To understand the role of water on zirconium passivation in n-butanol solutions containing Bun4NBr, composition and corrosion properties of the passive film were studied using cyclic voltammetry, X-ray photoelectron and electrochemical impedance spectroscopy. Zirconium undergoes spontaneous passivation followed by pitting corrosion as a result of passivity breakdown by the aggressive attack of bromide anions. The passive film consists mainly of ZrO2, ZrO2·2H2O and a small amount of zirconium butoxide. The pitting potential shifts positively and pitting corrosion is seriously inhibited with the addition of a small amount of water. Water improves the pitting corrosion resistance of the passive film by changing the thickness and the relative ratio of OH−/O2−. The result is helpful to electrosynthesize zirconium butoxide with high energy efficiency.
Quantum mechanical theory of electrochemical kinetics based on Fermi’s golden rule was formulated by introducing the concept of electron transfer distance. The expressions for the exchange current density and standard rate constant in electrochemistry were derived in analytical form, as well as exponential current overpotential dependence. The theory corresponds well to the electrode kinetics based on the transition state theory. It was applied to various kinds of electrode reactions to analyze the standard rate constants and the exchange current densities reported in past literature. The evaluated magnitudes of the electron exchange energy were very small, being in the order of 10−3 eV–10−5 eV. A new theory of transfer coefficient was constructed based on Debye-Hückel theory for electrolyte solutions could explain quantitatively the dependence of the transfer coefficient on the ionic strength of electrolyte solutions. It was demonstrated that the transfer coefficient represents electrostatic screening of the electrode potential by ions near the electrode and its magnitude was calculated quantitatively. Electron transfer distance was obtained by analyzing the dependence of the transfer coefficient on the ionic strength of electrolyte solutions. Our theory supported the ordinary electron transfer mechanism due to the overlap of wave functions between the electrode and redox species, denying tunneling mechanism.
Iron sulfide (FeSx) composite positive electrodes were prepared mechanochemistry and applied to all-solid-state lithium cells. The prepared composites, consisting of Fe, S, Li3PS4 solid electrolyte (SE), and vapor-growth carbon fiber (VGCF), were in amorphous state after ball milling for 10 h. An all-solid-state cell with an amorphous Fe-S-SE-VGCF composite as the positive electrode exhibits a high reversible capacity of 420 mAh per total weight of the composite electrode at a current density of 0.13 mA cm−2 at 25°C. The cell exhibited a capacity retention of 88% after 200 cycles at a current density of 0.64 mA cm−2 at 25°C. The all-solid-state cell using the Fe-S-SE-VGCF composite at a current density of 0.13 mA cm−2 at 100°C exhibited a higher reversible capacity of 550 mAh g−1. The Fe-S-SE-VGCF composite is thus a promising positive electrode material, with high capacity and good cycle performance for all-solid-state lithium secondary batteries.
Two types of the so-called “metal fog” were observed during Li electrodeposition in LiCl-KCl eutectic melt. One of them colored gray showed unique behaviors, and was hardly explained by the mechanisms reported before. In this study, the conditions to generate this “gray metal fog” have been investigated in detail, and its mechanism is discussed. A drop of the cathodic current and the subsequent current vibration were observed with the gray fog generation, and the in-situ observation indicated that a black film was formed on the surface of electrode just before this fog dispersion. The black film was formed only on an electrodeposited thin Li metal film, and spread from the edge of the electrode to the whole surface. The black film was formed and disappeared repeatedly, and the gray fog was seen only on the black film. These results suggest that the existence of the black film causes the gray fog generation, and that the film is formed electrochemically and decomposed spontaneously.
This paper reports unusual diffusion-controlled growth of TiO2 mesoporous anodic films on titanium in hot phosphate/glycerol electrolytes. The formation behavior was investigated by cyclic voltammetry (CV) between 0 and 5 V vs. Pt at 433 K. The current density became almost constant above 1.5 V vs. Pt during the positive potential sweep, and was maintained even during the negative potential sweep. This is contrast to a drastic decrease in current density in changing the direction of potential sweep from the positive to negative in fluoride-containing ethylene glycol electrolyte. The constant current density between 1.5 and 5 V vs. Pt increased with an increase in the basicity of the hot phosphate electrolyte, suggesting that the rate-determining step of the film formation in the hot phosphate electrolyte was diffusion process of oxygen sources in the electrolyte, not the ion migration in the thin barrier layer under the high electric field. When CV measurements were conducted to higher potentials up to 20 V vs. Pt, anatase was developed above 7 V vs. Pt, leading to generate oxygen gas. The film morphology was also potential-dependent and the diffusion current was also influenced by the film morphology as well as oxygen gas generation.
Ibuprofen (2-(p-isobutylphenyl)propionic acid) is adsorbed strongly onto the surface of commercial activated carbon and is never released thereafter by subsequent immersion in water. The electrochemical responses for the ibuprofen-adsorbed activated carbon electrode can be monitored via cyclic voltammetry and impedance spectrometry. The influence of adsorbed ibuprofen can be extracted. The adsorbed ibuprofen molecules passivate the pore surface of the activated carbon electrode and decrease its capacitance. The capacitance by ibuprofen adsorption remains decreased even after the exposure to a more positive potential.
Spinel lithium titanate (LTO; Li4Ti5O12) attracts much attention as a negative electrode material for a sodium-ion battery (SIB), while large volume changes in Na-insertion and extraction processes prevent practical applications of LTO-based electrodes. It is desirable to form a Na-substituted LTO phase as (Na3)8a(LiTi5)16d(O12)32e, which is expected to show excellent performance in a SIB, due to a small volume change from a Na-inserted phase, (Na6)16c(LiTi5)16d(O12)32e, analogous to a strain-free LTO electrode in a LIB. In this work, we have discovered that such a Na-substituted phase is really formed via the discharge (Na-extraction) process from a Na-inserted LTO electrode consisting of two phases as (Na6)16c(LiTi5)16d(O12)32e and (Li6)16c(LiTi5)16d(O12)32e. The Na-substituted phase generation occurs by the discharge with a high current density about 10 C rate, which induces high electrochemical polarization, exceeding the Li-extraction electrochemical potential in the discharging cell voltage. Thus both the Na-substituted LTO and pure LTO phases are formed due to the extraction of both Na+ and Li+ ions as an electrochemical equilibrium process. The present finding is a significant step toward practical application of the LTO-based electrode in a SIB.
Semiconductive silicon is widely used in solar cells, thyristors, and other important application. However, smelting and refining Si from silicon dioxide (SiO2) still require a large amount of energy, particularly for the reduction of SiO2 and removal of impurities. In this work, we designed an approach to prepare very fine Si powder from crystalline and/or amorphous SiO2 through the magnesiothermic reduction of SiO2 in molten salts. Moreover, the mechanism of reduction below 1273 K was elucidated. The composition of molten salts and the reaction temperature were varied, and their effect on the Si yield was investigated. The yield was lower in the molten NaCl-MgCl2 molten salt solvent than in LiCl-MgCl2, likely because of the Mg2Si by-product formation. The higher yield in LiCl-MgCl2 resulted from the better solubility of Mg in this molten salt and the suppression of Mg2Si formation.