The proton exchange membrane fuel cell performance of carbon-supported Pt, PtRu, and PtMo catalysts on reformate gas mixtures was investigated. The catalysts were tested as low Pt loaded electrodes in membrane electrode assemblies at 80°C using H 2 ; 10, 40, 100 ppm CO in H 2 ; 25% CO 2 in H 2 , and 40 ppm CO/25% CO 2 in H 2 reformate gas mixtures as the fuel stream. The PtMo catalyst showed better CO tolerance than the PtRu catalyst at 100 ppm, but showed poorer CO 2 tolerance to both Pt and PtRu. With 40 ppm CO/25% CO 2 in H 2 , PtMo showed inferior overall reformate tolerance to PtRu.The effective operation of proton exchange membrane fuel cells ͑PEMFCs͒ with impure H 2 , produced by the reforming of hydrocarbons or oxygenated hydrocarbons, depends on the use of anode electrocatalysts that are tolerant to the major contaminants in the resulting reformate ͑i.e., CO and CO 2 ͒. Even low levels of CO have a severe poisoning effect on the performance of Pt anodes. 1 Recently, it has been found that CO 2 also has a modest poisoning effect on Pt. 2,3 Currently, the use of carbon-supported PtRu bimetallic catalysts provides some tolerance to low levels of CO and reasonable concentrations of CO 2 . However, at higher levels of CO (Ͼ10 ppm), the tolerance of PtRu catalysts is not sufficient when used at economic electrode Pt loadings (Ͻ0.25 mg ͑Pt͒ cm Ϫ2 geometric). Practical reformate tolerance is currently achieved by the introduction of an air bleed to the fuel stream to remove the residual CO by selective oxidation within the anode. 4 A more elegant solution to reformate tolerance is the development of anode catalysts capable of operating effectively without loss in overpotential on reformate. Many Pt-based catalysts have been investigated for their resistance to poisoning toward CO. 5 The most promising systems are based on the addition of Mo and W to Pt, which can show greater CO tolerance than PtRu catalysts. Niedrach and Weinstock first showed the promotional effect of Mo and W oxides on the H 2 oxidation activity of Pt with CO/H 2 fuel mixtures, in 1965. 1 More recently, carbon-supported PtMo, PtW, PtCoMo, and PtCoW catalysts have all shown enhanced CO tolerance properties when compared to carbon-supported PtRu catalysts. 6-8 In particular, one recent report showed that a carbon-supported PtMo catalyst with a Pt:Mo ratio of 4:1 showed only a ca. 50 mV loss in cell voltage on 100 ppm CO in H 2 , compared to pure H 2 , when tested in a small PEMFC single cell at 85°C. In contrast, a carbon-supported PtRu catalyst (Pt:Ru ϭ 1:1) showed a ca. 160 mV loss. 9 Although the vast majority of reformate tolerance work has concentrated on the strongly poisoning effects of trace concentrations of CO, few reports have discussed the effect of the major impurity ͑in terms of concentration͒ of reformate, CO 2 . In particular, there have been no reports of operation of PtMo anodes on fuel mixtures that contain CO 2 . This paper reports the performance of a carbonsupported PtMo anode catalyst, together with standard Pt and PtRu c...
