Methanol diffusivities

Figure 5. Cartoon models of the reaction of methanol with oxygen on Cu(llO). 1 A methanol molecule arrives from the gas phase onto the surface with islands of p(2xl) CuO (the open circles represent oxygen, cross-hatched are Cu). 2,3 Methanol diffuses on the surface in a weakly bound molecular state and reacts with a terminal oxygen atom, which deprotonates the molecule in 4 to form a terminal hydroxy group and a methoxy group. Another molecule can react with this to produce water, which desorbs (5-7). Panel 8 shows decomposition of the methoxy to produce a hydrogen atom (small filled circle) and formaldehyde (large filled circle), which desorbs in panel 9. The active site lost in panel 6 is proposed to be regenerated by the diffusion of the terminal Cu atom away from the island in panel 7.

Wang et al240 reported the electrooxidation of MeOH in H2S04 solution using Pd well-dispersed on Ti nanotubes. A similar reaction was studied by Schmuki et al.232 (see above), but using Pt/Ru supported on titania nanotube which appear a preferable catalyst. Only indirect tests (cyclic voltammetry) have been reported and therefore it is difficult to understand the real applicability to direct methanol fuel cell, because several other aspects (three phase boundary to methanol diffusivity, etc.) determines the performance. 

Polar, Protic Water, Methanol Diffusion together of reactants... 

In the first cycle, methanol oxidation peaks are seen in both the anodic and cathodic sweeps around 0.7 V. As mentioned earlier, P -OH formation on Ptdll) does not occur to any substantial extent until 1.2 V. Therefore this current decrease over 0.7 V is not due to deactivation of platinum by the svuface Pt-OH formation. The cxirrent increase on the reversed sweep indicated that this current is not limited by methanol diffusion or active accumulated intermediates, either. It simply seems that platinum loses its catalytic activity over 0.7 V regardless whether platinvim is oxidized or not. Anion effects is not likely the reason because the same phenomena are found in percloric add also. Trace amount of impurities, such as chloride ions, may play some roles. 

Every, H. A., Hickner, M. A., McGrath, J. E. and Zawodzinski, T. A. 2005. An NMR study of methanol diffusion in polymer electrolyte fuel cell membranes. Journal of Membrane Science 250 183-188. 

Allcock et al. also have investigated the use of phosphonated polyphosphazenes as potential membrane materials for use in direct methanol fuel cells (Figure A2) Membranes were found to have lEC values between 1.17 and 1.43 mequiv/g and proton conductivities between 10 and 10 S/cm. Methanol diffusion coefficients for these membranes were found to be at least 12 times lower than that for Nafion 117 and 6 times lower than that for a cross-linked sulfonated polyphosphazene membrane. 

Figure 14. Solvent (water, methanol) diffusion coefficients of (a) Nafion 117 (EW =1100 g/equiv) and (b) sulfonated poly(arylene ether ketone)s, as a function of the solvent volume fraction. Self-diffusion data (AiaO. T eOi-i) are taken from refs 197, 224, 226, 255—263 and unpublished data from the laboratory of one of the authors) chemical diffusion coefficients (Z>h2o) are calculated from self-diffu-sion coefficients (see text), and permeation diffusion coefficients are determined from permeation coefficients. ...

A typical uptake curve for methanol diffusion in SAPO-34 at 373 K and a methanol partial pressure of 0.75 kPa is shown in Fig. 13. The transient diffusion equation for a slab geometry (Eq. (7)) for sorbate uptake was found to give the best fit to the experimental results, although SAPO-34 appears to have a typical cubic shape. 

The measured steady-state diffusivity of 3 x 10 m /s is comparable to the effective methanol diffusivity of 1.1 x 10 m /g obtained indirectly from the kinetics data characterizing crystals of various sizes. The consistency between steady-state diffusivity and that determined in reaction experiments is in good agreement with the results of Post et al. 105) and Garcia and Weisz 106). 

In this equation, r is the reaction rate of DME formation, CMeou denotes the equilibrium concentration of methanol, is the methanol diffusion coeffi-... 

Verdrugge, M.W. Methanol diffusion in perfluori-nated ion-conducting membranes. J. Electrochem. Soc. 1989, 136, 417 23. 

The flux of ethanol is just the negative of N. Composition profiles are shown in Figure 8.4. The arrows indicate the actual directions of mass transfer. Methanol diffuses from the interface to the bulk vapor, whereas ethanol diffuses in the opposite direction. In other words, the vapor is being enriched in the more volatile methanol. ... 

Estimate the gaseous diffusion coefficient for methanol diffusing through water vapor at 25°C and... 

Equimolar counterdiffusion can be assumed in this case (as will be shown in a later chapter, this is the basis of the McCabe-Thiele method of analysis of distillation columns). Methanol diffuses from the interface towards the bulk of the gas phase therefore, yM = 0.707 and yA1 = 0.656. Since they are not limited to dilute solutions,... 

Verbrugge and coworkers have employed a radioactive tracer method to measure the methanol diffusivity in a Nation membrane which was... 

H. A. Every, M. A. Hickner, J. E. McGrath, and T. A. Zawodzinski, Jr. Nafion versus sulfonated poly(arylene ether sulfone)s. A comparison of the methanol diffusion behavior. In Fuel Cells from Materials to Systems, The Electrochemical Society, Pennington, New Jersey 08534-2839, USA, 2003. 203rd Meeting of the The Electrochemical Society, Paris. 

Longer, flexible side-chains facilitating proton conductance, impede methanol diffusion [66]. This could be a very practical lesson. 

The most challenging task, to suggest chemical modifications of membranes, which minimize electro-osmosis and/or methanol diffusion, maximize proton conductance and membrane stability at a reduced price is only indirectly affected by theory and simulation. Most conclusions, once made, are rather obvious (such as higher channel connectivity, homogeneity of swelling, higher sidechain density, etc). Others are less trivial, such as the... 

The effect, mentioned earlier, of crossover rate decreasing with increasing working current density is because of the drastic decrease in methanol concentration in the anode s catalytic layer that occurs when methanol undergoes rapid consumption at high current densities. This, in turn, leads also to a decrease in methanol concentration in the membrane s surface layer adjacent to the catalytic layer. As methanol diffusion into the membrane starts precisely in this siuface layer, the diffusion rate also decreases. 

Schaffer T, Tschinder T, Hacker V, Besenhard JO (2006) Determination of methanol diffusion and electrosmotic drag coefficients in proton-exchange-membranes for DMFC. J Power Sources 153 210-216... 

Verbrugge MW (1989) Methanol diffusion in perfiuorinated ion-exchange membranes. J Electrochem Soc 136 417 23... 

From 2,5-bis(4-lluorophenyl)-1,3,4-oxadiazole and 2,2-bis(4-hydroxypheny)propane bisphenol A and 3, 3-disulfonate-4 4-dichloro diphenyl sulfone, a poly(aryl ether sulfone) containing 1,3,4-oxadiazole moieties was obtained [65]. From this polymer, proton exchange membranes have been fabricated. The sulfonated polyoxadiazole membranes exhibit excellent thermal, dimensional and oxidative stability, and a low methanol diffusion coefficient. Thus, these sulfonated polyoxadiazole membranes may be alternative materials for proton exchange membranes at medium to high temperature operations. 

The most widely studied conducting polymer support is polyaniline (PANl), which has been shown to decrease the poisoning of Pt by COads [88]. Gharibi et al. have recently explored the factors responsible for the enhanced formic acid oxidation activity of Pt supported on a carbon/PANI composite [89]. They concluded that improvements in both electron and proton conductivities, as well as the increased methanol diffusion coefficient and decreased catalyst poisoning, could be involved. A carbon nanotubes/PANI composite [90], poly(o-methoxyaniline) [91], and polyindole [92] have recently been reported as effective supports for formic acid oxidation at Pt nanoparticles, while polycarbazole [93] has also been used to support PtRu nanoparticles. 

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