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

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-ium1-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. H and C 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.


Introduction
2][3][4][5][6] For example, a nanosheet array of NiMoO 4 prepared by a hydrothermal method showed a specic capacity of 1483 Fg À1 and a nanopyramid array of metallic CoS 2 synthesized by a one-step solvothermal method showed a low over-potential of 0.14 V for H 2 evolution under acidic conditions. 4,5][9][10][11][12] POMs are anionic metal oxides derived from transition metal oxides, which have a robust structure and exhibit very useful functional physical and chemical properties, including electrical, magnetic, electrochemical and photonic properties. 135][16] 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 the former case, and organic cations and anions in the latter case.The POM-based IL salts offer several advantages, such as being solvent-free, possessing residual acidity and good thermal stability, and having much higher ionic conductivity than the corresponding anhydrous solid analogues.They also exhibit a low vapor pressure, which is benecial for hightemperature (above 100 C) fuel cells and catalytic applications.3][24][25][26][27][28][29][30] Several heteropoly acids (HPAs) and polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) hybrids were also recently reported. 28erein, we report the synthesis and structural, chemical and electrochemical characterization of hybrid gel electrolytes based on recently developed 3-(pyridin-1-ium-1-yl)propane-1sulfonate (PyPs) 24 IL and H 3 PW 11 MoO 40 , H 4 PMo 11 VO 40 and H 5 PMo 11 V 2 O 40 for potential electrochemical energy conversion and storage applications.The investigated POMs-ILs hybrid materials were found to be thermally stable to about 300 C. Hybrid [PyPs] 3 PW 11 MoO 40 showed the highest electrical conductivity of 0.1 S cm À1 at $90 C in ambient conditions, which is comparable to the proton conductivity of Naon under high relative humidity, and an electrochemical stability window of $3 V at room temperature.O (Fisher Scientic, 100%) and dissolved in 100 ml of deionized water and was added to the sodium molybdate solution.Dilute H 2 SO 4 (1 : 1) was used to adjust pH of the mixture to $2.5 and was then heated at 90 C for 2 h.It was allowed to cool down to room temperature and the solution was extracted using ether (50 ml) and dilute H 2 SO 4 (1 : 1) prior to drying in a vacuum oven at 50 C for 24 h to obtain the solid product.0.02 mol of NaVO 3 (BDH, 98%) was dissolved in 20 ml of deionized water.Na 2 MoO 4 (0.01 mol) (BDH, 99.5%) was mixed with 0.01 mol of Na 3 PO 4 $12H 2 O (Fisher Scientic, 100%) and dissolved in 100 ml of deionized water.The rest of the steps were the same as mentioned for H 3 PWMo synthesis.

Characterization
A PerkinElmer Series II C H N S/O Analyzer 2400 was used for the CHN analysis to understand the composition of PyPs and PyPs-POM hybrids.The purity of these compounds was checked by employing 1 H and 13 C NMR (nuclear magnetic resonance) (Bruker RDQ 400 NMR spectrometer) spectroscopy.The Fourier transform infrared spectra of the samples were recorded by employing a Varian 7000 FTIR spectrometer in the 600-2000 cm À1 range using KBr standard.The structural analysis was carried out using powder X-ray diffraction (PXRD) and a Bruker D8 Advance powder X-ray diffractometer (Cu-Ka radiation; 40 mA; 40 kV) was used for this purpose.The scanning was carried out at a 0.02 s À1 rate from 2q ¼ 2 to 40 at room temperature.The variable temperature (30-120 C) PXRD was also done using an Anton Par XRK 900 high temperature reactor chamber.The samples were held for 30 minutes at each temperature to ensure thermal equilibrium.A thermogravimetric analyzer (Setaram TAG 16 TGA/DSC dual chamber balance) was used to study the thermal stability of the samples under N 2 gas up to 650 C at a 10 C min À1 heating rate.The electrical conductivity measurements were done using a Solartron SI 1260 impedance and gainphase 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. A VersaSTAT 3 potentiostat was employed for the cyclic voltammetry using a 3-electrode sample set up under N 2 atmosphere.Pt electrodes were used as the working (WE) and counter electrodes (CE), and Ag/AgCl was used as the reference electrode (RE). 16emperature.The freshly made product was a dark green transparent semisolid at room temperature.PyPs-H 4 PMoV turned to liquid by heating it to nearly 80 C where it exhibited thermotropic liquid crystal behaviour.The melting point of the hybrid compounds can be related to the coulombic attraction (E c ) between the constituent ions, and is described by eqn (1):

Results and discussion
where M is the Coulomb's law constant, Z + and Z À are signed magnitudes of cations and anions charges, 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 a lower melting point when the POM has a lower charge. 30

Composition analysis
CHN analysis results of the as-prepared PyPs (IL), and all its hybrid compounds, PyPs-H 3 PWMo, PyPs-H 4 PMoV, and PyPs-H 5 PMoV are summarized in Table S1 (see the ESI †).The C, H, and N results were found to be consistent with the theoretical composition within 4%, and the discrepancy in the H percentage might be due to the uncertainty in quantifying the hybrids water content.Scheme 1 shows the idealized synthesis steps of the POMs-based IL hybrid gel electrolytes.The functional groups of PyPs and the hybrid POMs-ILs were further proven by NMR analyses (Fig. 1). 1 1 summarizes the 1 H NMR and 13 C NMR results with a comparison plot of both PyPs and the hybrid IL-POMs.

FTIR analysis
The FTIR spectra were used to identify structural and bonding changes in the polyoxoanion units present in POM-based materials.Fig. 2 presents the FTIR spectra of the hybrids PyPs-   table summarizing the major vibration bands is given in Table 1.

Powder XRD phase analysis
Fig. 3a shows the PXRD patterns of POM-ILs and their pristine solid acids, which reveal their crystalline and amorphous states.The Keggin-type of the polyoxoanions exhibited typical peaks in the range of 2q ¼ 7-11 in the X-ray diffraction (XRD) pattern. 31hese peaks in this range were also observed in the XRD patterns for all three hybrid materials.From these observations, it is clear that the hybrid materials were successfully prepared.However, the XRD peaks of the POM-ILs were signicantly different from that of the pristine acids.The PyPs-POM hybrids had two broad diffraction peaks at 2q ¼ 7-10 and 15-40 , which showed a smectic phase that was consistent with the liquid-state crystalline nature of the samples. 32   water loss appeared to be responsible for the decreased interplanar d value.A similar trend was observed for PyPs-H 4 PMoV hybrid and is shown in Fig. S2b (see the ESI †).Fig. 3b presents the PXRD pattern of PyPs-H 3 PWMo in the small angle region.Even though the intensities of peaks at 2q ¼ 3-5 were much weaker than the peak at 2q z 9 , they still could be observed.The diffraction patterns contain the fundamental reection in the small-angle region, which may be due to the regular arrangement of molecules in layers. 22As a result, we can speculate that at rst the molecules are organized into spheroidal aggregates, which have a core-shell structure, where each polyoxoanion is surrounded by PyPs cations due to electrostatic interactions between polyoxoanion and PyPs cation.Subsequently, IL-polyoxoanion aggregates may be regularly arranged in layers through the organic IL moieties.The schematic of the idealized crystal structure of the POM-IL hybrid gel electrolyte is shown in Fig. 4.

Electrical properties
Fig. 6 shows typical impedance spectra for the PyPs-POM hybrid, PyPs-H 3 PWMo at different temperatures in air (more  impedance spectra of all the hybrids and the tting results are provided in the ESI Fig. S3 and Table S2 †).The resistance was found to decrease with the increase in temperature in all the cases.The conductivity, estimated from the low-frequency intercept to real axis, of the materials were calculated to be $0.1, 0.01 and 0.0003 Scm À1 for PyPs-H 3 PWMo, PyPs-H 4 PMoV, and PyPs-H 5 PMoV, respectively, at around 90 C. The Arrhenius plots for the bulk conductivity (s) of PyPs-POM hybrids are presented in Fig. 7 and were determined according to eqn (2): where T is the temperature, A is the pre-exponential factor, E a is the activation energy and k is Boltzmann's constant.The conductivity of hybrid materials increased with increasing temperature.The conductivity sequence of the hybrid materials was in the ascending order i.e., The coulombic interaction between counter ions in the former case is lower than that in latter case, which resulted in higher conductivity for W containing PyPs-H 3 PWMo than that of V containing PyPs-H 4 PMoV, and PyPs-H 5 PMoV.However, in the case of N-methyl imidazolium-1-(3-sulfonic group) propyl (MIMPS) IL based hybrids, an increased conductivity was observed with H 4 PMoV compared to that of H 3 PWMo. 22This could be due to the variation in the ILs structure.The activation energy was found to be 0.45, 0.73 and 1.16 eV for [PyPs] 3 PW 11 -MoO 40 , [PyPs] 4 PMo 11 VO 40 and [PyPs] 5 PMo 10 V 2 O 40 , respectively, in the investigated temperature range and it was found to be consistent with the 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 a vehicle mechanism. 34Cyclic voltammetry analysis was used to understand the electrochemical stability window (ESW) of PyPs-H 3 PWMo at room temperature, and is shown along with electrical conductivity comparison of currently studied hybrids with other known promising proton conductors including Naon 22,35,36 (Fig. 8).The scanning was done at 10 mV s À1 between À4 and +4 V using a 3-electrode set up employing Pt and Ag/AgCl electrodes.The hybrid gel seemed to show a $3 V ESW, which is comparable to other IL-POM hybrids reported in the literature. 22,37

Conclusions
In summary, we successfully synthesized three POMs-based ionic liquid (IL) gels by adjusting the packing efficiency of the ions through introducing the ILs into polyoxoanions (POMs).IR spectra and XRD patterns conrmed interaction between the organic cation and inorganic heteropoly anion in the materials.The basic Keggin structure still remained in the hybrid materials, and TG analysis demonstrated that this kind of material is thermally stable up to $250 C. Bulk ionic conductivity values of 0.1, 0.01 and 0.0003 S cm À1 at $90 C were obtained for [PyPs]   Fig. 8 Cyclic voltammetry of the PyPs-H 3 PWMo hybrid gel electrolyte at room temperature between À4 to +4 V vs. Ag/AgCl with a 10 mV s À1 scan rate (inset) and comparison of electrical conductivity of PyPs-H 3 PWMo with other known promising proton conductors. 16,29,30

H 3
PWMo, PyPs-H 4 PMoV and PyPs-H 5 PMoV and their solid acid precursors.In the FTIR spectra, there are four characteristic vibrational bands resulting from the a-Keggin-type heteropolyanion (for e.g., PM 12 O 40 3À where M ¼ W, Mo, V), n(P-O a ), n(M-O d ), n(M-O b -M), and n(M-O c -M) appearing in the region between 700 and 1100 cm À1 (O d -terminal oxygen, O b -bridged oxygen of two octahedral sharing a corner, and O c -bridged oxygen of two octahedrals sharing an edge).The FTIR spectra of POM-ILs were found to be identical to that of corresponding pristine solid acids, which also showed splitting of the P-O a stretching, M-O d stretching, stretching of M-O b -M inter bridges between corner-sharing MO 6 octahedral and stretching of M-O c -M intra bridges between the edge-sharing MO 6 octahedral.In the IR spectra of the hybrid materials, those four well-known characteristic bands prove the presence of the core HPA clusters.The decrease in electrostatic anion-anion interactions leads to an increase in the stretching and bending vibrational frequencies.For heteropoly acid H 3 PW 11 MoO 40 and its hybrid material PyPs-H 3 PWMo, M-O b -M and M-O c -M asymmetrical stretching vibration were blue-shied, and the wavenumber increased from 887 and 820 for H 3 PW 11 MoO 40 to 914 and 855 cm À1 for PyPs-H 3 PWMo.There are also three characteristic peaks at 1221, 1185 cm À1 (S]O) and 1491 cm À1 (pyridine ring), indicating that [a-PW 11 MoO 40 ] 3À had been successfully immobilized on the ionic liquid (PyPs)-modied compound.

17
A Fig.3ashows the PXRD patterns of POM-ILs and their pristine solid acids, which reveal their crystalline and amorphous states.The Keggin-type of the polyoxoanions exhibited typical peaks in the range of 2q ¼ 7-11 in the X-ray diffraction (XRD) pattern.31These peaks in this range were also observed in the XRD patterns for all three hybrid materials.From these observations, it is clear that the hybrid materials were successfully prepared.However, the XRD peaks of the POM-ILs were signicantly different from that of the pristine acids.The PyPs-POM hybrids had two broad diffraction peaks at 2q ¼ 7-10 and 15-40 , which showed a smectic phase that was consistent with the liquid-state crystalline nature of the samples.32Fig. S2a (see ESI †) presents the HTXRD patterns of the PyPs-H 3 PWMo at different temperatures.This shows the inter-planar spacing d ( Å) of the IL-PMo 11 V at 2q z 9 versus temperature.The d value was almost stable from room temperature to 70 C. Upon increasing the temperature above 70 C, the absorbed

Fig. 5
Fig.5shows the TGA of the HPAs and their derivative hybrid materials (POMs-IL).As expected, there was signicant weight

Fig. 4
Fig.4The schematic illustration of the crystal structure of the POM-IL hybrid gel electrolyte.

Fig. 6
Fig. 6 Typical ac impedance spectra of PyPs-POM hybrid, PyPs-H 3 PWMo at (a) 23 C, and (b) 95 C measured in an air atmosphere.Impedance plots zoomed in the high frequency side is shown in the inset for clarity.
3 PW 11 MoO 40 , [PyPs] 4 PMo 11 VO 40 and [PyPs] 5 PMo 10 V 2 O 40 , with activation energies of 0.45, 0.73 and 1.16 eV, respectively.Because of the promising electrochemical properties, these gellike HPA and IL derived hybrid electrolytes have potential applications in electrochemical devices such as proton exchange membrane fuel cells.

Table 1
FTIR Bands of IL-H 3 PWMo, IL-H 4 PMoV and IL-H 5 PMoV and their precursors c -M)