Poly (vinyl alcohol) is used as raw material in this study. With adding graphene oxide, the PVA/GO membrane was prepared with solution casting method. To evaluate the effect of different casting method on the conductivity of PVA/GO membrane, knife casting method with different casting surface were conducted in this study. Ionic conductivity for PVA/GO prepared by solution casting method is about 0.0526 S/cm; whereas the ionic conductivity is sharply improved to 0.1439 S/cm for PVA/GO prepared by casting membrane on the glass with vacuum.

Keywords Poly (vinyl alcohol), polymer electrolyte membranes, ionic conductivity

Since alkaline polymer electrolytes have been prepared, the conductivity of alkaline electrolytes based on polyvinyl alcohol and polyethylene oxide had been reported.^1–4 Alkaline solid polymer electrolytes (ASPE) or alkaline solid polymer blend electrolytes (ASPBE) using potassium hydroxide (KOH) as ionic dopant had been reported and the results have shown considerable improvement in cell properties. It included poly (vinyl alcohol), poly (ethylene oxide), blends of PVA/PEO, copolymer of epichlorohydrin and ethylene oxide gels of hydrophonics and poly (acrylic acid).^3–13 Graphene oxide (GO) is generated from the oxidation of graphite. It is a nanomaterial with carbon, oxygen, and hydrogen in variable ratios to form a range of reactive groups such as carboxyl, hydroxyl, and carboxylic acid in the material. The typical structure of GO is shown in Figure 1. These reactive groups make GO a good candidate in the various applications and had attracted intensive study.¹⁴The polymer electrolyte films as well as the electrodes were often prepared by the casting technique.⁶ For example, PEO is swollen in water and then mixed with a concentrated aqueous solution of KOH at about 60 °C for 4 h to get viscous homogeneous solution. And then the solution is poured onto a Teflon plate and the water excess is evaporated slowly at ambient temperature for 48h and maintained in a vacuum oven before use. In general, the ionic conductivity increases with increasing the KOH concentration in the preparation. In our previous studies about modification of PVA for alkaline polymer electrolyte membranes and fuel cell,^14-19 the membranes prepared with solution casting method were immersed in 40wt% KOH solution to form the KOH containing polymer electrolyte membranes to evaluate the performance with measuring the KOH uptake, ionic conductivity, methanol permeability, and selectivity of these membranes. As there is not any report about the effect of casting method on the conductivity, the effect of casting method on the conductivity of graphene oxide containing poly (vinyl alcohol) membrane is reported in this study.

Poly (vinyl alcohol), (PVA, M_w=70,000-100,000g) was obtained from Sigma. KOH was purchased from Aldrich. Graphene oxide, (20% oxygen content, conductivity 10~30 S/cm, and specific volume higher than 350 m²g^-1), was obtained from Graphage. To evaluate the effect of graphene oxide on the membrane, pure poly (vinyl alcohol) is prepared as control in this study. The solutions of pure poly (vinyl alcohol) is prepared by adding PVA into distilled water at 85^oC with stirring for at least 3 h to make sure the solution was completely homogeneous. The solutions were slowly poured onto the prepared glass slides mold and dried in the ambient conditions for 24h. 0.05g graphene Oxide (GO) powder was added in distilled water and sonicated with ultrasonic bath for 1h. Then PVA powder is completely dissolved in the solution with stirring for 4 h at 85 °C to form the homogeneous solution. The solutions were slowly poured onto the prepared glass slides mold and dried in the ambient conditions for 24 h. To evaluate the effect of casting method on the conductivity of PVA/GO membrane, the preparation of the membrane was described in Figure 2. To prepare the alkaline solid polymer electrolyte, the membranes were immersed in 40wt% KOH solution to form the KOH containing polymer electrolyte membranes. Then the ionic conductivity in the KOH containing PVA/GO electrolyte membranes was determined by means of an ac impedance method.¹⁶ The procedure is the same as described in previous study.¹⁶ The membranes were sandwiched between SS316 stainless-steel (SS), ion-blocking electrodes in a spring-loaded glass holder. The impedance of the membranes was measured at different temperature. From the analysis of the experiments, the bulk resistance, R_b (Ω), was obtained. The electrolyte conductance, σ (S/cm) can be calculated according to the formula:

$\sigma =l/R_{b}A$ (1)

Where l is the thickness (cm) of the alkaline solid polymer electrolyte (ASPE), A is the area of the blocking electrode (cm²).

Figure 1 The typical structure of graphene oxide (GO).

Figure 2 Preparation of PVA/GO membrane with different casting method.

Ionic conductivity for different membranes had been reported in references.^1–28 It can be found that the researches emphasize the preparation of different materials with functional group to improve the conductivity. For example, proton conducting polymer derived from poly(etheretherketone) and poly(4-phenoxybenzoyl-1,4-phenylene)²¹, sulfonated-fluorinated poly(arylene ether)s membranes for a proton exchange membrane fuel cell²², solid polymer electrolytes based on statistical poly(ethylene oxide-propylene oxide) copolymers,²³ investigations on the effect of various plasticizers in PVA-PMMA solid polymer blend electrolytes,²⁴ ionomeric membranes based on partially sulfonated poly(styrene): synthesis, proton conduction and methanol permeation,²⁵ preparation and characterization of high ionic conducting alkaline non-woven membranes by sulfonation,²⁶ sulfonated polystyrene grafted polypropylene composite electrolyte membranes for direct methanol fuel cells,²⁷ sulfonated poly(2,5-benzimidazole) (SABPBI) impregnated with phosphoric acid as proton conducting membranes for polymer electrolyte fuel cells,²⁸ and so on. Most of Ionic conductivity (S/cm) reported in the references^1–28 is between 10⁻² to 10⁻⁵ (S/cm). In our previous studies about modification of PVA for alkaline polymer electrolyte membranes and fuel cell,^14–19 the ionic conductivity can be enhanced to about 0.11(S/cm). In this study, the ionic conductivity of PVA prepared as control is about 0.028(S/cm). With adding GO in to the membrane for PVA/GO membrane, the ionic conductivity is enhanced to 0.0526(S/cm) with solution casting method on glass. An intriguing result is found for the ionic conductivity through PVA/GO membrane prepared with knife casting on glass, Teflon, and metal (Table 1). Compared with the value of ionic conductivity of membrane prepared with solution casting method, the values of ionic conductivity of membranes prepared with knife casting are enhanced. It is about two times of the value of σ obtained from the solution casting membrane. The ionic conductivity was about 0.052(S/cm) for PVA/GO membrane prepared by solution casting method, whereas the ionic conductivity is enhanced to 0.129 and 0.1439(S/cm) with knife casting on the glass without or with vacuum treatment. It seems that the membrane thickness is higher than those prepared with knife casting method. There is not any effect of vacuum treatment of the thickness of membrane, although the ionic conductivity is higher for the membrane prepared with vacuum treatment. Compared with the value of ionic conductivity of membrane prepared with solution casting method, the values of ionic conductivity of membrane prepared with knife casting are enhanced.

With the different casting method and the different casting surface, the value of ionic conductivity of PVA/GO membrane is changed. It might be due to the effect of casting method on the morphology and resulted in the increased of the ionic conductivity by comparing with the membrane prepared with solution casting method.

This work was supported by Ministry of Science and Technology of the Republic of China with grant MOST-104-2211-E-182-054-MY3 and partial financial support of Chang Gung Memorial Hospital with grant CMRPD2F0011 and CMRPD2F0012.

Author declares that there is no conflict of interest.

1. JF Fauvarque, S Guinot, N Bouzir, et al. Alkaline poly (ethylene oxide) solid polymer electrolytes. Application to nickel secondary batteries. Electrochim Acta. 1995;40(13):2449–2453.
2. S Guinot, E Salmon, JF Penneau, et al. A new class of PEO-based SPEs: structure, conductivity and application to alkaline secondary batteries. Electrochimica Acta. 1998;43(10):1163‒1170.
3. A Lewandowski, K Skorupska. Alkaline Poly(vinyl alcohol)-KOH-H20 Solid Polymer Electrolyte. Polish J Chem. 1998;72:2635‒2638.
4. Lewandowski A, Skorupska K, J Malinska. Novel Poly (vinyl alcohol)-KOH-H20 Alkaline Polymer Electrolyte. Solid State Ionics. 2000;133(3):265‒271.
5. N Vassal, E Salmon, JF Fauvarque. Nickel metal hydride secondary batteries using an alkaline solid polymer electrolyte. J Electrochem Soc. 1999;146(1):20‒26.
6. A Lewandowski, M Zajder, E Frackowiak, et al. Supercapacitor Based on Activated Carbon and Polyethylene Oxide-KOH-H20 Polymer Electrolyte. Electrochim Acta. 2001;46(18):2777‒2780.
7. AA Mohamad, NS Mohamed, Y Alias, et al. Studies of alkaline solid polymer electrolyte and mechanically alloyed polycrystalline Mg2Ni for use in Nickel-Metal Hydride batteries. J. Alloys Comp. 2002;337(1):208‒213.
8. CC Yang, SJ Lin. Preparation of composite alkaline polymer electrolyte. Mater Let. 2002;57(4):873‒881.
9. CC Yang. Polymer Ni-MH battery based on PEO-PVA-KOH polymer electrolyte. J Power Sources. 2002;109(1):22-31.
10. CC Yang, SJ Lin. Alkaline composite PEO-PVA-glass-fibre-mat polymer electrolyte for Zn-air battery. J Power Sources. 2002;112(2):497‒503.
11. N Vassal, E Salmon, JF Fauvarque. Electrochemical properties of an alkaline solid polymer electrolyte based on P(ECH-co-EO). Electrochim Acta. 2000;45(8):1527‒1532.
12. R Othman, WJ Basirun, AH Yahaya, et al. Hydroponics gel as a new electrolyte gelling agent for alkaline zinc-air cells. J Power Sources. 2001;103(1):34‒41.
13. C Iwakura, S Nohara, N Furukawa, et al. The possible used of polymer gel electrolytes in nickel/metal hydride battery. Solid State Ionics. 2002;148(3):487‒492.
14. YC Cao, C Xu, X Wu, et al. A poly (ethylene oxide)/graphene oxide electrolyte membrane for low temperature polymer fuel cells. J Power Sources. 2011;196(20):8377– 8382.
15. JM Yang, HZ Wang, CC Yang. Modification and characterization of semi- crystalline poly(vinyl alcohol) with interpenetrating poly(acrylicacid) by UV radiation method for alkaline solid polymer electrolytes membrane. J Membr Sci. 2008;322(1):74–78.
16. JM Yang, CC Yuan, HZ Wang. Two step modification of poly (vinyl alcohol) by UV radiation with 2-hydroxyethylmethacrylate and sol–gel process for the application of polymer electrolyte membrane. J Membr Sci. 2009;341:186–194.
17. JM Yang, HC Chiu. Preparation and characterization of polyvinyl alcohol/ chitosan blended membrane for alkaline direct methanol fuel cells. J Membr Sci. 2012;419–420:65–71.
18. JM Yang, NC Wang, HC Chiu. Preparation and characterization of polyvinyl alcohol/sodium alginate blended membrane for alkaline solid polymer electrolytes membrane. J Membr Sci. 2014;457:139–148.
19. JM Yang, SA Wang, CL Sun, et al. Synthesis of size-selected Pt nanoparticles supported on sulfonated graphene with polyvinyl alcohol for methanol oxidation in alkaline solutions. J Power Sources. 2014; 254:298–305.
20. JM Yang, SA Wang, Preparation of graphene-based poly(vinyl alcohol)/chitosan nanocomposites membrane for alkaline solid electrolytes membrane. J Membr Sci. 2015;477:49‒57.
21. T Kobayashi, M Rikukawa, K Sanui, N Ogata. Proton conducting polymer derived from poly(etheretherketone) and poly(4-phenoxybenzoyl-1,4-phenylene). Solid State Ionics. 1998;106(3):219–225.
22. HC Lee, HS Hong, YM Kim, et al. Preparation and evaluation of sulfonated-fluorinated poly (arylene ether)s membranes for a proton exchange membrane fuel cell. Electrochim Acta. 2004;49(14):2315–2323.
23. X Andrieu, JF Fauvarque. Solid polymer electrolytes based on statistical poly (ethylene oxide-propylene oxide) copolymers. Electrochim Acta. 1995;40(13‒14):2295–2299.
24. S Rajendran, M Sivakumar. Investigations on the effect of various plasticizers in PVA-PMMA solid polymer blend electrolytes. Mater Lett. 2004;58(5):641–649.
25. N Carretta, V Tricoli, F Picchioni. Ionomeric membranes based on partially sulfonated poly(styrene): synthesis, proton conduction and methanol permeation. J Membr Sci. 2000;166(2):189–197.
26. GM Wu, SJ Lin, CC Yang. Preparation and characterization of high ionic conducting alkaline non-woven membranes by sulfonation. J Membr Sci. 2006;284(1):120–127.
27. B Bae, D Kim. Sulfonated polystyrene grafted polypropylene composite electrolyte membranes for direct methanol fuel cells. J Membr Sci. 2003;220(1‒2):75–87.
28. JA Asensio, S Borros, P Gomez-Romero. Sulfonated poly (2,5-benzimidazole) (SABPBI) impregnated with phosphoric acid as proton conducting membranes for polymer electrolyte fuel cells. Electrochim Acta 2004;49(25):4461–4466.