Direct membrane fuel cells methanol crossover

Jeon JD, Kim J, Kwak SY (2012) Nafion/microporous titanosilicate ETS-4 composite membranes for effective methanol crossover reduction in direct methanol fuel cells. J Membr Sci 415 16 353-359... 

Choi, W.C., Kim, J.D. and Woo, S.I. 2001. Modification of proton conducting membrane for reducing methanol crossover in a direct-methanol fuel cell. 96 ... 

However, DMFCs do suffer some drawbacks such as lower electrical efficiency and higher catalyst loadings as compared to H2 fuel cells. Efforts should be continued on developing anode catalysts with improved methanol oxidation kinetics, cathode catalysts with a high tolerance to methanol, membranes with lower methanol permeation rates, and strategies to reduce the methanol crossover rate. Attention should also be given to other direct-feed fuel cells using other liquid fuels (such as formic acid). 

Membrane prepared by blending sulfonated polybenzimidazole (PBI) with Nafion polymer showed a conductivity of 0.032 S cm The methanol permeability of the composite membrane was found to be 0.82 x 10 cm s as compared to Nafion, which is around 2.21 x 10 cm s [22]. Addressing the problem of methanol permeation, a composite membrane of Nafion with polyvinyl alcohol (PVA) for direct methanol fuel cell has been reported. It is concluded that at the weight ratio of 1 1 in PVA and Nafion, the thin film-coated Nafion membrane exhibited low methanol crossover, and the membrane protonic conductivity could be improved by the sulfonation treatment [23]. Recently, Zaidi et al. [24] prepared composite membranes of PFSA ionomer with boron phosphate and showed the conductivity of 6.2 X 10-2 S cm-i at 120°C. 

With the introduction of proton conducting membranes, interest in direct methanol fuel cells (DMFC) systems in the 1990s has been renewed with projects in North America, Japan, and Europe. Of particular significance has been the work of Los Alamos National Laboratory. If the power density required for vehicle applications is to be achieved, further improvements to anode performance are necessary. Existing membranes also allow methanol crossover, which in turn contributes to poor cell performance. hi this context, it is interesting to speculate on how high-temperature membranes such as that developed by Celanese would perform in a direct methanol fuel cell. 

HjSO, but the magnitude of the current density was not affected largely up to a concentration of lOmmol dm . With 2-propanol, the current due to the alcohol oxidation was superimposed on the ORR current, and the potential shifted by approximately 0.5 V at 0.1 mol dm . Both 2-propanol and acetone are MEA components because 2-propanol is added to the catalyst ink, and acetone is produced by oxidation of 2-propanol on the platinum cathode. The effects are, however, not serious because their concentration is less than lOmmol dm . Methanol is used as a fuel in direct methanol fuel cells, and crossover through the polymer electrolyte membrane is known to cause a degradation of the ORR at the cathode, when the concentration of the fuel is as high as 5 mol dm . 

The most widely studied fuel-ceU membrane is DuPont s Nafion , a copolymer of tetrafluoroethylene and perfluoro(4-methyl-3,6-dioxa-7-octene-l-sulfonic acid). Nafion is the membrane material of choice for most proton-exchange membrane fuel cells that operate at a temperature <80 °C. While Nafion offers high conductivity combined with exceptional chemical and mechanical stability [3], it suffers from several critical drawbacks. When used in a direct methanol fuel cell, Nafion shows significant methanol leakage (crossover from the anode to the cathode) with the resultant reduction in fuel-ceU performance. To overcome this shortcoming the methanol concentration in the anode feed is usuaUy reduced to 0.5-2.0 M, which necessitates... 

For an AEM fabricated in a fuel cell system that used methanol solution as the fuel, the methanol crossover should be considered, and the parameter of methanol permeability is often used. Majority of the developed aromatic AEMs exhibited one to three orders of magnitude lower methanol permeability than that for Nation series membranes, in the range of 10 -10 cm%, as shown in Table 11.1. Combing with the opposite direction of methanol diffusion and ion transport across the membrane, the rather low methanol permeability can ensure the stable performance of the fuel cells using methanol solutions as fuels. 

C. Xu and T. S. Zhao. In situ measurements of water crossover through the membrane for direct methanol fuel cells. Journal of Power Sources 168 (2007) 143-153. 

R Liu, G. Lu, and G. Y. Wang. Low crossover of methanol and water through thin membranes in direct methanol fuel cells. Journal of the Electrochemical Society 153 (2006) A543-A553. 

A particular version of the PEFC is the direct methanol fuel cell (DMFC). As the name implies, an aqueous solution of methanol is used as fuel instead of the hydrogen-rich gas, eliminating the need for reformers and shift reactors. The major challenge for the DMFC is the crossover of methanol from the anode compartment into the cathode compartment through the membrane that poisons the electrodes by CO. Consequently, the cell potentials and hence the system efficiencies are still low. Nevertheless, the DMFC offers the prospect of replacing batteries in consumer electronics and has attracted the interest of this industry. 

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