Ionic Liquid Supramolecular Gel

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Thermo-responsive Polyoxometalate/Ionic Liquid Supramolecular Gels Electrolytes for Supercapacitors: Fabrication, Structure, and Heteropolyanion Structure Effect Xuefei Wu, Wen Wu, Qingyin Wu, and Wenfu Yan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04603 • Publication Date (Web): 04 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017

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Thermo-responsive Polyoxometalate/Ionic Liquid Supramolecular Gels Electrolytes for Supercapacitors: Fabrication, Structure, and Heteropolyanion Structure Effect Xuefei Wu,a Wen Wu, a Qingyin Wu,*a and Wenfu Yanb a b

Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, PR China

Abstract: We report the fabrication, structure, and heteropolyanion structure effect of polyoxometalate (POM)/ionic liquid (IL) supramolecular gels firstly. These supramolecular gels owe ordered structures, creating by excellent reversible self-assembly and they show various physicochemical properties, determined by the heteropolyanion structure effect of polyoxometalate anions. Specifically, the formation of POM/IL supramolecular gels result a very orderly layer-shape structure, which have been calculated by XRD patterns and proved by TEM images for the first time. When these POM/IL supramolecular gels are heated, they become viscous liquid sols, with melting isotropic drops and even flower-like structures in microscopic scales, while to the outside seeming there is reversible gel-sol phase transformation from gel to sol. The heteropolyanion structure effect in these two ionic liquid gels, [TBTP]5PW10V2O40 and [TBTP]8P2W16V2O62, are demonstrated in physicochemical properties. The POM structures have strong structure effect on the physicochemical properties. As the size of heteropolyanions increases, there is a significant improvement in the conductivity, thermal performance and oxidizability, with lower phase

inversion

temperature,

which

means

Dawson-type

compound,

[TBTP]8P2W16V2O62, has higher conductivity, lower melting point, stronger oxidizability

and

better

thermal

performance

than

Keggin-type

[TBTP]5PW10V2O40, at the same condition.

1

Corresponding author: Tel: +86 571 88914042.

Fax: +86 571 87951895.

E-mail: [email protected] H

1

ACS Paragon Plus Environment

one,

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1. Introduction Ionic liquids (ILs), based on relatively large organic cations and inorganic anions through ionic supramolecular self-assembly, are a type of wonderful molten salts remaining the liquid phase at room temperatures. Recently lots of attention have focused on them because for their excellent physicochemical properties, just as stability in thermal and electrochemical application.1-3 Meanwhile, special ILs can also be regarded as gels somewhat, maintaining advantages of ILs with some significant improvements, bringing about widely applications. 4, 5 Heteropoly acids (HPAs) and polyoxometalates (POMs), are a type of metal oxide clusters, formed through inorganic metal–oxygen cluster anions, with a variety of structures, compositions, and functionalities. So these inorganic compounds have been widely applied such as medicine, catalysis and materials science, 6-12 owing to their perfect properties just as controllable oxidability, super-acidity and high proton-conductivity. Recently, in order to overcome the disadvantage of POMs such as bad machinability, POMs have been formed hybrid materials with organic polymer matrices

13

or various salts to prepare some significant kinds of gel-form hybrid soft

materials through ionic supramolecular self-assembly,14 which are easy to machine and reproduce. Recently, some POM/ILs formed through organic ammonium or phosphonium and polyoxometalates anion blocks have been explored, and they have been widely applied in catalysis, nanotechnology or electrochemistry,15-17 because of their some wonderful physical characteristics just as phase transformation.

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The cations in POM can greatly influence on the structure and characteristics of POM,

18

which have determined the properties of POM/ILs formed through ionic

supramolecular self-assembly. Meanwhile, POMs are wonderful inorganic anions in the construction of functional hybrid molecular materials, and they can be easily modified during formed practical hybrid materials through inorganic anions and organic cations,19 bringing about more perfect electrochemical properties,

20

meaning

that they are attractive candidates for some electrochemical applications. However, many researchers merely pay attention to cations’ effects on POM-based ILs,21 and there are few reports about POM anions’ influences on physicochemical properties of POM-based ILs. Additionally, the special structure of POM/IL supramolecular gels still lack direct proof although there are lots of indirect calculation and conclusion. Herein, we report fabrication, characterization and heteropolyanion structure effect of supramolecular POM/IL gels with highly ordered structures based on widely applied

organic

phosphonium,

tetraalkylphosphonium

(TBTP),

and

vanadium-substituted heteropolyanions (PW10V2O405- and P2W16V2O628-). Noteworthy these type of supramolecular POM/IL gels can form nano-size particles with obvious layer-shape structure by self-assembly and they will process phase transformation from the gel state to the liquid sol state below 100 °C with an increase in conductivity so that they can be classified as a type of gels.22 Moreover, a systematic study was conducted on structure effect of POM structures on these POM/ILs gels. It indicates that their properties (i.e., gelation, conductivity, thermal performance and oxidability) can be influenced by POM heteropolyanions. Our study show that these novel

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gel-form compounds are perfect candidates as electrolytes thanks to advantages of both gel and sol phases. What’s more, they will be functional materials in the practical area of electrochemistry.

2. Experimental Section Instruments and reagents Elemental analysis was determined by Inductively coupled plasma (ICP-MS) analysis on a Shimadzu V-1012 ICP-MS spectrometer. Infrared (IR) spectrum was conducted on a NICOLET NEXUS 470 FT/IR spectrometer during the wavenumber range 400–4000 cm−1 using KBr pellet. X-ray powder diffraction (XRD) pattern was conducted on a BRUKER D8 ADVANCE X-ray diffractometer using a Cu tube at the conditions: at 40 kV and 40 mA in the range of 2θ = 4-40° at a rate of 0.02°·s-1. The thermal stability of these samples was reported on a SHIMADZU thermal analyzer. The micro-texture of samples during phase transformation was observed on an Axioskop 40 polarizing microscope (Carl Zeiss Light Microscopy, Germany) equipped with a LINKAM THMS 600 hot stage and a LINKAM CI 94 temperature controller. Morphology was observed on a HF-3300 (Hitachi) transmission electron microscopy (TEM). Conductivity was measured through a DDS-11A conductivity meter using a Shanghai DJS-1 and DJS-10 electrode. Cyclic voltammetry studies were conducted on a CHI660E Electrochemical Workstation in a three-electrode electrochemical cell: glass carbon (5 mm in diameter) as the working electrode, platinum as the counter electrode, with a saturated calomel reference electrode. The density of substrate was 0.25 mM and 0.2 M NaClO4 was assigned as electrolyte. The

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background of solution has been deducted. All reagents were analysis-grade without further purification. Synthesis of POM-ILs H5PW10V2O40 (PW10V2) and H8P2W16V2O62 (P2W16V2) are synthesized based on recent

literatures.23

These

pre-prepared

intermediate

Na9PW9O34·nH2O

or

Na9P2W15O56·nH2O, Na2WO4·2H2O and NaVO3 just as the compose ratio, 1:1:2, were all dissolved in water through vigorous stirring and the pH of solution was adjusted to 2.0 by adding H2SO4. Then the pure acids are extracted with ether, drying in the air to produce orange powder. The organic phosphonium salt, TBTP-Cl and phosphorus-containing HPA, H5PW10V2O40 and H8P2W16V2O62, were taken in 5:1 and 8:1 molar ratio to give one mole of [TBTP]5PW10V2O40 and [TBTP]8P2W16V2O62. The ethanol solution of TBTP-Cl was slowly added to the aqueous solution of HPA, and the mixture solution was vigorous stirred for 24 h. Water was firstly evaporated at 50 °C and then the remaining was removed in vacuum to produce the orange oily products. These compounds are almost insoluble in water or ethanol (EtOH), but soluble in tetrahydrofuran (THF), acetone, ethyl acetate, dimethyl sulfoxide (DMSO) and N, N-dimethylformamide (DMF). Carbon, phosphorus, tungsten and vanadium were analyzed by elemental analysis through Inductively coupled plasma (ICP-MS). For [TBTP]5PW10V2O40 , Calcd: C: 33.87%; P: 4.04%; W: 39.90%; V: 2.22%. Found: C: 33.54%; P: 3.85%; W: 40.37%; V: 2.40%. For [TBTP]8P2W16V2O62 , Calcd: C: 34.25%; P: 4.25%; W: 40.35%; V: 1.40%. Found: C: 33.94%; P: 4.05%; W: 40.57%; V: 1.44%.

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3. Results and discussion

H P2W V2O 8 16 62

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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TBTP8P2W16V2O62

H PW V2O 5 10 40 TBTP5PW10V2O40

4000 3500 3000 2500 2000 1500 1000

500

-1 Wavenumber (cm )

Figure 1. IR spectra of POM-ILs and HPAs. Table 1. The assignment and wavenumber (cm-1) of the vibration modes in IR spectra of compounds Vibrations

[TBTP]5PW10V2O40

H5PW10V2O40

[TBTP]8P2W16V2O62

H8P2W16V2O62

- CH3 stretching

2963

-

2970

-

-CH2 stretching

2923

-

2930

-

-CH2 stretching

2860

-

2857

-CH2 scissoring

1467

-

1463

-

-CH2 twisting

1381

-

1384

-

P-Oa stretching

1095

1087

1087

1089

M-Od stretching

962

986

955

968

M-Ob-M stretching

889

885

915

909

M-Oc-M stretching

804

784

800

783

From Figure 1 and Table 1, where there are the detail about IR spectra of [TBTP]5PW10V2O40 and [TBTP]8P2W16V2O62, we can find that these feature frequencies, both POM-ILs and HPAs, fall in the stretching sequence of νas(P-Oa), νas(M-Od), νas(M-Ob-M) and νas(M-Oc-M), (M=W, V), referred to POM characteristic bands at 700–1100 cm−1, made up from the metal-oxide structure, respectively. Meanwhile, we can find it that the characteristic bands of polyoxoanions structure from POM/ILs have shifted somewhat when compared with pure acids. This matter can be explained by the vibrations frequency change because of the influence of the 6


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anion-anion interactions among polyoxoanions. When TBTP phosphonium have been added with HPA to produce gel-type compounds, the anion-anion electrostatic interaction among polyoxoanions are weakened because the distances among POM anions increase, 24 so asymmetrical characteristic stretching vibrations frequencies in POM structure decreased. The result reveals that these compounds still maintain POM structure without decomposition, while these TBTP cations also keep their structure in the compound. 25 It indicates this POM/ILs do not depolymerization or degradation but there is a successful assembly of both cation and anion, such as POM structure with organic phosphonium cations in these compounds. The structure of the phosphonium, TBTP, is shown in Figure 2(a), and TBTP cation can form gel-type compounds with polyoxometalates by self-assembly (Figure 2 (b)). In fact, at room temperature these compounds are layer-shape structure, as the XRD patterns shown in Figure 2 (c) and inspired from recent articles.26 There are layer structure in these POM/ILs which can be calculated from the strong peak in the small angle region of XRD patterns. The organic phosphonium cations and polyoxometalate anions have taken up in each layer. Furthermore, there are very weak intense in the large region of POM/ILs’ XRD patterns while there are very strong peaks in the similar region of pure acids’ XRD patterns, meaning that these POM/ILs do not have crystal shape but they owe smectic gel-form appearance instead.27

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Figure 2. The structure of tetraalkylphosphonium (TBTP) (a); The layer-shape structure of [TBTP]5PW10V2O40 and [TBTP]8P2W16V2O62 (b); XRD patterns of POM-ILs and pure acids (c).

In fact, the layer-shape structure can be proved by the TEM image. Figure 3 shows the TEM image of the compounds, and there are layer-shape structures in Figure 3 (c) and Figure 3 (e), which are almost 2nm, really corresponded to the conclude and

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calculation from Figure 2 and related articles.26 So it is certain there are layer-shape structures existing in these supramolecular POM/ILs and we have proved it by the direct TEM images for the first time.

Figure 3. TEM images of POM-IL: (a)~(c) are TEM images of [TBTP]5PW10V2O40, (d)~(f) are TEM images of [TBTP]8P2W16V2O62, the scale bar of (a), (c), (d), (f) is 20nm while the scale bar of (b) is 100nm and the scale bar of (e) is 50nm.

Figure 4. The supramolecular self-assembly of POM-ILs.

Meanwhile, from Figure 3 we can also find these POM-ILs also form many nano particles by supramolecular self-assembly, just like similar compounds reported by 9


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related articles, where POM-based compounds can form nano-size structure through supramolecular self-assembly.28 So it is obviously these compounds are formed through supramolecular self-assembly from polyoxometalate anions and TBTP cations and these ions formed layer-shape structure and nano-size particles, just like the process in Figure 4.

Figure 5. Photographs of the reversible phase transformation about [TBTP]8P2W16V2O62 at room temperature (a) and 90℃ (b). Similar thermo responsive phenomena were observed for [TBTP]5PW10V2O40 (a); PM images of [TBTP]5PW10V2O40 at different temperature: (a) 65 °C, (b) 75 °C, (c) 85 °C and (d) 95 °C, and [TBTP]8P2W16V2O62 at (e) 55 °C, (f) 65 °C, (g ) 75 °C and (h) 90 °C (b), (magnification: × 100).; Schematic drawing of different packing model of [TBTP]5PW10V2O40 during phase transformation. Similar thermo-responsive phenomena were observed for [TBTP]8P2W16V2O62 (c).

What’s more, these compounds can both show temperature-response behaviors with phase transformation from the quasi-solid-state gel phase to the liquid sol state just 10


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like in Figure 5 (a), meaning a novel type of gels. Additionally, Figure 5 (b) shows the detail in the gel-sol transition process, observing by the polarized optical microscopy (PM) equipment at different temperature. Just like recent related articles phenomena, we can find that during heating process, there is plenty of melting isotropic drops, even forming flower-like or fiber-like structures.29 At high temperature the melting drops show obvious birefringence texture, suggesting that the compounds can be regarded as novel liquid crystal, an extremely viscous fluid state in Figure 5 (a). This phenomenon can be explained as the disassembly of layer structure system among the organic phosphonium and POM anions upon heating, forming ions pair, which plays significant roles in forming the gel-appearance of the compound. In fact, the lattice energy is the major factor in influencing the phase transformation process of these POM-IL gels, determined by the symmetry among the cation and anion structure, also influence the conformational degree of freedom and the intermolecular forces among the molecular.30,31 Upon heating, the layer-shape structure begins to break up and form ion pairs just as shown in Figure 5 (c),32 bringing about liquid-crystal appearance of isotropic sol. In another word, these compounds form nano particles formed from layer-shape structure at room temperature but these structures break and become ion pair at high temperature on the micro, which is semi-solid gel at room temperature and isotropic sol at high temperature viewed from the outside appearance. Meanwhile, from Figure 6, we can also find that the [TBTP]8P2W16V2O62 shows lower melting point than [TBTP]5PW10V2O40 at the same conditions, (50℃ for [TBTP]8P2W16V2O62 and 61℃ for [TBTP]5PW10V2O40 ), which can be concluded by the bigger size of the

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POM anion in the system, bringing about a decrease of the lattice energy of the compound, then resulting the lower phase transition point.

Figure 6. TG-DTA plots of [TBTP]5PW10V2O40 (a) and [TBTP]8P2W16V2O62 (b).

Additionally,

the

TG

and

DTA

plots

of

[TBTP]5PW10V2O40

and

[TBTP]8P2W16V2O62 are shown in Figure 6, where organic ammonium cations and POM anions begin to decompose. Weight loss (42.77 wt % and 44.42 wt %) can be corresponded to the decomposition of TBTP phosphonium cations contained in these POM-type compounds (43.28 wt % and 43.79 wt %). From Figure 8 and consider recent articles,

33

we can find that Dawson-type compounds seem to be more stable

than Keggin-type ones because of the higher decomposition temperature, which can be concluded as the heteropolyanion structure effect in thermal performance.

Figure 7. DTA and conductivity-temperature plots of [TBTP]5PW10V2O40 [TBTP]8P2W16V2O62 (b) at the same condition.

(a) and

We have recorded the conductivity of these compounds under different temperatures with various phases. Figure 7 shows DTA and conductivity-temperature 12


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plots of POM/ILs. During the heating process, there is a broad peak at about 61°C when it comes to [TBTP]5PW10V2O40 in upon heating and a similar one happening at about 50°C by [TBTP]8P2W16V2O62

that can both be referred to a phase

transformation from a quasi-solid-state gel phase to an isotropic liquid sol phase, referred to the phase transformation above. Above this temperature the conductivity of POM/ILs increase very fast but below the temperature the conductivity increases very slow, almost retaining contact. In fact, [TBTP]5PW10V2O40 owes a conductivity as high as 7.20×10-4 S·cm-1 at 110 °C while the conductivity of [TBTP]8P2W16V2O62 is 1.35×10-3 S·cm-1 at the same condition. Compared with other POM-based solid electrolyte system,34,

35

this POM/ILs can be concluded as a type of phase

transformation ionic liquids with fast ionic conductivity, which are good electrolytes and exhibit much higher conductivity at high temperature than their quasi-solid-state gel phase. It can be explained as an increase during the migrating of ions in POM/ILs.36 For this matter, these POM/ILs are a type of ionic liquid indeed just like other series of similar compounds.37

Figure 8. Conductivity-temperature plots of [TBTP]5PW10V2O40 and [TBTP]8P2W16V2O62 at the same condition (a); the conductive Arrhenius plots of POM/ILs: [TBTP]5PW10V2O40 and [TBTP]8P2W16V2O62 (b).

Moreover, from Figure 8 (a) we can find that [TBTP]8P2W16V2O62 shows higher 13


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conductivity than [TBTP]5PW10V2O40 at the same conditions. The reason may be as follows: [TBTP]8P2W16V2O62 has larger molecular volume than [TBTP]5PW10V2O40 because the anion of the former is Dawson structure and the latter one is Keggin-type, just as the results in the XRD patterns. In fact, considering the empirical equation blow:

 117.3  Upot / kJ  mol 1  2  1/3  51.9   Vm  Where Upot is regarded as the lattice potential energy and Vm is referred to the molecular volume in units of nm3, meaning the larger Vm will bring about a smaller Upot.38 So the value of Upot of [TBTP]8P2W16V2O62 is lower than [TBTP]5PW10V2O40, bringing about lower electrostatic attractions between cationic and polyanions in [TBTP]8P2W16V2O62 than [TBTP]5PW10V2O40, suggesting weaker Coulombic interactions inside the former ones. When the Coulombic interactions among the phosphonium cations and the POM anions of POM/ILs tend to bring about a decrease in mobility,39 [TBTP]8P2W16V2O62 is expected to have higher conductivity than [TBTP]5PW10V2O40. Figure 8 (b) shows a conductive Arrhenius plot of these POM/ILs. From these slopes, the conductive activation energy, usually abbreviated as Ea, can be evaluated by the Arrhenius relation:

σ = σ0exp〔-Ea/RT〕 Where σ is conductivity, σ0 is the pre-exponential factor, R is the gas constant and T is the absolute temperature. Meanwhile, Ea is usually considered as the energy barrier in POM/ILs that must be overcome indeed for the phosphonium cations and the POM 14


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anions to migrate, so a smaller Ea will bring about a much easier method for the ions to move.

40

So we can calculate Ea of POM/ILs, which is 35.74 kJ·mol-1

[TBTP]5PW10V2O40

and

31.66

kJ·mol-1

for

[TBTP]8P2W16V2O62.

for As

[TBTP]8P2W16V2O62 shows smaller activation energy than [TBTP]5PW10V2O40, which corresponds to the result of the conductivity for the two compounds.

3





0 I/

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-3

' '

-6 TBTP5PW10V2O40

-9

TBTP8P2W16V2O62

-12 -0.50 -0.25 0.00 0.25 0.50 0.75 E/V vs Hg2Cl2/Hg

Figure 9. The cyclic voltammetry scanning of [TBTP]5PW10V2O40 and [TBTP]8P2W16V2O62 with a scanning rates of 50mv·s-1 in DMF.

Figure 9 shows the cyclic voltammetry scanning of [TBTP]5PW10V2O40 and [TBTP]8P2W16V2O62, indicating that these compounds can process electrochemical reductions:

PW10V2O405  2e  PW10V2O407 

α/α’

8 10  PW  2e   PW α/α’ 2 16V2O62 2 16V2O62

In fact, based on the recent articles,41 these electrochemical reactions can be concluded as the reduction of the vanadium atoms:

V (V)  e  V (IV) We can find that [TBTP]8P2W16V2O62 shows stronger oxidability than [TBTP]5PW10V2O40 because [TBTP]8P2W16V2O62 has higher formal potentials, Ef, 15


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than [TBTP]5PW10V2O40 (47mV vs -43mV according to saturated calomel reference electrode) at the same conditions, when the formal potentials (Ef) of the redox couples in the cyclic voltammetry are regarded as average values of anodic (Epa) and cathodic (Epc) peak potentials, and they obey this relation: Ef = (Epa + Epc)/2. 42 It can be explained by the structure effect of the POM structure, bringing about stronger oxidability.

Figure 10. Cyclic voltammetry of [TBTP]5PW10V2O40 and [TBTP]8P2W16V2O62 at different scanning rates; dependence of cathodic and anodic peak currents of the first reduction and oxidation waves as a function of the scan rate in low right corner.

Meanwhile, the electrochemical reaction above is invertible reactions. 43 According to the Figure 10, the scanning rates (v) and reduction current (I) usually follow this

16


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equation below:

I  v 0.5 So these electrochemical reactions can be indicated as a type of reversible reaction somewhat, a useful message in the particular situation such as cell, supercapacitors or catalysis. 44

4. Conclusions In conclusion, the fabrication, structure, and heteropolyanion structure effect of POM-ILs, [TBTP]5PW10V2O40 and [TBTP]8P2W16V2O62 have been reported. Two compounds have a very orderly layer-shape structure, which have been calculated by XRD patterns and proved by TEM images for the first time. When these POM/IL supramolecular gels are heated, they become viscous liquid sols, with melting isotropic drops and even flower-like structures on the micro, where they process reversible gel-sol phase transformation from quasi-solid-state gel to isotropic liquid sol, with an obvious increase in conductivity. Meanwhile, there is clearly structure effect in these POM gels about physicochemical properties, as the size of heteropolyanions increase, there is a significant improvement in the conductivity, thermal performance and oxidizability, with lower melting point for this POM-ILs. In other words, Dawson-type compound, [TBTP]8P2W16V2O62, has higher conductivity, lower melting point, stronger oxidizability and better thermal performance than Keggin-type one, [TBTP]5PW10V2O40.

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Acknowledgments This work is financially supported by the National Key Research and Development Program of China (2016YFB0901600), the National Natural Science Foundation of China (21173189), the Zhejiang Provincial Natural Science Foundation of China (LY14B030005) and the Foundation of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jilin University (2016-03).

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Table of Contents: The

fabrication,

structure,

and

heteropolyanion

structure

effect

of

two

polyoxometalate (POM)/ionic liquid (IL) supramolecular gels are investigated firstly. The formation of nano-particles POM/IL supramolecular gels is made from a very orderly layer-shape structure, calculated by XRD patterns and proved by TEM images. These structures are break to melting isotropic drops and even flower-like structures in the micro views while in fact POM-IL gels can process reversible gel-sol phase transformation indeed, with an obvious increase in conductivity during the phase transformation. There is clearly structure effect in these POM gels about physicochemical properties, for with the increase in the size of heteropolyanions, there is a significant improvement in the conductivity, thermal performance and oxidizability

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Ionic Liquid Supramolecular Gel

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