Methanol diffusion coefficient

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 T eOi-i) are taken from refs 197, 224, 226, 255—263 and unpublished data from the laboratory of one of the authors) chemical diffusion coefficients (Z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. ...

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. 

In this section, the variables related to oxygen and methanol are equipped with the subscripts ox and mt, respectively. Here, iox, imt are the ORR and MOR volumetric exchange current densities, Cox, cj are the local and reference (inlet) oxygen concentrations, Cmt, c t are the local and reference methanol concentrations, r]mt are the ORR and MOR polarization potentials (overpotentials), box, bmi are the ORR and MOR Tafel slopes, and Dox, Dmt are the oxygen and methanol diffusion coefficients, respectively. 

Methanol diffusion coefficient in the CCL (cm s ), Equation 4.200 Oxygen diffusion coefficient in the CCL (cm s ), Equation 4.200 Long-range diffusion coefficient of water in membrane (cm s ) Local diffusion coefficient of water in membrane (cm s )... 

Scott et al. [56] tested radiation-grafted manbranes in DMFC. PVDF-g-PSSA membranes showed superior performance to Nafion under identical conditions at high current density in addition to lower methanol diffusion coefficient by an ordCT of magnitude. However, such membranes suffered performance deterioration after few hundreds of hours due to poor MEA interfacing. 

However, proton conductivity of many SPEEK membranes is lower than that of Nafion membranes, but SPEEK membranes, with 39% and 47% degrees of sulfonation, have been found to have proton conductivity values close to that of Nafion 115 membranes [26]. Their methanol diffusion coefficient value is an order lower than that of Nafion 115 membranes under the same conditions. At 80°C, the overall DMFC performance of the SPEEK membranes is better than that of Nafion 115 membrane. Furthermore, SPEEK membranes with methanol diffusion coefficient values of 5 X 10" -3 x 10" cm s" at 25°C—which is lower compared with that of Nafion membranes (10" -10 cm s" )—have been developed [28]. 

Fig. 15.5 OCV as a function of the cathode pressure for Nafion 115 membrane at different DMFC operation times. The dotted hnes correspond to the model equation circle) first measurement (filled circle) second measurement and (rectangle) third measurement [2]. (This figure was pubhshed in Journal of Power Sources, Vol. 104, V. M. Barragan, Estimation of the Membrane Methanol Diffusion Coefficient from Open Circuit Voltage Measurements in a Direct Methanol Fuel Cell, page 71, Copyright Elsevier, 2002.)...

Barragan, V. M., and Heinzel, A., 2002, Estimation of the membrane methanol diffusion coefficient from open circuit voltage measurements in a direct methanol fuel cell , J. Power Sources 104 (l) 66-72. 

Tang et al. [30] and Guo et al. [31] found that sulfonated poly[bis(3-methylphenoxy)phosphazene] (denoted as SPB3MPP) (Fig. 7b) offered an attractive combination of good proton conductivity, crosslinkability and low methanol permeability features especially important for direct methanol fuel-cell membranes. UV-crosslinked membranes prepared from SPB3MPP of lEC 1.4 mmol/g exhibited a methanol diffusion coefficient on the order of 1.0 X lO cm /s, which was significantly smaller than that in a Nation... 

Zhang, G., Fu, T., Shao, K., Li, X.F., Zhao, C.J., Na, H., Zhang, H., Novel sulfonated poly(ether ether ketone ketone)s for direct methanol fuel cells usage Synthesis, water uptake, methanol diffusion coefficient and proton conductivity, J. Power Sources, 2009, 189,875-881. 

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