Methanol oxidation reaction anode electrocatalysts

Research on the direct conversion of chemical energy to electricity via fuel cells has received considerable attention in the past decades. Fuel cells are indeed attractive alternatives to combustion engines for electrical power generation in transportation applications and also as promising future power sources, especially for mobile and portable applications. Thus, the search for excellent electrocatalysts for the electro-catalytic oxygen reduction and methanol oxidation reactions, which are the two important cathodic and anodic reactions in fuel cells, is intensively pursued by scientists... 

Figure 12.13. (a) Peak current densities and (b) onset potentials of methanol oxidation reaction electrocatalyzed by the combinatorial (A) Pt-Ru-W and (B) Pt-Ru-Co systems. [22]. (Reprinted from Journal of Power Sources, 163(1), Cooper JS, McGinn PJ. Combinatorial screening of thin film electrocatalysts for a direct methanol fuel cell anode, 330-8, 2006, with permission from Elsevier.)... 

Due to the faeile poisoning effect of CO on Pt, many Pt-based binary alloys, such as Pt-Ru, Pt-Os, Pt-Sn, Pt-W, Pt-Mo, and so on, have been investigated as electrocatalysts for the methanol oxidation reaction (MOR) on flic DMFC anode. Among them, the Pt-Ru alloy has been found to be the most active binary alloy catalyst, and is commonly used in state-of-the-art DMFCs [32]. 

Role of Anode Electrocatalysts—Methanol Oxidation Reaction (MOR)... 

This chapter summarized the fundamental aspects and recent advances in electrocatalysts for the oxidation reactions of methanol and ethanol occurring at fuel cell anodes. Pt-based electrocatalysts are still considered to be the most viable for the anodic reactions in acidic media. The major drawback, however, is the price and limited reserves of Pt. To lower the Pt loading, the core-shell structure comprising Pt shells is more beneficial than the alloy structure, since aU the Pt atoms on the nanoparticle surfaces can participate in the catalytic reactions (and those in cores do not) particularly, the Pt submonolayer/monolayer approach would be an ultimate measure to minimize the Pt content. The architectures in nanoscale also have a significant effect on the reactivity and durability [119, 120] and thus should be explored continuously in a future. As for the ethanol oxidation, Rh addition is shown to enhance the selectivity towards C-C bond splitting however, Rh is even more expensive than Pt, and thus, the Rh content has to be very low, or less expensive constituents replacing Rh are necessary to be found. 

Fluang et al. carried out in situ solution electrochemical C NMR spectroscopy to study the reactions on commercial fuel cell electrocatalysts (Pt and PtRu blacks) used for ethanol oxidation reaction. It was concluded that the complete oxidation of ethanol to CO2 only took place dominandy at the very beginning of a potentiostatic chronoamperometric measurement and the PtRu had a much higher activity in catalyzing oxygen insertion reaction that leads to acetic acid [202]. Han et al. investigated the electrochemical oxidation of methanol on Pt and PtRu anode catalysts using in situ NMR spectroscopy and revealed the role of Ru in both Faradaic and non-Faradaic reactions and the spatial distributions of chemicals [203]. 

So far, the Pt-Ru alloy has shown the most promising performance for the oxidation of methanol and hydrogen oxidation reaction in the presence of CO. Carbon black has been used as support for the metal nanoparticles, particularly Vulcan XC-72 (Cabot), which has a surface area of 240 m g . Methanol oxidation starts at lower potential values for all the Pt-Ru/C catalysts than for Pt/C anodes. Comparison of electrodes prepared with Pt and Pt-Ru as the electrocatalyst supported on nanotubes and those prepared with the most usual support, Vulcan XC-72 showed that multi-wall carbon nanotubes produce catalysts (Pt-Ru/MWNT) with better performance than on other supports, particularly with respect to those prepared with the traditional Vulcan XC-72 carbon powder [18],... 

In the case of direct methanol fuel cells, compared with oxygen reduction, methanol oxidation accounts for the main activation loss because this process involves six-electron transfer per methanol molecule and catalyst self-poison when Pt alone was used from the adsorbed intermediate products such as COads-From the thermodynamic point of view, methanol electrooxidation is driven due to the negative Gibbs free energy change in the fuel cell. On the other hand, in the real operation conditions, its rate is obviously limited by the sluggish reaction kinetics. In order to speed up the anode reaction rate, it is necessary to develop an effective electrocatalyst with a high activity to methanol electrooxidation. Carbon-supported (XC-72C, Cabot Corp.) PtRu, PtPd, PtW, and PtSn were prepared by the modified polyol method as already described [58]. Pt content in all the catalysts was 20 wt%. 

In Table 1 the ancxlic reactions that have been studied so far in small cogenerative solid oxide fuel cells are listed. One simple and interesting rule which has emerged from these studies is that the selection of the anodic electrocatalyst for a selective electrocatalytic oxidation can be based on the heterogeneous catalytic literature for the corresponding selective catalytic oxidation. Thus, the selectivity of Pt and Pt-Rh alloy electrocatalysts for the anodic NH3 oxidation to NO turns out to be comparable (>95%) to the selectivity of Pt and Pt-Rh alloy catalysts for the corresponding commercial catalytic oxidation. The same applies for Ag, which turns out to be equally selective as an electrocatalyst for the anodic partial oxidation of methanol to formaldehyde, ... 

For these low-temperature fuel cells, the development of catalytic materials is essential to activate the electrochemical reactions involved. This concerns the electro-oxidation of the fuel (reformate hydrogen containing some traces of CO, which acts as a poisoning species for the anode catalyst methanol and ethanol, which have a relatively low reactivity at low temperatures) and the electroreduction of the oxidant (oxygen), which is still a source of high energy losses (up to 30-40%) due to the low reactivity of oxygen at the best platinum-based electrocatalysts. 

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