Synthesis and performance of solid proton conductor molybdovanadosilicic acid

A molybdovanadosilicic acid H5SiMo11VO40·8H2O was synthesized and investigated in this work. The structure features and hydration degree of this acid were characterized by IR, UV, XRD and TG-DTA. Its proton conductivity was studied by electrochemical impedance spectroscopy (EIS). The EIS measurements demonstrated that H5SiMo11VO40·8H2O showed excellent proton conduction performance with proton conductivity reaching 5.70 × 10−3 S cm−1 at 26 °C and 70% relative humidity. So, it is a new solid high proton conductor. The conductivity enhances with the increase of temperature, and it exhibits Arrhenius behavior. The activation energy value for proton conduction is 21.4 kJ mol−1, suggesting that the proton transfer in this solid acid is dominated by Vehicle mechanism.


Introduction
Heteropoly acids (HPAs) are a unique class of nanosized polynuclear clusters composed of transition metals and oxygen atoms. 1,2 During the past decades, HPAs have attracted huge interest owing to their huge potential in many elds, for instance, catalysts, biomedicine and material science. [3][4][5][6][7] In particular, due to their high proton conductivity and proton transfer/storage abilities, HPAs have become one of the best candidate electrolyte materials for the development of fuel cells and supercapacitors. [8][9][10][11][12] So, a further investigation of the role and function of these compounds in electrochemical devices will give insight into developing solid electrolytes based on HPAs. 13 HPAs contain two kinds of protons: (1) dissociated hydrated protons connected to one heteropolyanion as a whole; (2) non-hydrated protons located on the bridge-oxygen or terminal-oxygen atoms of the polyanion. 14 Keggin-type HPAs, which are an important branch of HPAs, occupy an unique place at the forefront of the HPAs eld because of their many advantages, including chemical stability and convenience of synthesis. 15,16 Its chemical formula can be expressed as [XM 12 O 40 ] nÀ , with mainly X ¼ P, As, Si and Ge, and M ¼ W, Mo and V. What attracts us most is that the structure and proton conductive property of HPAs vary with the component elements of heteropolyanion changes. According to our recent researches, 17 vanadium have great impact on the thermal stability and proton conductivity of heteropoly acids. Hence, in this work, based on Keggin-type HPAs, a mono-vanadium-substituted molybdovanadosilicic acid H 5 SiMo 11 VO 40 $8H 2 O is synthesized, and its structure, hydration and proton conductive properties have also been investigated.

Instructions and reagents
Element content was measured on a THERMO ELECTRON PQ3 inductively coupled plasma mass spectrometer (ICP-MS). The Fourier-transform infrared spectroscopy (FTIR) spectrum was performed on a NICOLET NEXUS470 FT/IR spectrometer using KBr as pellets, and the resolution is 4 cm À1 . UV spectrum was got using SHIMADZU UV-2550 UV-Vis spectrophotometer.
Powder X-ray diffraction (XRD) pattern was recorded on a BRUKER D8 ADVANCE X-ray diffractometer in the range of 2q ¼ 3-40 at the rate of 0.02 s À1 . The crystal data was collected using graphitemono-chromatic Mo-K radiation (0.71073 A) at 293 K, and the data sets were corrected by empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. The thermal stability of the sample was carried out using simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) technique, measurements were performed using a Shimadzu thermal analyzer in a nitrogen stream from room temperature to 600 C, with the scanning rate of 10 C min À1 .
All chemicals were of analytical grade and used without further purication.

Measurement of proton conductivity
At ambient condition, the crushed sample was pressed at 25 MPa into a compacted pellet. The diameter of the obtained tablet is 10.00 mm and the thickness of it is 1.02 mm. The proton conductivity was measured using a cell: copper | sample | copper, in which copper slices were attached to the two sides of the tablet as electrodes. Complex impedance measurements were carried out on a VMP2 multichannel potentiostat electrochemical impedance analyzer over a frequency range from 9.99 Â 10 4 to 0.01 Hz.

Results and discussion
IR is a useful method for investigating the structure information of heteropoly compounds. There are four kinds of oxygen atoms in [SiM 12 O 40 ] nÀ . The SiO 4 tetrahedron is located in the center of the twelve MO 6 octahedra, which can be split into four groups of three edge-shared octahedra, M 3  For comparison, these four peaks of H 4 SiMo 12 O 40 are 957 cm À1 , 904 cm À1 , 855 cm À1 , 770 cm À1 . 18 The similarity of vibration bands demonstrates that the heteropoly acid H 5 SiMo 11 VO 40 has the Keggin structure as its parent acid H 4 SiMo 12 O 40 does. 19 Obviously, all of these peaks have slightly shied, which caused by the more negative charge carrier and the shi symmetry of molecular structure of H 4 SiMo 11 VO 40 $8H 2 O. Furthermore, there are two strong absorption peaks at 3410 cm À1 and 1627 cm À1    We have also got some information about crystal structure of H 5 SiMo 11 VO 40 $8H 2 O by powder X-ray diffraction (XRD). Fig. 4 is the powder X-ray diffraction pattern of H 5 SiMo 11 VO 40 $8H 2 O. From the data of XRD in Table 1, we get to know that the characteristic peaks of this HPA mainly exist at the range of 8-10 , 17-23 , 25-32 and 33-38 , and the most intensive peak is at about 9.25 . They are the characteristic peaks of HPAs with Keggin structure. 21 Up to now, XRD, combined with UV and IR spectra, allows us to verify that the heteropoly acid H 5 SiMo 11 -VO 40 possesses the typical Keggin structure as shown in Fig. 5.
The hydration degree and thermal stability of H 5 SiMo 11 -VO 40 $8H 2 O was investigated by thermogravimetric (TG) and differential thermal analysis (DTA). Generally, heteropoly acids contain three kinds of crystallographic water: crystal water, protonized water and structure water. As illustrated in Fig. 6, The TG curve shows the total percent of the weight loss below 383 C is 9.30%, which indicates that 10.5 molecules of water calculated are lost. Firstly, the loss of 4.9 molecules crystal water happen with corresponding to the weight loss of 4.61%, then, The proton conductivity is one of the most important properties of heteropoly acids. Fig. 7 is the electrochemical impedance spectrum of H 5 SiMo 11 VO 40 $8H 2 O, and inset is the equivalent circuit, where R 1 is the bulk resistance, C 1 represents a constant phase element of the double layers, R 2 denotes the charge transfer resistance and W 1 is the nite length Warburg element of solid diffusion. The proton conductivity of HPA is calculated using the following equation: s ¼ L/(RS) (R is the resistance, L is the thickness, and S is the area of the tablet). By calculation, the proton conductivity of H 5 SiMo 11 VO 40 $8H 2 O is   . Color legend: VO 6 , orange octahedra; MoO 6 , green octahedra; SiO 4 , purple tetrahedra. Fig. 6 The TG-DTA curves of H 5 SiMo 11 VO 40 $8H 2 O. 5.70 Â 10 À3 S cm À1 at 26 C and 70% relative humidity. It shows a higher conductivity than H 4 SiMo 12 O 40 $12H 2 O (2.03 Â 10 À4 S cm À1 ), 24 which illustrated that the incorporation of V element can increase hugely the proton conductivity of silicomolybdic acid. So, this heteropoly compound is a high proton conductor. Additionally, CPE for double layers from the equivalent circuit is 3.59 Â 10 À3 F, indicating that this acid has the potential for application in energy storage device, such as supercapacitors.
To investigate the relationship between proton conductivity and temperature, we measured the conductivity of this heteropoly acid at the temperature range of 26-60 C. It is found that the conductivity value of this heteropoly acid enhances with higher temperature because the mobility of conducting species accelerates with the increase of temperature, which increases to 1.35 Â 10 À2 S cm À1 at 60 C and 70% relative humidity. The relationship between proton conductivity and temperature is consistent with Arrhenius equation: s ¼ s 0 exp (E a /kT) . In this formula, E a is the activation energy of proton conductivity, s 0 is the pre-exponential factor and k presents the Boltzmann constant. As shown in Fig. 8, the activation energy of proton conductivity calculated from the slope of Arrhenius curve is 21.4 kJ mol À1 .
So far, there are two major recognized proton conduction mechanisms: Vehicle and Grotthuss mechanism. 25,26 In Vehicle mechanism, proton interacts with water molecules, which transfers in the form of hydrated hydrogen ions, such as H 3 O + , H 5 O 2 + and H 9 O 4 + species, similar to molecular diffusion. It differs from Grotthuss mechanism, in which a large amount of water can assist proton hopping from one proton carrier to a neighboring one down a chain of hydrogen-bonded network. Therefore, water plays a fairly important role in the process of proton mobility, and proton conductivity and Arrhenius parameters are strongly dependent on the water content. 27,28 Generally, we distinguish them by the numerical value of activation energy and the type of hydrated hydrogen ions. The activation energy of Grotthuss mechanism is oen less than 15 kJ mol À1 , which is lower than that of Vehicle mechanism, whose is normally more than 20 kJ mol À1 . 29,30 The activation energy of this acid is 21.4 kJ mol À1 . The acid protons and water molecules form H 5 O 2 + that bridges the Keggin units owing to the low hydration levels (less than 10 water molecules of hydration per Keggin unit). 31 Hence, we can speculate that the proton migration of H 5 SiMo 11 VO 40 $8H 2 O occurs by a mixing mechanism and Vehicle mechanism is predominant.

Conclusions
In this work, we have reported the synthesis of a vanadiumsubstituted heteropoly acid H 5 SiMo 11 VO 40 $8H 2 O, which was characterized by IR, XRD of powder, TG-DTA. This acid shows a good proton conductivity of 5.70 Â 10 À3 S cm À1 at 26 C and 70% relative humidity. The proton conductivity enhances with the increase of temperature, and the proton conduction mechanism is dominated by Vehicle mechanism due to the activation energy of 21.4 kJ mol À1 . It is a novel solid high proton conductor, which may be applied as solid electrolyte in the elds of fuel cell and supercapacitors.

Conflicts of interest
There are no conicts to declare.