Crossover flux

Fuels other than methanol have been contemplated, such as formic acid, which exhibits lower crossover fluxes than methanol and thus may be an alternative for use e.g., in small systems contemplated for portable applications (Ha et al, 2004 Zhu et al, 2004). 

Crossover flux - DMeOH, mem x ( MeOhl) membrane/anode x mem... 

Crossover flux = Ciyi oH, mem ( MeOn) membrane/anode mem... 

Methanol Limiting Crossover Flux and Current Density at Open Circuit through PEM with Different Thicknesses... 

Fig. 14.6 The effect of methanol concentration and membrane thickness on DMFC performance curves. Methanol crossover fluxes are given relative to Nafion 117 (215 um) at 1.0 M and 60°C. Crossover is expressed as a relative fraction of the methanol flux observed for a Nafion 117 membrane at the fuel cell operation conditions with Nafion 117 and 1.0 M methanol...

Typical voltage-current density fuel cell performance plots are shown in Fig. 14.12 for membranes with different FEP contents. For FEP contents of 20,40, and 50 wt%, the V-i enrves lie above that for Nafion 112 and 117. These membranes had an acceptably low sheet/areal resistance and a low methanol permeability. When the membrane contained >50% FEP, fuel cell performance fell below that of Nafion 112 and 117, dne to excessively high resistance losses (the membrane conductivity was too low). The effect of methanol feed concentration (1.0, 5.0, and 10.0 M) on the performance of MEAs with a 50/50 wt% Nafion/PFA membrane (70 J,m thickness) is contrasted to MEAs with Nafion 117 in Fig. 14.13. At 1.0 M, the blended membrane worked as well as Nafion 117, but at the two higher methanol feed concentrations, the Nafion/PFA membrane produced more power due to lower methanol crossover (see Table 14.1 of open circuit methanol crossover fluxes). 

Table 14.1 Methanol crossover flux through Nafion 117 and a Nafion/PFA-blended membranes (50/50 wt%, 70 pm thickness) in a fuel cell MEA at open circuit, as measured by CO, in the cathode air exhaust ...

Methanol crossover flux and diffusivity can be determined by measuring the limiting current in a DMFC. Assuming the methanol crossover to be due to diffusion only, then the diffusivity is given as [37] ... 

However, corrections for electto-osmotic drag effects are necessary even for low methanol concentrations for accurate methanol crossover flux measurement in a DMFC at open circuit voltage. With correction, the diffusivity becomes ... 

If CJS) is known, then from the slope of linear plot of (15.16), the permeability DK) of the membrane could be obtained. Average methanol crossover flux can be determined using dC / values and equation (15.10). Cj(t)is expressed as ... 

Mass spectrometry of liquid samples of the cathode outlet stream is another way of determining the methanol crossover flux. For mass spectrometric measurements of methanol crossover, a clear description of the respective system conld be achieved by measuring the background methanol signal of a cell filled with distilled water and equipped with the membrane sample, and subseqnently adding well-adjusted portions of aqueous or pure methanol to this liquid [25]. The slopes of mass signal vs. time curves are typical for diffusion-controlled processes and with the help of the calibration lines, the diffusion coefficient of methanol through the membrane can be calculated. Online analysis of the cathode exhaust gas with multipurpose electrochemical mass spectrometry can also be employed to determine methanol permeability. However, as mentioned, the assumptions that the entire permeated methanol is converted to CO and that there is no anodic CO contribution are contentious. 

The output of working electrode current density (current normalized by the active area of the working electrode) versus potential is used to determine the hydrogen crossover flux (mol/cm /s) from Faraday s law. 

Fig. 16 Direct methanol fuel-cell performance curves with blended membranes composed of PVDF and either sulfonated poly[(3-methylphenoxy)(4-ethylphenoxy)phosphazene] (SP3MP4EPP) or sulfonated poly[(4-ethylphenoxy)(phenoxy)phosphazene] (SP4EPPP). 1.0 M methanol feed, 60 °C, air at ambient pressure and 500 seem. Cross denotes the methanol crossover flux (mol/cm min) at open circuit, relative to that in Nafion 117...

Proton conductivity measurements were performed by AC impedance technique. The proton conductivities for all the membranes increased with increase in temperature from 30°C to 100°C as seen in Figure 10.16. It is noteworthy that the proton conductivity increases with increase in SWA content from 5 to 10 wt% and decreases at 15 wt% SWA. Higher content of SWA (>10 wt%) in cross-linked CS-PVA blend disrupts the proton conduction path by blocking the voids of polymer matrix and decreases its proton conductivity, which may be also attributed to the water sorption data. The methanol crossover for the membranes is measured in situ in fuel cells under OCV condition at 70°C. The methanol crossover flux is lower for CS-PVA-SSA-SWA (10 wt%) (about 3.3 x 10" mol s" cm- ) hybrid membranes in comparison with Nation 117 and other membranes. In addition, the electrochemical selectivity of CS-PVA-SSA-SWA (10 wt%) has reached to 2.69 x 10 S cm- s. 

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