Kazuhisa Tamura

Dynamic structural changes at LiMn2O4/electrolyte interface during lithium battery reaction.

Gaining a thorough understanding of the reactions on the electrode surfaces of lithium batteries is critical for designing new electrode materials suitable for high-power, long-life operation. A technique for directly observing surface structural changes has been developed that employs an epitaxial LiMn(2)O(4) thin-film model electrode and surface X-ray diffraction (SXRD). Epitaxial LiMn(2)O(4) thin films with restricted lattice planes (111) and (110) are grown on SrTiO(3) substrates by pulsed laser deposition. In situ SXRD studies have revealed dynamic structural changes that reduce the atomic symmetry at the electrode surface during the initial electrochemical reaction. The surface structural changes commence with the formation of an electric double layer, which is followed by surface reconstruction when a voltage is applied in the first charge process. Transmission electron microscopy images after 10 cycles confirm the formation of a solid electrolyte interface (SEI) layer on both the (111) and (110) surfaces and Mn dissolution from the (110) surface. The (111) surface is more stable than the (110) surface. The electrode stability of LiMn(2)O(4) depends on the reaction rate of SEI formation and the stability of the reconstructed surface structure.

Dissolution of the anodic oxide film on aluminium in a sulphuric acid solution

Abstract The time-variation of the amount of Al ions dissolved from anodic oxide films on aluminium was followed after interrupting the anodizing current in a 10 % sulphuric acid solution. The amount of Al ions included in the film before the dissolution experiment, W0a, was estimated by measuring the charge passed and the amount of Al ions in the solution during anodization. It was found that the rate of dissolution increases with time until a transient time tR is reached beyond which the rate decreases rapidly to nearly zero. In this final stage of film dissolution, the surface appeared to be covered with a thin oxide layer of a highly protective nature. For films of different thickness formed at a constant anode potential of 11·9 V, the total amount of dissolution always agreed well with W0a. This means that there is no self-corrosion of the underlying metal during the dissolution experiment. It was recognized that the transient time of 120 min is almost independent of the initial film thickness or time of anodization. The results are quantitatively explained by assuming a constant rate of dissolution of the film from the inner surface of its pore structure, for which Kellers model was assumed. This rate, from the experimental data, is 7·5 × 10−9 cm/min. The dissolution behaviour of Al during anodization is also discussed; the rate of dissolution from the bottom of the pores is equivalent to 1·04 × 10−4 cm/min, for anodizing at 11·9 V, 9·4 mA/cm2.

Characterization of Electrode/Electrolyte Interface with X-Ray Reflectometry and Epitaxial-Film LiMn2O4 Electrode

Structural changes at electrode/electrolyte interface of a lithium cell were studied by X-ray reflectometry and two-dimensional model electrodes with a restricted lattice plane of LiMn 2 O 4 . The electrodes were constructed with an epitaxial film synthesized by the pulsed laser deposition method. The orientation of the film depends on the substrate plane; the (111), (110), and (100) planes of LiMn 2 O 4 grew on the (111), (110), and (100) planes of the SrTiO 3 substrates, respectively. The ex situ reflectometry indicated that a thin impurity layer covered the lattice plane of the as-grown film. The impurity layer was dissolved and a solid-electrolyte-interface-like phase appeared after the electrode was soaked into the electrolyte. A defect layer was formed in the (111) plane, whereas no density changes were detected for the other lattice planes. The in situ observation clarified that the surface reactivity depended on the lattice planes of the spinel; the defect layer at the (111) plane was stable during the electrochemical reaction, whereas a slight decrease in the film thickness was observed for the (110) plane. Our surface characterization of the intercalation electrode indicated that the surface structure changes during the pristine stage of the change-discharge processes and these changes are dependent on the lattice orientation of LiMn 2 O 4 .

On the mechanism of dissolution of porous oxide films on aluminium during anodizing

Abstract The temperature distribution along the vertical pores of the anodic oxide films on aluminium was calculated for usual conditions of anodizing in 10% sulphuric acid solution by taking into account the accumulation of both Joule heat and the heat of formation of oxide at the pore-base and the outward conduction of heat through the solution-filled pore. The results show that a steady state of distribution is established at any instant and at any film thickness during anodizing and that the temperature rise at the pore-base is always negligibly small. This conclusion was confirmed by some experiments on the dissolution behaviour of oxide films. It was demonstrated that small changes in the bulk solution temperature significantly affect the dissolution rate and anodizing current. It was also shown that the dissolution rate of oxide at the pore-base is almost constant irrespective of the film thickness and of the time of anodizing. This rate was evaluated in the manner reported previously by subtracting the dissolution rate at the side walls of the pores from the total dissolution rate. These facts are in direct opposition to the ‘thermal mechanism’ and suggest that the extra-high rate of dissolution during anodizing almost certainly results from the high electrical field applied across the barrier layer.

Reaction pathway of four-electron oxidation of formaldehyde on platinum electrode as observed by in situ optical spectroscopy

Abstract When a platinum electrode is dipped into aqueous sulfuric acid solution containing formaldehyde, the surface is covered by CO, which is formed by the dissociative adsorption of formaldehyde. A second harmonic generation voltammogram pumped by a 532 nm laser pulse and photo-reflectance voltammogram probed by a 670 nm laser beam are sensitive to the surface CO and surface oxide, respectively. The experiments demonstrate that the anodic oxidation pathway of the formaldehyde through CO adsorption proceeds by coupling with the surface oxide formation and that the CO controls the dual reaction pathway, the anodic and cathodic path of the reaction, and induces a non-linear self-sustained oscillation. The kinetics of the dissociative adsorption is discussed based on the time course of the current induced by the laser desorption of CO.

A novel spectroelectrochemical cell for in situ surface X-ray scattering measurements of single crystal disk electrodes

A newly designed spectroelectrochemical cell was constructed for surface X-ray scattering (SXS) to study single crystal electrodes. Electrochemical characteristics of a specific face of a single crystal electrode can be investigated in the meniscus mode, and SXS measurement can be easily carried out using the spectroelectrochemical cell. The usefulness of the present cell was demonstrated by studying the electrochemical deposition of thin Pd layers on Au(111) and Au(100) that require precise amounts of Pd deposits.

LOCAL STRUCTURES OF ISOVALENT AND HETEROVALENT DILUTE IMPURITIES IN SI CRYSTAL PROBED BY FLUORESCENCE X-RAY ABSORPTION FINE STRUCTURE

Local structures of dilute isovalent and heterovalent impurity atoms in Si crystal (Si:X, X=Ga, Ge, As) have been studied by fluorescence x-ray absorption fine structure. The distortion of local lattice around the impurity atoms was evaluated from the Si–X bond length determined by extended x-ray absorption fine structure. The results demonstrate that the local lattice deformation is strongly dependent on the electronic configuration of impurity atoms, i.e., we find an anomalous expansion (0.09±0.01 A) along the [111] direction for donor (As) atoms but much smaller magnitude (0.03±0.01 A) for isovalent (Ge) atoms and acceptor (Ga) atoms. The results suggest that the local lattice distortions are strongly affected by the Coulomb interactions between the localized charge, which piles up to screen the ion core and the bond charge, and the ion-core repulsion. Absence of anomaly in case of negatively charged Ga atoms suggests that the former mechanism is a dominant factor for anomalous lattice expansion.

Mechanistic studies on lithium intercalation in a lithium-rich layered material using Li2RuO3 epitaxial film electrodes and in situ surface X-ray analysis

The surface structure of a lithium-rich layered material and its relation to intercalation properties were investigated by synchrotron X-ray surface structural analyses using Li2RuO3 epitaxial-film model electrodes with different lattice planes of (010) and (001). Electrochemical charge–discharge measurements confirmed reversible lithium intercalation activity through both planes, corresponding to three-dimensional lithium diffusion within the Li2RuO3. The (001) plane exhibited higher discharge capacities compared to the (010) plane under high rate operation (over 5 C). Direct observations of surface structural changes by in situ surface X-ray diffraction (XRD) and surface X-ray absorption near edge structure (XANES) established that an irreversible phase change occurs at the (010) surface during the first (de)intercalation process, whereas reversible structural changes take place at the (001) surface. These experimental findings suggest that the surface reconstructed phase limits lithium intercalation between the electrode and the electrolyte, leading to the poor rate capability of the (010) film. Surface structural changes at the initial cycling therefore have a pronounced effect on the power characteristics and stability of lithium-rich layered materials during battery operation.

Orientation dependence of Pd growth on Au electrode surfaces

The structure of thin Pd films grown on Au(111) and Au(001) electrodes in a solution containing PdCl(4)(2 -) and SO(4)(2 -) has been investigated by surface x-ray scattering. This technique provided structural information on the Pd films in the lateral direction as well as in the surface normal direction. Comparison of Pd/Au(111) and Pd/Au(001) growth modes shows similarity in the first layer deposition. On Au(111) and Au(001) substrates, Pd follows the crystal structure of the substrates and forms a pseudomorphic monolayer. Beyond 2 ML, however, Pd films grown on Au(111) are relaxed, although there are still pseudomorphic layers at the interface. In contrast, Pd films on Au(001) continue to grow pseudomorphically over 10 ML. The difference in the growth mode between (111) and (001) surfaces is not ascribable only to anisotropy in the elasticity of the film. The relationship between a growing surface and an allowed gliding plane in misfit dislocations is presented as a crucial factor determining the critical thickness of the film.

Origin of the enhancement of electrocatalytic activity and durability of PtRu alloy prepared from a hetero bi-nuclear Pt–Ru complex for methanol oxidation reactions

PtRu alloy, prepared by pyrolysis of hetero bi-nuclear Pt–Ru complexes, showed a higher electrocatalytic activity and durability in the methanol oxidation reaction (MOR) than PtRu alloy prepared by pyrolysis of a mixture of K2PtCl4 and RuCl3. The origin of this activity enhancement was investigated by XPS and Ru K edge XAFS measurements. Although both alloys were composed of metallic Pt and RuO2, the RuO2 in the PtRu alloy prepared from the Pt–Ru complex had greater oxygen deficiency than that in the PtRu alloy prepared from the mixture of K2PtCl4 and RuCl3. XAFS measurements showed that the Ru–O distance of the former was longer than that of the latter, suggesting the presence of the weakly coordinated oxygen species, i.e., H2O, in the PtRu alloy prepared from the Pt–Ru complex. The H2O coordinated to Ru can become an active oxidant, i.e., surface Ru–OH, under electrochemical conditions and enhance MOR activity and durability.