Reformate-Tolerant Anode Catalysts

The definition of reformate tolerance is that, compared to running on pure H2, a fuel cell stack can run on reformate and show no change in performance, apart from that expected for dilution effects (of H2 due to CO2, N2, H2O). This requires the development of reformate-tolerant anode catalysts capable of tolerating the remaining levels of CO and CO2 in the fuel feed. 

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Under reformate-feed conditions, carbon-supported PtRu alloys are widely used as reasonably reformate-tolerant anode catalysts [254-257]. However, the CO toleranee of PtRu is still unsatisfactory for the higher CO concentrations expected at system start-up or during changes in load. Moreover, the limited availabihty of... 

However, the Pt anode is seriously poisoned by trace amounts of carbon monoxide in reformates (fuel gas reformed from hydrocarbon), because CO molecules strongly adsorb on the active sites and block the HOR [Lemons, 1990 Igarashi et ah, 1993]. Therefore, extensive efforts have been made to develop CO-tolerant anode catalysts and cell operating strategies to suppress CO poisoning, such as anode air-bleeding or pulsed discharging. 

CO (6). Another approach is to develop a CO tolerant anode catalyst such as the platinum/ruthenium electrodes currently under consideration. Platinum/ruthenium anodes have allowed the cells to operate, with a low level air bleed, for over 3,000 continuous hours on reformate fuel containing 10 ppm CO (23). 

Figure 3-6 shows that performance equivalent to that obtained on pure hydrogen can be achieved using this approach. It is assumed that this approach would also apply to reformed natural gas that incorporate water gas shift to obtain CO levels of 1% entering the fuel cell. This approach results in a loss of fuel, that should not exceed 4 percent provided the reformed fuel gas can be limited to 1 percent CO(l). Another approach is to develop a CO-tolerant anode catalyst such as the platinum/ruthenium electrodes currently under consideration. Platinum/ruthenium anodes have allowed cells to operate, with a low-level air bleed, for over 3,000 continuous hours on reformate fuel containing 10 ppm CO (27). 

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. 

Carbon-supported Pt can also be used as the anode catalyst. However, this requires pure H2. Contaminants such as carbon monoxide (CO) poison the catalyst, because CO can strongly adsorb on Pt, blocking the catalytic sites and reducing platinum s catalytic activity. In H2 produced from the reforming of other fuels, CO is always present. Thus, to improve contaminant tolerance, carbon-supported PtRu was developed and now is always used as the anode catalyst. Ru can facilitate the oxidation of CO, releasing the catalytic sites on Pt through the following reactions ... 

Develop anode catalyst compositions and structures with higher reformate tolerance and/or a nonprecious metal replacement for Ru, using a catalyst deposition process that easily generates new compositions and structures. 

Studies have shown that PtMo is a more CO-tolerant catalyst than PtRu in the absence of CO2 [1]. However, in the presence of CO2, PtMo showed inferior CO-tolerance to both PtRu and Pt [1]. Therefore, PtRu is a more universal anode catalyst for reformate-type firels. 

As with the platinum anode catalyst in the PEM fuel cell, the anode of the PAFC may be poisoned by carbon monoxide in the fuel gas. The CO occupies catalyst sites. Such CO is produced by steam reforming and for the PAFC the level that the anode can tolerate is dependent on the temperature of the cell. The higher the temperature, the greater is the tolerance for CO. The absorption of CO on the anode electrocatalyst is reversible and CO will be desorbed if the temperature is raised. Any CO has some effect on the PAFC performance, but the effect is not nearly so important as in Ihe PEMFC. At a working temperature above 190°C, a CO level of up to 1% is acceptable, but some quote a level of 0.5% as the target. The methods used to reduce the CO levels are discussed in the next chapter, especially in Section 8.4.9. 

Natural gas and petroleum liquids contain organic sulphur compounds that normally have to be removed before any further fuel processing can be carried out. Even if suphur levels in fuels are below 0.2 ppm, some deactivation of steam reforming catalysts can occur. Shift catalysts are even more intolerant to sulphur (Farrauto, 2001), and to ensure adequate lifetimes of fuel processors the desulphurisation step is very important. Even if the fuel processor catalysts were tolerant to some sulphur, it has been shown that levels of only 1 ppb are enough to permanently poison a PEM anode catalyst. 

Anodes are usually very similar to, if not identical to those that serve as cathodes. Anodes that operate on reformed-hydrocarbon fuels, which contain some carbon monoxide, generally utilize a platinum-alloy catalyst to enhance co-tolerance. The catalyst-layer structure is sometimes altered between anodes and cathodes to adjust their respective hydrophobicity and reactant-diffusion properties. The thickness of the catalyst layer typically ranges from 10 to 20 tm, that of the substrate from 0.1 To 0.5 mm (uncompressed). 

CO tolerance is still an issue for residential PEFC cogeneration systems in which hydrogen-rich gas reformed from city gas or oil is utilized as a fuel for the PEFC. Although the allowable CO concentration for a Pt-Ru anode catalyst, as low as 10 ppm, is stably achieved by a recent fuel processing system (Echigo et al. 2006) and so CO tolerance is not an apparent problem for achievement of the required performance, there are quite a few reports on the long-term durability of CO tolerance. Furthermore, it has not been clarified how the addition of breed air to the fuel which is widely applied for a reformed-gas-fueled PEFC influences the long-term durability of the anode and the membrane. 

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