Nafion/inorganic composite membranes

More interesting results were obtained by Xiao et al. [200] using the PPA sol- gel process shown in Fig. 6.11. This procedure leads to very high acid doping levels (2a > 10-20), never reached by PBl membranes prepared with standard casting procedures. The conductivity of a membrane prepared with the PA A sol-gel method and 2a 16, raises from 15 mS.cm at 20 °C up to 270 mS.cm at 200 °C. The last value is comparable with those observed for the best proton conducting Nafion/inorganic composite membranes at temperatures above 100 °C. 

Lee W, Kim H, Kim TK, Chang H (2007) Nafion based organic/inorganic composite membrane for air-breathing direct methanol fuel cells. J Membr Sci 292 29-34... 

Many efforts have been made to find the proper alternative to Nafion at elevated temperature operation for PEMFCs, as Nafion degrades at temperatures higher than 110-130 °C. It was found that the organie-inorganic composite membrane can improve its own mechanical strength and thermal stability at... 

The primary drawbacks of the Nafion membranes are poor conductivity at low relative humidities (and consequently at temperatures >100 C and ambient pressure) and large crossover of methanol in direct methanol fuel ceU (DMFC) applications. As a result, considerable efforts have been made in recent years to overcome these drawbacks. Peihaps the most widely employed approach is the addition of inorganic additives to Nafion m branes to yield organic/inorganic composite membranes. Three major types of inorganic additives that have been studied (zirconium phosphates, heteropolyadds, metal hydrogen sulfates and metal oxides) are reviewed in the following. 

Park, Y.-S., and Yamazaki, Y., 2006, Low water/methanol permeable Nafion/CHP organic-inorganic composite membrane with high crystaUinity, Eur. Polym. J. 42 375-387. 

In order to achieve more control of methanol crossover, composite membranes are synthesized. Organic-inorganic composite membranes comprising Nafion with inorganic materials silica, mesoporous zirconium phosphate (MZP) and mesoporous titanium phosphate (MTP) are made as proton-exchange-membrane electrolytes for direct methanol fuel cells (DMFCs) [206] with increase in proton conductivity and low methanol crossover. Composite membranes with mordenite incorporated in polyvinyl alcohol-polystyrene sulfonic acid blend tailored with varying degree of sulfonation also retards the methanol release kinetics considerably [199]. 

Nafion-based hybrid membranes are traditionally used electrolytes for both PEM fuel cell and DMFCs. " Besides Nafion, many composite engineering thermoplastic polymers based on poly(etheretherketone) (PEEK), polyvinyl alcohol (PVA), polysulfone, " polybenzimidazole (PBI), polyimide, and other organic-inorganic composite membranes have been employed as alternative membranes for both PEM fuel cells and DMFCs due to their lower cost, comparable conductivity, high mechanical and thermal stabilities, and easy modification as well. 

For high temperature membranes, three avenues are being pursued a) synthesis of new ionomers, b) covalent modification of Nafion , and c) pol mier-inorganic composites. Some of the approaches aim to increase the attractive forces holding water within the membrane, enabling operation at low RH. Others seek to embody Bronstead bases as an alternative to water, which allow for proton mobility in the absence of water. This mechanism is exemplified by proton conductivity in e.g. H3PO4 or CSHSO4, in which the protons hop and the protonated base need not move macroscopic distances. 

Firstly, Nafion and other perfluorinated sulfonic acid ionomers will be discussed, along with inorganic- and organic-Nafion based composites. Secondly, we will introduce non-fluorinated single and composite membranes, including membranes for high temperature DAFC. Finally we will discuss anion conducting membranes for alkaline DAFC. 

Most of the reported inorganic fillers used to modify Nafion are composite where the inorganic particles (usually nanoparticles) are located in the membrane bulk. Most of them are prepared using the recast method, where the filler nanoparticles dispersed in a solvent are mixed with Nafion ionomer dispersion in the same solvent or a compatible one. The solution is cast on a Petri dish or a plane surface at high temperature to form the recast composite membrane. An alternative method adopted to prepare Nafion composites with silica [31, 32, 41, 95], functionalized silica [35], and zirconium and titanium phosphate [41], is the in situ sol-gel reaction method, schematized in Fig. 6.5. 

Other reported method to prepare inorganic/Nafion composite membranes is the colloidal [56] or ionic impregnation [75-78, 87, 93], where the inorganic filler is introduced a colloidal dispersion into the membrane or by exchange of by the salt cation, like Zr" ", followed by salt precipitation with the corresponding acid. 

A few examples of Nafion composite membranes with polymers and inorganic compotmds has been reported, such as poly(ethylene glycol) (PEG)/Si02 [155], PTFE/S102 [156], chitosan/phosphotungstic acid [157], PVI/Pd [158], and PBI/zirconium phosphate [159]. 

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