SPEEK membranes proton conductivity

M.L. Di Vona, A. D Epifanio, D. Marani, M. Trombetta, E. Traversa, S. Licoccia, SPEEK/PPSU-based organic-inorganic membranes proton conducting electrolytes in anhydrous and wet environments, J. Membrane Sci., 279 (2006) 186-191... 

Proton conductivity as a function of lEC for ETFE-g-PSSA = polyethylenetetrafluoroethylene-gra/t-polystyrene sulfonic acid, BAM membrane = substituted poly(trifluorostyrene) sulfonic acid, SPEEK = sulfonated poly(ether ether ketone) and Nafion. (From Peckham, T. J. et al. 2007. Journal of Materials Chemistry 17 3255-3268, and Dolye, M. et al. 2001. Journal of Physical Chemistry B 105 9387-9394.)... 

Zhang et al. [128] synthesized a self-humidifying membrane based on a sulfonated poly(ether ether ketone) (SPEEK) hybrid with a sulfated zirconia (SO / ZrO2, SZ) -supported platinum catalyst (Pt-SZ catalyst). This type of composite membrane has a higher proton conductivity than plain SPEEK, due to the effect of the Pt-SZ catalyst the membrane also provided excellent single cell performance at low humidity. 

Fig.15 Temperature dependence of proton conductivity at 100% RH of sPEEK (a) and sPEEK-silica membranes containing 20 (b) and 45 wt. % (c) silica...

The reported proton conductivity and methanol permeability of sPEEK- and sPAEK-based membranes [364—389] are summarized in Fig. 6.30 as a relative selectivity plot. 

There are several sPEEK membranes with very good relative selectivity and several sPEEK composites with silica, zirconia and their mixtures with heteropolyacids [370, 371, 377] which exhibit remarkable selectivities with high proton conductivities. Composites of sPAEK with epoxy resin [383] and crosslinked with PVA [386] also show excellent selectivities and high conductivities. Unfortunately, the performances of these promising membranes in DMFC have not been reported yet. 

In order to make use of the low methanol crossover, multilayer membranes consisting of the copolymer as a methanol-barrier are used. This material is used as the central layer and SPEEK is used as the proton conducting outer layers. These multilayer membranes have been found to exhibit a better performance and higher power density than plain sulfonated PEEK and Nafion 115 membranes [129],... 

Nafion membranes, as discussed, have been intensively used for fuel cells because they show high proton conductivity and chemical stability, but their methanol permeability is too high. However, the critical aspect of Nafion is still its high cost. Several nonfluorinated membranes, with potentially lower costs, have been tested for fuel cells. Sulfonated PSF, sulfonated poly(ether ether ketone) (SPEEK), sulfo-nated polyphosphazene, and sulfonated polyamides (PA) with good performance for hydrogen fuel cells are described in several reports (Savadogo 1998 Zaidi et al. 2000 Guo et al. 1999 Vallejo et al. 1999). However, the methanol permeability in many cases is still relatively high. 

The sulfonation of poly(ether ether ketone) is similar to the sulfonation technique for PSSA using sulfonation agents such as high concentration of sulfuric acid. The DS of SPEEK can be controlled by varying the reaction time and temperature (Xing et al. 2004). The work of Lee et al. showed that the proton conductivity of the SPEEK membrane increased with DS (Lee et al. 2007). The increase in DS will increase the hydrophilicity and water uptake and, consequently, gives an opportunity for the formation of water-mediated pathways for protons. Methanol permeability measurement indicates that its rate increases with the increase of DS but at a much lower rate than the Naflon membrane. This is due to the difference in their microstructures (Li et al. 2003a). 

Water uptake of the SPEEK membrane is known to increase with the increase in DS due to enhancement of the hydrophilicity when introdncing the snlfonic acid groups in the aromatic PEEK polymer. These sulfonic acid groups, in the presence of water molecules, facilitate proton transfer through the membrane, thus increasing the proton conductivity. Therefore, water uptake is a crucial characteristic of the SPEEK membrane, since it is proportional to DS and proton conductivity. However, as methanol transfer is also facilitated by the water molecules, increasing the water uptake normally increases the methanol permeability as well. 

The proton conductivity of SPEEK membrane can be improved by incorporating a fast proton conductor in the polymer matrix as studied by Silva et al. (2005). They utilized zirconium phosphate (ZrPh) pretreated with -propylamine and then with PBI. The pretreatment was to improve the compatibility between ZrPh and SPEEK. Even though the compatibility of the inorganic was good, the proton conductivity and water uptake of the composite membrane were reduced, consequently reducing the methanol permeability. 

Earlier, Mikhailenko et al. have introduced boron orthophosphate (BPO4) as an inorganic material (Mikhailenko et al. 1998). The composite SPEEK/BPO4 was able to increase the proton conductivity as the water uptake was also increased. There were no reported data on methanol permeability, but it was mentioned that the porosity of the membrane was increased. This chapter continues with more details on the SPEEK modification by using various types of inorganics such as Cloisite 15A, boron orthophosphate, and tungstosilicic acid supported on silica-aluminum oxide. 

FIGURE 13.7 Ratio of the proton conductivity to the methanol permeability for SPEEK nanocomposite membranes and Nafion 112 membrane. (Reprinted from Jaafar, J., Ismail, A.E., and Matsuura, T., J. Memb. Sci., 345, 119—127, 2009. With permission.)... 

FIGURE 13.11 Proton conductivity of SPEEK/BPO4 composite membranes as a function of BPO4 loading at room temperature. (Reprinted from Othman, M.H.D., Ismail, A.P., and Mustafa, A., J. Memb. ScL, 299, 156-165, 2007. With permission.)... 

Ren et al. introduced a layer technique where they immersed the SPEEK membrane in a Naflon casting solution, as a result of which a layer membrane of Nafion-SPEEK-Naflon was prepared (Ren et al. 2005). The proton conductivity of the modified membrane was greater than the original SPEEK, but, unfortunately, the methanol permeability also increased. This shows that Nation, which has high methanol permeability and is on the top and bottom layers of the modified SPEEK, has some influence in ino easing the methanol permeability. 

Another important characteristic of the membrane for DMFC application is the proton conductivity of the membrane. Table 13.3 also shows the proton conductivity at room temperature and 100% RH for Nafion 112, SPEEK, and SPEEK/cSMM membranes. The proton conductivity of Nafion 112 is 1.20 x 10 S/cm, highest of all the three membranes. The proton conductivity for SPEEK is 3.3 x 10 S/cm. Interestingly, the proton conductivity of the SPEEK/cSMM blend membrane is 6.4 X 10 S/cm, about twice as high as that of SPEEK. This result is expected from the simultaneous increase in the water uptake, since the proton conductivity relies... 

Reprinted from M.L. Di Vona, Z. Ahmed, S. BeUitto, A. Lenci, E. Traversa, S. Licoccia, SPEEK-Ti02 nanocomposite hybrid proton conductive membranes via in situ mixed sol-gel process, Journal of Membrane Science 296 (2007) 156-161. 

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