Methanol in fuel cell

The interest in the use of methanol in fuel cell has re-activated fundamental and applied research in the last decade. Approaching the conditions for practical systems, Hamnett and coworkers have obtained FTIR spectra during the adsorption of methanol on finely divided Pt catalysts deposited on graphite [96]. These authors suggest that the activity of the fine-particle catalyst toward the dehydrogenation of methanol is enhanced. Correspondingly, spectra obtained using this catalyst show the formation of adsorbed carbon monoxide at potentials as low as 50 mV vs. RHE. 

Emerging energy focusing on the use of methanol in fuel cells, Methanex has entered into strategic alliances with Petro-Canada and Statoil ... 

In the above reactions, the oxidation process takes place in the anode electrode where the methanol is oxidized to carbon dioxide, protons, and electrons. In the reduction process, the protons combine with oxygen to form water and the electrons are transferred to produce the power. Figure 9-1 is a reaction scheme describing the probable methanol electrooxidation process (steps i-viii) within a DMFC anode [1]. Only Pt-based electrocatalysts show the necessary reactivity and stability in the acidic environment of the DMFC to be of practical use [2], This is the complete explanation of the anodic reactions at the anode electrode. The electrodes perform well due to the presence of a ruthenium catalyst added to the platinum anode (electrode). Addition of ruthenium catalyst enhances the reactivity of methanol in fuel cell at lower temperatures [3]. The ruthenium catalyst oxidizes carbon monoxide to carbon dioxide, which in return helps methanol reactivity with platinum at lower temperatures [4]. Because of this conversion, carbon dioxide is present in greater quantity around the anode electrode [5]. 

Yet, using methanol in fuel cells is associated with certain difficulties and inconvenient features ... 

Interest in fuel cells has stimulated many investigations into the detailed mechanisms of the electrocatalytic oxidation of small organic molecules such as methanol, formaldehyde, formic acid, etc. The major problem using platinum group metals is the rapid build up of a strongly adsorbed species which efficiently poisons the electrodes. 

Figure 2. Current-voltage characterization of the 40-module methanol-air lliel cell developed by Allis-Chalmers. (From J. N. Murray and P. G. Grimes in Fuel Cells, p. 57, 1963 reproduced with permission of the AICHE).

Formates and carbon monoxide are products of a shallow reduction of carbon dioxide (two electrons per one CO2 molecule). In the last few years, much attention has been paid the problem of obtaining products of deeper reduction (e.g., methanol or methane) which may be used as fuels in engines or in fuel cells. 

Very early during research into the anodic oxidation of methanol in the 1960s, it was repeatedly attempted to build experimental models of methanol-oxygen or methanol-air fuel cells. Most of these studies were conducted in snlfnric acid solntions... 

Gottesfeld, S., and T. A. Zawodzinski, Direct methanol oxidation fuel cells from a 20th century electrochemist s dream to a 21st century emerging technology, in Electrochemical Science and Engineering, R. C. AUdre et al., Eds., Vol. 5, WUey, New York, 1988. 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

Leger JM, Rousseau S, Coutanceau C, Hahn E, Lamy C. 2(X)5. How bimetallic electrocatalysts does work for reactions involved in fuel cells Example of ethanol oxidation and comparison to methanol. Electrochim Acta 50 5118-5125. 

An opportunity exists to apply to the study of reducing agent reactions at surfaces some of the analytical techniques successfully used to study the intermediates and poisons in fuel cell reactions, e.g. methanol. 

One way to illustrate the effect of the EDL is to compare in situ electrochemical reactions with their equivalent UHV counterparts. Due to their roles in fuel cells, the methanol oxidation reaction and the oxygen reduction reaction are two such reactions for which numerous in situ and UHV experiments have been performed. 

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