Poly(vinyl alcohol)–Functionalized Silica Nanocomposite Electrolyte Membranes for Elevated-Temperature PEM Fuel Cells

Poly(vinyl alcohol)–functionalized silica nanocomposite electrolyte membranes for elevated-temperature PEM fuel cells

M. Safak Boroglu* ¹, A. Bozkurt ², I. Boz ¹

¹Istanbul University, Turkey; ²Fatih University,Turkey

mehtap@istanbul.edu.tr

The proton exchange membrane fuel cell (PEMFC) is considered as one of the most promising power sources due to its wide applications in vehicular transportation and portable electronic devices. Perfluoro sulfonic acid ionomer Nafion^® which is initially developed by DuPont and successfully used in fuel cell, is currently the most commonly utilized proton exchange membrane (PEM) for PEMFC due to its good chemical and thermal stability and high ionic conductivity. However, the expensive cost, loss of conductivity at high temperature (>80 °C) and difficulty in synthesis and processing have limited their wide commercialization. To overcome the temperature limitations of the current electrolyte materials, several organic/inorganic hybrid membranes have become the focus of intense research recently [1]. So far, many kinds of inorganic fillers, such as TiO₂, SiO₂, ZrO₂, zeolite nanoparticles as well as mesoporous silica (MCM41) have been employed for use in these organic/inorganic hybrid membranes for PEMFC applications at high temperatures [2-3]. Another approach was doping of the acidic polymers with aromatic heterocyclic structures such as imidazole [4], pyrazole [5], benzimidazole and triazole [6]. The introduction of azole groups has generated much excitement in previous years with the hope that the azole-based electrolytes may meet the requirements for a new generation of PEM fuel cells [7]. In this study, a silica nanoparticle is synthesized as an inorganic filler to modify a PVA membrane. Silica was functionalized by using 3-mercaptopropyltri-methoxysilane (MPTMS 98 wt.%) as a precursor for the formation of sulfonate groups. The functionalization of these inorganic fillers can provide sulfonate groups which will serve to increase the proton conductivity of these materials. Functionalized silica groups were introduced into PVA matrix at various molar ratios. Also we have prepared the acid–base composite materials by mixing of a sulfonated Silica-poly(vinyl alcohol) (PVA)  with the high proton exchange capacity heterocycles (1,2,4-triazole, 3-amino-1H,1,2,4-triazole, 5-amino-1H-tetrazole, 1H-1,2,3-benzotriazole). The polymers were characterized with Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The proton conductivity of the anhydrous samples was studied by an impedance spectrometer, and the results are compared with the previously reported systems.

Fourier transform infrared spectroscopy (FT-IR) results confirmed that the sulfonic acid groups were successfully grafted onto the silica nanomaterials. The PVA-F-Silica composite membranes were thermally stable up to 200 °C, and they had suitable thermal stability as electrolyte membranes for the high-temperature fuel cells under dry condition. Differential scanning calorimetry results illustrated the homogeneity of the materials and show the restrictions of segmental motions of the polymer chains.

                     

            (a)                                                                     (b)

Figure 1. AC conductivity versus frequency of (a) PVA-F-Silica-5% and (b) PVA-F-Silica-BTri at several temperatures.

AC conductivity, sac, versus frequency curves are shown in Fig. 1 (a, b) for anhydrous PVA-F-Silica-5% and PVA-F-Silica-BTri at several temperatures. The protonic conductivity of the composite membranes decreases with increasing silica content and the maximum proton conductivity was obtained for the sample PVA-F-Silica-5% as 4.78x10^-4 S/cm at 100 °C under 100% humidity. In the anhydrous state, the proton conductivity of PVA-F-Silica-BTri is found to be 2.45x10^-3 S/cm at 180 °C. Proton conductivity of these membranes are also measured and compared with previous studies. The results suggest that proton conduction is occurred between surfaces of the nano-silica particles with the aid of sulfonic acid units. The conductivities of all the polymers steadily increased with temperature, which would be due to the increased segmental motion of the proton transport groups at higher temperatures, facilitating rapid formation and breaking of hydrogen bonding, resulting in fast proton transfer dynamics. The higher number of nitrogen sites in the 1H-1,2,3-benzotriazole groups may facilitate easier proton hopping in the PVA-F-Silica-BTri blend membranes compared to that in the azole based systems.

REFERENCES

1.      Lin, Y.F., Yen, C.Y., Ma, C.C. M., Liao, S.H., Hung, Y.H., Hsiao, Y.H., J. Power Sources, 14, 692 (2007).

2.      Elena C., Michael B.P., Mark V.F., David J.W., Serguei N.L., J Electrochem Soc, 152, A1035 (2005).

3.      Aparicio M, Klein LC., J. Electrochem. Soc., 152, A493 (2005).

4.      Sevil F., Bozkurt A., J. Phys. Chem. Solids, 65, 1659 (2004).

5.      Yamada M., Honma I., Electrochim. Acta. 48, 2411 (2003).

6.      Gunday S.T., Bozkurt A., Meyer W.H., Wegner G., J. Polym. Sci. Part B: Polym. Phys., 44, 3315 (2006).

7.      Schuster M.F.H., Meyer W.H., Schuster M., Kreuer K.D., Chem. Mater. 16, 329 (2004).