The synthesis and electrical properties of hybrid gel electrolytes derived from Keggin-type heteropoly acids and 3-(pyridin-1-ium-1-yl)propane-1-sulfonate (PyPs)

Article abstract of DOI:10.1039/c6ra23082b

Herein, we report the effect of the proton concentration in polyoxometalates (POMs) upon hybrid formation with ionic liquids (ILs), and their ionic conductivity relationship to optimize their ionic conductivity. The hybrid gels were derived from Keggin-type heteropoly acids containing different proton concentrations, such as H₃PW₁₁MoO₄₀, H₄PMo₁₁VO₄₀ and H₅PMo₁₀V₂O₄₀, and 3-(pyridin-1-ium-1-yl)propane-1-sulfonate (PyPs) IL. Elemental C, H, and N analysis was found to be consistent with the theoretical composition within 4% for C and N, whereas H content was found to be slightly higher than the anticipated value, which may be due to potential uptake of water during the sample preparation. ¹H and ¹³C nuclear magnetic resonance and Fourier transform infrared spectroscopy (FTIR) confirmed the presence of functional groups of PyPs in the hybrids. In situ variable temperature powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), electrochemical AC impedance spectroscopy and cyclic voltammetry studies showed excellent thermal (up to ～300 °C) and electrochemical (3 V at room temperature) stability of [PyPs]₃PW₁₁MoO₄₀. The structural characterizations confirmed the interaction between the organic cation and Keggin-type inorganic heteropoly anion in the hybrid material. The bulk ionic conductivity of 0.1, 0.01 and 0.0003 S cm^-1 at ～90 °C was obtained for [PyPs]₃PW₁₁MoO₄₀, [PyPs]₄PMo₁₁VO₄₀ and [PyPs]₅PMo₁₀V₂O₄₀, respectively.

Full text of DOI:10.1039/c6ra23082b

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Synthesis and electrical properties of hybrid gel electrolytes derived from Keggin-type heteropoly
acids and 3-(pyridin-1-ium-1-yl) propane-1-sulfonate (PyPs)
*
Sumaletha Narayanan, Xia Tong and Venkataraman Thangadurai
Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta,
T2N 1N4 Canada *For correspondence (vthangad@ucalgary.ca) Phone: 1 403 210 8649
Abstract: Here, we report the effect of concentration of protons in polyoxometalates (POMs) on hybrid
formation with ionic liquids (ILs) and their ionic conductivity relationship to optimize their ionic
conductivity. The hybrid gels are derived from the Kegginꢀtype heteropoly acids containing different
proton concentration, but contains similar transition metals, such as H PW MoO , H PMo VO and
3
11
40
4
11
40
H PMo V O and 3ꢀ(pyridinꢀ1ꢀiumꢀ1ꢀyl) propaneꢀ1ꢀsulfonate (PyPs) IL. Elemental C, H, and N
5
10
2
40
analysis was found to be consistent with theoretical composition within 4 % for C and N, while H
content was found to be slightly higher than anticipated value, which may be due to potential uptake of
13
water during the sample preparation. Proton and C nuclear magnetic resonance and Fourier transform
infrared spectroscopy (FTIR) confirmed the presence of functional groups of PyPs in hybrids. Inꢀsitu
variable temperature powder Xꢀray diffraction (PXRD), thermogravimetric analysis (TGA),
electrochemical AC impedance spectroscopy and cyclic voltammetry studies showed excellent thermal
(
up to ~ 300 °C) and electrochemical (3V at room temperature) stability of [PyPs] PW MoO . The
3
11
40
structural characterizations confirm the interaction between the organic cation and Kegginꢀtype
inorganic heteropoly anion in the hybrid material. The bulk ionic conductivity of 0.1, 0.01 and 0.0003
ꢀ1
Scm at ~ 90 °C was obtained for [PyPs] PW MoO , [PyPs] PMo VO and [PyPs] PMo V O ,
3
11
40
4
11
40
5
10
2
40
respectively.
1

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1
. Introduction
A lot of efforts have been made recently to prepare hierarchical structured materials for alternative
electrochemical energy storage (for e.g., batteries and capacitors) and conversion (for e.g., fuel cells)
1
ꢀ3
technologies. These hierarchical materials are being prepared by softꢀchemical synthesis methods and
1
ꢀ6
4
they exhibit superior physical and chemical properties. For example, nanosheet array of NiMoO4
−
1
prepared by hydrothermal method showed specific capacity of 1483 Fg and nanopyramid array of
5
2
metallic CoS synthesized by oneꢀstep solvothermal method showed low overꢀpotential of 0.14V for H
2
evolution under acidic condition. Hybrid polyoxometalates (POMs) and ionic liquids (ILs) have drawn a
wide attention in the material research due to their applications in catalysis, medicine, and
7
ꢀ12
electrochemical energy conversion and storage devices.
POMs are anionic metal oxides derived from
transition metal oxides, which have a robust structure and exhibit very useful functional physical and
1
3
chemical properties, including electrical, magnetic, electrochemical and photonic properties. ILs are
low melting point salts, which are typically comprised of organic cations and inorganic or organic
anions possessing very low vapour pressure, nonꢀflammability, high ionic conductivity, and high
1
4ꢀ16
chemical, thermal and electrochemical stability window.
These unique properties of POMs and ILs
are combined in their respective hybrid compounds, which ensure the tunability of desired physical and
chemical properties by varying the transition metal cations in formal case and organic cations and anions
in the later case. The POMꢀbased IL salts offer several advantages, such as solventꢀfree, residual acidity
and good thermal stability, and much higher ionic conductivity than the corresponding anhydrous solid
analogues. They also exhibit a low vapor pressure, which is beneficial for highꢀtemperature (above 100
o
C) fuel cells and catalytic applications. The commonly used organic cations are substituted
imidazolium, pyridinium, and pyrrolidinium and alkyl ammonium and phosphonium cations and anions
1
7ꢀ21
include tetrafluoroborate, hexafluorophosphates and bis(trifluoromethanesulfonyl)imide.
Various
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hybrid gels were developed using POMs and ILs such as Nꢀmethyl imidazoliumꢀ1ꢀ(3ꢀsulfonic group)
propyl (MIMPS), 3ꢀ(pyridineꢀ1ꢀiumꢀ1ꢀyl)propaneꢀ1ꢀsulfonate (PyPs), 1ꢀ(3ꢀsulfonic group) triethylamine
2
2ꢀ27
(TEAPS) and NꢀpropylꢀNꢀmethylpiperidinium hexafluorophosphate.
Several heteropoly acids
(HPAs) and polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) hybrids were also recently
2
8
reported.
Here, we report the synthesis, structural, chemical and electrochemical characterization of hybrid
2
4
gel electrolytes based on recently developed 3ꢀ(pyridinꢀ1ꢀiumꢀ1ꢀyl)propaneꢀ1ꢀsulfonate (PyPs) IL and
H PW MoO , H PMo VO and H PMo V O for electrochemical energy conversion and storage
3
11
40
4
11
40
5
11
2
40
applications. The investigated POMsꢀILs hybrid materials were found to be thermally stable to about
ꢀ1
3
00 °C. Hybrid [PyPs] PW MoO showed the highest electrical conductivity of 0.1 Scm at ~ 90 °C in
3
11
40
ambient condition, which is comparable to Nafion proton conduction under high relative humidity, and
electrochemical stability window of ~ 3V at room temperature.
2
2
. Experimental section
.1 Synthesis of solid acid H PW MoO (H PWMo). Heteropoly acid (HPA) or polyoxometalate
3
11
40
3
(
POM) of the nominal chemical composition H PW MoO was synthesized according to the
3 11 40
2
2
literature. Na MoO (0.01 mol) (BDH, 99.5%) was dissolved in 20 ml of deionized water.
2
4
Na WO .2H O (0.1 mol) (Sigma Aldrich, 99%) was mixed with 0.02 mol of Na PO .12H O (Fisher
2
4
2
3
4
2
Scientific, 100%) and dissolved in 100 ml of deionized water and is added to the sodium molybdate
solution. Dilute H SO (1:1) was used to adjust pH of the mixture to 2.5 and was then heated at 90 °C
2
4
for 2h. It was allowed to cool down to room temperature and the solution was extracted using ether (50
ml) and dilute H SO (1:1) prior to drying in vacuum oven at 50 °C for 24h, to obtain the solid product.
2
4
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2
.2 Synthesis of solid acid H PMo VO (H PMoV). 0.01 mol of NaVO (BDH, 98%) was dissolved
4 11 40 4 3
in 20 ml of deionized water. 80 mL of Na MoO (0.1 mol) (BDH, 99.5%) was mixed with 20 mL of
2
4
Na PO .12H O (0.01 mol) (Fisher Scientific, 100%) solution. The rest of the steps are the same as
3
4
2
mentioned for H PWMo synthesis.
3
2
.3 Synthesis of solid acid H PMo V O (H PMoV). 0.02 mol of NaVO (BDH, 98%) was dissolved
5 10 2 40 5 3
in 20 ml of deionized water. Na WO .2H O (0.1 mol) (Sigma Aldrich, 99%) was mixed with 0.01 mol
2
4
2
of Na PO .12H O (Fisher Scientific, 100%) and dissolved in 100 ml of deionized water. The rest of the
3
4
2
steps are the same as mentioned for H PWMo synthesis.
3
2
.4 Synthesis of PyPs ionic liquid. Synthesis of ionic liquid, 3ꢀ(pyridineꢀ1ꢀiumꢀ1ꢀyl) propaneꢀ1ꢀ
2
9
sulfonate (PyPs) was carried out as per the procedure reported elsewhere. As received 4.4 ml of 1,2ꢀ
oxathiolane 2,2ꢀdioxide (Alfa Aesar, 99%) was added to 50 ml of Acetone. 4.0 ml Pyridine (Alfa Aesar,
9
9%) was added to this solution and was continuously heated at 50 °C for 4h while stirring. A white
precipitate was formed which was filtered, washed with ether and dried in vacuum.
2
.5 Synthesis of hybrid gels. PyPs (3ꢀ(pyridineꢀ1ꢀiumꢀ1ꢀyl)propaneꢀ1ꢀsulfonate) and POM hybrid gel
electrolytes were synthesized by taking 3:1, 4:1, and 5:1 mole ratio of PyPs and H PW MoO ,
3
11
40
H PMo VO , and H PMo V O , respectively. PyPs was added to aqueous solution of the heteropoly
4
11
40
5
10
2
40
acids, H PW MoO , H PMo VO , and H PMo VO under constant stirring for overnight at room
3
11
40
4
11
40
5
11
40
temperature. Water was removed from the solution by vacuum drying to get the desired hybrid gel
products
[PyPs] PW MoO
(PyPsꢀH PWMo),
[PyPs] PMo VO
(PyPsꢀH PMoV)
and
3
11
40
3
4
11
40
4
4

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[
PyPs] PMo V O (PyPsꢀH PMoV). Schematic representation of the formation of hybrid gel
5 10 2 40 5
electrolytes starting from PyPs and POMs is shown in Scheme 1.
2
.6 Characterization. PerkinElmer Series II C H N S/ O Analyzer 2400 was used for the CHN analysis
to understand the composition of PyPs and PyPsꢀPOM hybrids. The purity of these compounds were
1
13
checked by employing H and C NMR (nuclear magnetic resonance) (Bruker RDQ 400 NMR
spectrometer) spectroscopy. The Fourier transform infrared spectra of the samples were recorded by
ꢀ1
employing Varian 7000 FTIR spectrometer in the range of 600ꢀ2000 cm using KBr standard. The
structural analysis was carried out using Powder Xꢀray diffraction and a Bruker D8 Advance powder Xꢀ
ray diffractometer (CuꢀKα radiation) was used for this purpose. The scanning was carried out at a rate of
−1
0
.02° s from 2θ = 2 to 40° at room temperature. The variable temperature (30ꢀ120 °C) PXRD was also
done using Anton Par XRK 900, a high temperature reactor chamber. The samples were hold for 30
minutes at each temperature to ensure the thermal equilibrium. Thermogravimetric analyzer (Setaram
TAG 16 TGA/DSC dual chamber balance) was used to study the thermal stability of the samples under
N gas up to 650 °C at a heating rate of 10 °C/min. The electrical conductivity measurements were done
2
using a Solartron SI 1260 impedance and gainꢀphase analyzer in the frequency range of 0.01 Hz to 1
MHz (AC amplitude 100 mV) in air by heating the sample from room temperature to 100 °C.
VersaSTAT 3 potentiostat was employed for the cyclic voltammetry for the 3ꢀelectrode sample set up
and Pt electrodes were used as the working (WE) and counter electrodes (CE) and Ag/AgCl was used as
1
6
the reference electrode (RE) under N atmosphere.
2
5

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3
3
. Results and discussion
.1 Phase analysis. Fig. S1 shows the photographs of heteropoly acid, H PMo VO and its derivative
4
11
40
hybrid PyPsꢀH PMoV compound at room temperature. The freshly made product was a dark green
4
transparent semisolid at room temperature. PyPsꢀH PMoV turned to liquid by heating it up to nearly 80
4
°
C, exhibiting thermotropic liquid crystal behaviour. The melting point of the hybrid compounds can be
3
0
related to the Coulombic attraction (E ) between the constituent ions, and is described by equation 1,
c
+
−
MZ Z
E =
c
2
r
(1)
+
-
where M is the Coulomb’s law constant, Z and Z are magnitudes of cations and anions charges, ε0 is
the permittivity of free space, and r is the interionic separation. For larger ions possessing lower ionic
charge, the formation of low melting salts is favoured. This is because the larger size enables the
effective delocalization of the charge as well as enhances the charge separation. Rickert et al. proposed
that the ionic salts made from a polyoxometalate anion and an organic cation will have lower melting
3
0
point when the POM has a lower charge.
3
.2 Composition analysis. CHN analysis results of the asꢀprepared PyPs (IL), and all its hybrid
compounds, PyPsꢀH PWMo, PyPsꢀH PMoV, and PyPsꢀH PMoV are summarized in Table S1 (see
3
4
5
supporting information). The C, H, and N results were found to be consistent with theoretical
composition within 4 % and the discrepancy in % H might be due to the uncertainty in quantifying the
water content of the hybrids. Scheme 1 shows the idealized synthesis steps of POMsꢀbased IL hybrid gel
electrolytes.
6

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Scheme 1: Synthesis of POMsꢀbased IL hybrid gel electrolytes.
The functional groups of PyPs and the hybrid POMsꢀILs are further proved by NMR analyses (Fig. 1).
1
H NMR results show the chemical shift (ppm) values corresponding to the structure, PyPs: 2.46 (p,
2
H), 2.98 (t, 2H), 4.77 (t, 2H), 8.08 (t, 2H), 8.56 (t, 1H), 8.88 (d, 2H); PyPsꢀH PWMo: 2.47 (s, 2H), 2.98
3
(
m, 3H), 4.81(s, 1H), 8.13 (d, 2H), 8.60 (s, 1H), 8.89 (s, 2H); PyPsꢀH PMoV: 2.44 (m, 2H), 2.96 (t, 2H),
4
4
.76 (s, 1H), 8.08 (s, 2H), 8.55 (s, 1H), 8.86 (d, 2H); and PyPsꢀH PMoV: 2.45 (m, 2H), 2.96 (t, 2H),
5
1
3
4
.77 (s, 1H), 8.08 (s, 2H), 8.55 (s, 1H), 8.87 (d, 2H). The corresponding C NMR chemical shift (ppm)
1
13
results are also consistent with the H NMR results. The C NMR chemical shift values are for PyPs:
2
1
6
6.20, 47.12, 59.98, 128.48, 144.47, 145.99; PyPsꢀH PWMo: 26.33, 47.22, 60.14, 128.79, 144.42,
3
46.26; PyPsꢀH PMoV: 26.19, 47.09, 59.96, 128.50, 144.42, 146.01; and PyPsꢀH PMoV: 26.23, 47.13,
4
5
1
13
0.01, 128.45, 144.42, 146.03. Fig. 1 summarizes the H NMR and C NMR results as a comparison
plot of both PyPs and the hybrid ILꢀPOMs.
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3
.3 FTIR analysis. The FTIR spectra are used to identify structural and bonding changes in the
polyoxoanion units present in POMꢀbased materials. Fig. 2 presents the FTIR spectra of hybrids, PyPsꢀ
H PWMo, PyPsꢀH PMoV and PyPsꢀH PMoV and their solid acid precursors. In the FTIR spectra, there
3
4
5
are four characteristic vibrational bands resulting from the αꢀKegginꢀtype heteropolyanion (for e.g.,
3
40
ꢀ
PM O
where M = W, Mo, V), ν(PꢀO ), ν(MꢀO ), ν(MꢀO ꢀM), and ν(MꢀO ꢀM) appearing in the
1
2
a
d
b
c
ꢀ1
region between 700 and 1100 cm (O ꢀterminal oxygen, O ꢀbridged oxygen of two octahedral sharing a
d
b
corner, and O ꢀbridged oxygen of two octahedral sharing an edge). The FTIR spectra of POMꢀILs are
c
found to be identical to that of corresponding pristine solid acids, which also shows the splitting of the
PꢀO stretching, MꢀO stretching, stretching of MꢀO ꢀM inter bridges between cornerꢀsharing MO
6
a
d
b
octahedral and stretching of MꢀO ꢀM intra bridges between edgeꢀsharing MO octahedral. In the IR
c
6
spectra of the hybrid materials, those four wellꢀknown characteristic bands prove the presence of the
core HPA clusters. The decrease in electrostatic anionꢀanion interactions leads to an increase in the
stretching and bending vibrational frequencies. Heteropoly acid, H PW MoO and its hybrid material
3
11
40
PyPsꢀH PWMo MꢀO ꢀM and MꢀO ꢀM asymmetrical stretching vibration are blueꢀshifted and the wave
3
b
c
ꢀ1
numbers increase from 887 and 820 for H PW MoO to 914 and 855cm for PyPsꢀH PWMo. There
3
11
40
3
ꢀ1
ꢀ1
are also three characteristic peaks at 1221, 1185cm (S=O) and 1491 cm (pyridine ring), indicating that
3
ꢀ
[
αꢀPW MoO ] had been successfully immobilized on ionic liquid (PyPs)ꢀmodified compound. A
11
40
table summarizing the major vibration bands is given in Table 1.
.4 Power XRD phase analysis. Fig. 3a shows the PXRD patterns of POMꢀILs and their pristine solid
acids, which reveal their crystalline and amorphous state. The Keggin type of the polyoxoanions exhibits
3
3
1
typical peaks at the range of 2θ = 7ꢀ11° in the Xꢀray diffraction (XRD) pattern. These peaks at this
range are also observed in XRD patterns of all three hybrids materials. From these observations, it is
clear that the hybrid materials have been successfully prepared. However, the XRD peaks of these
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POMꢀILs are significantly different from that of the pristine acids. The PyPsꢀPOM hybrids have two
broad diffraction peaks at 2θ =7ꢀ10° and 15ꢀ40°, which show a smectic phase and are consistent with the
3
2
liquidꢀstate nature of the samples. Fig. S2a (see supporting information) presents the HTXRD patterns
of the PyPsꢀH PWMo under different temperatures. It shows the interꢀplanar spacing d (Å) of the ILꢀ
3
PMo V at 2θ ≈ 9° vs temperature. The d value is almost stable from room temperature to 70 °C. On
11
increasing the temperature after 70 °C, the absorbed water loss seems to be responsible for the decrease
of interꢀplanar d value. A similar trend was observed for PyPsꢀH PMoV hybrid as shown in Fig. S2b
4
(
see supporting information). Fig. 3b presents the PXRD pattern of PyPsꢀH PWMo in the small angle
3
region. Even though the intensities of peaks at 2θ = 3ꢀ5° are much weaker than the peak at 2θ ≈ 9°, they
still can be observed. The diffraction patterns contain the fundamental reflection in the smallꢀangle
2
2
region, which may be due to the regular arrangement of molecules in layers. As a result, we can
speculate that at first, the molecules are organized in spheroidal aggregates, which have a coreꢀshell
structure, where each polyoxoanion is surrounded by PyPs cations due to electrostatic interactions
between polyoxoanion and PyPs cation. Subsequently, ILꢀpolyoxoanion aggregates may be regularly
arranged in layers through the organic IL moieties. The schematic illustration of the idealized crystal
structure of POMꢀIL hybrid gel electrolyte is shown in Fig. 4.
3
.5 Thermal analysis. Fig. 5 shows the TGA of the HPAs and their derivative hybrid materials (POMsꢀ
IL). As expected, there is significant weight loss observed for the ILꢀhybrids compared to the
corresponding HPAs. H PMoV and its IL derivative (PyPsꢀH PMoV) show the initial low temperature
4
4
weight loss of 3.8 wt. % for the H PMoV and 3.4 wt. % for the PyPsꢀH PMoV due to 4 and 5 moles of
4
4
water, respectively. For the PyPsꢀH PMoV salt, the weight loss beginning at 250 °C is primarily due to
4
the decomposition of PyPs organic moieties, indicating that the material is stable below 250 °C. The
9

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huge weight loss of PyPsꢀH PMoV, nearly 27%, occur between 250 °C and 650 °C corresponds to the
4
proportion of PyPs contained in the PyPsꢀH PMoV (30 wt. %). The TGA of PyPs provided as inset also
4
shows the decomposition after 250 °C. The TGA curves showed the presence of water in the ILꢀPOMs
hybrid samples; hence, the potential uptake of water might have occurred during the sample preparation,
which is consistent with the result of C, H, and N elemental analysis. Similar observation was found for
H PWMo and its hybrid with lower weight loss as compared to that of H PMoV.
3
4
3
.6 Electrical properties. Fig. 6 shows typical impedance spectra of PyPsꢀPOM hybrid, PyPsꢀH PWMo
3
at different temperatures in air (more impedance spectra of all the hybrids and the fitting results are
provided in supporting information Fig. S3 and Table S2). The resistance was found to decrease with
increase in temperature in all the cases. The conductivity, estimated from the lowꢀfrequency intercept to
ꢀ1
real axis, of the materials were calculated to be ~ 0.1, 0.01 and 0.0003 Scm for PyPsꢀH PWMo, PyPsꢀ
3
H PMoV, and PyPsꢀH PMoV, respectively at around 90 °C. The Arrhenius plots for the bulk
4
5
conductivity (σ) of PyPsꢀPOM hybrids are presented in Fig. 7 according to the equation 2:

−
Ea 
kT 



σ
T = Aexp
(2)
where T is the temperature, A is the preꢀexponential factor, E is the activation energy and k is the
a
Boltzmann’s constant. The conductivity of hybrid materials increases with increase in temperature. The
conductivity sequence of the hybrid materials is in the ascending order i.e., [PyPs] PMo V O <
40
5
10
2
6
+
[
PyPs] PMo VO < [PyPs] PW MoO . It has been suggested that the larger radius of W increases
4 11 40 3 11 40
the molecular volume of [PyPs] PW MoO compared to corresponding V based hybrids such as
3
11
40
26
[
PyPs] PMo VO and [PyPs] PMo V O . As the lattice potential energy (UPOT) and molecular
4 11 40 5 10 2 40
3
3
volume (V ) are inversely related, i.e., (equation 3)
m
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


117.3



−1
UPOT / kJmol = 2
+ 51.9
1/3
(3)
Vm
The Coulombic interaction between counter ions in the former case is lower than that in later case,
which results in higher conductivity for W containing PyPsꢀH PWMo than that of V containing PyPsꢀ
3
H PMoV, and PyPsꢀH PMoV. However, in the case of Nꢀmethyl imidazoliumꢀ1ꢀ(3ꢀsulfonic group)
4
5
propyl (MIMPS) IL based hybrids, an increased conductivity was observed with H PMoV compared to
4
2
2
than that of H PWMo. It could be due to the variation in the structure of ILs. The activation energy
3
was found to be 0.45, 0.73 and 1.16 eV for [PyPs] PW MoO , [PyPs] PMo VO and
3
11
40
4
11
40
[
PyPs] PMo V O , respectively in the investigated temperature range. It is consistent with the
5 10 2 40
observed trend for conductivity of the hybrid materials. Based on the higher activation energy (> 0.20
eV), it can be assumed that the proton conduction in these types of hybrids appears to be following
3
4
vehicle mechanism. The cyclic voltammetry analysis was used to understand the electrochemical
stability window (ESW) of PyPsꢀH PWMo at room temperature and is shown along with electrical
3
conductivity comparison of currently studied hybrids with other known promising proton conductors,
2
2, 35, 36
ꢀ1
including Nafion
(Fig. 8). The scanning was done at 10 mVs between ꢀ4 to +4 V using a 3ꢀ
electrode set up employing Pt and Ag/AgCl electrodes. The hybrid gel seemed to show an ESW of ~ 3V
2
2, 37
which is comparable to other ILꢀPOM hybrids reported in the literature.
4
Conclusions
In summary, we successfully synthesized three POMsꢀbased ionic liquid (IL) gels by adjusting the
packing efficiency of the ions through introducing the ILs into the polyoxoanions (POMs). IR spectra
and XRD patterns confirm interaction between the organic cation and inorganic heteropoly anion in the
material. The basic Keggin structure is still remained in the hybrid materials, and TG analysis
demonstrates that this kind of materials is thermally stable up to about 250 °C. The bulk ionic
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ꢀ1
conductivity of 0.1, 0.01 and 0.0003 Scm at ~ 90 °C was obtained for [PyPs] PW MoO ,
3
11
40
[
PyPs] PMo VO and [PyPs] PMo V O , with activation energy of 0.45, 0.73 and 1.16 eV,
4 11 40 5 10 2 40
respectively. Because of the promising electrochemical properties, these gelꢀlike HPA and IL derived
hybrid electrolytes have potential applications in the electrochemical devices such as proton exchange
membrane fuel cells.
Acknowledgements
This research was supported through Discovery Grants from the Natural Science and Engineering
Research Council of Canada (NSERC) (Grant Number: RGPINꢀ2016ꢀ03853). We thank NSERC for
funding through the Research Tools and Instruments and Infrastructure grants (RTI) (cat. 1) and Canada
Foundation for Innovation (CFI) for financial support. We thank Shahrukh Shamim for his help with
experimental work. We also would like to thank Dr. Todd Sutherland for providing the access to FTIR
instrument and Jian Li for all his help with CHN, and NMR analysis.
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Table 1. FTIR Bands of ILꢀH PWMo, ILꢀH PMoV and ILꢀH PMoV and their Precursors.
3
4
5
-1
Wavenumber (cm )
Vibrations
H PWMo
3
PyPs ꢀ
H PMoV
4
PyPsꢀ
H PMoV
5
PyPs ꢀ
H PWMo
3
H PMoV
4
H PMoV
5
1
077
1098
1001
914
1064
959
871
787
ꢀ
ꢀ
ꢀ
ʋ(PꢀOa)
9
80
87
20
942
888
825
971
859
ꢀ
967
862
ꢀ
ʋ(MꢀOd)
8
ʋ(MꢀO ꢀM)
b
8
855
ʋ(MꢀO ꢀM)
c
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Figure Captions
1
13
Fig. 1. (a) H NMR and (b) C NMR of PyPs, PyPsꢀH PWMo, PyPsꢀH PMoV, and PyPsꢀH PMoV. The
3
4
5
chemical shift values are represented against TMS dissolved in D O.
2
Fig. 2. FTIR spectra of heteropoly acids H PW MoO (H PWMo), H PMo VO (H PMoV) and
3
11
40
3
4
11
40
4
H PMo V O (H PMoV) and their ILꢀhybrids (PyPsꢀH PWMo, PyPsꢀH PMoV, and PyPsꢀH PMoV.
5
10
2
40
5
3
4
5
Fig. 3. PXRD patterns of (a) PyPs, heteropoly acids H PW MoO (H PWMo), H PMo VO
40
3
11
40
3
4
11
(
H PMoV) and H PMo V O (H PMoV) and their PyPsꢀPOM hybrids (PyPsꢀH PWMo, PyPsꢀ
4 5 10 2 40 5 3
H PMoV, and PyPsꢀH PMoV) and (b) PyPsꢀH PWMo in the small angle region.
4
5
3
Fig. 4. The schematic illustration of the crystal structure of POMꢀIL hybrid gel electrolyte.
Fig. 5. The TGA of heteropoly acids H PW MoO (H PWMo) and H PMo VO (H PMoV), their
3
11
40
3
4
11
40
4
PyPsꢀPOM hybrids (PyPsꢀH PWMo, and PyPsꢀH PMoV and PyPs which is shown as inset.
3
4
Fig. 6. Typical ac impedance spectra of PyPsꢀPOM hybrid, PyPsꢀH PWMo at (a) 23 °C, and (b) 95 °C
3
measured in air atmosphere. Impedance plots zoomed at high frequency side is shown as inset for
clarity.
Fig. 7. Arrhenius plots for ionic conductivity of PyPsꢀPOM gel electrolytes, PyPsꢀH PWMo, PyPsꢀ
3
H PMoV, and PyPsꢀH PMoV.
4
5
Fig. 8. Cyclic voltammetry of PyPsꢀH PWMo hybrid gel electrolyte at room temperature between ꢀ4 to
3
ꢀ1
+
4 V vs. Ag/AgCl with a scanning rate of 10 mVs (inset) and comparison of electrical conductivity of
1
6, 29, 30
PyPsꢀH PWMo with other known promising proton conductors
.
3
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1
H NMR
13
(a)
C NMR
(
b)
PyPs-H PMoV
5
PyPs-H PMoV
5
PyPs-H PMoV
PyPs-H PMoV
4
4
PyPs-H PWMo
3
PyPs-H PWMo
3
PyPs
PyPs
10
8
6
4
2
200
150
100
50
0
Chemical shift (ppm)
Chemical shift (ppm)
Fig. 1.
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PyPs-H PMoV
5
H PMoV
5
PyPs-H PMoV
4
H PMoV
4
PyPs-H PWMo
3
H PWMo
3
1
800 1500 1200
900
600
-1
Wavenumber (cm )
Fig. 2.
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(a)
PyPs-H PMoV
5
H PMoV
5
PyPs-H PMoV
(b)
4
4.72
H PMoV
4
PyPs-H PWMo
3
H PWMo
3
PyPS
4
6
8
10
12
14
Two theta (degrees)
1
0
20
30
40
Two theta (degrees)
Fig. 3.
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+
+
+
+
+
+
+
+
+
+
N
N
N
N
N
N
N
N
N
N
POM
self-assembly
SO
3
H
SO3
H
SO3
H
SO3
H
SO3
H
SO3
SO
H
SO3
H
SO
3
H
SO
3
H
SO
3
H
H
SO3
H
SO
3
H
SO
3
H
SO
3
H
SO
3
H
SO
3
H
SO
3
H
SO
3
H
3
SO H
3
+
SO
3
H
N
N
+
N
N
N
N
N
N
N
N
N
+
+
+
+
+
+
+
+
+
IL
Fig. 4.
Fig. 5.
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Fig. 6.
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Temperature (°C)
0
0
0
8
0
0
4
1
6
-
-
-
-
-
-
-
1
2
3
4
5
6
7
E
=
0.45
a
eV
Ea
Ea
PyPs-H PWMo
3
PyPs-H PMoV
4
PyPs-H PMoV
5
2.6
2.8
3.0
000/T
3.2
3.4
-1
1
(K )
Fig. 7.
Fig. 8.
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TOC page Figure and Abstract
Cyclic voltammetry of PyPsꢀH PWMo at room temperature between ꢀ4 to +4 V vs. Ag/AgCl at scan rate
3
ꢀ1
of 10 mVs and comparison of electrical conductivity of PyPsꢀH PWMo with other known promising
3
proton conductors.
24