Silica/Nafion membranes

Heteropolyacid-modified polymer silica membranes for Direct Methanol Fuel Cells have been prepared and tested under high temperature operation conditions (145°C) in single cell configuration. A maximum power density of 0.4 W/cm in oxygen with 2 M methanol has been obtained with air at the cathode, this value decreased to 0.25 W/cm. The higher performance of the heteropolyacid-Nafion-silica membrane, with respect to Nafion-silica, is attributed to its better ion transport properties, since the measured cell resistance value is similar for both membranes. 

By comparison of the dif actograms, an increase of the ratio between the crystalline peak and the amorphous scattering, as well a shift of the crystalline peak to higher Bragg angles, is apparent in the Nafion-silica membrane with respect to Nafion 117. 

Heteropolyacid-modified silica-Nafion membranes showed suitable properties for operation at 145°C in direct methanol fuel cell. Since the cell resistance is similar to that of Nafion-silica membrane and the main improvement in polarization is observed at high current density, it is thought that the excellent oxygen solubility characteristics at the electrode/PWA interface are responsible for the significantly higher limiting current density. 

P. Staiti, A. S. Arico, V. Baglio, F. Lufrano, E. Passalacqua, V. Anto-Nucci, Hybrid Nafion-silica membranes doped with heteropolyacids for application in direct methanol fuel cells. Solid State Ionics 145 (2001)... 

Shao, Z. G., Joghee, R, and Hsing, 1. M. 2004. Preparation and characterization of hybrid Nafion-silica membrane doped with phosphotungstic acid for high temperature operation of proton exchange membrane fuel cells. Journal of Membrane Science 229 43-51. 

Miyake, N., Wainright, J. S. and Savinell, R. E 2001. Evaluation of a sol-gel derived Nafion/silica hybrid membrane for proton electrolyte membrane fuel cell applications. I. Proton conductivity and water content. Journal of the Electrochemical Society 148 A898-A904. 

Figure 20. Electro-osmotic drag coefficients of diverse membranes based on perfluorinated polymers (Dow - and Nafion/silica composites ) and polyarylenes (S—PEK/ PSU blends, ionically cross-linked S—PEK/PBP ), as a function of the solvent (water/methanol) volume fraction Xy (see text for references). Lines represent data for Nafion and S—PEK (given for comparison) for data points, see Figure 15. Dashed lines correspond to the maximum possible electro-osmotic drag coefficients for water and methanol, as indicated (see text).

Inorganic membranes are very resistant and quite stable at hard-operating conditions. Several materials are available. Different membranes have been successfully tested for separations involving supercritical fluids such as tubular carbon membranes [ 1 ], mbular silica membranes [2-5], silica hollow fibber membranes [6], zeolite membranes [7-10], titane-nafion membranes [11], polycarbonate membrane [12], nanofilter having a thin layer of Zr02-Ti02 [12], and silicalite membranes [4]. 

N. Miyake, J.S. Wainright and R.F. Savinell, Evaluation of a sol-gel derived Nafion/ silica hybrid membrane for polymer electrolyte membrane fuel cell applications. II. Methanol uptake and methanol permeability, J. Electrochem. Soc., 2001, 148, A905-A909. 

The electrode-electrolyte assembly was investigated in a single cell test station. After installing the MEA in the fuel cell housing, water was supplied to the anode and cathode backing layers and the cell was warmed-up step-wise from room temperature to 145°C. The polarization curves obtained for the fuel cells equipped with the Nafion-silica and Nafion-silica-PWA membranes, under same conditions in presence of oxygen feed at cathode and 2M methanol solution at anode, are reported in Fig. 6. 

Fig. 6. Polarization and current density curves for MEAs equipped with Nafion-silica ( ) and Nafion-silica-PWA ( ) membranes. Conditions 145°C, oxygen, 2M methanol.

It is interesting to analyze the effect of inorganic fillers or blending with other polymers on the methanol uptake of Nafion-based membranes. Kim et al. [46] measured methanol uptake in composite of Nafion with organic modified silica (ORMOSIL) and they observed that the methanol uptake decrease proportionally to the amount of inorganic phase in the membrane, reaching a half of its value in Nafion when the filler content is close to 40 wt%. The connectivity between the... 

Yen CY, Lee CH, Lin YF, Lin HL, Hsiao YH, Liao SH, Chuang CY, Ma CCM (2007) Sol-gel derived sulfonated-silica/Nafion composite membrane for direct methanol fuel cell. J Power Sources 173 36 14... 

Jin Y, Qiao S, Zhang ZP, Xu ZP, Smart S, Diniz da Costa JC, Lu GQ (2008) Novel Nafion composite membranes with mesoporous silica nanospheres as inorganic fillers. J Power... 

Park CH, Kim HK, Lee CH, Park HB, Lee YM (2009) Nafion nanocomposite membranes effect of fluorosurfactants on hydrophobic silica nanoparticle dispersion and direct methanol fuel cell performance. J Power Sources 194 646-654... 

Sahu AK, Meenakshi S, Bhat SD, Shahid A, Sridhar P, Pitchumani S, Shukla AK (2012) Meso-structured silica-Nafion hybrid membranes for direct methanol fuel cells. J Electrochem Soc 159 F702-F710... 

Tang, H., Wan, Z., Pan, M. and Jiang, S.P. 2007. Self-assembled Nafion-silica nanoparticles for elevated-high temperature polymer electrolyte membrane fuel cells. F.lp.ct.rnrhi>m. Commun. 9(8) 2003-2008. 

Xu, W., Lu, T., Liu, C. and Xing, W. 2005. Low methanol permeable composite Nafion/silica/ PWA membranes for low temperature direct methanol fuel cells. Electrochim. Acta 50(16-17) 3280-3285. 

Ren et al. also reported that the water uptake by the membrane containing oxide is higher than that by the pristine Nafion (Ren et al. 2005). Nafion-silica man-branes have been prepared employing several methods by casting mixtures such as silica powder, diphenylsilicate (DPS), sol-gel reaction with tetraethylorthosilicate (TEOS), followed by solution casting of the Nafion solution, phosphotungstic acid (PTA)-doped composite silica/Nafion/PTA, and silica oxide (Wang et al. 2007). 

The conductivity tests were carried out for a number of P(VDF-CTFE)/ Nafion/Inorganic membranes 3D phase zirconium phosphate, 3D porous titanosili-cate, amorphous zirconium phosphate (AZP), mesoporous alumina, and a silica-based commercial molecular sieve (MS). Figure 12.12 shows the conductivity of various PEM composite membranes under various humidity conditions. The inorganic phase... 

Composite PSFA—silica membranes have been demonstrated for water electrolysis. Composite membrane containing 3 wt% Si02 (Cab-O-Sil EH5, Cabot) for water electrolysis were prepared by using a combination of recast and thermal treatment procedures. The thickness of the composite membrane was comparable to Nafion 115, i.e., 120 jim. The membrane forming procedure is exemplified in Fig. 2.17. 

Mixing Solutions. Nafion/Si02 membranes also can be prepared using Nafion solutions and silicon alkoxides. The mixed solution is cast into shallow containers and dried at ambient temperature. This procedure allows the incorporation of continuously increasing amounts of sihca. Since the composites are formed from a mixed solution, instead of a Nafion film, the silica phase is not restricted to the clusters ofNafion. Transparent but brittle films are obtained when TEOS is used, showing a lamellar structure by transmission electron microscopy (TEM). When part of the TEOS is substituted by 1,1,3,3-tetramethyl-l, 3-diethoxy disiloxane (TMDES), phase separation, higher flexibility and higher ionic conductivity are observed (Zoppi, 1997, 1998). 

---