Poly sulfonated, DMFC

Fu, Y. Z., Manthiram, A. and Guiver, M. D. 2007. Blend membranes based on sulfonated poly(ether ether ketone) and polysulfone bearing benzimidazole side groups for DMFCs. Electrochemical and Solid State Eetters 10 B70-B73. 

Sulfonated poly(arylene ether)s (sPAE)s, such as sulfonated poly(ether ether ketone) (sPEEK) (see Fig. 6.7) are promising ionomer for DMFC [181] due to their chemical stability, possibility of structural variants, and sulfonatiOTi procedures, as discussed by Kim et al. [7]. 

Hammond and coworkers [167] prepared several LBL assembled films onto Nafion using different polyanions and polycations. The best results were obtained with a membrane produced assembling poly(ethylene imine) (LPEI) and sulfonated poly(2,6-dimethyl 1,4-phenylene oxide) (sPPO), having k, 47 and a DMFC performance over 50 % compared to unmodified Nafion. Moon and Rhim [168] prepared LBL assembled membranes using poly(allylamine hydrochloride) (PAH) and poly (estyerene sulfonic acid) (PSSA), where both, methanol and conductivity decrease with the number of layers, in such a way that methanol selectivity does not improve significantly in comparison with unmodified Nafion. However, Nafion membranes coated with a layer of PAH exhibit a DMFC performance at 80 °C much better (150 mW.cm ) than Nation (70 mW.cm ) when fed with 5 M methanol. PAH and poly (vinyl sulphate) (PVS) were LBL assembled onto Nafion achieving a reasonable methanol selectivity = 3 ) [170], without test in DMFC. 

Some PVA-based membranes showing modest selectivities, like sulfonated PVA [339] and PVA/PSSA/mordenite [361] exhibit unexpected high MPD (81 and 74 mW.cm , respectively) at 70 °C. PVA/poly(ether sulfone)/PWA membranes [354], with very low selectivity, show MPD in the range 95-117 mW.cm at 80 °C. In summary, the performance of DMFC with PVA-based membranes could not yet overpass the results obtained with Nafion composite membranes, although some promising results have been obtained during the last years. Indeed, the results shown in Table 6.3 forNafi(Mi/PVA blends [114] and PVA-coated Nafion membranes are superior to those obtained with PVA-based membranes. 

Other polysulfone membranes for DMFC were prepared by sulfonation of commercial polyethersulfone with Cardo group (sPES-C) [473] (Fig. 6.9), by sulfonation of a commercial poysulfone and the use of silica as a filler [474], by "click" cycloaddition of alkyne sulphonate to polysulfone containing azide moieties and crosslinked with 1,7-octadiyne [475], and by polycondensation of 4,4 -difluorodiphenyl sulfone (DFDPS) and l,3-bis(4-fluorobenzoyl)benzene,6,7-dihydroxy-2-naphtha lene sulfonate with bisphenol [476]. Commercial poly(phtalazinone ether sulfone ketone (PPESK) [477], Udel (Solvay) polysulfone [478-480], and Lasuf Lati SPA) polysulfone [481] were sulfonated to prepare membranes for DMFC. Relative selectivity larger than 7 have been obtained with some of these membranes [477,478], as indicated in Fig. 6.37. 

Poly(vinylidene fluoride) (PVDF) membranes, on the other hand, have much better chemical and thermal stabilities than PSSA, but is not a proton conducting polymer. A composite membrane of polyvinylidene fluoride -hexafluoropropyl ene (PVDF-HFP) with sulfonated silica was prepared [485] with proton conductivity up to 40 mS.cm and good relative selectivity (fi, >3), which delivers 43 mW.cm in a DMFC at 80 °C. However, the most studied PVDF composites membranes, are those prepared with PSSA [486—488], poly(2-acrylamido-2-methyl propylene sulfonic acid) (PAMPS) [489]. 

A PVDF-HFP/PSSA composite membrane was tested in DMFC [490] reaching 80 mW.cm at 60 °C. When Si02 was added to the composite the MPD increased up to 110 mW.cm at the same temperature, which represents the best DMFC performance for a PVDF-based membrane. Recently, a composite of PVDF with sulfonated poly(styrene-b-ethylene butylenes-b-styrene) (sPSEBS) was reported to deliver up to 72 mW.cm at 60 °C in a DMFC, although the characteristics of the catalysts and the MEA were not described [491]. 

Zhang N, Zhang G, Xu D, Zhao C, Ma W, Li H, Zhang Y, Xu S, Jiang H, Sun H, Na H (2011) Cross-linked membranes based on sulfonated poly (ether ether ketone) (SPEEK)/Nafion for direct methanol fuel cells (DMFCs). Int J Hydrogen Energ 36 11025-11033... 

Gosalawit R, Chirachanchai S, Shishatskiy S, Nunes SP (2008) Sulfonated montmorillonite/ sulfonated poly(ether ether ketone) (SMMT/SPEEK) nanocomposite membrane for direct methanol fuel cells (DMFCs). J Membr Sci 323 337-346... 

Most of the commonly used membranes used in DMFC exhibit acceptable performance stability after thousands of hours under steady-state operation, including perfluorinated sulfonic acid, PTFE-co-HFP radiation-grafted poly- styrenes, and sPAE [85]. However, membrane durability results under unsteady-state operation are stUl scarce. Siroma et al. [114] have shown that a significant fraction of Nation dissolved after 1 week of DMFC operation with methanol solution. 

Feng S, Shang Y, Xie X, Wang Y, Xu J. Synthesis and characterization of crosslinked sulfonated poly(arylene ether sulfone) membranes for DMFC applications. J Membr Sci 2009 335(l-2) 13-20. 

Lee, C.H., Min, K.A., Park, H.B., Hong, Y.T., Jung, B.O. and Lee, YM. 2007a. Sulfonated poly(arylene ether sulfone)-silica nanocomposite membrane for direct methanol fuel cell (DMFC). J. Memh. ScL 303(1-2) 258-266. 

Modification of Sulfonated Poly(Ether Ether Ketone) for DMFC Application 413... 

Nafion (Fu et al. 2008). However, the corresponding proton conductivity value is lower. The methanol crossover, however, is only one-third of that found in Nafion 115, compared at the same thickness. Although the PSf membrane has lower proton conductivity than Nafion, the lower production cost and methanol crossover make it a promising alternative for DMFC. Sulfonated poly(phthalazinone ether ketone) (SPPEK) has been discovered as a new kind of PEM for DMFC due to its superior performance in terms of chemical and oxidative resistances, mechanical strength, and thermal stability (Gao et al. 2003). Tian et al. reported that SPPEK prepared from direct polymerization of presulfonated monomer has better performance than that of postsulfonation (Tian et al. 2005). Unfortunately, with direct polymerization it was hard to control the DS and location of sulfonation. 

H. Pan, X. Zhu, J. Chen, X. Jian, Synthesis of fluoro-containing sulfonated poly(phthalazinone ether ketone ketone)s and their properties as PEM in PEMFC and DMFC, J. Memb. Sci. 326 (2) (2009) 453 59. 

The effects of side chain architectures on the properties and proton conductivities of graft copolymer membranes for DMFCs were studied by Su et al. [126]. Poly(vinylidene fluoride)-g-poly(styrene sulfonic acid) (PVDF-g-PSSA) copolymers with either linear graft (LG) or arborescent graft (AG) PSSA side chains were prepared. Scheme 6.29 shows the structures of LG and AG polymers. 

The group of GKSS Research Center Germany extensively researched the effect of a variety of inorganic nanoparticles on the properties of aromatic ionomer membranes [86-93]. Composite membranes were prepared from silicates and sulfonated poly(ether ketone)s or sulfonated poly(ether ether ketone)s. For DMFC applications, the composite membranes showed promising properties with lower methanol and water permeability and comparable (or higher) proton conductivity compared to the parent polymer membranes. The flux of water and methanol decreased with the increase in content of silicates. 

Yoon SJ, Choi JH, Hong YT, Lee SY (2010) Synthesis and characterization of sulfonated Poly(arylene ether sulfone) ionomers incorporating perfluorohexylene units for DMFC membranes. Macromol. Res. 18 352-357. 

Block copolymers combing PBI with other types of macromolecular units have also been developed for superior membrane properties, as shown in Fig. 7.6. Two types of copolymers have been prepared, characterized, and evaluated as fuel cell electrolytes. One is the sulfcmated copolymer containing PBI and snUrmated polymer moieties for low temperature PEMs in both PEM fuel cells and direct methanol fuel cells (DMFCs) [123-126]. The other is the random copolymer containing PBI and poly(imine/ amide) moieties [127, 128]. For the sulfonated PBI copolymers, benzimidazole monomers... 

Development of Sulfonated Poly(ether-ether ketone)s for PEMFC and DMFC... 

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