Catalysts carbon monoxide-tolerant anode

Holleck GL, Pasquariello DM, Clauson SL. Carbon monoxide tolerant anodes for proton exchange membrane (PEM) fuel cells. II. Alloy catalyst development In proceedings of the 2nd international symposium on proton conducting membrane fuel cells 1998 2 150. 

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. 

One of the drawbacks of the DMFC is that the low-temperature oxidation of methanol to hydrogen ions and carbon dioxide requires a more active catalyst, which typically means that a larger quantity of expensive platinum catalyst is required than in conventional PEMFCs. In addition, the anode has a limited carbon monoxide tolerance. Further, the overall effrdency is smaller than for a PEMFC. 

The operation of PEM at temperature higher 100 °C is receiving much attention because it could enhance reaction kinetics at both electrodes, improve the carbon monoxide tolerance of the platinum catalyst at the anode, and simplify heat and water managements of the fuel cells. 

Carbon Monoxide. Carbon monoxide, a fuel in high-temperature cells (MCFC and SOFC), is preferentially absorbed on noble metal catalysts that are used in low-temperature cells (PAFC and PEFC) in proportion to the hydrogen-to-CO partial pressure ratio. A particular level of carbon monoxide yields a stable performance loss. The coverage percentage is a function of temperature, and that is the sole difference between PEFC and PAFC. PEFC cell limits are < 50 ppm into the anode major U.S. PAFC manufacturers set tolerant limits as < 1.0% into the anode MCFC cell limits for CO and H20 shift to H2 and C02 in the cell as the H2 is consumed by the cell reaction due to a favorable temperature level and catalyst. 

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 ... 

The high-cost of materials and efficiency limitations that chemical fuel cells currently have is a topic of primaiy concern. For a fuel cell to be effective, strong acidic or alkaline solutions, high temperatures and pressures are needed. Most fuel cells use platinum as catalyst, which is expensive, limited in availability, and easily poisoned by carbon monoxide (CO), a by-product of many hydrogen production reactions in the fuel cell anode chamber. In proton exchange membrane (PEM) fuel cells, the type of fuel used dictates the appropriate type of catalyst needed. Within this context, tolerance to CO is an important issue. It has been shown that the PEM fuel cell performance drops significantly with a CO con-... 

Develop a new catalyst with a considerably lower Pt loading, to reach the 2004 DOE target of 300 xg/ cm total noble metal loading (150 pg/cm for anode) and improved carbon monoxide (CO) tolerance compared with the commercial platinum Ruthenium (Pt-Ru) catalysts. 

What we have seen is, that electrocatalysts are vital components of fuel cell systems. Much progress has been made over the years in improving their effectiveness both for anode and cathode reactions. There is nevertheless scope for considerable improvement in the performance of the electrocatalysts, particularly at the air cathode, where large activation overpotentials should be overcome. With the anode reaction also, electrocatalysts more tolerant to carbon monoxide should allow the use of less pure hydrogen and stimulate performance. There is much room for further improvement in the design of catalysts for use in fuel cells, by increasing both activity and durability. 

Carbon monoxide (CO), even at trace amounts such as a few ppm levels, can poison Pt catalysts because it can strongly adsorb on the Pt surface, leaving a very small percentage of the Pt surface (e.g., less than 5% at 80°C in the presence of 10 ppm CO) for the HOR. Pt alloys with ruthenium (Ru) and tin (Sn) possess higher CO tolerance, and are thus popular (especially PtRu) as the anode catalysts when H2 is not CO-free. The mechanisms are mainly the accelerated oxidation of CO on PtRu and the reduction of CO adsorption strength on PtSn, respectively. Reaction 1.23 shows how Ru accelerates the... 

Classical phosphoric add fuel cells use phosphoric add as the electrolyte, which is immobilized in a Teflon bonded silicon carbide matrix. Phosphoric acid fuel cells usually work at temperatures around 200 °C and are able to tolerate carbon monoxide levels of up to 2 vol.% [1]. Platinum/ruthenium as the anode catalyst may improve the performance in presence of carbon monoxide, similar to PEM fuel cells [33]. 

Molten carbonate fuel cells operate at temperatures around 650 °C and are tolerant to unlimited amounts of carbon monoxide. In most instances mixtures of lithium carbonate and potassium carbonate act as the electrolyte. The electrolyte is suspended in an insulating and chemically inert lithium aluminate ceramic. Nickel or nickel-chromium alloys serve as the anode catalysts, while nickel oxide is used as the cathode catalysts. 

This failure mechanism can have significant impact on the ability of the anode to tolerate adsorbed contaminants. Similar to the impact of carbon corrosion on the cathode, the reduced electrochemically active catalyst surface area becomes very sensitive to the presence of contaminants. This is very important, for example, for operation on reformate where even small amounts of carbon monoxide can result in significant performance loss. 

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. 

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). 

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