Fuel cells anodic hydrogen oxidation catalysts

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

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... 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

Some key adsorbates and reaction intermediates relevant to fuel-cell anodes are H2 as the fuel, CO and CO2 as poisons in hydrogen reformate feeds, and water as a co-adsorbate and potential oxidant. In the case of the cathode, oxygen is clearly the most important reactant. In the case of a number of these molecules, such as H2, O2, and H2O, not only is the molecular adsorption important on platinum (or promoted platinum catalysts), but the dissociative adsorption of the molecules is important as well. With this in mind, some details concerning the dynamics of adsorption of these molecules, the associated dissociation barriers, molecular degrees of freedom, and energy partition are important to the overall catalytic processes. In addition to the... 

What, if any, relevance do such results have when predicting the influence of adsorbed bismuth on the CO of supported platinum nanoparticle catalysts In order to test the transferrability of results obtained on single crystals to practical fuel-cell anode catalysts, a series of experiments was performed [77] on a gas diffusion electrode of carbon-supported platinum (0.22 mg cm ) catalyst (Johnson Matthey). Figure 10 shows the results of polarization measurements for hydrogen oxidation at clean and bismuth-modified (0.65-ML) catalysts. In order to establish the CO tolerance of the electrodes, in addition to experiments involving pure H2,... 

Platinum has been the most widely used catalyst, since it (and its alloys) is the only sufficiently efficient catalyst material for oxygen reduction in low temperature (< 120 °C) fuel cells. For fuel cell anodes, Pt-Ru alloys provide better tolerance to CO in the fuel stream (hydrogen from reformed methane or methanol) and have been found to be most effective for methanol oxidation. 

One of the main challenges for the commercialization of PEFCs is the long-term stability. As was explained above, under fuel cell operation, hydrogen is oxidized at the anode while oxygen is reduced at cathode catalyst layer. 

Hydrogen is the choice of fuel for most applications due to its high reactivity with a suitable catalyst, its ability to be produced from wide range of other energy sources, and its high energy density. However, in theory, any substance capable of chemical oxidation can be used as fuel at the anode of a fuel cell. Similarly, the oxidant can be any substance that can be reduced. The oxygen is the most common oxidant as it is economically available in air [4]. 

At the fuel cell anode side, the main contaminants are CO, HjS, and NH3. These can originate from the reformate gas and hydrogen production processes. Both CO and H2S reduce fuel cell performance by strong adsorption on the Pt catalyst surface, thus poisoning the catalyst and retarding H2 oxidation. NH3 causes a decrease in membrane conductivity by forming NH and then replacing H+ in the Nafion membrane and ionomer in both anode and cathode catalyst layers. 

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