Tatsuo Nishina

Determination of Chemical Diffusion Coefficients in Metal Hydride Particles with a Microelectrode Technique

Microelectrochemical studies for hydrogen storage materials have been carried out by using a microelectrode technique. A carbon fiber filament was contacted with single particles of LaNi{sub 4.5}Al{sub 0.5}, LmNi{sub 4.0}Cr{sub 0.4}Mn{sub 0.3}Al{sub 0.3}, ZrNi{sub 1.4}, ZrMn{sub 0.6}Co{sub 0.1}V{sub 0.2}Ni{sub 1.2}, and ZrMn{sub 0.4}Cr{sub 0.4}Ni{sub 1.2} in 1 M KOH solutions by using a micromanipulator, where Lm indicates the lanthanum-rich rare-earth metal. In this way, cyclic voltammetry and potential step experiments, together with the in situ x-ray diffraction experiments, were conducted and the apparent hydrogen diffusion coefficients in solid phases were determined. The current responses recorded as a function of time were analyzed with a spherical diffusion model and the results are reported herein.

In Situ Conductivity Measurements of LiCoO2 Film during Lithium Insertion/Extraction by Using Interdigitated Microarray Electrodes

Thin films of LiMn2O4 have been prepared by RF magnetron sputtering on interdigitated microarray electrodes. In situ conductivity–potential profiles and cyclic voltammograms during extraction/insertion processes of Li ions were obtained simultaneously in nonaqueous and aqueous electrolyte solutions (1 M LiClO4/propylene carbonate and 1 M LiCl/water). The electronic conductivity of Li1– x Mn2O4 was found not to show metallic transition and maintain its semiconducting state during the extraction/insertion of Li ion. A slight decrease in conductivity was observed with increasing the anodic potential, i.e., with increasing x (lithium extraction) and recovered reversibly when the potential returned to the cathodic side (re-insertion of Li ions). Similar results were obtained in both aqueous and nonaqueous electrolyte solutions.

Electrochemical measurements of single particles of Pd and LaNi5 with a microelectrode technique

Abstract Microelectrochemical studies of hydrogen storage materials have been carried out using a microelectrode technique. A micromanipulator was used to contact single particles of Pd and LaNi 5 in alkaline solutions with a carbon fiber filament. In this way cyclic voltammetry and potential step experiments were conducted and the apparent hydrogen diffusion coefficients in solid phases were determined. The current responses recorded as a function of time were analyzed with a spherical diffusion model: the diffusion coefficients were D = (4.3 ± 1.9) × 10 −8 cm 2 s −1 for a LaNi 5 alloy particle and D = (2.8 ± 0.2) × 10 −7 cm 2 s −1 for a Pd particle, which were in good agreement with values reported in literature.

Electrochemical characterization of thin-film LiCOO2 electrodes in propylene carbonate solutions

Thin films of LiCo0 2 (0.22-1.15 μm) were prepared by oxidation of metallic cobalt in molten alkali carbonates containing Li + ions. We report here the behavior of the thin film of LiCoO 2 by cyclic voltammetry in propylene carbonate solvent systems, the kinetic analysis using electrochemical impedance spectroscopy (EIS), and potential step chronoamperometry (PSCA). The chemical diffusion coefficients for Li + ions in the LiCoO 2 electrodes were determined; D = (2.3-9.0) X 10 -12 cm 2 /s by PSCA, and D = 0.9 X 10 -9 -2 X 10 -8 cm 2 /s by EIS.

In situ Raman spectroscopic study of LixCoO2 electrodes in propylene carbonate solvent systems

Abstract Simultaneous measurements of in situ Raman spectroscopy and cyclic voltammetry have been carried out for thin film electrodes of Li x CoO 2 in propylene carbonate containing 1 M LiClO 4 . The Raman lines of 485 and 587 cm −1 observed at Li x CoO 2 electrodes were attributed to the A 1g and E g modes of the LiCoO 2 , respectively. The Raman intensity of two lines changed drastically during the insertion/extraction of lithium ions. This effect can be explained by the reduction of the optical skin depth due to the conductivity change of Li x CoO 2 . Phase transition from Raman-active phase to Raman-inactive phase is also conceivable.

Gas electrode reactions in molten carbonate media Part I. Exchange current density of oxygen reduction in (Li + K)CO3 eutectic at 650°C

Abstract Exchange current density (i0) measurements for oxygen reduction have been carried out by using ac impedance (AC), potential step (PS) and coulostatic relaxation (CS) methods on smooth gold electrodes in (42.7 + 57.3) mol% (Li + K)CO3 eutectic under PO2/PCO2 = 0.9/0.1 atmosphere at 650°C. A comparative study of the io values obtained by these techniques has demonstrated that they are of the order of 10−2 A cm−2 assuming n = 2; typical results are io = 38.5 mA cm−2 from the AC method (activation resistance = 1.03Ω cm2, Warburg coefficient = 147Ω cm2 s 1 2 , with double layer capacity (Cd) = 114μF cm−2, io = 26.9 mA cm−2 from the CS method (Cd = 111 μF cm−2) and io = 9.55 mA cm−2 from the PS method (α = 0.56.)

Electrochemical characterization of in situ NiO formation in a molten carbonate

Abstract The electrochemical oxidation of nickel electrodes in (62+38) mol% (Li+K)CO3 in different gas atmospheres has been studied using an ac impedance method. It was demonstrated that the impedance changes during oxidation under open-circuit conditions characterize well the in situ formation of lithiated NiO. Lithiation takes place with oxygen at a potential around −0.4 V vs. (1:2) O2/CO2 according to the parabolic rate law, involving the oxidation of Ni(II) to Ni(III) and the diffusion of Li+ into the solid phase. The parabolic rate law constant for lithiation and its activation energy were determined in the temperature range 650–750 °C. The lithiation process characteristic of a molten carbonate system is presented in comparison with the gas-phase oxidation of nickel. The characteristic features of NiO formation in pure He and CO2 atmospheres are also reported.

Oxide electrodes in molten carbonates Part 1. Electrochemical behaviour of nickel in molten Li + K and Na + K carbonate eutectics

Abstract Spontaneous electrochemical processes occurring on Co and CoO in molten Li + K and Na + K carbonate eutectics saturated with a 0.9 O 2 + 0.1 CO 2 atmosphere were investigated at 1000 K. It was shown that oxidation of Co to CoO proceeds via formation of an unstable compound in Li 2 CO 3 + K 2 CO 3 . There is no indication that such a compound is formed in Na 2 CO 3 + K 2 CO 3 . CoO undergoes further oxidation in molten carbonates producing LiCoO 2 in Li 2 CO 3 + K 2 CO 3 and NaCoO 2 (presumably) in Na 2 CO 3 + K 2 CO 3 . The oxidation processes increase the active area of the electrode as indicated by the measurements of exchange current densities, double-layer capacitances and impedances of electrodes.

Gas Electrode Reactions in Molten Carbonate Media IV . Electrode Kinetics and Mechanism of Hydrogen Oxidation in Eutectic

The hydrogen oxidation reaction in mole percent at 650°C has been studied at Au, Ag, Cu, Pt, Ir, Pd, Ni, Co, and Fe electrodes by using cyclic voltammetry, ac impedance, potential step, and chronocoulometric methods. Impedance data were analyzed by computer fitting with an equivalent circuit, taking hydrogen adsorption into account. The exchange current density decreased, under the gas composition of , in the following series: . The chronocoulometric method was employed to determine the stoichiometric number ν using a new technique described in this paper, and the value obtained was . The apparent transfer coefficient, α, and exchange current density were also determined from an Allen‐Hickling plot, and found to be for Au, Ir, and Ag, and for Ni and Pt. The concentrations and diffusion coefficients of the reactant and the product species, including the diffusion process of hydrogen dissolved in the Ni lattice, were estimated, based on an estimated value for from the Wilke‐Chang equation. Based on these results, a detailed reaction mechanism is proposed in this paper.

Characterization of a 100 cm2 Class Molten Carbonate Fuel Cell with Current Interruption

A current-interrupter method has been employed to measure the potential relaxation from output to open-circuit voltage (OCV). The relaxation process represents the voltage losses of molten-carbonate fuel cells which are composed of ohmic loss ({eta}{sub IR}), reaction overpotential ({eta}{sub Re}), and Nernst loss ({Delta}E{sub loss}). The responses of current interruption for a 100 cm{sup 2} class Li-Na carbonate fuel cell were measured as functions of temperature, gas utilization, and oxidant gas composition (O{sub 2}/CO{sub 2}). The single cell showed three different relaxation patterns of time regions during potential decay to OCV; the shortest time region (less than 20 {micro}s) is due to {eta}{sub IR}, an intermittent time region (20 {micro}s to 150 ms) is due to {eta}{sub Re}, and the residual time region is due to {Delta}E{sub loss}. The further analysis of {eta}{sub Re} data is consistent with the argument that the oxygen reduction reaction in the single cell is controlled by a mixed diffusion process of superoxide ion (O{sub 2}{sup {minus}}) and CO{sub 2}.