Nafion membranes inorganic additive addition

The proton conductivities of Nafion and inorganic additive-containing Nation composite membranes under various temperature and RH are also partly summarized in Table 9.5. 

Nearly all of the commercially available membranes are based on Nafion. Nafion also has the largest body of literature devoted to its study because of its demonstrated industrial importance and availability. Nafion composite systems also have already become significant in both industrial and academic research. In composite structures, Nafion can be impregnated into an inert Teflon-like matrix (i.e. W. L. Gore membranes ), or inorganic additives can be added to a supporting Nafion matrix for improved physical or electrochemical properties (i.e. lon-omem °). Some critical aspects of Nation s molecular structure and physical properties will be briefly highlighted to provide a baseline for comparison with the other alternative materials discussed in this review. 

Composite Nafion membranes have been made by the incorporation of inorganic nanoparticles as methanol barrier components, which include silicon oxide, titanium oxide, or mixed silicon-titanium oxides (Arico et al. 2003 Dimitrova et al. 2002 Jung et al. 2003). One drawback of this approach is a decrease in conductivity due to the addition of the nonconductive oxide (Aparicio et al. 2005). 

FIGURE 12.12 Conductivity of P(VDF-CTFE)/Nafion/inorganic membranes with different inorganic additives at 120°C and different RH. (1-30% P(VDF-CTFE)/30%Nafion/40% H30Zr2(P04)3 2-20% P(VDF-CTFE)/20%Nafion/60% AZP 3-30% P(VDF-CTFE)/ 30%Nafion/40% H2(SiTi207)1.5H20 4-30% P(VDF-CTFE)/30%Nafion/40% MS.)... 

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. 

Overall, AB-PBI and sAB-PBI are possible alternative to Nafion-type membranes, because of the nearly zero dependence of conductivity on water. These membranes also have the requisite mechanical and thermal stability, while achieving moderate levels of proton conductivity that can be further modified with inorganic additives, such as heteropolyacids. 

Direct methanol fuel cells (DMFCs) face two major problems of methanol crossover and low proton conductivity. In order to overcome these problems, modified Nafion membranes containing HPAs have been casted by sonicating the inorganic additives namely silicon dioxide particles and molybdophosphoric acid in commercially available Nafion solution in high boiling point solvent (dimethylsulfoxide or 1-methyl-2-pyrrolidone) [95]. Additives were effectively immobilized in the cast membranes, which did not leach out on boiling in sulfuric... 

Quaternary Ammonium Ions. In a recent study (17), 1200 EW Nafion has been used to construct a membrane ion selective electrode. The electrode was placed in both the tetrabutylammonium ion and cesium ion forms, and the response characteristics of each form were measured. These electrodes show Nernstian responses, and the tetrabutylammonium ion electrode has no interference from inorganic cations such as Na" ", K" ", and Ca2" ". However, this electrode shows a marked interference with decyltri-methylammonium ion. In addition the cesium ion electrode response is sensitive to the presence of tetrabutylammonium ion and especially dodecyltrimethylammonium ion. Although membrane electrode sensitivities are not in general proportional to thermodynamic selectivity coefficients, the results do indicate that these large, hydrophobic cations are preferred over smaller inorganic cations by the polymer. The authors suggest that the surfactant character of the two asymmetric tetraalkylammonium ions may lead to non-electrostatic interactions with the fluorocarbon regions of the polymer, which would enhance their affinities (17). 

Thus, the addition of silica and heteropolyacid modifies the relative ratio between crystalline and amorphous structure of ctist Nafion with respect to bare Ntifion. Yet, due to the low loading of silica and heteropolyacid in the membrane (3% silica, less than 1.5% PWA), it appears unlikely that the inorganic components are responsible of such significant modification in the Nafion structure. More probably, the observed changes have to be attributed to the final thermal treatment (160°C). 

Figure 13.13 illustrates the dependence of the proton conductivity on the incorporation of SiWA and Si02-Al203 composite at room temperature. The result for Nafion 112 is also presented for comparison with the membranes prepared. Only pure SPEEK56 membrane without any addition of inorganic materials possessed lower proton conductivity than Nafion 112 membrane while other membranes showed higher values of proton conductivity as shown in Eigure 13.14. 

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