PW₉V₃/rGO/SPEEK hybrid material: an excellent proton conductor

Abstract

To improve the proton conductive performance of heteropoly acids, sulfonated polyether ether ketone (SPEEK) and reduced graphene oxide (rGO) were introduced into a tungstovanadophosphoric acid (H₆PW₉V₃O₄₀, abbreviated as PW₉V₃) to prepare a hybrid film material. The results indicate that the Keggin framework of the PW₉V₃O₄₀⁶⁻ anion still remains in the hybrid material and confirm the homogeneous dispersion of PW₉V₃ on the surface of graphene sheets, which results in a better stability of PW₉V₃. The obtained PW₉V₃/rGO/SPEEK hybrid material exhibits appreciable proton conductivity of 6.2 × 10⁻² S cm⁻¹ at 17 °C and 70% relative humidity because the introduction of rGO and SPEEK into PW₉V₃ can help to form more hydrogen bonds in the HPA-based material. The conductivity of the PW₉V₃/rGO/SPEEK hybrid material enhances with the increase of temperature, and it shows Arrhenius behavior with the activation energy value of 18.7 kJ mol⁻¹, indicating that the proton conduction in this film occurs by a mixing mechanism. It is an alternative hybrid film material which may be applied in the field of proton exchange fuel cells.

---

1. Introduction

Heteropoly acids (HPAs) are an important class of nanosized polynuclear clusters composed of transition metals and oxygen atoms.^1,2 HPAs are a versatile candidate as building blocks for the construction of hybrid materials due to their multiple charge transfer reactions and unique structural, electronic, and optical properties.^3,4 Especially, owing to their high proton conductivity and excellent proton transfer/storage abilities, HPAs have become one of the best solid inorganic electrolytes for the development of fuel cells and supercapacitors.^5,6 Despite these attractive features, the drawbacks of HPAs, for instance, high solubility in aqueous solutions, diffusional problems and continuous leakage, will result in the decrease of HPAs-based electrochemical devices, so thus, a pathway to address this problem is to construct HPAs-based hybrid materials.^7,8

Graphene has inspired great enthusiasm due to its excellent physical and chemical properties.⁹ For the large scale production of graphene-based materials, a widely adopted strategy is to use graphene oxide (GO) as a low cost precursor and convert it to reduced GO (rGO).¹⁰ Owing its exceptionally high specific surface area, rGO could fix HPAs even at very high loading by the electron transfer and electrostatic interaction between HPAs and the residual oxygen-containing groups of rGO;¹¹ what's more, residual hydrophilic sites of rGO, such as –O–, –OH and –COOH functional groups, could contribute to the proton conductivity by forming hydrogen-bonds.^12–15

Hence, based on HPAs with excellent conductivity, we chose SPEEK, which has been widely used as electrolyte film material in the field of fuel cell due to its good mechanical strength and high chemical stability,^16,17 as the polymer matrix to prepare the PW₉V₃/rGO/SPEEK hybrid film material. The conductive properties of this film have been investigated.

2. Experimental

2.1. Instruments and reagents

Fourier-transform infrared spectroscopy (FTIR) was recorded on a NICOLET NEXUS470 FT/IR spectrometer using KBr as pellet, and the resolution is 4 cm⁻¹. X-ray powder diffraction (XRD) was carried out on a BRUKER D8 ADVANCE X-ray diffractometer in the range of 2θ = 3–40° at the rate of 0.02° s⁻¹. Morphology was observed by a Hitachi S-4800 (Japan) scanning electron microscope (SEM) and HF-3300 (Hitachi) transmission electron microscopy (TEM). Cyclic voltammetry (CV) was performed with a CHI660E electrochemical workstation in a conventional three-electrode electrochemical cell. Conductivity measurement was taken by a four-point-probe method using AC impedance spectroscopy over a frequency range of 100 mHz to 100 kHz, 10 mV AC perturbation. A sheet of film (4.5 cm × 1.5 cm) was placed on the test cell. Polyether ether ketone (PEEK) was obtained from College of Chemistry at Jilin University. Graphene oxide (GO) was synthesized by a new scalable and effective method.¹⁸ PW₉V₃ and SPEEK were synthesized by a modified method according to our literature procedures.^19,20 The procedure from PEEK to SPEEK is shown in Fig. 1. The sulfonation degree (DS) was determined by an acid–base titration method. It is found that the DS is about 75%. All reagents are analysis grade.

Fig. 1 The sulfonation of PEEK.

2.2. Preparation of PW₉V₃/rGO/SPEEK hybrid film material

0.35 g of PW₉V₃ powder was reduced by hydrazine hydrate to obtain the reduced PW₉V₃, which is often called ‘heteropoly blue’ (HB), and then dried at 55 °C. 10 mg of GO was dissolved in 30 mL deionized water, added the above reduced PW₉V₃ to GO solution and stirred for about 2 h. The solution turned black because the consecutive electron has transferred from HB to GO to produce the rGO. Afterwards, the solution was dried at 60 °C to get the PW₉V₃/rGO solid composite. 0.14 g of SPEEK was first dissolved in dimethylformamide (DMF) and added the above obtained PW₉V₃/rGO composite to the solution. The resulting mixture was mixed with the assistance of sonication and stirring for 4 h to form a suspension. After evaporation of most of the solvent, the mixture was cast onto a glass plate using a casting knife. Then the cast material was dried at room temperature for 48 h. The thickness of the obtained PW₉V₃/rGO/SPEEK hybrid film is 107 μm, and the film is flexible, black and homogeneous. The weight ratio of the film material is about 28% (SPEEK), 70% (PW₉W₃) and 2% (graphene). The procedure is depicted in Scheme 1.

Scheme 1 Preparation procedure of PW₉V₃/rGO/SPEEK hybrid material.

3. Results and discussion

Infrared spectrum is fairly useful for studies on properties of materials. Fig. 2 shows the IR spectra of pure GO nanosheets, PW₉V₃, PW₉V₃/rGO and PW₉V₃/rGO/SPEEK hybrid material. The spectrum of the GO shows the presence of O–H (ν_O–H at 3420 cm⁻¹), C=O (ν_C=O at 1720 cm⁻¹ from carboxyl group) and C–O (ν_C–O at 1060 cm⁻¹ in alkoxy groups, at 1230 cm⁻¹ in epoxy groups). While in PW₉V₃/rGO composite, the peak intensities of these oxygen-containing groups were decreased as a consequence of the deoxygenation process,²¹ indicating that GO, to some extent, has been reduced by heteropoly blue. The characteristic peaks of pure PW₉V₃ appear at 1070 cm⁻¹, 979 cm⁻¹, 881 cm⁻¹ and 789 cm⁻¹, which are assigned to ν_as(P–O_a), ν_as(M–O_d), ν_as(M–O_b–M), and ν_as(M–O_c–M) of heteropolyanion PW₉V₃O₄₀⁶⁻. There are four similar peaks observed in the spectrum of SPEEK/PW₉V₃/rGO, they are 1079 cm⁻¹, 962 cm⁻¹, 901 cm⁻¹ and 795 cm⁻¹, respectively. It suggests that the Keggin framework of PW₉V₃O₄₀⁶⁻ still maintain in the hybrid materials. What is worth notified is the peaks of ν_as(M–O_d) in PW₉V₃/rGO/SPEEK have a red shift, from 979 to 962 cm⁻¹. The reason is that the M–O_d stretching is a proportional function of the anion–anion interaction. The addition of other material into PW₉V₃ would undoubtedly weaken the anion–anion interaction. Besides, the peaks observed at 1232 cm⁻¹, 1019 cm⁻¹ and 709 cm⁻¹ are assigned to the stretching vibration of sulfonic acid groups (–SO₃H) of SPEEK.²²

Fig. 2 FT-IR spectra of (a) GO, (b) PW₉V₃, (c) PW₉V₃/rGO and (d) PW₉V₃/rGO/SPEEK.

To test the electrochemical stability of PW₉V₃/rGO hybrid material, we have assembled the modified glassy carbon electrode (GCE) by depositing 5 μL of PW₉V₃/rGO dispersion (0.5 mg mL⁻¹) on the surface of GCE, and casting 5 μL chitosan dispersion (0.01 g mL⁻¹) on the surface of GCE to fix PW₉V₃/rGO. The result shows that the decay of the first reduction peak current was found to be only 4% after 100 circles (as shown in Fig. 3), and for comparison, the pure PW₉V₃ modified electrode is unstable when applied during the electrochemical study because of the high solubility in water of PW₉V₃. These observations indicate PW₉V₃ has been immobilized because of the strong interaction between PW₉V₃ and graphene. So graphene could act as a stabilizer to prevent PW₉V₃ from leaching out. It is more favorable when PW₉V₃/rGO rather pure PW₉V₃ applied in SPEEK/PW₉V₃/rGO hybrid material.²³

Fig. 3 CV stability test of PW₉V₃/rGO for 100 cycles at the scan rate of 90 mV s⁻¹ in 0.025 M H₂SO₄ electrolyte.

TEM image was used to characterize the morphology of the as-obtained PW₉V₃/rGO/SPEEK hybrid material. Fig. 4a illustrated the wrinkled and flake-like shape at high magnification, which is the feature structure of graphene nanosheets, indicating that graphene disperses well in the hybrid matrix. In Fig. 4b, the presence of the inorganic PW₉V₃ clusters in hybrid film is clearly detected as small dots. This homogeneous distribution will be beneficial to the proton mobility as it minimizes the distance between particles.

Fig. 4 TEM of PW₉V₃/rGO/SPEEK hybrid material.

Fig. 5 represents the X-ray powder diffraction patterns of PW₉V₃ and PW₉V₃/rGO/SPEEK. Compared with PW₉V₃, the most intense characteristic peaks still identified in PW₉V₃/rGO/SPEEK composites at the range of 2θ = 7–11°. It indicates the existence of Keggin anion in the hybrid materials. This is consistent with the results of the IR. The broad diffraction peaks at 15–38° were observed for PW₉V₃/rGO/SPEEK, suggesting that the hybrid material is considered amorphous.

Fig. 5 XRD patterns of (a) PW₉V₃ and (b) PW₉V₃/rGO/SPEEK.

The proton conductivity of hybrid materials has been investigated by electrochemical impedance spectroscopy. Fig. 6 shows Nyquist plots of SPEEK/PW₉V₃/rGO at room ambient of 17 °C and 60% relative humidity. Inset is the equivalent circuit. The high-frequency semicircle of Nyquist plots is modeled by parallel of a resistor R with a constant phase element CPE₁. R is the actual resistance of the film material, and the low-frequency inclined line of Nyquist plots represents the impedance of the film/electrodes interfaces and is modeled by CPE₂. The proton conductivity (σ, S cm⁻¹) of film material was calculated using the following equation: σ = L/(RS) (L is the distance between the electrode pairs, S is the cross-sectional area of the film and R is the resistance of the film). By calculation, conductivity of PW₉V₃/rGO/SPEEK is 6.2 × 10⁻² S cm⁻¹. For comparison, we have also got the conductivities of pure SPEEK film and PW₉V₃/SPEEK (70 wt% PW₉V₃) film, which are 1.1 × 10⁻⁴ S cm⁻¹ and 3.76 × 10⁻² S cm⁻¹ at the ambient condition. So it is obvious PW₉V₃/rGO/SPEEK hybrid shows the highest conductivity. The –O–, –OH and –COOH functional groups of rGO have hydrophilic sites,^24,25 and SPEEK is composed of a hydrophobic backbone and ionic domains filled with –SO₃H in the hydrated state (see the structure of SPEEK in Fig. 1). So the introduction of rGO and SPEEK into PW₉V₃ can help to form more hydrogen bonds in the system to enhance the water retention in the HPA-based materials and accelerate the proton conduction.

Fig. 6 Electrochemical impedance spectrum of PW₉V₃/rGO/SPEEK film material at 17 °C and 70% relative humidity.

To investigate the relationship between proton conductivity with temperature, we have measured the conductivity in the range of 17–70 °C. It is found that the proton conductivity of SPEEK/PW₉V₃/rGO increases with higher temperature as the mobility of conducting species increase with higher temperature, which increases to 2.36 × 10⁻¹ S cm⁻¹ at 70 °C. As shown in Fig. 7, the relationship between proton conductivity and temperature is consistent with Arrhenius equation: σ = σ₀ exp(E_a/κT). In this formula, E_a is the activation energy of proton conductivity, σ₀ is the pre-exponential factor and κ is the Boltzmann constant. The activation energy of PW₉V₃/rGO/SPEEK calculated from the slope is 18.7 kJ mol⁻¹. It is lower than pure PW₉V₃, whose is 25.68 kJ mol⁻¹.¹⁹ It indicates the influence of temperature on the conductivity decreases when pure PW₉V₃ hybrids with rGO and polymer.

Fig. 7 Arrhenius plots of PW₉V₃/rGO/SPEEK film material.

So far, there are two major recognized proton conduction mechanisms: vehicle mechanism and Grotthuss mechanism.²⁶ In vehicle mechanism, protons interact with water molecules, which transfer in the form of hydrated hydrogen ions, such as H₃O⁺, H₅O₂⁺ and H₉O₄⁺ 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.²⁷ Generally, we distinguish them by the value of activation energy, because the activation energy of vehicle mechanism is often higher than 20 kJ mol⁻¹, while the activation energy of Grotthuss mechanism is usually less than 15 kJ mol⁻¹. The activation energy of this hybrid material is 18.7 kJ mol⁻¹, indicating the proton conduction of PW₉V₃/rGO/SPEEK film material is a share of Grotthuss mechanism and vehicle mechanism. Just as shown in Fig. 8, the blue arrow is the track of proton-hopping in the network structure, and the red arrow represents the pathway of proton movement with the assistance of water. It is the mixing mechanism that accelerates the proton conduction in this hybrid film.

Fig. 8 Schematic illustration of proton conduction in PW₉V₃/rGO/SPEEK matrix.

4. Conclusions

In this work, a proton-conducting hybrid film material PW₉V₃/rGO/SPEEK was formed. The results indicate that the Keggin framework of PW₉V₃O₄₀⁶⁻ anion still remain in the hybrid film and confirm the homogeneous dispersion of PW₉V₃ on the surface of graphene sheet. This hybrid film material shows a superior proton conductivity of 6.2 × 10⁻² S cm⁻¹ at ambient condition and the activation energy of proton conduction are 18.7 kJ mol⁻¹, indicating it is the mixing mechanism that accelerates the proton conduction in this hybrid film. So it is a high proton conductor. This work lays a solid foundation in the field of HPAs-based materials, especially in the special fields of proton exchange membrane fuel cells and supercapacitors.

Acknowledgements

This work was 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 State Key Laboratory of Inorganic Synthesis, Preparative Chemistry of Jilin University (2016-03).

Notes and references

1. Z. X. Sun, Y. Z. Zhang, N. Li, L. Xu and T. Q. Wang, J. Mater. Chem. C, 2015, 3, 6153–6157.
2. J. P. Tessonnier, S. Goubert-Renaudin, S. Alia, Y. S. Ya and M. A. Barteau, Langmuir, 2013, 29, 393–402.
3. A. Rubinstein, P. Jiménez-Lozanao, J. J. Carbó, J. M. Poblet and R. Neumann, J. Am. Chem. Soc., 2014, 136, 10941–10948.
4. X. F. Wu, X. Tong, Q. Y. Wu, H. Ding and W. F. Yan, J. Mater. Chem. A, 2014, 2, 5780–5784.
5. S. Y. Oh, T. Yoshida, G. Kawamura, H. Muto, M. Sakai and A. Matsuda, J. Power Sources, 2010, 195, 5822–5828.
6. M. H. Yang, B. G. Choi, S. C. Jung, Y. K. Han, Y. S. Huh and S. B. Lee, Adv. Funct. Mater., 2014, 24, 7301–7309.
7. Y. Kim, K. Ketpang, S. Jaritphun, J. S. Park and S. Shanmugam, J. Mater. Chem. A, 2015, 3, 8148–8155.
8. M. H. Yang, D. S. Kim, T. J. Lee, S. J. Lee, K. G. Lee and B. G. Choi, J. Colloid Interface Sci., 2016, 468, 51–56.
9. H. L. Li, S. P. Pang, S. Wu, X. L. Feng, K. Müullen and C. Bubeck, J. Am. Chem. Soc., 2011, 133, 9423–9429.
10. S. Wang, H. L. Li, L. Y. Zhang, B. Li, X. Cao, G. H. Zhang, S. L. Zhang and L. X. Wu, Chem. Commun., 2014, 50, 9700–9703.
11. W. H. Guo, X. H. Cao, Y. H. Liu, X. L. Tong and X. S. Qu, J. Electrochem. Soc., 2014, 161, 248–255.
12. Y. W. Liu, S. M. Liu, X. Y. Lai, J. Miao, D. F. He, N. Li, F. Luo, Z. Shi and S. X. Liu, Adv. Funct. Mater., 2015, 25, 4480–4485.
13. Y. H. Lee, K. H. Chang and C. C. Hu, J. Power Sources, 2013, 227, 300–308.
14. Y. Y. Shao, J. Wang, M. Engelhard, C. Wang and Y. H. Lin, J. Mater. Chem., 2010, 20, 743–748.
15. X. F. Lei, Y. H. Chen, M. T. Qiao, L. D. Tian and Q. Y. Zhang, J. Mater. Chem. C, 2016, 4, 2134–2146.
16. N. Zhang, G. Zhang, D. Xu, C. J. Zhao, W. J. Ma and H. T. Li, Int. J. Hydrogen Energy, 2011, 36, 11025–11033.
17. T. Yang, J. Membr. Sci., 2009, 342, 221–226.
18. L. Peng, Z. Xu, Z. Liu, Y. Y. Wei, H. Y. Sun, Z. Li, X. L. Zhao and C. Gao, Nat. Commun., 2015, 6, 5716,  DOI:10.1038/ncomms6716.
19. (a) X. Tong, N. Q. Tian, W. Wu, W. M. Zhu, Q. Y. Wu, F. H. Cao, W. F. Yan and A. B. Yaroslavtsev, J. Phys. Chem. C, 2013, 117, 3258–3263; (b) Q. Y. Wu, X. Y. Qian and S. M. Zhou, J. Xuzhou Inst. Technol., Nat. Sci. Ed., 2012, 27, 1–4.
20. (a) N. Q. Tian, X. F. Wu, B. H. Yang, Q. Y. Wu, F. H. Cao, W. F. Yan and A. B. Yaroslavtsev, J. Appl. Polym. Sci., 2015, 132, 42204; (b) Q. Y. Wu, X. Tong and X. F. Wu, J. Xuzhou Inst. Technol., Nat. Sci. Ed., 2011, 26, 1–8.
21. M. H. Yang, B. G. Choi, H. S. Park, W. H. Hong, S. Y. Lee and T. J. Park, Electroanalysis, 2010, 22, 1223–1228.
22. R. Kumar, M. Mamlouk and K. Scott, RSC Adv., 2014, 4, 617–623.
23. Y. Kim, K. Ketpang, S. Jaritphun, J. S. Park and S. Shanmugam, J. Mater. Chem. A, 2015, 3, 8148–8155.
24. A. Khan, R. Asmatulu and G. Hwang, ECS Trans., 2015, 69, 569–577.
25. J. L. Zhang, H. J. Yang, G. X. Shen, P. Cheng, J. Y. Zhang and S. W. Guo, Chem. Commun., 2010, 46, 1112–1114.
26. K. Cheückiewicz, G. Zúukowska and W. Wieczorek, Chem. Mater., 2001, 13, 379–384.
27. A. Eguizábal, J. Lemus, M. Urbiztondo, O. Garrido, J. Soler, J. A. Blazquez and M. P. Pina, J. Power Sources, 2011, 196, 8994–9007.

---