Direct methanol fuel cells modification

Abstract There have been numerous studies on modifying DuPont s Nafion (a perfluorosulfonic acid polymer) in order to improve the performance of this membrane material in a direct methanol fuel cell. Modifications focused on making Nafion a better methanol barrier, without sacrificing proton conductivity, so that methanol crossover during fuel cell operation is minimized. In this chapter, a brief literature survey of such modifications is presented, along with recent experimental results (membrane properties and fuel cell performance curves) for (1) thick Nafion films, (2) Nafion blended with Teflon-FEP or Teflon-PFA, and (3) Nafion doped with polybenzimidazole. 

S.P. Nunes, B. Ruffmann, E. Rikowski, S. Vetter, and K. Richau. Inorganic modification of proton conductive polymer membranes for direct methanol fuel cells. Journal of Membrane Science 203, 215-225 2002. 

The modification of platinum catalysts by the presence of ad-layers of a less noble metal such as ruthenium has been studied before [15-28]. A cooperative mechanism of the platinurmruthenium bimetallic system that causes the surface catalytic process between the two types of active species has been demonstrated [18], This system has attracted interest because it is regarded as a model for the platinurmruthenium alloy catalysts in fuel cell technology. Numerous studies on the methanol oxidation of ruthenium-decorated single crystals have reported that the Pt(l 11)/Ru surface shows the highest activity among all platinurmruthenium surfaces [21-26]. The development of carbon-supported electrocatalysts for direct methanol fuel cells (DMFC) indicates that the reactivity for methanol oxidation depends on the amount of the noble metal in the carbon-supported catalyst. 

Brandao L, Rodrigues J, Madeira LM, Mendes A (2010) Methanol crossover reduction by Nafion modification with palladium composite nanoparticles application to direct methanol fuel cells. Int J Hydrogen Energ 35 11561-11567... 

Ren S, Li C, Zhao X, Wu Z, Wang S, Sun G, Xin Q, Yang X (2005) Surface modification of sulfonated poly(ether ether ketone) membranes using Nafion solution for direct methanol fuel cells. J Membr Sci 247 59-63... 

Woong JC, Venkataramani SD, Kim SC (2006) Modification of Nafion membrane using poly (4-vinyl pyridine) for direct methanol fuel cell. Polym Int 55 491-499... 

Li L, Zhang Y (2008) Chemical modification of Nafion membrane with 3,4-ethylene dioxythiophene for direct methanol fuel cell application. J Power Sources 175 256-260... 

Zhang J, Lan F, Liang D, Xiao Y, Lu S, Xiang Y (2011) Bulk modification of Nafion with purple membrane for direct methanol fuel cell applications. J Membr Sci 382 350-354... 

Fang Y, Wang T, Miao R, Tang L, Wang X (2010) Modification of Nafion membranes with ternary composite materials for direct methanol fuel cells. Electrochim Acta 55 2404—2408... 

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 ... 

Nunes, S., Ruffinann, B., Rikowski, E., Vetter, S. and Richau, K. 2002. Inorganic modification of proton conductive polymer membrane for direct methanol fuel cells. 

Serov AA, Cho S-Y, Han S, Min M, Chai G, Nam KH, Kwak C (2007) Modification of palladium-based catalysts by chalcogenes for direct methanol fuel cells. Elecfrochem Commun 9(8) 2041-2044... 

As reported so far, one of the best platinum-free ORR catalysts of chalcogenide-type structure is a selenium-modified mthenium catalyst (RuScx/C) [9-20], State-of-the-art catalysts are composed of carbon-supported nano-scaled ruthenium particles whose surface was modified with selenium [9-14], The modification leads to 10 times higher ORR activity, protects the ruthenium particles against electrooxidation, and suppresses the H2O2 formation. As RuSe /C is insensitive to methanol, it might be particularly suitable as an alternative cathode material in direct methanol fuel cells (DMFC) where platinum shows potential losses due to the methanol crossover [15-18]. However, ruthenium is still a costly and rare noble metal and seems not to be a feasible alternative to platinum. Therefore, readers who are interested in this type of catalyst are referred to the cited literature. 

Despite advancement in the development of direct methanol fuel cells (DMFCs), some restrictions still inhibit their large-scale commercialization. This chapter has discussed one of the primary constraints, that is, identification of appropriate membrane materials. Nafion membranes that dominate the market of polymer electrolyte membranes allow methanol permeation from the anode to the cathode side of a DMFC. This results in serious negative consequences. Three approaches have been pursued in order to resolve the methanol permeation problem. These include Nafion membranes modification, development of alternative membranes and provision of high activity anode catalysts or methanol tolerant cathode catalysts. All the three options have achieved certain degree of success in solving the problan. Of particular interest are the Nafion membranes modification and development of alternative membranes in which membranes with permeability values of 10 to 70 times lower than the pure Nafion membranes have been developed. In general, based on the tremendous research efforts being made to develop DMFCs membranes with the best qualities, we are optimistic that very soon the issue of methanol permeation shall become a history. 

Direct alcohol fuel cells (DAFC) are very attractive as power sources for mobile and portable applications. The alcohol is fed directly into the fuel cell without any previous chemical modification and is oxidized at the anode while oxygen is reduced at the cathode. Methanol has been considered the most promising fuel because it is more efficiently oxidized than other alcohols. Among different electrocatalysts tested in the methanol oxidation, PtRu-based electrocatalysts were the most active [1-3]. In Brazil ethanol is an attractive fuel as it is produced in large quantities from sugar cane and it is much less toxic than methanol. On the other hand, its complete oxidation to CO2 is more difficult than that of methanol due to the difficulty in C-C bond breaking and to the formation of CO-intermediates that poison the platinum anode catalysts. Thus, more active electrocatalysts are essential to enhance the ethanol electrooxidation [3],... 

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