Methanol permeation

Reeve RW, Christensen PA, Dickinson AJ, Hamnett A, Scott K (2000) Methanol-tolerant oxygen reduction catalysts based on transition metal sulfides and their appheation to the study of methanol permeation. Electrochim Acta 45 4237-4250... 

Direct-methanol fuel cells (DMFCs) have attracted considerable attention for certain mobile and portable applications, because of their high specific energy density, low poison emissions, easy fuel handling, and miniaturization [129,130], However, the methanol permeation through electrolyte membranes (usually called methanol cross-over) in DMFCs still is one of the critical problems hindering the commercialization [131,132], Nafion , a... 

Kim, Y. J., Choi, W. C., Woo, S. I. and Hong, W. H. 2004. Proton conductivity and methanol permeation in Nafion/ORMOSIL prepared with various organic silanes. Journal of Membrane Science 238 213-222. 

Gogel et al. [118] compared two CFPs, one untreated and one treated (25 wt% PTFE) as the anode DL (both were TGP-H-120). The fuel cell was operated at a cell temperature of 110°C, and it was observed that the DL without any treatment performed better. However, the difference between both materials was very small and the methanol permeation was actually reduced (increased Faradaic efficiency) with the treated DL. A possible explanation for this is that methanol is oxidized more effectively at the anode due to the formation and stabilization of carbon dioxide bubbles in the active area. As a consequence, the methanol concentration gradient across the membrane is reduced. 

V. Gogel, T. Frey, Z. Yongsheng, et al. Performance and methanol permeation of direct methanol fuel cells Dependence on operating conditions and on electrode structure. Journal of Power Sources 127 (2004) 172-180. 

The DMFC, based on a polymer electrolyte fuel cell (PEFC), uses methanol directly for electric power generation and promises technical advantages for power trains. The fuel can be delivered to the fuel cell in a gaseous or liquid form. The actual power densities of a DMFC are clearly lower than those of a conventional hydrogen-fed polymer electrolyte fuel cell. In addition, methanol permeates through the electrolyte and oxidizes at the cathode. This results in a mixed potential at the cathode (Hohlein et al., 2000). 

Another finding reported by DuPont is that the equivalent weight (EW) exhibits a pronounced influence on performance and methanol permeation. Whereas the highest performances were found with low EW membranes, the membranes with high EW had the lowest relative methanol permeation (Figure 27.24). According to DuPont, a 2 mil experimental membrane is in development which exhibits better performance for DMFCs compared to the 7 mil commercial membrane. 

The methanol permeation measurement illustrates the relationship between the methanol permeation rate and the temperature versus the thickness of the membrane. For thinner samples, the permeation rate changes by a factor of 4 or 5 with a temperarnre change from 25°C to 65°C, while in the thicker membranes this factor is about 3. At lower temperatures, the variation of the methanol flux through the membranes versus thickness is small compared to that at 65°C, where this dependence is pronounced. As anticipated, a strong decrease in permeation with thickness is observed in all cases. For the reciprocal quantity, a linear relationship between permeation rate and thickness can be derived approximately as shown in the inset of Figure 27.64. The comparison between the composite membranes and the commercial Nafion (Nafion 112, 115, and 117) reveals a similar methanol permeation rate at elevated temperature with slightly higher values of the composites. 

Fig. 52. Variation of methanol permeation rate in a polymer electrolyte fuel cell at elevated temperature with cell current density for different methanol feed concentrations. The results show that, for methanol concentrations under 1 m, methanol is effectively consumed at the anode, thus minimizing the permeation rate [117], (Reprinted by permission of the Electrochemical Society).

M. Walker, K.-M. Baumgartner, M. Kaiser, J. Kerres, A. Ullrich and E. Rauchle, Proton-conducting polymers with reduced methanol permeation, J. Appl. Polym. Sci., 1999, 74, 67-73. 

M solutions and the penalty of methanol permeation is not as great due to the increased consumption at the anode. 

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