Evidence for Reversible Formation of Metallic Cu in Cu0.1V2O5 Xerogel Cathodes during Intercalation Cycling of Li+ Ions as Detected by X-ray Absorption Spectroscopy
Abstract
Vanadium pentoxide materials prepared through sol-gel processes ~xerogel, aerogel, and aerogel-like! act as excellent intercalation hosts for lithium as well as polyvalent cations. The large lithium insertion capacity of these materials makes them attractive for use as cathodes in high-capacity lithium batteries. Copper-doped V2O5-based cathodes have excellent properties in terms of high intercalation rate and very good reversibility upon cycling with no capacity fading after more than 450 cycles. This paper reports in situ X-ray absorption spectroscopy ~XAS! investigations of copper-doped V2O5 xerogel. The K-edge X-ray absorption thresholds of V and Cu were studied. The local structure around copper is found to change dramatically during individual insertion/release cycles for Li1 ions. The XAS analysis indicates that the lithiation of the Cu0.1V2O5 xerogel cathode proceeds with concurrent formation of copper metal. However, the starting compound is regenerated upon releasing the inserted lithium ~charging the cathode!, thus revealing the foundation for excellent structural and electronic reversibility of the material.
6Li and 7Li Magic Angle Spinning Nuclear Magnetic Resonance and In situ X-ray Diffraction Studies of the Charging and Discharging of LixMn2O4 at 4 V
Abstract
6Li and 7Li magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and in situ X-ray diffraction (XRD) have been used to study lithium manganate cathode materials (LixMn2O4, 0 < x # 1) during and following charging and discharging. Only one major local environment is observed by 6Li MAS NMR from 0 to > 50% charging, from lithium in the tetrahedral sites of the spinel structure, the resonance shifting by no more than 8 ppm in this range (from its original position at ca. 520 ppm). When the cell is charged above 50%, a new resonance is observed at <645 ppm, due to Li1 in a spinel local environment with nearby manganese ions in oxidation states close to 14 (e.g., in a composition such as Li0.1Mn2O4). The two resonances at <520 and 645 ppm are both observed in the range 70 to <80 and 80 to 90% for the samples annealed at 850 and 6508C, respectively. These coexistences occur over the same ranges as the two-phase coexistence, as observed by XRD during the first charging cycle. Resonances were observed at 830 and 930 ppm, for the samples annealed at 650 and 8508C, respectively, at $90% charging. These same resonancesvwere seen after multiple charging cycles and are assigned to defect spinels with high manganese oxidation states. Additional resonances are observed for electrodes with high carbon contents at 22 to 213 ppm, from nonspinel phases, which grow in intensity with the number of charging cycles. The presence of the defect spinels and the additional impurity phases contribute to the reduced capacity of these materials. Three-phase behavior is observed by in situ XRD during the first discharging cycle. These results are discussed in terms of previous in situ XRD charging/discharging results.
In situ X-ray Absorption and Diffraction Study of the Li Reaction with a Tin Composite Oxide Glass
Abstract
We have measured the X-ray absorption fine structure spectra of a sample of tin-based composite oxide (TCO) material with a nominal composition of Sn1.0B0.56P0.40Al0.42O3.47 during the discharge and charge cycles in an in situ configuration. Our results confirm the amorphous nature of TCO material and show that Sn in the pristine state of TCO is coordinated with three oxygen atoms at a distance of 2.12 Å. Upon discharging, initially Li interacts with the electrochemically active Sn-O center, forming metallic Sn in a highly dispersed form (i.e., clusters containing just a few atoms). Upon further discharge, Li alloys with Sn, forming highly dispersed forms of LixSn alloys with x being dependent on the degree of Li intercalation. The structural nature of the highly dispersed alloys differs from those of the corresponding crystalline phases such as Li2Sn5, LiSn, Li7Sn3, Li5Sn2, Li13Sn5, and Li7Sn2. Upon charging, Li dealloys from LixSn, forming metallic Sn in a highly dispersed form with a Sn-Sn distance intermediate to those of gray and white Sn.
Electrocatalysis of CO Tolerance in Hydrogen Oxidation Reaction in PEM Fuel Cells
S. J. Lee, S. Mukerjee, E. A. Ticianelli, and J. McBreen Electrochimica. Acta.,44, 3283 (1999)
Abstract
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.
Reaction Kinetics and X-ray Absorption Spectroscopy Studies of Yttrium-Containing Metal Hydride Electrodes
Abstract
This is a study of electrode degradation mechanisms and reaction kinetics of LaNi4.7Sn0.3, La(12x)YxNi4.7Sn0.3 (x 5 0.1, 0.2, and 0.3) and La0.7Y0.3Ni4.6Sn0.3Co0.1 metal hydride electrodes. Alloy characterization included X-ray diffraction, X-ray absorption (XAS), hydrogen absorption in a Sieverts apparatus, and electrochemical cycling of alloy electrodes. The atomic volume of H was determined for two of the alloys. Electrochemical kinetic measurements were made using steady-state galvanostatic measurements, galvanodynamic sweep, and electrochemical impedance techniques. XAS was used to examine the degree of corrosion of the alloys with cycling. Alloying with Y decreased the corrosion rate. The results are consistent with corrosion inhibition by a Y-containing passive film. The increase in the exchange current density of the hydrogen oxidation reaction with increasing depth of discharge was much greater on the Y-containing alloys. This may be due to the dehydriding of the catalytic species on the surface of the metal hydride particles.
In situ X-ray Absorption Spectroscopy Characterization of V2O5 Xerogel Cathodes upon Li Intercalation
Abstract
Vanadium pentoxide materials prepared through sol-gel processes act as excellent intercalation hosts for lithium as well as for polyvalent cations. Previous ex situ X-ray absorption spectroscopy and X-ray diffraction characterizations have shown that the electrochemical performance of vanadium pentoxide xerogels depends inversely on the long-range order of the V2O5-layered structure. Recently, new ways to prevent the self-organization of the dry materials, which takes place upon water removal from the starting hydrogel, have been introduced. In the present paper we report on the in situ X-ray absorption spectroscopy characterization of a spray-coated V2O5 (freeze-dried) xerogel cathode upon lithium intercalation.
In situ Synchrotron X-ray Diffraction Studies of the Phase Transitions in LiXMn2O4 Cathode Materials
Abstract
In situ X-ray diffraction studies of LixMn2O4 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 (²C/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 l-MnO2-like cubic phase, rather than through a continuous lattice constant contraction in a single phase.
LiNiXCu0.5-XMn1.5O4 Spinel Electrodes, Superior High Potential Cathode Materials for Li Batteries. I. Electrochemical and Structural Studies
Y. Ein Eli, J. T. Vaughey, M. M. Thackeray, S. Mukerjee, X. Q. Yang, and J. McBreen J. Electrochemical Society, 146, 908 (1999)
Abstract
LiNixCu0.52xMn1.5O4 (0 # x # 0.5) spinel materials that are of interest as electrodes for lithium-ion batteries have been studied by crystallographic and electrochemical methods. Electrochemical results show that the inclusion of Ni into the Cu-modified spinel increases the overall reversible capacity from 72 mAh/g with LiCu0.5Mn1.5O4 (x 5 0) at 4.95 V to 120 mAh/g with LiNi0.5Mn1.5O4 (x 5 0.5) at 4.6 V. The Cu-rich spinels in the LiNixCu0.52xMn1.5O4 system show enhanced stability upon cycling. These spinel products are difficult to prepare in single-phase form. Neutron diffraction data show that the end members LiNi0.5Mn1.5O4 and LiCu0.5Mn1.5O4 contain small amounts of lithium-nickel-oxide and lithium-copper-oxide impurity phases, respectively; these impurities affect the composition of the spinel component. The composition of the spinel component in LiNi0.5Mn1.5O4 was determined from a combined X-ray and neutron diffraction profile refinement to be Li0.97Ni0.42Mn1.61O4.
Investigation of Enhanced CO Tolerance in Proton Exchange Membrane Fuel Cells by Carbon Supported PtMo Alloy Electrocatalysts
Abstract
We report a two- to threefold enhancement of CO tolerance in a proton exchange membrane (PEM) fuel cell, exhibited by carbon supported nanocrystalline PtMo/C as compared to the current state of the art PtRu/C electrocatalysts. The bulk of these nanocrystals 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 absorption fine structure measurements. Rotating disk electrode measurements and cyclic voltammetry in a PEM fuel cell indicate the onset of CO oxidation at potentials as low as 0.1 V. Further, the oxidation of CO exhibits two distinct peaks, indicating redox behavior involving oxyhydroxides of Mo. This is supported by in situ X-ray absorption near edge structure measurements at the Mo K edge.
An In situ X-ray Absorption Spectroscopy Investigation of the Effect of Sn Additions to Carbon-Supported Pt Electrocatalysts. Part I
Abstract
Carbon-supported platinum (Pt/C) with an adsorbed layer of underpotential deposited (upd) Sn is a much better catalyst for the methanol oxidation reaction (MOR) than a carbon-supported platinum-tin (PtSn/C) alloy. In situ X-ray absorption (XAS) was used to determine the differences in the effects that the two methods of Sn addition have on the electronic properties and the structural properties of the catalyst. X-ray diffraction and XAS at the Pt L3 and L2 edges indicate that the PtSn/C catalyst has a Pt3Sn L12 structure, and alloying with Sn causes partial filling of the Pt d band vacancies and an increase in the Pt–Pt bond distance from 2.77 to 2.8 Å. However, upd Sn does not perturb Pt structurally or electronically. XAS at the Sn K edge indicates that both the upd Sn on Pt/C and the surface Sn on PtSn/C are associated with oxygenated species at all potentials, and that the nature and strength of the Sn–O bonds are potential dependent. The differences in the activity of the two catalysts for the MOR are due to the effects of alloying on the Pt electronic structure that inhibit the ability of the Pt to adsorb methanol and dissociate C–H bonds. The ability of PtSn/C to adsorb oxygen at low potentials enhances its activity for CO oxidation.
Effects of Nafion Impregnation on Performance of PEMFC Electrodes
S. J. Lee, S. Mukerjee, J. McBreen, Y. W. Rho, Y. T. Kho, and T. H. Lee, Electrochimica Acta,43, 3693 (1998)
Abstract
The effect of Nafion loading on the electrode polarization characteristics of a conventional proton exchange membrane (PEM) fuel cell electrode has been investigated in terms of both H2/O2 and H2/air performance. Correlation of Nafion loading with the activation polarization characteristics shows an initial increase of activity upto a loading of 1.3 mg/cm2 followed by a more gradual change with maxima at 1.9 mg/cm2 for both oxygen and air. This trend correlated well with the decrease in charge transfer resistance and increase in the electrochemically active surface area. The contributions to the linear ohmic polarization region of both the H2/O2 and H2/air performance are predominantly from ionic resistance as well as diffusional contributions in the catalyst layer. Among all the polarization losses those due to mass transport were the highest. Fits using a thin film agglomerate model showed a rapid increase in the film thickness with Nafion loading in the pores of the carbon of the catalyst layer followed by an equilibrium of 800 Å thickness at a Nafion loading of 1.9 mg/cm2. Further additions caused deeper penetration of this Nafion film into the catalyst layer increasing the diffusional pathways for the reactant gases. These results correlate well with the mass transport characteristics in O2 and air as well as morphological characterization of the electrode based on SEM and pore volume distributions.
LiCuxMn2-xO4 Spinels (0.1£ x £ 0.5): A New Class of Cathode Material for Li Batteries. II In situ Measurements
Abstract
Electrochemical data obtained from lithium cycling in LiCuMn2..O4 spinels (0.1 _ x _ 0.5) show a remarkable, highly reversible, electrochemical cycling behavior at 4.9 V. In situ X-ray absorption near edge structure spectroscopy revealed that this ultrahigh voltage is due to the existence of the Cu2t/Cu2 redox couple. The origin of the high stability at the 5 V region was studied with the in situ X-ray diffraction technique. It was found that the unit cell dimension undergoes minimal change during charge-discharge cycles at the 5 V plateau.
LiMn2-xCuxO4 Spinel (0.1£x£0.5)-A new Class of 5 V Cathode Materials for Li Batteries I. Electrochemical, Structural and Spectroscopic Studies
Abstract
A series of electroactive spinel compounds, LiMn2CurO4 (0.1 x a 0.5), has been studied by crystallographic, spectroscopic, and electrochemical methods and by electron microscopy. These LiMn2_Cu04 spinels are nearly identical in structure to cubic LiMn2O4 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 Cu2 to Cu. The inclusion of Cu in the spinel structure enhances the electrochemical stability of these materials upon cycling. The initial capacity of LiMn2Cu04 spinels decreases with increasing x from 130 mAh/g in LiMn2O4 (x = 0) to 70 mAh/g in "LiMn1 ,Cu1504" (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 Li1Mn204 electrodes (1 _ x _ 0) corresponding to the oxidation of MnS* to Mn44. Although the powder X-ray diffraction pattern of "LiMn1 5Cu04" 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 LiMn1 -Cu0 504 was determined from X-ray diffraction and X-ray absorption near-edge spectroscopy to be Li1 01Mn, 67Cu0 3204, 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 LiMn2O4.
Structural Evolution of LixMn2O4 in Lithium Ion Battery Cells Measured In situ using Synchrotron X-ray Diffraction techniques
Abstract
We describe synchroton based X-ray diffraction techniques and issues related to in situ studies of intercalation processes in battery electrodes. We then demonstrate the utility of this technique, through a study of two batches of LiMn2O4 cathode materials. The structural evolution of these spinel materials was monitored in situ during the initial charge of these electrodes in actual battery cells. Significant differences were observed in the two batches, particularly in the intercalation range of x = 0.45 to 0.20. The first-order structural transitions in this region indicated coexistence of two cubic phases in the batch 2 material, whereas the batch 1 material showed suppressed two-phase coexistence. Batch 2 cells also indicated structural evolution in the low-potential region below 3.0 V in contrast to the batch 1 material. Differences in structural evolution between batches of LiMn2O, could have important ramifications in their cycle life and stability characteristics.
Effect of Particle Size on the Electrocatalysis of Carbon Supported Pt Electrocatalysts
S. Mukerjee and J. McBreen, J. Electroanalytical Chemistry,448, 163 (1998)
Abstract
In situ X-ray absorption studies were done in 1 M HClO4, with and without 0.3 M MeOH, on several well-defined carbon-supported Pt electrocatalysts with particle sizes in the range of 25 to 90 Å. Data were obtained at several potentials in the range of 0.0 to 1.14 V vs. RHE. The results show that as the particle size is reduced below 50 Å, the strength of adsorption of H, OH and C1, moieties such as CO is increased. The strong adsorption of OH explains the reduced specific activity for oxygen reduction on small particles. The reduced activity for methanol oxidation on the small particles is due to a combination of the increased strength of adsorption of both CO and OH. The strong adsorption of H at negative potentials on small Pt particles is sufficient to induce reconstruction and morphological changes in the Pt particles. Both XANES and EXAFS data on a 53 Å particle at 0.84 V indicate that formation of PtOH is the rate determining step in the oxidation of methanol. All these affects are due to an increase in the number of Pt sites with low coordination on the small particles.
Effect of Zn Additives to the Electrolyte on the Corrosion and Cycle Life of some AB5Hx metal Hydride Electrodes
Abstract
Investigation on a series of ABs-type metal hydride electrodes reveals significant improvement of cycle life in the presence of zincate electrolytes (0.5 M ZnO in 6 M KOH) for alloys without substituents such as Ce. For alloys containing Ce, which are known to impart passive protection against corrosion no additional advantage was derived by using zincate electrolytes. X-ray absorption near-edge structure investigation at the Ni K edge, due to its relative high abundance and catalytic importance in the hydriding process, was used to elicit the corrosion characteristics as a function of cycling. The spectra averaged over the bulk of the sample (transmission mode) and those from the top 200 to 250 A (electron yield mode) indicate significant lowering of corrosion and buildup of Ni(OH)2 in non-Ce substituted alloys when using zincate electrolytes. In Ce substituted samples this effect was marginal.
Function of Cobalt in AB5Hx Electrodes
G. D. Adzic, J. R. Johnson, S. Mukerjee, J. McBreen, and J. J. Reilly J. Alloys and Compounds,253-254, 579 (1997)
Abstract
The role of cobalt in the behavior of AB5Hx electrodes has been investigated. Alloy compositions were as follows, LaNi4.3−x CoxMn0.4Al0.3 (x=0, 0.2, 0.4, 0.75) and MmNi4.3−xCoxMn0.4Al0.3 (x=0, 0.75). Cobalt was found to decrease the molar volume of hydrogen, VH, in the hydride phase. Both LaNi4.3−xCoxMn0.4Al0.3 and MmNi4.3−xCoxMn0.4Al0.3 alloy electrodes were subjected to repeated electrochemical cycling and corrosion rates measured. The corrosion rate was found to be inversely proportional to the cobalt content of the electrode.
Synchrotron X-ray Diffraction Studies of the Structural Properties of Electrode Materials in Operating Battery Cells
T. R. Thurston, N. M. Jisrawi, S. Mukerjee, X. Q. Yang, J. McBreen, M. L. Daroux and X. Xing Applied Physics Letters, 69, 194 (1996)
Abstract
Hard x rays from a synchrotron source were utilized in diffraction experiments which probed the bulk of electrode materials while they were operating insitu in battery cells. Two technologically relevant electrode materials were examined; an AB2-type anode in a nickel–metal–hydride cell and a LiMn2O4 cathode in a Li-ion ``rocking chair'' cell. Structural features such as lattice expansions and contractions, phase transitions, and the formation of multiple phases were easily observed as either hydrogen or lithium was electrochemically intercalated in and out of the electrode materials. The relevance of this technique for future studies of battery electrode materials is discussed.
Hydrogen Electrocatalysis by Carbon Supported Pt and Pt Alloys: An in situ X-ray Absorption Study
Abstract
In situ x-ray absorption spectroscopy (XAS) in 1 M HC1O4 was used to examine the electronic and structural effects of hydrogen adsorption on carbon supported Pt (Pt/C) and Pt alloyed with first row transition metals (Cr, Mn, Fe, Co, and Ni). In the case of Pt/C, potential excursions from the double layer region (0.54 V vs. RHE) to 0.0 V caused significant changes in the XAS spectra whereas none was observed for the alloys. The L, and L2 x-ray absorption near edge structure indicated the generation of empty electronic states in the vicinity of the Fermi level due to adsorption of hydrogen, and the L, extended x-ray absorption fine structure indicated an increase in the coordination number of the first Pt-Pt shell from 9 to 11. The latter was attributed to a reversible surface restructuring process. Alloying of the Pt suppresses both the electronic and structural effects at 0.0V. A comparison of the electrochemical kinetics for hydrogen oxidation by these electrocatalysts in a proton exchange membrane fuel cell indicated that alloying of the Pt had insignificant effects on the kinetics.
Cerium Content and Cycle Life of Multicomponent AB5 Hydride Electrodes
Abstract
Multicomponent AB5 hydrides are attractive replacements for the cadmium electrode in nickel-cadmium batteries. The archetype compound of the AB~ alloy class is LaN%, but in a typical battery electrode mischmetal is substituted for La and Ni is substituted in part by various metals. While the effects of Ni substitution have been widely studied, relatively little effort has been focused on the effect of La substitution. Cerium is the predominant rare earth in normal misehmetal, and this paper deals with the effect on cycle life and storage capacity due to the increasing presence of Ce in the alloy series La1_x Ce~ Ni3.~5 Co0.7~Mn0.~Al0.3. Alloys were characterized by the determination of pressure-composition relationships, molar volume of H in the hydride phase and electrode cycle life. The effects due to lattice expansion are taken into account. It was concluded that the rate of loss of electrochemical capacity per charge-discharge cycle due to electrode corrosion was significantly decreased by the presence of Ce.
In-situ X-ray Absorption Studies of a Pt-Ru Electrocatalyst
Abstract
X-ray absorption studies (XAS) were done on a carbon supported Pt-Ru electrocatalyst in 1 M HC1Q. Results at the Pt L3 and L~ 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 P t. The Pt XAS indicated that the R u increased the Pt d band vacancies and decreased the Pt-Pt bond distances from 2.77 A to values between 2.71 and 2.73 A. 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.
Abstract In situ x-ray absorption spectroscopy (XAS) studies were done on three metal hydride electrodes, LaNi~, LaNi~.sSns.2, La0.sCe02Ni48Sn0.2, in 6M KOH. Ex situ measurements were also made on dry uncycled electrodes and on material from an La0.sCe0.2Ni48Sn0.2 electrode that had been cycled 25 times. Comparison of the in situ XAS at the Ni K and at the La L3 edge of charged and discharged electrodes indicates large changes in the electronic and structural characteristics on introduction of hydrogen. Results at the Ce L2 edge in La~.sCe~.2Ni4.~Sn0.~ show a transition from a mixed valent c~ to a -/-like Ce state as the lattice expands during charge. Ex situ x-ray absorption near-edge structures (XANES) at the Ni K edge indicate that the additions of either Ce or Sn fill empty Ni 3d states. The Ni K edge extended x-ray absorption fine structures (EXAFS) for all three alloys in the dry uncharged state were similar, indicating that minor substitutions for either the A or B component do not substantially change the structure. The Sn substitution causes an increase both in a and c axis as evidenced from increase in the Ni-Ni and the Ni-La distances. Partial substitution of La by Ce causes a slight contraction in the Ni-La distance. The Ni XANES and EXAFS indicate that about 6 % of the Ni in the La0.sCe0.2Ni~.sSn02 corroded after 25 cycles. Ce XANES on the cycled electrode indicates some corrosion of Ce and the formation of Ce (III) state. The results indicate that XAS is a very useful technique for the study of alloy hydrides, particularly the role of electronic structure, the environment around minor constituents, and the corrosion of individual components.
Effect of Preparation Conditions of Pt Alloys on their Electronic, Structural and Electrocatalytic Activities for Oxygen Reduction-XRD, XAS and Electrochemical Studies
Abstract
The effect of different alloying conditions (alloying temperature, annealing period) on the electrocatalytic activities for the oxygen reduction reaction (ORR) by three carbon-supported Pt alloy electrocatalysts (WCr, WCo, m i ) was investigated and correlated with electronic and structural parameters determined by in-situ XAS. The results indicate that all the Pt alloys show enhanced ORR activities relative to a WC electrocatalyst. However, the electrocatalytic activity and activation energy for ORR in the case of Pt/Ni and Pt/Co alloys show marked effect due to different alloying conditions. This was in contrast to WCr alloy, where both parameters remained unchanged over the range of alloying conditions. Those electrochemical results were correlated with those obtained from in-situ X-ray absorption spectroscopic (XAS) investigations, which provided information on the electronic (Pt 5d-orbital vacancy, from the X-ray absorption near-edge structure) and geometric (Pt-Pt bond distances, from the extended X-ray absorption fine structure) factors. In-situ XAS results indicate that the supported alloys possess higher Pt Sd-orbital vacancies and shorter Pt-Pt bond distances. In addition, the XAS results showed that alloying inhibited chemisorption of oxygenated species (OH) on the Pt at potentials above 0.8 V vs RHE. Correlation of electrocatalytic activities and activation energies for ORR with parameters obtained from in-situ XAS studies indicates that, in the case of mi and Pt/Co alloys, higher alloying temperature and longer annealing periods result in higher Pt 5d-orbital vacancies with the geometric parameters remaining unchanged. The WCr alloy on the other hand revealed no dependence of either the Pt d-orbital vacancies or the geometric parameters on alloying temperature. These observations indicate that the dependence of electrocatalytic activities and activation energy for Pt/Co and Pt/Ni alloys on the thermal history and the absence of such an effect in the WCr alloy could be related to the differences in the Pt Sd-orbital vacancies.
Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction: an In-situ XANES and EXAFS Investigation
Abstract
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.
Enhanced Electrocatalysis of Oxygen Reduction on Platinum Alloys in Proton Exchange Membrane Fuel Cells
S. Mukerjee and S. Srinivasan J. Electroanalytical Chemistry, 357, 201 (1993)
Abstract
Enhanced electrocatalysis of the oxygen reduction reaction (ORR) on carbon-supported binary and ternary alloys of Pt in phosphoric acid fuel cells has been reported previously. This investigation focuses on the electrocatalysis of the ORR on some binary alloys of Pt (Pt+Ni, Pt+Cr and Pt+Co) at interfaces with proton exchange membranes (Dow perfluorinated sulfonic acids). Comparison of the results of these studies with those on carbon-supported Pt electrocatalysts (electrodes containing same Pt loading of 0.3 mg/cm2) revealed enhanced activities, lower activation energies and different reaction orders for all the alloys. X-ray powder diffraction showed lattice contractions for the alloys, the predominant phase being Pt3M (LI2) f.c.c. crystalline. X-ray photoelectron spectroscopy studies on the constituent elements of the electrocatalyst showed no chemical energy shifts owing to alloying and/or the presence of oxides on the surface. Lifetime evaluations of proton exchange membrane fuel cells, using both electrochemical as well as scanning electron microscopy/energy-dispersive X-ray analysis techniques, revealed only small amounts of dissolution of the more oxidizable component during the testing periods, which ranged from 400 to 1200 h. Therefore, the enhanced electrocatalysis exhibited by the binary Pt alloys appears to originate primarily as a result of changes in the lattice structure owing to alloying and the unique environment of the supported catalyst in the particle size range 35–75 Å.
Effect of Sputtered Film of Platinum on Low Platinum Loading Electrodes on Electrode Kinetics of Oxygen Reduction in Proton Exchange Membrane Fuel Cells
S. Mukerjee, S. Srinivasan and A. J. Appleby Electrochimica Acta, 38, 1661 (1993)
Abstract
Localization of Pt electrocatalyst by sputter deposition of a thin film (500 Å) on the front surface of a fuel cell electrode containing a supported electrocatalyst (20% Pt/C, 0.4 mg cm-2 loading) has been known to exhibit higher fuel cell performance as compared to that on the electrode without the sputtered film. This study compares the electrode kinetic parameters, electrochemically active surface areas, activation energies and reaction orders for the oxygen reduction reaction (ORR) in the sputtered and unsputtered electrodes in proton exchange membrane fuel cells as functions of temperature and pressure. Comparison of the cell performance at 5 atm and 95oC indicates an almost 4 fold improvement in ORR activity at 0.9 V vs. rhe and a similar 3.6 fold improvement in the exchange current densities
Particle Size Effects in Pt Electrocatalysis
S. Mukerjee J. Applied Electrochemistry, 20, 537 (1990)
Abstract
Of the several factors which influence electrocatalytic activity, particle size and structural effects are of crucial importance, but their effects and mechanism of interaction,vis-a-vis overall performance, have been, at best, vaguely understood. The situation is further aggravated by the use of a wide range of experimental conditions resulting in non-comparable data. This paper attempts systematically to present the developments to date in the understanding of these structural interactions and to point out areas for future investigation. The entire content of this review has been examined from the context of the highly dispersed Pt electrocatalyst, primarily because it has been examined in the greatest detail. In the first two sections a general idea on the correlations between surface microstructure and geometric model is presented. Subsequently, indicators of a direct correlation between particle size and catalyst support synergism are considered. The structural and particle size effect on electrocatalysis is examined from the point of view of anodic hydrogen oxidation and cathodic oxygen reduction reactions. The hydrogen and oxygen chemisorption effects, presented with the discussion on the anodic and cathodic electrocatalytic reactions, provide important clues toward resolving some of the controversial findings, especially on the dependence of particle size on the anodic hydrogen oxidation reaction. Finally, the effect of alloy formation on the cathodic oxygen reduction reaction is discussed, providing insights into the structural aspect.
800 Å thickness at a Nafion loading of 1.9 mg/cm2. Further additions caused deeper penetration of this Nafion film into the catalyst layer increasing the diffusional pathways for the reactant gases. These results correlate well with the mass transport characteristics in O2 and air as well as morphological characterization of the electrode based on SEM and pore volume distributions.