The CO tolerance mechanism of the hydrogen oxidation reaction was investigated on several highly dispersed carbon-supported nanocrystalline Pt and binary Pt alloys. For this purpose, current/potential behavior was derived from half-cells under actual proton exchange membrane fuel cell operating conditions and correlated with expressions derived from kinetic models. Kinetic analyses have shown that the CO poisoning effect on Pt/C, PtRu/C, and PtSn/C catalysts occurs through a free Pt site attack mechanism, involving bridge- and linear-bonded adsorbed CO. For all catalysts, the onset of CO oxidation occurs via the bridge-bonded species, but for PtRu/C and PtSn/C, the reaction starts at smaller potentials. Under this condition, the hydrogen oxidation currents are generated on the vacancies of a carbon monoxide adsorbed layer created when some of the bridge-bonded CO molecules are oxidized. The linearly adsorbed CO is oxidized at higher overpotentials, leading to an increase of the holes on the CO layer and thus of the rate of the hydrogen oxidation process. © 2002 The Electrochemical Society. All rights reserved.

This paper is a full version of an earlier short communication, where significantly higher ͑up to threefold͒ CO tolerance was reported for PtMo/C ͑atomic ratio, Pt:Mo, 3:1͒ relative to the current state-of-the-art PtRu/C ͑1:1͒ in a proton exchange membrane fuel cell ͑PEMFC͒ under standard operating conditions ͑85°C, 100% humidification, with H 2 ϩ 100 pm CO//O 2 ). We report significantly different behavior for PtMo/C in contrast to PtRu/C, wherein there is negligible variation in CO tolerance ͑100 ppm CO in H 2 ) with variations in alloying compositions ͑Pt:Mo, 1:1 to 5:1͒. Further, in contrast to Pt/C and PtRu/C, significantly lower variations in overpotential losses is observed for PtMo/C as a function of temperature ͑55-115°C͒ and CO concentrations ͑5-100 ppm, balance H 2 ). In addition, excellent long-term stability is reported for PtMo/C ͑1:1͒ under steady-state conditions ͑constant potential conditions at 0.6 V͒ for a total duration of 1500 h, with anode gas composition varied between pure H 2 and those with 100 ppm CO, with or without the presence of other reformate gases ͑primarily CO 2 and N 2 ). These are discussed in the context of detailed physicochemical characterization of the nanoparticles using a combination of X-ray diffraction, transmission electron microscopy, and in situ synchrotron X-ray absorption spectroscopy.CO tolerance in reformer-based low-and medium-temperature proton exchange membrane fuel cells ͑PEMFCs͒ is crucial for the viability of this technology for transportation and portable power applications. The choice of an appropriate anode electrocatalyst with low susceptibility to CO poisoning and a high kinetic rate for hydrogen oxidation is therefore paramount. Despite its superior activity for anodic, hydrogen oxidation, and interfacial stability under acidic pH conditions and the operating temperatures of an actual PEMFC, electrocatalysis by Pt/C suffers from the problem of high polarization losses due to CO poisoning. This is especially true for temperatures below 115°C.The large affinity for CO chemisorption at potentials lower than ϳ0.65 V on Pt/C necessitates the search for other nanophase Ptbased electrocatalysts capable of initiating CO oxidation, preferably close to the hydrogen oxidation potential. This need manifested in the ''bifunctional'' 1,2 approach, where a second, more oxidizable element, present either as an admetal or as an alloying element, initiates the CO oxidation at lower potentials. The result is enough bare surface on Pt crystallites to efficiently oxidize hydrogen at lower overpotentials.Prior literature, involving several decades of research, is replete with investigations of alloys such as PtSn, 3,4 PtRh, 5 PtRu, 6-8 and Pt with adsorbing adatoms such as Ge, Sb, and Sn, 2,9 etc. In recent years nanophase PtRu electrocatalysts have received renewed attention as promising candidates for CO oxidation in PEMFCs. 10-12 A recent report by Oetjen et al., 13 using steady-state polarization data, indicates a fourfold performance enhancement with highly dispersed ...

In situ X-ray diffraction studies of Li x Mn 2 O 4 spinel cathode materials during charge-discharge cycling were carried out using a synchrotron as the X-ray source. Lithium-rich (x = 1.03-1.06) spinel materials, obtained from two different sources, were studied. Three cubic phases with different lattice constants were observed during charge-discharge cycles in all of the samples when a sufficiently low charge-discharge rate (≤C/10) was used. There were two regions of two-phase coexistence, which indicates that both phase transitions are first order. The separation of the Bragg peaks representing these three phases varied from sample to sample and also depended on the charge-discharge rate. These results show that the deintercalation of lithium in lithium-rich spinel cathode materials proceeds through a series of phase transitions from a lithium-rich phase to a lithium-poor phase and finally to a λ-MnO 2 -like cubic phase, rather than through a continuous lattice constant contraction in a single phase.The Li x Mn 2 O 4 spinel is one of the most promising cathode materials for lithium rechargeable batteries because of its low cost and low toxicity. Recent studies have focused on the problem of capacity fading of this material during cycling, especially at elevated temperature. This fading has been attributed to two sources, the dissolution of Mn 2+ into the nonaqueous electrolytes 1 and the inhomogeneity of the spinel local structure. 2,3 Early ex situ X-ray diffraction studies of Li x Mn 2 O 4 were performed by Ohzuku et al. 4 They found that two cubic phases coexist for 0.60 > x > 0.27, and a single cubic phase is present for 1.0 > x >0.6. Xia and Yoshio 5 reported in their studies that the two-phase coexistence was suppressed in cathode materials which were prepared lithium rich (x = 1.04) or oxygen rich. They also claimed that the two-phase coexistence is one of the key factors for the capacity fading during cycling. By suppressing this phase transition, the capacity fading of the lithium-rich cathode materials was significantly improved at the expense of lower initial capacity. In their later X-ray diffraction (XRD) studies, 6 an in situ technique was used, and the same conclusion that there was a onephase structure for lithium-rich spinel was presented. This interesting work raised an important issue: the relationship between the structural change and the capacity fading of these spinel materials during cycling. However, there are experimental data published by other research groups that show two-phase coexistence in lithiumrich spinels. For example, Richard et al. 7 have reported their in situ XRD studies with the observation of two-phase coexistence in the 0.60 > x > 0.27 range for Li x Mn 2 O 4 material, where the x value before charge was similar to that in the studies of Ref. 5 (x = 1.02 vs. x = 1.04). The main difference between these two studies is the charge rate (C/40 in Ref. 7 vs. C/3 in Ref. 5). The first issue to address in this article is whether the two-phase coexistence region is being suppre...

Partial substitution of Mn in lithium manganese oxide spinel materials by Cu and Ni greatly affects the electrochemistry and the cycle life characteristics of the cathode. Substitution with either metal or a combination of both shortens the 4.2 V plateau associated with the conversion of Mn3+ to Mn4+. A higher voltage plateau associated with oxidation of the substituted transition element is also observed. These substituent also significantly alter the onset of Jahn Teller distortions in the 3 V plateau. Synchrotron based in situ x-ray absorption (XAS) is used to determine the exact nature of the oxidation state changes in order to explain the overall capacities at the different voltage plateaus. Synchrotron based in situ x-ray diffraction (XRD) studies on LiCu0.5Mn1.504 shows single phase behavior in the 4-5 V region and a good cycle life. Lower cycle life characteristics for LiNi0.5Mn1.504 and LiNi0.25Cu0.25Mn1.504 are accounted for on the basis of several phase coexistence in this potential region. In the 3 V plateau however, the LiCu0.5Mn1.504 shows onset of the Jahn Teller distortions, in contrast to the LiNi0.5Mn1.504 and LNi0.25Cu0.25WMn1.504.