Direct methanol fuel cells active

PEMFC)/direct methanol fuel cell (DMFC) cathode limit the available sites for reduction of molecular oxygen. Alternatively, at the anode of a PEMFC or DMFC, the oxidation of water is necessary to produce hydroxyl or oxygen species that participate in oxidation of strongly bound carbon monoxide species. Taylor and co-workers [Taylor et ah, 2007b] have recently reported on a systematic study that examined the potential dependence of water redox reactions over a series of different metal electrode surfaces. For comparison purposes, we will start with a brief discussion of electronic structure studies of water activity with consideration of UHV model systems. 

The Pt/Ru catalyst is the material of choice for the direct methanol fuel cell (DMFC) (and hydrogen reformate) fuel cell anodes, and its catalytic function needs to be completely understood. In the hrst approximation, as is now widely acknowledged, methanol decomposes on Pt sites of the Pt/Ru surface, producing chemisorbed CO that is transferred via surface motions to the active Pt/Ru sites to become oxidized to CO2... 

The same group, in a previous work, reported on the realization of a hybrid anode electrode [197]. An appreciable improvement in methanol oxidation activity was observed at the anode in direct methanol fuel cells containing Pt-Ru and Ti02 particles. Such an improvement was ascribed to a synergic effect of the two components (photocatalyst and metal catalyst). A similar behavior was also reported for a Pt-Ti02-based electrode [198]. Another recent study involved the electrolysis of aqueous solutions of alcohols performed on a Ti02 nanotube-based anode under solar irradiation [199]. 

Despite the uncertainty regarding the exact nature of the active site for oxygen reduction, researchers have managed to produce catalysts based on heat-treated macrocycles with comparable activities to state-of-the-art platinum catalysts. In numerous cases researchers have shown activity close to or better than platinum catalysts.64,71,73,103,109 Since the active site for the ORR in these materials is not fully understood, there is still potential for breakthrough in their development. Another advantage of this class of materials that should be mentioned is their inactivity for methanol oxidation, which makes them better suited than platinum for use in direct methanol fuel cell cathodes where methanol crossover to the cathode can occur.68,102,104,122-124 While the long-term activity of heat treated materials is... 

Lee, S-A. et al., Nanoparticle synthesis and electrocatalytic activity of Pt alloys for direct methanol fuel cells, J. Elect. Chem. Soc., 149, A1299, 2002. 

Fuel cells o fer important advantages as a power source, such as the potential for high efficiency, clean exhaust gases and quiet operation. In addition, the direct methanol fuel cell offers special benefits as a power source for transportation, such as potential high energy density, no need for a fuel reformer and a quick response. These advantages, however, have not been fully realized yet. One of the problems is the poor performance of the fiiel electrode. Even platimun, which seems the most active single element for methanol oxidation in add media, loses its electrocatalytic activity rapidly by the accumulation of adsorbed partially oxidized products. 

Anode Investigations using cyclovoltammetry confirm an important effect of surface oxides (see Vols. 3, 4). A known example of the different anodic activity is the poisoning of platinum by adsorbed carbon monoxide species, for example, in the direct methanol fuel cell (DMFC),... 

Another important future area for diffusion layers is the use of three-dimensional catalyzed diffusion layers for liquid-based fuel cells. This allows the three-phase active zone to be extended into the diffusion layer to increase performance and utilization and reduce crossover [276,277]. Recent work by Lam, Wilkinson, and Zhang [278] has shown the scaleable use of this concept to create a membraneless direct methanol fuel cell. In other work by Fatih et al. [279], the... 

There is increased interest in the use of Ru-based systems as catalysts for oxygen reduction in acidic media, because these systems have potential applications in practicable direct methanol fuel cell systems. The thermolysis of Ru3(CO)i2 has been studied to tailor the preparation of such materials [123-125]. The decarbon-ylation of carbon-supported catalysts prepared from Ru3(CO)i2 and W(CO)6, Mo(CO)is or Rh(CO)is in the presence of selenium has allowed the preparation of catalysts with enhanced activity towards oxygen reduction, when compared with the monometallic ruthenium-based catalyst [126],... 

Pt-based electrocatalysts are usually employed in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMSC). In direct-methanol fuel cells (DMFCs), aqueous methanol is electro-oxidized to produce COj and electrical current. To achieve enhanced DMFC performance, it is important to develop electrocatalysts with higher activity for methanol oxidation. Pt-based catalysts are currently favored for methanol electro-oxidation. In particular, Pt-Ru catalysts, which gave the best results, seem to be very promising catalysts for this application. Indeed, since Pt activates the C-H bounds of methanol (producing a Pt-CO and other surface species which induces platinum poisoning), an oxophilic metal, such as Ru, associated to platinum activates water to accelerate oxidation of surface-adsorbed CO to... 

Recent work by Lukehart et al. has demonstrated the applicability of this technique to fuel-cell catalyst preparation [44g,h]. Through the use of microwave heating of an organometallic precursor that contains both Pt and Ru, PtRu/Vulcan carbon nanocomposites have been prepared that consist of PtRu alloy nanoparticles highly dispersed on a powdered carbon support [44g]. Two types of these nanocomposites containing 16 and 50 wt.% metal with alloy nanoparticles of 3.4 and 5.4 nm, respectively, are formed with only 100 or 300 s of microwave heating time. The 50 wt.% supported nanocomposite has demonstrated direct methanol fuel-cell anode activity superior to that of a 60 wt.% commercial catalyst in preliminary measurements. 

Besides the effects of the typical carbon functional groups, the role of nitrogen and sulfur functionalities, introduced on carbons by chemical and thermal treatments, on the electrochemical performance of Pt catalysts for oxygen reduction in direct methanol fuel cells was investigated [47]. Once again, the metal-support interaction influences the size and chemical state of platinum particles and, as a consequence, the electrocatalytic activity. The introduction of nitrogen and sulphur functionalities was reported to improve the catalytic activity, but this result was mainly ascribed to the Pt particle size. 

Palladium electroless deposition was used for coating of Nafion 117 for the application as a membrane in direct methanol fuel cell.59 After the activation of the polymer membrane (Nafion 117) with Pd(II) complexes, reduction was carried out with sodium boro-hydride, NaBH4. Further, autocatalytic deposition of palladium was performed using a commercially available solution. Compared to bare Nafion, the Nafion/Pd composites considerably reduced methanol crossover. This resulted in enhanced cell performance, which was attributed to the existence of the Pd layer at the surface of polymer. 

The only deviation from this pattern is the DMFC (direct methanol fuel cell) which uses methanol as a fuel without intermediate reforming and microbial fuel cells that use sugar as a fuel and derive current from the metabolic activity of yeast. Both types use a solid ion exchange membrane type electrolyte (proton exchange membrane). 

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. 

C. K. Witham, T. I. Valdez, and S. R. Narayanan, Methanol Oxidation Activity of Co-Sputter Deposited Pt-Ru Catalysts , Proceedings of the Symposium on Direct Methanol Fuel Cells, Electrochemical Society, PV 2001-4, p.l23... 

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