Electrode Kinetics of the Anode Reaction

The electro-oxidation of methanol to carbon dioxide involves the transfer of six electrons, and it is highly unlikely that these electrons will transfer simultaneously. It is also unlikely that partial electron transfer will lead to a range of stable solution intermediates. Clearly, surface-adsorbed species must be present on the platinum catalyst surface throughout its useful potential range, and these species must be responsible for the poor catalytic activity of platinum towards methanol electro-oxidation.

The postulated mechanisms for methanol electro-oxidation were reviewed comprehensively by Parsons and Vandernoot (1988) and can be summarized as follows.

Step 1: Electrosorption of methanol onto the catalyst surface to form carbon-containing intermediates Step 2: Addition of oxygen (from water) to the electrosorbed carbon-containing intermediates to generate CO2

This corresponds to the following electrochemical reactions:

With respect to the first process (step 1), very few materials are able to electrosorb methanol. In acidic electrolytes, only platinum-based electrocatalysts have shown the required activity and chemical stability. The adsorption mechanism is believed to take place through the sequence of steps shown in Fig. 7.5 (Kazarinov et al., 1975; Mundy et al., 1990; Christensen et al., 1990). The mechanism shows the elec-trosorption of methanol on the surface of platinum with sequential proton and electron stripping, leading to the main catalyst poison, linearly bonded carbon monoxide (Pt-CO). Subsequent reactions are believed to involve oxygen transfer to the Pt-CO species to produce CO2.

The most recent work has been covered by Burstein et al. (1997), Anderson and Grantscharova (1995), Chrzanowski et al. (1998), Arico et al. (1994), and Liu et al. (1998).

At potentials below about 450 mV, the surface of pure platinum is poisoned by a layer of strongly bonded COads. Further electrosorption of methanol cannot take place until the surface-bound COads is oxidized to CO2, which desorbs from the platinum surface. At potentials below about 450 mV, this process occurs at an insignificant rate (compare CO poisoning in H2 PEMFCs — Chapter 6) and hence, the surface of the pure platinum remains poisoned throughout its useful (low) potential range. This has led to an intensive search for alternative materials that can electro-oxidize methanol at lower overpotentials, and in particular materials that might combine with platinum to promote the above processes (Eqs. 7.4 and 7.5). A number of possible explanations may account for the enhanced activities seen for some of these advanced materials. The most likely are:

1. The binary metal element (e.g., ruthenium) modifies the electronic properties of the catalyst, weakening the chemical bond between platinum and the surface intermediate (intrinsic effect).

2. The binary element (e.g., ruthenium, tin, lead, or rhodium) is unstable and leaches out of the alloy to leave a highly reticulated and active surface. This leads to a higher number of extended step sites, which have been associated with the methanol electrosorption process. In addition, these low coordination sites may be much more easily electro-oxidized, giving rise to Pt-OHads species at potentials far below that at which planar platinum is oxidized.

3. The binary metal element (e.g., ruthenium, tin, or tungsten) is able to provide an adjacent platinum site with -OHads through a spillover process (promotion effect). Hence, the catalytic activity is governed by the potential at which the binary metal elevctro-oxidizes and delivers OHads to adjacent platinum sites. For materials such as Ru, this occurs at significantly lower potentials (<250 mV) than is possible on a platinum surface (Gasteiger et al., 1994; Franaszczuk and Sobkowski, 1992; Hamnett and Kennedy, 1988; Ticanelli et al., 1989). By virtue of this process, at present the most active methanol electro-oxidation catalysts are based on Pt-Ru alloy materials.

FIGURE 7.5 Methanol electrosorption mechanism in H2SO4 on pure Pt surfaces.

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