Anodic Oxidation of Methanol

The first studies of methanol oxidation s special features and of the kinetics and mechanism of anodic methanol oxidation at platinum electrodes began in the early 1960s, in the period known as the first boom of work in fuel cells. In the years after that, this reaction was the subject of countless studies by many groups in different countries. In summary one can say of all this work that, by now, the mechanism of this reaction has been established rather reliably (for reviews see Bagotsky et al., 1977 Iwasita and Vielstich, 1990 Kauranen et al., 1996), while conflicting views persist on certain detailed aspects. Work on these questions is continuing even now. 

The major reaction product is carbon dioxide, but in certain cases, the transient production of small amounts of other oxidation products, such as formaldehyde, formic acid, and so on, is seen. Six electrons are given off in the complete oxidation of methanol to carbon dioxide, so that the specific capacity of methanol is close to 0.84 Ah/g. 

Methanol oxidation is a reaction with several consecutive stages. At the first stage, the methanol molecules undergo dehydrogenation  

At the next stage, these species are oxidized by way of chemical interaction with oxygen-containing species OH ds adsorbed on neighboring sites of the platinum surface  

Two possibilities exist for the use of methanol in the fuel supply for fuel cells (1) its prior catalytic or oxidizing conversion to technical hydrogen, and (2) its direct anodic oxidation at the electrodes in the fuel cell. The former possibility implies that additional unwieldy equipment for the conversion of methanol to technical hydrogen and for subsequent purification of this hydrogen is needed. The second possibility is more attractive but involves certain difficulties related to the relatively slow anodic oxidation of methanol even at highly active platinum electrodes. In the face of these difficulties, much attention is given at present, to the development of such direct oxidation methanol fuel cells. 

The electrode reactions taking place at the electrodes of DMFCs, the overall current-producing reactions and the corresponding thermodynamic values of equilibrium electrode potentials and EMF of the fuel cell are as follows  

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The direct anodic oxidation of methanol became much more attractive after it was shown that platinum-ruthenium alloys are catalytically much more active in this reaction than pure platinum (pure ruthenium is totally inactive in this reaction). 

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... 

In the late 1960s it was discovered (Entina, 1968 Binder et al., 1972) that a strong synergy effect exists in the platinum-ruthenium system. Alloys of these two metals are two to three orders of magnitude more active catalytically for the anodic oxidation of methanol than pure platinum, whereas pure ruthenium is altogether inactive for this reaction. Prolonged exploitation of such anodes is attended by a gradual decrease in catalytic activity of the alloys because of slow anodic dissolution of ruthenium from the surface layer. A similar simation is seen for platinum-tin alloys, which... 

Ota K-I, Nakagawa Y, Takahashi M. 1984. Reaction products of anodic oxidation of methanol in sulfuric acid solution. J Electroanal Chem 179 179-186. 

M. Morita, Y. Iwanaga, and Y. Matsuda, Anodic-oxidation of methanol at a gold-modified platinum electrocatalyst prepared by RE-sputtering on a glassy-carbon support, Electrichim. Acta 36, 947-951 (1991). 

Combined with methanol crossover, slow anode kinetics lead to a power density of a DMFC that is three to four times lower than that of a hydrogen fuel cell. Much work has been focused on the anodic oxidation of methanol. The mechanism of the... 

The Cat, or its product of electrode oxidation or reduction Cat, is immobilized at the electrode surface and decreases the overpotential for oxidation or reduction of the S, without being involved in the chemical redox reaction with the S. Typical example is the catalytic effect of underpotential deposited layer of lead on a platinum electrode, on anodic oxidation of methanol [v]. 

One of the drawbacks of DMFCs is the relatively slow rate of the anodic oxidation of methanol even on highly active platinum electrodes. It was shown that the Pt-Ru system is much more catalytically active than pure platinum (pure ruthenium is inert towards this reaction) (-> platinum-ruthenium -> electrocatalysis). The so-called bifunctional mechanism is broadly accepted to describe this synergistic effect, according to which organic species are chemisorbed predominantly on platinum centers while ruthenium centers more readily adsorb species OH, required for the oxidation of the organic intermediates. Usually the composition of such alloys is Pto.sRuo.s and the metal loading is 2-4 mg cm-2. 

In this section, recent advances in the field of polymer electrolyte direct methanol fuel cells, i.e., PEFCs based on direct anodic oxidation of methanol are discussed. A schematic of such a ceU is shown in Fig. 48, together with the processes that take place in the cell. The DMFC has many facets, electrocatalysis materials and components which deserve a detailed treatment. The discussion here will be confined, however, to the very significant performance enhancement demostrated recently with polymer electrolyte DMFCs, and, as a result, to possible consideration of DMFCs as a nearer term technology. 

K.-I. Machida, A. Fukuoka, M. Ichikawa, M. Enyo, Preparation of chemically modified electrodes attachement of platinum carbonyl clusters, and their efficient electrocatalytic action in anodic oxidation of methanol. J. Chem. Soc. Chem. Commun. 1987, 1486-1487. 

At high potentials S04 anions cover the surface of anode which hinders the neutral methanol from reaching anode surface and hence at high potentials lower currents are observed. It is clear from Fig.2 that for a given concentration of electrolyte as methanol concentration increases anodic oxidation of methanol increases and for a given concentration of methanol as electrolyte concentration increases anodic oxidation of methanol increases. Similar behaviour is seen for sequentially deposited Pt-Ru- WO3/C electrode. 

The PEMFC (see Table 17.2 for identification) has the greatest potential to reach high power densities. DMFCs suffer from the high activation potential of the cathodic reduction of oxygen and anodic oxidation of methanol. MCFCs operate at 650°C and SOFCs at 1000°C, their electrolytes being, respectively, molten carbonates and solid metal oxides. Their activation overpotentials are small, but ohmic overpotentials at the... 

Rauhe BR, Mclamon FR, Cairns EJ (1995) Direct anodic-oxidation of methanol on supported platinum ruthenium catalyst in aqueous cesium carbonate. J Electrochem Soc 142(4) 1073-1084... 

In the 1920 s, E. MQller and his co-workers made a series of studies on the anodic oxidation of methanol, formaldehyde, and formic acid which represent the first extensive mechanistic investigation of these compounds, although the principles of electrode kinetics had not yet been formulated. Muller did not establish mechanisms for these reactions however, many of his observations have been later confirmed and his studies were among the first with a comparison of polarization curves on several noble metals including platinum, palladium, rhodium, iridium, osmium, rubidium, gold, and silver (cf. Figure 1). As was usual at that time, Muller discussed his results in terms of polarization, rather than in terms of current or reaction rate. 

None of the mechanisms suggested for the anodic oxidation of methanol can explain all of the observed kinetic parameters and a definite mechanism cannot at present be established. 

Considerable effort was directed toward finding efficient co-catalysts or promoters that, together with Pt, could enhance the rate of anodic oxidation of methanol and formic acid. The oxidation of ethanol received comparatively less attention in the 1970s and 1980s, most likely due to the more complex electrode kinetics and catalysis imposed by the C-C bond. 

The role of organometallie eomplexes as co-catalysts with Pt for methanol electrooxidation was mentioned earlier. Can they also act alone and replace Pt completely For the anodic oxidation of methanol, ethanol, and formic acid, studies looking at CO oxidation by various porphyrin complexes with Ir, Rh, and Co in aqueous electrolytes (both acid and alkaline) are relevant [90, 203, 204]. The mechanism proposed by Shi and Anson for the activity Co-octaethylporphyrin considers the oxidation of Co(II) to Co(III) in conjunction with coordination of CO to the Co(III) centres, followed by nueleophilic H2O attack leading to catalytic oxidation forming CO2 (see Equations 4.15-4.18) [90]. 

Sandstede G, Binder H, Koehling A. Electrodes, especially for the anodic oxidation of methanol. France patent 1481338. 1967. 

Shen PK, Tseung ACC. Anodic oxidation of methanol on Pt/W03 in acidic media. J Electrochem Soc 1994 141 3082-90. 

Liao S, Hohnes K-A, Tsaprailis H et al (2006) High performance PtRuIr catalysts supported on carbon nanotubes for the anodic oxidation of methanol. J Am Chem Soc 128 3504—3505... 

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