James McBreen

Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction An In Situ XANES and EXAFS Investigation

The electrocatalysis of the oxygen reduction reaction (ORR) on five binary Pi alloys (PtCr/C, PtMn/C, PtFe/C, PtCo/C, and PtNi/C) supported on high surface area carbon in a proton exchange membrane fuel cell was investigated. All the alloy electrocatalysts exhibited a high degree of crystallinity with the primary phase of the type Pt3M (LI2 structure with fcc type lattice) and a secondary phase (only minor contribution from this phase) being of the type PtM (LIo structure with tetragonal lattice) as evidenced from x-ray powder diffraction (XRD) analysis. The electrode kinetic studies on the Pt alloys at 95~ and 5 atm pressure showed a two- to threefold increase in the exchange current densities and the current density at 900 mV as well as a decrease in the overvoltage at i0 mA em -2 relative to Pt/C eleetrocatalyst. The PtCr/C alloy exhibited the best performance. In situ EXAFS and XANES analysis at potentials in the double-layer region [0.54 V vs. reversible hydrogen electrode (RHE)] revealed (i) all the alloys possess higher Pt d-band vacancies per atom (with the exception of PtMn/C alloy) relative to Pt/C electrocatalyst and (it) contractions in the Pt-Pt bond distances which confirmed the results from ex situ XRD analysis. A potential excursion to 0.84 V vs. RHE showed that, in contrast to the Pt alloys, the Pt/C electrocatalyst exhibits a significant increase in the Pt d-band vacancies per atom. This increase, in Pt/C has been rationalized as being due to adsorption of OH species from the electrolyte following a Temkin isotherm behavior, which does not occur on the Pt alloys. Correlation of the electronic (Pt d-band vacancies) and geometric (Pt-Pt bond distance) with the electrochemical performance characteristics exhibits a volcano type behavior with the PtCr/C alloy being at the top of the curve. The enhanced electrocatalysis by the alloys therefore can be rationalized on the basis of the interplay between the electronic and geometric factors on one hand and their effect on the chemisorption behavior of OH species from the electrolyte. The role of Pt/C and Pt alloys on the mechanism of the oxygen reduction reaction (ORR) has been investigated previously, 1-4 however the mechanism still remains elusive. One of the first investigations I of the ORR on Pt alloy electrocatalysts was in phosphoric acid; the effect of changes in the Pt-Pt interatomic distances, caused by alloying, was examined. The strength of the [M-HO2]aas bond, the intermediate formed in the rate-determining step of the molecular dioxygen reduction, was shown to depend on the Pt-Pt bond distance in the alloys. A plot of the electrocatalytic activity vs. adsorbate bond strength exhibited a volcano type behavior. 5 It was shown that the lattice contractions due to alloying resulted in a more favorable Pt-Pt distance (while maintaining the favorable Pt electronic properties) for dissociative adsorption of 02. This view was disputed by Glass et al. ~ in their investigation on bulk alloys of PtCr (the binary alloy at the top of the volcano plot) of different compositions. The latter investigation showed no activity enhancement for the ORR in phosphoric acid. This study therefore suggested the possibility of differences in electrochemical properties of bulk vs. supported alloy electrocatalysts (small particles of 35-85 A). A recent study on supported PtCo electrocatalysts ~ revealed the possibility that particle termination, primarily at the vicinal planes in the supported alloy electrocatalyst, is the reason for the enhanced ORR electrocatalysis (i.e., vicinal planes are more active than ). Paffett et al., 3 attributed higher activities for the ORR on bulk PtCr alloys in phosphoric acid to surface roughening, and hence increased Pt surface area, caused by the dissolution of the more oxidizable alloying component Cr. In contrast to these findings on bulk alloys, the supported alloy electrocatalysts have been reported to retain their nonnoble alloying element in the electrode during long periods (6000-9000 h) of operation in phosphoric acid fuel cells (PAFCs) 6 and proton exchange membrane fuel ceils (PEMFCs). 7 Based on these previous investigations and in the context of the ORR mechanisms, the principle explanations for the

Effects of Nafion impregnation on performances of PEMFC electrodes

0800 Athickness at a Nafion loading of 1.9 mg/cm 2 . Further additions caused deeper penetration of this Nafion film into the catalyst layer increasing the diAusional pathways for the reactant gases. These results correlate well with the mass transport characteristics in O2 and air as well as morphological characteriz- ation of the electrode based on SEM and pore volume distributions. # 1998 Elsevier Science Ltd. All rights reserved

Electrocatalysis of CO tolerance in hydrogen oxidation reaction in PEM fuel cells

The electrocatalysis of CO tolerance in the hydrogen oxidation reaction was investigated for Pt/C, PtSn/C and PtRu/C electrocatalysts in proton exchange membrane (PEM) fuel cells. Both half and single cell polarization characteristics were studied at several temperatures and CO partial pressures. It is proposed that the CO adsorption step occurs predominantly through a displacement path for PtSn/C and through a free site attack path for CO on both Pt/C and PtRu/C. The data are more consistent with the participation of linear (PtSn/C, and PtRu/C [T≤55°C]) and bridged bonded adsorbed CO on Pt/C and PtRu/C [T≥70°C]. The CO oxidation process occurs at different potentials depending on the nature of the electrode material. The oxidation of CO by the alloy catalysts is not the only contributor to CO tolerance. Changes in the thermodynamics and the kinetics of the CO adsorption process, induced by the alloy catalysts, also contribute to CO tolerance.

Surface changes on LiNi0.8Co0.2O2 particles during testing of high-power lithium-ion cells

LiNi0.8Co0.2O2 particles from high-power lithium-ion cells were examined to determine material changes that result from accelerated aging tests. X-ray absorption spectroscopy (XAS) and transmission electron microscope (TEM) data indicated a LixNi1−xO-type layer on the particle surfaces. The greater thickness on particles from high-power fade cells indicate that these surface layers are a significant contributor to cathode impedance rise observed during cell tests.

Correlating Capacity Fading and Structural Changes in Li1 + y Mn2 − y O 4 − δ Spinel Cathode Materials: A Systematic Study on the Effects of Li/Mn Ratio and Oxygen Deficiency

Several series of Li 1-x Mn 2 O 4+δ samples with the spinel structure were synthesized. These samples had different Li/Mn ratios (by varying the Li/Mn ratio used in starting materials) and various oxygen contents (by controlling synthesis conditions, including temperature, heat-treatment time, and purging gas during both the solid-state reaction and annealing). In systematic studies of charge-discharge cycling behavior and in situ X-ray diffraction (XRD) at room temperature, it was found that both the charge/ discharge profile and the structural changes during cycling are closely related to the degree of oxygen deficiency created in the synthesis process. Their effects on the capacity fading are much more important than the Li/Mn ratio or other factors. A higher degree of oxygen deficiency is accompanied with a faster fading of capacity during cycling. In cells using spinet cathodes with an oxygen deficiency, the capacity lading during cycling occurs on both the 4.2 and 4 0 V plateaus. This behavior is quite different from that found in cathodes without an oxygen deficiency, where most of the capacity fading occurs on the 4.2 V plateau region only. Our in situ XRD results indicate clearly that the capacity fading on the 4.2 V plateau is related to the phase transition between the cubic II and cubic III (λ-MnO 2 ) structure, while the capacity fading on the 4.0 V plateau is related to the phase transition between the cubic I and cubic II spinel structures. The effects of oxygen deficiency on the structural phase transition of Li 1±y Mn 2 O 4±δ -type materials at temperatures around 10°C were also studied. It was found that this phase transition is closely related to the degree of oxygen deficiency of the material. In samples with no oxygen deficiency, this phase transition disappeared.

In Situ X‐Ray Absorption Studies of a Pt‐Ru Electrocatalyst

X-ray absorption studies (XAS) were done on a carbon supported Pt-Ru electrocatalyst in 1 M HClO{sub 4}. Results at the Pt L{sub 3} and L{sub 2} and edges confirmed that the Pt was alloyed with Ru and that the Ru content was about 25 atomic percent. There was a large excess of unalloyed Ru, with only about 10% of the Ru alloyed with the Pt. The Pt XAS indicated that the Ru increased that Pt d band vacancies and decreased the Pt-Pt bond distances from 2.77 {angstrom} to values between 2.71 and 2.73 {angstrom}. The bifunctional mechanism for methanol oxidation on Pt-Ru electrocatalysts needs to be modified to account for the effect of these electronic changes on the adsorption of H and CO residues from methanol decomposition. There are significant changes in the Pt XAS in going from the reversible hydrogen potential to 0.24 V. This may be due to the onset of the formation of RuOH species on the alloy. Further fine tuning of the electronic structure and the electrocatalysis may be possible through the use of ternary alloys.

Investigation of Enhanced CO Tolerance in Proton Exchange Membrane Fuel Cells by Carbon Supported PtMo Alloy Catalyst

E-TEK, Incorporated, Natick, Massachusetts 01760, USAWe report a two- to threefold enhancement of CO tolerance in a proton exchange membrane (PEM) fuel cell, exhibited by carbonsupported nanocrystalline PtMo/C as compared to the current state of the art PtRu/C electrocatalysts. The bulk of these nanocrys-tals were comprised of Pt alloyed with Mo in the ratio 8.7:1.3 as shown by both X-ray diffraction and in situ extended X-ray ab sorp-tion fine structure measurements. Rotating disk electrode measurements and cyclic voltammetry in a PEM fuel cell indicate theonset of CO oxidation at potentials as low as 0.1 V. Further, the oxidation of CO exhibits two distinct peaks, indicating redox b ehav-ior involving oxyhydroxides of Mo. This is supported by in situ X-ray absorption near edge structure measurements at the Mo Kedge.© 1999 The Electrochemical Society. S1099-0062(98)08-029-8. All rights reserved.Manuscript submitted August 10, 1998; revised manuscript received September 28, 1998. Available electronically October 30, 1998.

Microscopy and spectroscopy of lithium nickel oxide-based particles used in high power lithium-ion cells

Structural and electronic investigations were conducted on lithium nickel oxide-based particles used in positive electrodes of 18650-type high-power Li-ion cells. K-edge X-ray absorption spectroscopy (XAS) revealed trivalent Ni and Co ions in the bulk LiNi{sub 0.8}Co{sub 0.2}O{sub 2} powder used to prepare the high power electrode laminates. Using oxygen K-edge XAS, high resolution electron microscopy, nanoprobe diffraction, and electron energy-loss spectroscopy, we identified a <5 nm thick modified layer on the surface of the oxide particles, which results from the loss of Ni and Li ordering in the layered R{bar 3}m structure. This structural change was accompanied by oxygen loss and a lowering of the Ni- and Co-oxidation states in the surface layer. Growth of this surface layer may contribute to the impedance rise observed during accelerated aging of these Li-ion cells.

LiMn2 − x Cu x O 4 Spinels (0.1 ⩽ x ⩽ 0.5): A new Class of 5 V Cathode Materials for Li Batteries I. Electrochemical, Structural, and Spectroscopic Studies

A series of electroactive spinel compounds, LiMn 2-x Cu x O 4 (0.1 ≤ x ≤ 0.5), has been studied by crystallographic, spectroscopic, and electrochemical methods and by electron microscopy. These LiMn 2-x Cu x O 4 spinels are nearly identical in structure to cubic LiMn 2 O 4 and successfully undergo reversible Li intercalation. The electrochemical data show a remarkable reversible electrochemical process at 4.9 V which is attributed to the oxidation of Cu 2+ to Cu 3+ . The inclusion of Cu in the spinel structure enhances the electrochemical stability of these materials upon cycling. The initial capacity of LiMn 2-x Cu x O 4 spinels decreases with increasing x from 130 mAh/g in LiMn 2 O 4 (x = 0) to 70 mAh/g in LiMn 1.5 Cu 0.5 O 4 (x = 0.5). The data also show slight shifts to higher voltage for the delithiation reaction that normally occurs at 4.1 V in standard Li 1-x Mn 2 O 4 electrodes (1 ≥ x ≥ 0) corresponding to the oxidation of Mn 3+ to Mn 4+ . Although the powder X-ray diffraction pattern of LiMn 1.5 Cu 0.5 O 4 shows a single-phase spinel product, neutron diffraction data show a small but significant quantity of an impurity phase, the composition and structure of which could not be identified. X-ray absorption spectroscopy was used to gather information about the oxidation states of the manganese and copper ions. The composition of the spinel component in the LiMn 1.5 Cu 0.5 O 4 was determined from X-ray diffraction and X-ray absorption near-edge spectroscopy to be Li 1.01 Mn 1.67 Cu 0.32 O 4 , suggesting to a best approximation that the impurity in the sample was a lithium-copper-oxide phase. The substitution of manganese by copper enhances the reactivity of the spinel structure toward hydrogen: the compounds are more easily reduced at moderate temperature (∼200°C) than LiMn 2 O 4 .

The CO Poisoning Mechanism of the Hydrogen Oxidation Reaction in Proton Exchange Membrane Fuel Cells

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.