4 a
n d c ) ] n
a n d 4 . 1 8
×
1 0 − 4
S
c m − 1 f o r
Zn3(H2PO4)6(H2O)3(Hbim))6a among the anhydrous proton
conducting MOF and COF materials reported to date.
Moreover, the proton conductivity of 2 within the temperature
range from room temperature to 90 °C is the highest among all
imidazole incorporated porous materials without the contribu-
tions from lattice water.5c This may possibly be due to the large
space provided by the mesopores in 2 favors to the movements
of the imidazole then to improve the proton mobility. On the
basis of the Arrhenius equation (Figure 3), the activation
energy of 1 and 2 is 0.33 and 0.30 eV, respectively, clearly
smaller than the reported anhydrous imidazole-loaded MOFs.5
The low activation energy reflects the Grotthuss mechanism of
proton conduction for 1 and 2.12a,e Notably, the observation of
the high proton conductivity and low energy values of 2 in the
temperature range from 40 to 90 °C suggests that it belongs to
anhydrous fast-ion conductors.12a
Figure 4. Arrhenius plot for 1 and 2 in comparison with other
representative proton-conducting MOFs and COF materials under
anhydrous conditions.
CONCLUSIONS
■
In conclusion, the strategy based on tetrahedral polyimides
(Td-PNDI and Td-PPI) with controllable mesopores and good
stability via imidazole loading is proposed to realize the first
examples of porous organic polymer (POP) materials (Im@
Td-PNDI 1 and Im@Td-PPI 2) with high anhydrous proton
conductivity over a wide temperature range from subzero to 90
°C. Obvious proton conducting capability at low temperature
down to −40 °C for 1 and 2 has never been shown in other
MOF or COF materials. Td-PPI has larger mesopores than Td-
(Supporting Information), and Table S2 (Supporting Informa-
tion). Anhydrous proton conductivities for most of the MOF
and COF materials reported are remarkably reduced below
room temperature, similar to the hydrated PEMs suffering from
significant damage by freeze. Even under subzero temperature
(−40 °C), obvious proton conductivity for 1 and 2 has been
observed with respective values of 4.58 × 10−7 and 2.23 × 10−6
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PNDI to accommodate the imidazole guests with the lower
density and higher mobility, resulting in the higher proton
conductivity for 2. The conductivity reaches a maximum (9.04
× 10−5 S cm−1 for 1 and 3.49 × 10−4 S cm−1 for 2) at 90 °C.
Among all anhydrous proton conductors, the conductivity of 2
is the highest at the temperatures below 40 °C, and 2 belongs
to anhydrous fast-ion conductors in the temperature range from
40 to 90 °C with the values comparable with those for the best
m a t e r i a l s , H i s @ [ A l ( O H ) ( 1 , 4 - n d c ) ] n a n d
[Zn3(H2PO4)6(H2O)3](Hbim). We will continue working on
the design of new POP-based proton conducting materials and
their fuel cell applications. We believe that our findings will
encourage further work in subzero-temperature proton
conducting materials in the near future.
AUTHOR INFORMATION
■
Corresponding Authors
*zzhang@fjnu.edu.cn
*scxiang@fjnu.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (21207018, 21273033, and
21203024) and the Fujian Science and Technology Depart-
ment (2014J06003). S.X. gratefully acknowledges the support
of the Recruitment Program of Global Young Experts, Program
for New Century Excellent Talents in University (NCET-10-
0108), and the Award “MinJiang Scholar Program” in Fujian
Province.
EXPERIMENTAL SECTION
■
Preparation of Imidazole-Loaded Frameworks. After syn-
theses of TAPM, Td-PNDI, and Td-PPI (Supporting Information),
Td-PNDI and Td-PPI were finely grounded and then degassed by
heating to 120 °C under reduced pressure for 12 h to remove guest
molecules. Imidazole was vaporized into guest-free Td-PNDI (0.151
g) and Td-PPI (0.151 g) at 120 °C for 36 h to yield 1 and 2. The
amount of loaded imidazole was determined by thermogravimetric
analysis.
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⎛
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