Synthesis and Electrochemical Performance of Keggin-type

Polyoxometalate Gels

Q Y Wu

, W S Dai, S B Cui and F W He

School of Biomedical and Chemical Engineering, Liaoning Institute of Science and

Technology, Benxi 117004, Liaoning, China

Corresponding author and e-mail: Q Y Wu, qywu@lnist.edu.cn

Abstract. Three Keggin-type vanadium-substituted polyoxo metalate gels, [PyPS]

[PyPS]

and [PyPS]

PMo

have been synthesized and characterizated. The

relationship between the component elements of the vanadium-substituted polyoxo metalate

gels and their electrochemical performance has been investigated. The results show that

[PyPS]

PMo

has stronger oxidability than [PyPS]

and [PyPS]

1. Introduction

Polyoxometalates (POMs), a class of nano-sized inorganic transition-metal oxide clusters with a

diverse range of fascinating properties, have attracted special interest in the fields of catalysis,

medicine, biology and materials science [1-6]. POMs can be modified by some other series of cations,

such as quaternary ammonium cations, to prepare many novel types of gel-type hybrid materials [7-

10]. Such POM-based gel-type materials can be easily shaped, and can maintain some significant

physical characteristics such as temperature-responsive behavior. Therefore, they have remarkable

potential applications, such as electrochemical supercapacitors and fuel cells [11, 12].

Herein, we report the synthesis and electrochemical performance of three Keggin-type vanadium-

substituted polyoxometalate gels, [PyPS]

, [PyPS]

and [PyPS]

PMo

40.

2. Experimental

2.1. Instruments and reagents

Infrared (IR) spectra were recorded on a NICOLET NEXUS 470 FT/IR spectrometer over the

wavenumber range 400–4000 cm

−1

using KBr pellet. X-ray powder diffraction analysis was obtained

on a BRUKER D8 ADVANCE X-ray diffractometer using a Cu tube operated at 50 kV and 200 mA

in the range of 2θ =4–40° at a scanning rate of 0.02° s

−1

. Inductively coupled plasma mass

spectrometry (ICP-MS) analysis was determined on a Shimadzu V-1012 ICP-MS spectrometer.

Electrochemical experiments were performed with a CHI660E Electrochemical Workstation in a

conventional three-electrode electrochemical cell using glass carbon (5 mm in diameter) as the

working electrode, platinum as the counter electrode, and a saturated calomel reference electrode in

organic media. The density of substrate was 0.25 mM and 0.2 M NaClO

was assigned as electrolyte.

All reagents were analysis grade and purchased from Aladdin, without further purification.

Wu, Q., Dai, W., Cui, S. and He, F.

Synthesis and Electrochemical Performance of Keggin-type Polyoxometalate Gels.

In Proceedings of the International Workshop on Materials, Chemistry and Engineering (IWMCE 2018), pages 625-629

ISBN: 978-989-758-346-9

625

2.2. Synthesis of polyoxometalate gels

1-(3-sulfonic group) propyl-pyridine (PyPS) was synthesized according to the literature [13].

, H

and H

PMo

were synthesized by modification of the method

according to the literatures [14-16]. The pre-synthesized PyPS and phosphorus-containing HPA,

, H

and H

PMo

were taken in 6:1, 4:1 and 4:1 mole ratio to give one

mole of [PyPS]

, [PyPS]

and [PyPS]

PMo

. PyPS was added to an

aqueous solution of HPA, and then the mixture was stirred for 10 h at room temperature. Water was

first evaporated in a 40°C water bath and then removed under vacuum to give highly viscous even

gel-state products. The obtained compounds are highly insoluble in tetrahydrofuran, acetone or ethyl

acetate, but soluble in water, N, N-dimethylformamide and dimethyl sulfoxide.

Carbon, nitrogen, sulfur, phosphorus, tungsten, molybdenum and vanadium were analyzed by

elemental analysis. The results indicate that the actual measurement values are consistent with the

calculated values, which confirms the composition of three POM-Gels.

3. Results and discussion

3.1. IR spectra

Table1. The assignment of the vibration modes in the IR spectra of the gels.

Vibration modes

Wavenumber (cm

-1

)

[PyPS]

PMo

O-H stretching

3420

3413

3425

-CH

stretching

2953

2951

2935

H-O-H bending

1633

1637

1635

-CH

scissoring

1468

1488

1482

S=O bending

1171

1163

1148

P-O

stretching

1055

1053

1051

M-O

stretching

971

963

956

M-O

-M stretching

892

891

878

M-O

-M stretching

806

808

798

Figure 1.IR spectra of the products: (a) [PyPS]

, (b) [PyPS]

, (c)

[PyPS]

PMo

40.

IWMCE 2018 - International Workshop on Materials, Chemistry and Engineering

626

Compared with the pure acid, the characteristic bands of polyoxoanions have shifted (Figure 1). The

pure stretching vibration, M-O

vibrations, where the vibration frequency is influenced by the anion-

anion interactions, have decreased when sulf-group grafted ammoniums have been added with HPA

to make gel-type compounds. This is due to the weaker anion-anion electrostatic interaction as the

anion-anion distance increases. As M-O

-M and M-O

-M vibrations are not pure and cannot be free

from bending character, there is perhaps a competition of the opposite effects, which leads to an

increase in the vibratation comparing to the pure acid. The result reveals that this series of

compounds still maintain POM structures, which is consistent with those reported in the literature.

In the high wavenumber region, each spectrum of the gels exhibits two other peaks at around

3420cm

-1

and 1630cm

-1

. These are assigned to the stretching vibration of O-H bonds and the bending

vibration of H-O-H bonds, respectively.

In addition, there are some other characteristic peaks of sulf-group grafted ammoniums such as

S=O

,ν

C-H

of CH

. These sulf-group grafted ammoniums cations also maintain their structure, which

indicates the successful assembly and existence of the POM structure units and organic ammonium

cations in the compounds without depolymerization or degradation.

3.2. XRD

patterns

Figure 2.XRD patterns (a) [PyPS]

, (b) [PyPS]

, (c) [PyPS]

PMo

40.

Figure 3.Schematic illustration of organized structures of the POM-type layered materials.

Synthesis and Electrochemical Performance of Keggin-type Polyoxometalate Gels

627

The XRD patterns (Figure 2) of [PyPS]

, [PyPS]

and [PyPS]

PMo

are in

marked contrast to that of the pure heteropoly acids and consistent with the gel-state appearance.

According to the recent paper and considered intense peaks in the XRD patterns of these compounds

in small angles area, we can assume that an organized layer-type structure exists in this series of

compounds, as illustrated in Figure 3, and the height of each layer can be calculated by the intense

peaks in small angles area. Meanwhile, the strong diffraction peak at 7~10° can be considered as the

POM anion structure and a wide diffraction peak appears in the wide-angle region, indicating that

these compounds have a gel-type phase at room temperature, which is caused by weak connections of

the layers, rather than an identified shape in total like the pure acid.

3.3. The cyclic voltammetry

The cyclic voltammetry studies of [PyPS]

, [PyPS]

and PyPS]

PMo

are

shown in Figure 4 and Table 2.

-0.3 0.0 0.3 0.6

-18

-12

-6

Ⅰ'

Ⅰ

Ⅰ'

Ⅰ

Ⅰ'

Ⅰ

I/

E/V vs Hg

/Hg

PyPS

PMo

Figure 4. Cyclic voltammetry of [PyPS]

, [PyPS]

and [PyPS]

PMo

with

a scanning rates of 50mv·s

-1

in DMF.

Table 2. The half-wave potentials, E

1/2

, for the redox couples observed in Figure 4.

Compounds

1/2

(I/ I’) [mv]

[PyPS]

131

[PyPS]

206

[PyPS]

PMo

245

In [PyPS]

[PyPS]

and [PyPS]

PMo

, the reduction of the vanadium

is shown as follows: .

11 40 11 40

(V) (IV)PW V O e PW V O

  



Ⅰ /Ⅰ ’

9 3 40 9 3 40

(V) 3 (IV)PWV O e PWV O

  



Ⅰ /Ⅰ ’

IWMCE 2018 - International Workshop on Materials, Chemistry and Engineering

628

11 40 11 40

(V) (IV)PMo V O e PMo V O

  



Ⅰ /Ⅰ ’

Table 2 reveals that [PyPS]

PMo

has stronger oxidability than [PyPS]

and

[PyPS]

while the tri-substituted compound, [PyPS]

, has stronger oxidability

than the mono-substituted one, [PyPS]

. In other words, it can be concluded that when it

comes to the vanadium-substituted molybdophosphorates and tungstophosphorates, it is the POM

component elements that mainly determine the oxidability rather than the number of the substituted

atoms, as [PyPS]

PMo

has stronger oxidability than [PyPS]

and [PyPS]

while there are more vanadium atoms in [PyPS]

4. Conclusions

In this paper, we have mainly reported the synthesis and electrochemical performance of a series of

POM-type gels, [PyPS]

, [PyPS]

and [PyPS]

PMo

. The relationship

between the component elements of the vanadium-substituted polyoxometalate gels and their

electrochemical performance has been investigated. The results show that [PyPS]

PMo

has

stronger oxidability than [PyPS]

and [PyPS]

. They can be promising materials

for supercapacitors.

Acknowledgements

This work was supported by the Liaoning Provincial Natural Science Foundation of China

(201602404) and the Scientific Research Foundation of Liaoning Institute of Science and

Technology (RXYJ2015001).

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