- Email: [email protected]
Synthesis and conductive performance of quaternary molybdotungstovanadophosphoric heteropoly acid with Keggin structure
Materials Letters 157 (2015) 109–111
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and conductive performance of quaternary molybdotungstovanadophosphoric heteropoly acid with Keggin structure Yunyan Li a, Tianpei Huang a, Qingyin Wu a,n, Lin Xu b a b
Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China Key Laboratory of Polyoxometalate Science of Ministry of Education, Northeast Normal University, Changchun 130024, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 March 2015 Accepted 10 May 2015 Available online 16 May 2015
A new vanadium and molybdenum-substituted Keggin-type quaternary heteropoly acid H5PW9MoV2O40 10H2O has been synthesized by the stepwise acidification and the stepwise addition of element solutions. The product is characterized by the IR, UV, XRD, TG–DTA and electrochemical impedance spectroscopy (EIS). EIS measurement shows the conductivity value of H5PW9MoV2O40 10H2O is 8.92 10 3 S cm 1 at 22 1C and 80% relative humidity, which increases with higher temperature. Its mechanism of proton conduction is Vehicle mechanism for its conductive activation energy of 23.45 kJ mol 1. & 2015 Elsevier B.V. All rights reserved.
Keywords: Heteropoly acid Nanocrystalline materials Functional Thermal analysis Conductivity Mechanism
1. Introduction
2. Experimental section
Heteropoly acids (HPAs), an important kind of solid acids with polynuclear and oxygen bridges, have attracted special interest in the fields of catalysis, medicine, biology and materials science, due to their excellent properties such as simple composition, structure determination, the structural features of both complex and metaloxide, and acidic oxidation–reduction [1–5]. They are formed by inorganic metal–oxygen cluster anions. Because the HPAs have discrete and mobile ionic structures, they possess strong Brønsted acidity and “pseudoliquid phase” behavior. Hence, HPAs have been regarded as attractive candidates for proton conduction in fuel cells [6]. Unfortunately, most of the heteropoly acids reported are binary ones, including H3PW12O40, H3PMo12O40, H4SiW12O40 and H4SiMo12O40. We have prepared a series of vanadium-substituted ternary Keggin-type and Dawson-type HPAs while studies of quaternary HPAs are rarely reported [7]. In this paper, we report the synthesis, characterization, conductivity and proton conduction mechanism of a vanadium-substituted quaternary Keggin-type heteropoly acid H5PW9MoV2O40 10H2O (abbreviated as PW9MoV2).
Preparation: Na9PW9O34 7H2O (PW9) was synthesized according to the literature available [8]. PW9MoV2 was synthesized by a modification of the method according to the procedure described in our previous report [9]. 20 mL aqueous solution of sodium metavanadate (1.58 g, NaVO3 2H2O) was mixed with 15 mL aqueous solution of sodium molybdate (1.21 g, Na2MoO4 2H2O). The mixture was added dropwise to 100 mL aqueous solution of PW9 (12.81 g). The final solution was adjusted to pH 2.8 and continuously heated at 90 1C for 2 h under stirring. The cooled solution was extracted with ether (30 mL) in sulfuric acid medium. After the concentrated etherate solution was dried in vacuum, the orange–red power was obtained. Reagents and instruments: FTIR spectrum was recorded on a Nicolet Nexus 470 FT/IR spectrometer over the wave number range 400–4000 cm 1 using KBr pellets. UV spectrum was measured on a SHIMADZU U-2550 UV–vis spectrophotometer. XRD pattern was recorded on a BRUKER D8 ADVANCE X-ray diffractometer using a Cu tube operated at 40 kV and 40 mA in the range of 2θ¼4–401 at a rate of 0.021 s 1. The thermal stability of the sample was studied using simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) techniques from room temperature to 600 1C. Measurements were performed using a Shimadzu thermal analyzer in a Nitrogen stream, with a scanning rate of 10 1C min 1. Impedance measurements of the HPA were performed on a VMP2 Multichannel potentiostat electrochemical impedance analyzer over a frequency ranges from 9.99 104 to 0.01 Hz.
n
Corresponding author. Tel.: þ 86 571 88914042; fax: þ86 571 87951895. E-mail address: [email protected] (Q. Wu).
http://dx.doi.org/10.1016/j.matlet.2015.05.026 0167-577X/& 2015 Elsevier B.V. All rights reserved.
The IR spectrum of PW9MoV2 was investigated, as shown in Fig. 1. PW9MoV2 exhibits characteristic peaks of heteropoly acid, which demonstrates that this acid possesses Keggin strcture [10]. The characteristic band at 1078 cm 1 is attributed to υ(P–Oa), the band at 979 cm 1 is attributed to υ(M–Od), and peaks at 880 cm 1 and 788 cm 1 are corresponding to the υ(M–Ob) and υ(M–Oc) modes of the polyoxoanion (M¼ W, Mo, V) [11]. The bands at about 3400 and 1600 cm 1 of the HPA are assigned to the stretching vibration of O–H bonds and the bending vibration of H–O–H bonds. For PW9MoV2, the UV absorption band appears at 259 nm, which is assigned to charge-transfer of bridge-oxygen (Ob, Oc) to metal atoms (Mo, V) in the heteropolyanion cages [12]. Fig. 2 shows the XRD pattern of PW9MoV2, which shows its phase and structure. The data of the XRD are listed in Table 1. Its intense peaks mainly show in the regions of 7–101, 16–221, 25–301 and 33–381, which indicates that the acid has crystal structure [13]. The peaks in the region of 2θ¼ 7–101 are the characteristic peaks of HPAs with Keggin structure. The content of crystal water, protonated water and structure water of PW9MoV2 can be calculated by its TG curve (Fig. 3). The TG curve shows that the total percent of weight loss before 330 1C of PW9MoV2 is 6.60%, indicating that 9.9 water molecules are lost. The loss of 7.9 molecules of crystal water happen at first, the second is the loss of 2 molecules of protonized water, and 2 molecules of structural water are lost at last. Thus, the accurate molecular formula of the product is (H5O2þ )H4[PW9MoV2O40] 8H2O. The temperature of exothermic peak in the DTA curve can be used to characterize the thermal stability of the HPAs [14]. The endothermic peaks and the first exothermic peak in the DTA curve of the HPA are attributed to the processes of their dehydration and irreversible decomposition. Thus, the process of the dehydration of PW9MoV2 occurs at 116 1C and 297 1C. The exothermic peak appears at 467 1C is due to the irreversible decomposition of Keggin-type anion. The electrochemical impedance spectrum (EIS) of PW9MoV2 is shown in Fig.4. Proton conductivity of the HPA is calculated according to the relation, σ ¼L/(S R) (where R is the resistance, L is the thickness, and S is the area of the tablet). The proton conductivity of PW9MoV2 is 8.92 10 3 S cm 1 at 22 1C and 80% relative humidity (RH). In the range of measured temperature, its proton conductivity increases with higher temperature. Fig. 4 indicates the Arrhenius plot of proton conduction of PW9MoV2. The relationship between proton conductivity and activation energy is consistent with Arrhenius equation. The activation energy of proton conduction of PW9MoV2 is 23.45 kJ mol 1. There are two predominant mechanisms of proton conduction: Grotthuss mechanism [15] and Vehicle mechanism [16]. In Grotthuss mechanism, a large amount of water can assist proton transport through a hydrogen-bonded network. It is different from Vehicle mechanism, in which water assists proton movement by facilitating transport as an H3O þ species. And, the activation energy of Grotthuss mechanism is lower (less than 15 kJ mol 1) than that of Vehicle mechanism (more than 20 kJ mol 1). So, the mechanism of proton conduction of PW9MoV2 is Vehicle mechanism for its activation energy is more than 20 kJ mol 1.
1078 979 788 880
4000
3200
2400
1600
800
-1
Wavenumber (cm ) Fig. 1. IR spectrum of H5PW9MoV2O40 10H2O.
5
10
15
20
25
30
35
40
2θ (degree) Fig. 2. XRD pattern of H5PW9MoV2O40 10H2O.
Table 1 Data of X-ray powder diffraction of H5PW9MoV2O40 10H2O. 2θ (1)
I/I0
d (Å)
8.06 8.94 9.32 17.84 18.64 19.50 20.54 26.16 27.80 28.98 33.42 36.44 37.96
0.73 0.69 0.49 0.31 0.37 0.34 0.39 0.53 1.00 0.78 0.32 0.31 0.34
10.96 9.88 9.48 4.97 4.75 4.55 4.32 3.40 3.21 3.08 2.68 2.46 2.37
467 o C
100
DTA (μV/mg)
3. Results and discussion
Intensity (a.u.)
All chemicals were of analytical grade and used without further purification.
Transmittance (%)
Y. Li et al. / Materials Letters 157 (2015) 109–111
Weight loss (%)
110
98 116 o C
96
297 o C
94 92
en
4. Conclusion
100
200
300
400
500
600
o
In this paper we have reported the syntheses, characterization and conductive performance of a new vanadium and
Temperature ( C) Fig. 3. TG–DTA curve of H5PW9MoV2O40 10H2O.
Y. Li et al. / Materials Letters 157 (2015) 109–111
2
Z" (Ohm cm )
-8
.
-6 -4 -2 0 2 25
30
35
40
45
50
Z' (Ohm.cm ) 2
Fig. 4. Electrochemical impedance spectrum of H5PW9MoV2O40 10H2O. Inset: Arrhenius plot of conductivity for H5PW9MoV2O40 10H2O.
molybdenum-substituted Keggin-type quaternary heteropoly acid H5PW9MoV2O40 10H2O. IR, UV and XRD patterns indicate that it possesses Keggin structure. Its accurate molecular formula is (H5O2þ )H4[PW9MoV2O40] 8H2O. The compound has high proton conductivity. The mechanism of proton conduction of H5PW9MoV2O40 10H2O is Vehicle mechanism. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (21071124, 21173189), the Zhejiang Provincial Natural Science Foundation of China (LY14B030005) and the Foundation of Key Laboratory of Polyoxometalate Science, the Ministry of Education of Northeast Normal University. References [1] Khenkin AM, Efremenko I, Martin JML, Neumann R. Polyoxometalatecatalyzed insertion of oxygen from O-2 into tin-alkyl bonds. J Am Chem Soc 2013;135:19304–10. [2] Li YY, Huang TP, Wu QY, Ding H, Yan WF, Yaroslavtsev AB. A reversible phase transformation monovanadium-substituted Keggin polyoxometalate-based ionic liquid. Mater Lett 2014;121:159–61.
111
[3] Suárez-Guevara J, Ruiz V, Gomez-Romero P. Hybrid energy storage: high voltage aqueous supercapacitors based on activated carbon–phosphotungstate hybrid materials. J Mater Chem A 2014;2:1014–21. [4] Busche C, Vilà-Nadal L, Yan J, Miras HN, Long DL, Georgiev VP, et al. Design and fabrication of memory devices based on nanoscale polyoxometalate clusters. Nature 2014;515:545–9. [5] Zhang H, Xie AJ, Shen YH, Qiu LG, Tian XY. Layer-by-layer inkjet printing of fabricating reduced graphene-polyoxometalate composite film for chemical sensors. Phys Chem Chem Phys 2012;14:12757–63. [6] Miao J, Liu YW, Tang Q, He DF, Yang GC, Shi Z, et al. Proton conductive watery channels constructed by Anderson polyanions and lanthanide coordination cations. Dalton Trans 2014;43:14749–55. [7] Wu QY, Sang XG, Liu B, Ponomareva VG. Synthesis and performance of highproton conductor undecatungstochromoindic heteropoly acid. Mater Lett 2005;59:123–6. [8] Domaille PJ, Hervéa G, Téazéa A. Vanadium (V) substituted dodecatungstophosphates. Inorg Synth 2007;27:96–104. [9] Tong X, Tian NQ, Wu W, Zhu WM, Wu QY, Cao FH, et al. Preparation and electrochemical performance of tungstovanadophosphoric heteropoly acid and its hybrid materials. J Phys Chem C 2013;117:3258–63. [10] But S, Lis S. Spectroscopic studies of Eu (III) Keggin's and Dawson's polyoxotungstates substituted by acetato and oxalato ligands. J Alloys Compd 2008;451:384–7. [11] Kato CN, Hara K, Hatano A, Goto K, Kuribayashi T, Hayashi K, et al. A Dawsontype dirhenium (V)-oxido-bridged polyoxotungstate: X-ray crystal structure and hydrogen evolution from water vapor under visible light irradiation. Eur J Inorg Chem 2008;3134–41. [12] Gao Q, Li FY, Wang YC, Xu L, Bai J, Wang Y. Organic functionalization of polyoxometalate in aqueous solution: self-assembly of a new building block of {VMo6O25} with triethanolamine. Dalton Trans 2014;43:941–4. [13] Zhang MJ, Zhao PP, Leng Y, Chen GJ, Wang J, Huang J. Schiff base structured acid-base cooperative dual sites in an ionic solid catalyst lead to efficient heterogeneous knoevenagel condensations. Chem Eur J 2012;18:12773–82. [14] Wei ML, Wang YX, Wang XJ. Two proton-conductive hybrids based on 2-(3pyridyl) benzimidazole molecules and Keggin-type heteropolyacids. J Solid State Chem 2014;209:29–36. [15] Dippel T, Kreuer KD, Lassegues JC, Rodriguez D. Proton conductivity in fused phosphoric acid; A1H/31P PFG-NMR and QNS study. Solid State Ionics 1993;61:41–6. [16] Kreuer KD, Weppner W, Rabenau A. Vehicle mechanism, a new model for the interpretation of the conductivity of fast proton conductors. Angew Chem Int Ed 1982;21:208–9.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and conductive performance of quaternary molybdotungstovanadophosphoric heteropoly acid with Keggin structure Yunyan Li a, Tianpei Huang a, Qingyin Wu a,n, Lin Xu b a b
Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China Key Laboratory of Polyoxometalate Science of Ministry of Education, Northeast Normal University, Changchun 130024, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 March 2015 Accepted 10 May 2015 Available online 16 May 2015
A new vanadium and molybdenum-substituted Keggin-type quaternary heteropoly acid H5PW9MoV2O40 10H2O has been synthesized by the stepwise acidification and the stepwise addition of element solutions. The product is characterized by the IR, UV, XRD, TG–DTA and electrochemical impedance spectroscopy (EIS). EIS measurement shows the conductivity value of H5PW9MoV2O40 10H2O is 8.92 10 3 S cm 1 at 22 1C and 80% relative humidity, which increases with higher temperature. Its mechanism of proton conduction is Vehicle mechanism for its conductive activation energy of 23.45 kJ mol 1. & 2015 Elsevier B.V. All rights reserved.
Keywords: Heteropoly acid Nanocrystalline materials Functional Thermal analysis Conductivity Mechanism
1. Introduction
2. Experimental section
Heteropoly acids (HPAs), an important kind of solid acids with polynuclear and oxygen bridges, have attracted special interest in the fields of catalysis, medicine, biology and materials science, due to their excellent properties such as simple composition, structure determination, the structural features of both complex and metaloxide, and acidic oxidation–reduction [1–5]. They are formed by inorganic metal–oxygen cluster anions. Because the HPAs have discrete and mobile ionic structures, they possess strong Brønsted acidity and “pseudoliquid phase” behavior. Hence, HPAs have been regarded as attractive candidates for proton conduction in fuel cells [6]. Unfortunately, most of the heteropoly acids reported are binary ones, including H3PW12O40, H3PMo12O40, H4SiW12O40 and H4SiMo12O40. We have prepared a series of vanadium-substituted ternary Keggin-type and Dawson-type HPAs while studies of quaternary HPAs are rarely reported [7]. In this paper, we report the synthesis, characterization, conductivity and proton conduction mechanism of a vanadium-substituted quaternary Keggin-type heteropoly acid H5PW9MoV2O40 10H2O (abbreviated as PW9MoV2).
Preparation: Na9PW9O34 7H2O (PW9) was synthesized according to the literature available [8]. PW9MoV2 was synthesized by a modification of the method according to the procedure described in our previous report [9]. 20 mL aqueous solution of sodium metavanadate (1.58 g, NaVO3 2H2O) was mixed with 15 mL aqueous solution of sodium molybdate (1.21 g, Na2MoO4 2H2O). The mixture was added dropwise to 100 mL aqueous solution of PW9 (12.81 g). The final solution was adjusted to pH 2.8 and continuously heated at 90 1C for 2 h under stirring. The cooled solution was extracted with ether (30 mL) in sulfuric acid medium. After the concentrated etherate solution was dried in vacuum, the orange–red power was obtained. Reagents and instruments: FTIR spectrum was recorded on a Nicolet Nexus 470 FT/IR spectrometer over the wave number range 400–4000 cm 1 using KBr pellets. UV spectrum was measured on a SHIMADZU U-2550 UV–vis spectrophotometer. XRD pattern was recorded on a BRUKER D8 ADVANCE X-ray diffractometer using a Cu tube operated at 40 kV and 40 mA in the range of 2θ¼4–401 at a rate of 0.021 s 1. The thermal stability of the sample was studied using simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) techniques from room temperature to 600 1C. Measurements were performed using a Shimadzu thermal analyzer in a Nitrogen stream, with a scanning rate of 10 1C min 1. Impedance measurements of the HPA were performed on a VMP2 Multichannel potentiostat electrochemical impedance analyzer over a frequency ranges from 9.99 104 to 0.01 Hz.
n
Corresponding author. Tel.: þ 86 571 88914042; fax: þ86 571 87951895. E-mail address: [email protected] (Q. Wu).
http://dx.doi.org/10.1016/j.matlet.2015.05.026 0167-577X/& 2015 Elsevier B.V. All rights reserved.
The IR spectrum of PW9MoV2 was investigated, as shown in Fig. 1. PW9MoV2 exhibits characteristic peaks of heteropoly acid, which demonstrates that this acid possesses Keggin strcture [10]. The characteristic band at 1078 cm 1 is attributed to υ(P–Oa), the band at 979 cm 1 is attributed to υ(M–Od), and peaks at 880 cm 1 and 788 cm 1 are corresponding to the υ(M–Ob) and υ(M–Oc) modes of the polyoxoanion (M¼ W, Mo, V) [11]. The bands at about 3400 and 1600 cm 1 of the HPA are assigned to the stretching vibration of O–H bonds and the bending vibration of H–O–H bonds. For PW9MoV2, the UV absorption band appears at 259 nm, which is assigned to charge-transfer of bridge-oxygen (Ob, Oc) to metal atoms (Mo, V) in the heteropolyanion cages [12]. Fig. 2 shows the XRD pattern of PW9MoV2, which shows its phase and structure. The data of the XRD are listed in Table 1. Its intense peaks mainly show in the regions of 7–101, 16–221, 25–301 and 33–381, which indicates that the acid has crystal structure [13]. The peaks in the region of 2θ¼ 7–101 are the characteristic peaks of HPAs with Keggin structure. The content of crystal water, protonated water and structure water of PW9MoV2 can be calculated by its TG curve (Fig. 3). The TG curve shows that the total percent of weight loss before 330 1C of PW9MoV2 is 6.60%, indicating that 9.9 water molecules are lost. The loss of 7.9 molecules of crystal water happen at first, the second is the loss of 2 molecules of protonized water, and 2 molecules of structural water are lost at last. Thus, the accurate molecular formula of the product is (H5O2þ )H4[PW9MoV2O40] 8H2O. The temperature of exothermic peak in the DTA curve can be used to characterize the thermal stability of the HPAs [14]. The endothermic peaks and the first exothermic peak in the DTA curve of the HPA are attributed to the processes of their dehydration and irreversible decomposition. Thus, the process of the dehydration of PW9MoV2 occurs at 116 1C and 297 1C. The exothermic peak appears at 467 1C is due to the irreversible decomposition of Keggin-type anion. The electrochemical impedance spectrum (EIS) of PW9MoV2 is shown in Fig.4. Proton conductivity of the HPA is calculated according to the relation, σ ¼L/(S R) (where R is the resistance, L is the thickness, and S is the area of the tablet). The proton conductivity of PW9MoV2 is 8.92 10 3 S cm 1 at 22 1C and 80% relative humidity (RH). In the range of measured temperature, its proton conductivity increases with higher temperature. Fig. 4 indicates the Arrhenius plot of proton conduction of PW9MoV2. The relationship between proton conductivity and activation energy is consistent with Arrhenius equation. The activation energy of proton conduction of PW9MoV2 is 23.45 kJ mol 1. There are two predominant mechanisms of proton conduction: Grotthuss mechanism [15] and Vehicle mechanism [16]. In Grotthuss mechanism, a large amount of water can assist proton transport through a hydrogen-bonded network. It is different from Vehicle mechanism, in which water assists proton movement by facilitating transport as an H3O þ species. And, the activation energy of Grotthuss mechanism is lower (less than 15 kJ mol 1) than that of Vehicle mechanism (more than 20 kJ mol 1). So, the mechanism of proton conduction of PW9MoV2 is Vehicle mechanism for its activation energy is more than 20 kJ mol 1.
1078 979 788 880
4000
3200
2400
1600
800
-1
Wavenumber (cm ) Fig. 1. IR spectrum of H5PW9MoV2O40 10H2O.
5
10
15
20
25
30
35
40
2θ (degree) Fig. 2. XRD pattern of H5PW9MoV2O40 10H2O.
Table 1 Data of X-ray powder diffraction of H5PW9MoV2O40 10H2O. 2θ (1)
I/I0
d (Å)
8.06 8.94 9.32 17.84 18.64 19.50 20.54 26.16 27.80 28.98 33.42 36.44 37.96
0.73 0.69 0.49 0.31 0.37 0.34 0.39 0.53 1.00 0.78 0.32 0.31 0.34
10.96 9.88 9.48 4.97 4.75 4.55 4.32 3.40 3.21 3.08 2.68 2.46 2.37
467 o C
100
DTA (μV/mg)
3. Results and discussion
Intensity (a.u.)
All chemicals were of analytical grade and used without further purification.
Transmittance (%)
Y. Li et al. / Materials Letters 157 (2015) 109–111
Weight loss (%)
110
98 116 o C
96
297 o C
94 92
en
4. Conclusion
100
200
300
400
500
600
o
In this paper we have reported the syntheses, characterization and conductive performance of a new vanadium and
Temperature ( C) Fig. 3. TG–DTA curve of H5PW9MoV2O40 10H2O.
Y. Li et al. / Materials Letters 157 (2015) 109–111
2
Z" (Ohm cm )
-8
.
-6 -4 -2 0 2 25
30
35
40
45
50
Z' (Ohm.cm ) 2
Fig. 4. Electrochemical impedance spectrum of H5PW9MoV2O40 10H2O. Inset: Arrhenius plot of conductivity for H5PW9MoV2O40 10H2O.
molybdenum-substituted Keggin-type quaternary heteropoly acid H5PW9MoV2O40 10H2O. IR, UV and XRD patterns indicate that it possesses Keggin structure. Its accurate molecular formula is (H5O2þ )H4[PW9MoV2O40] 8H2O. The compound has high proton conductivity. The mechanism of proton conduction of H5PW9MoV2O40 10H2O is Vehicle mechanism. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (21071124, 21173189), the Zhejiang Provincial Natural Science Foundation of China (LY14B030005) and the Foundation of Key Laboratory of Polyoxometalate Science, the Ministry of Education of Northeast Normal University. References [1] Khenkin AM, Efremenko I, Martin JML, Neumann R. Polyoxometalatecatalyzed insertion of oxygen from O-2 into tin-alkyl bonds. J Am Chem Soc 2013;135:19304–10. [2] Li YY, Huang TP, Wu QY, Ding H, Yan WF, Yaroslavtsev AB. A reversible phase transformation monovanadium-substituted Keggin polyoxometalate-based ionic liquid. Mater Lett 2014;121:159–61.
111
[3] Suárez-Guevara J, Ruiz V, Gomez-Romero P. Hybrid energy storage: high voltage aqueous supercapacitors based on activated carbon–phosphotungstate hybrid materials. J Mater Chem A 2014;2:1014–21. [4] Busche C, Vilà-Nadal L, Yan J, Miras HN, Long DL, Georgiev VP, et al. Design and fabrication of memory devices based on nanoscale polyoxometalate clusters. Nature 2014;515:545–9. [5] Zhang H, Xie AJ, Shen YH, Qiu LG, Tian XY. Layer-by-layer inkjet printing of fabricating reduced graphene-polyoxometalate composite film for chemical sensors. Phys Chem Chem Phys 2012;14:12757–63. [6] Miao J, Liu YW, Tang Q, He DF, Yang GC, Shi Z, et al. Proton conductive watery channels constructed by Anderson polyanions and lanthanide coordination cations. Dalton Trans 2014;43:14749–55. [7] Wu QY, Sang XG, Liu B, Ponomareva VG. Synthesis and performance of highproton conductor undecatungstochromoindic heteropoly acid. Mater Lett 2005;59:123–6. [8] Domaille PJ, Hervéa G, Téazéa A. Vanadium (V) substituted dodecatungstophosphates. Inorg Synth 2007;27:96–104. [9] Tong X, Tian NQ, Wu W, Zhu WM, Wu QY, Cao FH, et al. Preparation and electrochemical performance of tungstovanadophosphoric heteropoly acid and its hybrid materials. J Phys Chem C 2013;117:3258–63. [10] But S, Lis S. Spectroscopic studies of Eu (III) Keggin's and Dawson's polyoxotungstates substituted by acetato and oxalato ligands. J Alloys Compd 2008;451:384–7. [11] Kato CN, Hara K, Hatano A, Goto K, Kuribayashi T, Hayashi K, et al. A Dawsontype dirhenium (V)-oxido-bridged polyoxotungstate: X-ray crystal structure and hydrogen evolution from water vapor under visible light irradiation. Eur J Inorg Chem 2008;3134–41. [12] Gao Q, Li FY, Wang YC, Xu L, Bai J, Wang Y. Organic functionalization of polyoxometalate in aqueous solution: self-assembly of a new building block of {VMo6O25} with triethanolamine. Dalton Trans 2014;43:941–4. [13] Zhang MJ, Zhao PP, Leng Y, Chen GJ, Wang J, Huang J. Schiff base structured acid-base cooperative dual sites in an ionic solid catalyst lead to efficient heterogeneous knoevenagel condensations. Chem Eur J 2012;18:12773–82. [14] Wei ML, Wang YX, Wang XJ. Two proton-conductive hybrids based on 2-(3pyridyl) benzimidazole molecules and Keggin-type heteropolyacids. J Solid State Chem 2014;209:29–36. [15] Dippel T, Kreuer KD, Lassegues JC, Rodriguez D. Proton conductivity in fused phosphoric acid; A1H/31P PFG-NMR and QNS study. Solid State Ionics 1993;61:41–6. [16] Kreuer KD, Weppner W, Rabenau A. Vehicle mechanism, a new model for the interpretation of the conductivity of fast proton conductors. Angew Chem Int Ed 1982;21:208–9.