Ultra CO Tolerant PtMo/PtRu anodes for PEMFCs
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Ultra CO Tolerant PtMo/PtRu anodes for PEMFCs Sarah C Ball and David Thompsett Johnson Matthey Technology Centre Blounts Court Sonning Common Reading, RG4 9NH UK ABSTRACT Progress has been made in designing an anode electrode that can tolerate CO levels of several thousand ppm which are a typical output of a fuel processor consisting of a reformer and water-gas-shift unit only. The combination of a PtMo catalyst that is capable of electrochemically oxidising CO and a PtRu catalyst that can tolerate high levels of CO2 arranged in a bilayer configuration, has been shown to tolerate 2000 ppm CO in H2 with a relatively small loss in fuel cell performance. Further improvements in the ability to tolerate higher levels of CO with lower performance losses are expected due to improved PtMo catalyst design.
INTRODUCTION Difficulties associated with the storage and distribution of hydrogen means that the first generation of commercially available Proton Exchange Membrane Fuel Cells (PEMFCs) for either stationary or transportation use, are likely to operate using a fuel processor that converts hydrocarbons to hydrogen-rich fuel. A key feature to the successful introduction of PEMFCs will be the effectiveness of the fuel cell stack to tolerate the impurities present in hydrogen derived from reformed hydrocarbons such as methane and gasoline. However, existing present day anode catalyst technology based on PtRu bimetallic particles cannot tolerate even relatively low levels of CO (e.g. > 50 ppm) without significant performance loss at current operating temperatures (c. 80oC). This low tolerance to CO by the fuel cell places a demand on the fuel processor system to produce hydrogen feeds with low CO levels. In general, primary reforming of fuels such as methane produce a hydrogen rich gas containing up to 10% CO. The use of the water-gas-shift reaction can reduce this down to 0.5% CO, as well as increasing the hydrogen concentration. However, the lowering of CO down to ppm levels requires either further selective oxidation or methanation steps. Although effective in lowering CO levels to c. 10 ppm, these extra stages do increase system complexity, system weight and volume, as well as consuming hydrogen. Complementary to this technology is the further use of selective oxidation at the electrocatalyst level within the Membrane Electrode Assembly (MEA) itself. The addition of a low concentration of air into the fuel stream has the effect of further reducing the CO levels to a few ppm. However, this practise is limited to CO levels of less than 100 ppm, due to the exothermic nature of the CO oxidation reaction. It has been shown previously that anode performance can be degraded with air bleed operation over time. This effect can be alleviated by the presence of a separate selective oxidation layer within the anode electrode structure, however higher CO levels cannot be successfully lowered as the catalysts become inhibited with CO at these low temperatures [1].
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One alternative approach to dealing with the presence
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