Direct methanol fuel cell catalytic processes

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. 

A serious candidate for transportation application is also the direct methanol fuel cell (DMFC) which has been realized already on a laboratory scale. A catalytic burner is requited to evaporate the methanol/water mixture and to bum the exhaust gas at the anode [43]. Considering the complete energy chain, a PEFC is by 50 % more efficient than a diesel engine which consumes 4 1 per 100 km this is also valid for a natural gas driven engine [37]. Fig. 7-6 presents the processing schematics of both IMFC and DMFC. The DMFC offers a much simpler system than the PEFC. The DMFC is currently at an early development stage. It is perceived to offer improved solutions to the need for a small-scale power supply. A program for the construction of a 30 kW stack has recently started [29]. 

Comprehensive EXAFS reviews on catalysis have been recently published. These encompass studies concerning surface mediated electrochemical processes and catalytic solids under environmental conditions. The first set of studies examined the local structural changes in the metal occurring in tandem with redox processes under voltage control. " Recent work in this area examines the stability of bimetallic Pt-Ru electrodes and their performance in the hydrogen oxidation reaction in the presence of CO or direct methanol fuel cells. Studies reviewed involving supported metal catalysts concern, in the first place, the reduction process " while other studies also consider the oxidation process and reduction under inert gases. All of these studies make an attempt to understand the relevance of the metal support interface... 

The generated potential via exergonic reactions in direct fuel cells is partly used to promote electrode reactions (kinetic overpotential), while in indirect fuel cells, the fuel is first processed into simpler fuels (to reduce the kinetic overpotential) via conventional catalytic reactors, or CMRs, in which temperature is used as the key operating parameter to accomplish the desired kinetics and equilibrium conversions. Among the direct fuel cells, e.g., PEMFCs use hydrogen, direct methanol fuel cell (DMFC) uses methanol, while the SOFC can operate directly on natoral gas (Figiue 15.3). However, in indirect fuel cells, a complex fuel must be suitably reformed into simpler molecules such as H2 and CO before it can be used in a fuel ceU. 

Direct Methanol Fuel Cell (DMFC) is very attractive as energy source for portables, mobiles and stationary applications. PtRu/C electrocatalyst has been considered the best electrocatalyst and the catalytic activities depend on the preparation method [1]. Studies have been shown that the use of carbon nanotubes and mesoporous carbon as support increase the performance of the PtRu/C eleetrocatalysts, however, the synthesis of these supports are normally complex or involve harsh conditions. Recently, the synthesis of metal/carbon nanoarchitectures by a one-step and mild hydrothermal carbonization process was reported using starch or glucose and metals salts [2]. We have studied the synthesis of PtRu/C eleetrocatalysts by hydrothermal carbonization and focused especially on the effects of different carbon somces on the electrocatalytic performance for methanol oxidation. 

The electrochemical oxidation of methanol has been extensively studied on pc platinum [33,34] and platinum single crystal surfaces [35,36] in acid media at room temperature. Methanol electrooxidation occurs either as a direct six-electron pathway to carbon dioxide or by several adsorption steps, some of them leading to poisoning species prior to the formation of carbon dioxide as the final product. The most convincing evidence of carbon monoxide as a catalytic poison arises from in situ IR fast Fourier spectroscopy. An understanding of methanol adsorption and oxidation processes on modified platinum electrodes can lead to a deeper insight into the relation between the surface structure and reactivity in electrocatalysis. It is well known that the main impediment in the operation of a methanol fuel cell is the fast depolarization of the anode in the presence of traces of adsorbed carbon monoxide. 

The left half of Figure 15.3, thus, depicts the catalytic processes that different fuels imdergo, such as reforming, high-temperature shift and low-temperatore shift, and preferential oxidation (PrOx), which are individually conducted at different temperatures so as to make the most of their thermodynamic limitations and kinetics. In direct fuel ceUs with relatively complex fuels, such as methanol, in fact, many of these reaction steps occur electrocatalylicaUy within the fuel cell, thus accounting for the large oveipotentials, e.g., in a DMFC. 

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