Reformate Tolerant Electrocatalysts

A consequence of the steam reformation process and the subsequent clean up steps of high and low temperature shift converters and a selective oxidizer (called the prox unit) the typical levels of CO at the inlet stream of a PEMFC s are expected to be in the range of 50 and 100 ppm, higher levels in the range of 500 to 

000 ppm is expected with partial or full absence of selective oxidizer. Besides CO, the major constituent of the reformate gas outlet is CO2. The levels of CO2 however can vary between 35-40% (dry gas composition) for the high temperature iso-octane system (gasoline) and 75 % (dry gas composition) for the relatively low temperature methanol reformation. The anode feed streams therefore typically contains a mole fraction of hydrogen in the range of 0.4 to 0.75, with CO in the range of 50-100 ppm and CO2 making up the balance. 

The central issues related to reformate tolerance as pointed out recently by us are  

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Cooper SJ, Gunner AG, Hoogers G, Thompsett D. Reformate tolerance in proton exchange membrane fuel cells electrocatalyst solutions. In Savadogo O, Roberge PR, editors. Proceedings of the second international symposium on new materials for fuel cell and modem battery systems 1997 July 6-10 Montreal Ecole Polytechnique de Montreal, 1997 286-96. 

A viable electrocatalyst operating with minimal polarization for the direct electrochemical oxidation of methanol at low temperature would strongly enhance the competitive position of fuel ceU systems for transportation appHcations. Fuel ceUs that directiy oxidize CH OH would eliminate the need for an external reformer in fuel ceU systems resulting in a less complex, more lightweight system occupying less volume and having lower cost. Improvement in the performance of PFFCs for transportation appHcations, which operate close to ambient temperatures and utilize steam-reformed CH OH, would be a more CO-tolerant anode electrocatalyst. Such an electrocatalyst would reduce the need to pretreat the steam-reformed CH OH to lower the CO content in the anode fuel gas. Platinum—mthenium alloys show encouraging performance for the direct oxidation of methanol. 

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

Commercial application of H2 is generally based on the gas obtained by hydrocarbons reforming and therefore, it is necessary to dispose of an anode electrocatalyst able to tolerate a limited amoimt of CO, either rmder steady state operation or under transient conditions of high CO content. In addition, CO2 adversely affects the catalyst performance, because a reverse gas-shift reaction yielding CO could take place depending on the fuel composition [42]. It seems that Pt-Ru better tolerates CO2 than Pt [43]. The preferred electrocatalysts for HOR in the presence of CO include Pt and several Pt alloys or Pt mixtures with other noble or non-noble metals [44]. The most used include ruthenium, molybdenum and tin. 

Figure 12.12. Representative CO-tolerance hydrogen oxidation polarization for the six binary systems studied. The measurements were made with (gray hnes) 10 ppm and (black lines) 50 ppm CO in reformate fuel (40% H2, 21% CO2, 39% N2) [25]. (Reproduced by permission of ECS—The Electrochemical Society, from Journal of the Electrochemical Society, Yang R, Bonakdarpour A, Bradley Eaton E, Stoffyn-Egli P, Dahn JR. Characterization and PEMFC testing of Pti- M (M=Ru,Mo,Co,TaAu,Sn) anode electrocatalyst composition spreads.)...

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. 

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