Yong-Kook Choi

Thin Titanium Dioxide Film Electrodes Prepared by Thermal Oxidation

Effects of temperatures, at which TiO 2-x electrodes were prepared by thermal oxidation from titanium sheet metals, as well as platinum loading, on electrochemical and photoelectrochemical properties have been studied and the results are reported. Titanium dioxide electrodes prepared at higher temperatures were found to have slightly more negative flat-band potentials and significantly higher donor densities than their low temperature counterparts. Platinum loading showed a similar effect to a lesser extent

Catalytic effect of transition metal(II)-N,N′-bis(naphthaldehyde)diimines on reduction of thionyl chloride

Abstract A series of transition metal(II) complexes, such as 1,2-bis(naphthylideneimino)ethane [M(II)(NAPET)], 1,3-bis(naphthylideneimino)propane [M(II)(NAPPR)], 1,4-bis(naphthylideneimino)butane [M(II)(NAPBU)] and 1,5-bis(naphthylideneimino)pentane [M(II)(NAPPE)] (M=Co, Cu, Ni), are synthesized. Catalytic effects of these complexes for the reduction of thionyl chloride on a glassy carbon electrode are evaluated by determining kinetic parameters with cyclic voltammetry. The charge transfer process during the reduction of thionyl chloride is affected by the concentration of catalysts. Some tetradentate Schiff base M(II) complexes show catalytic activities for the reduction of thionyl chloride. The catalytic effects are demonstrated from a shift of the reduction potential for thionyl chloride towards a more positive direction and an increase in peak current. Significant improvements in the cell performance have been noted in terms of both thermodynamic and kinetic parameters for the thionyl chloride reduction. An exchange rate constant, k o , of 1.24×10 −8 cm/s was observed at a bare electrode, while larger values of 0.23–1.69×10 −7 cm/s were observed at the catalyst-supported glassy carbon electrode. Thermodynamic and kinetic parameters for thionyl chloride reduction are affected by the chelate ring size of ligands in the metal(II)–Schiff base complexes used as a catalyst.

Electrochemical properties of passivation film on mesophase pitch-based carbon fiber electrode

Abstract We studied the irreversible capacity in negative carbon electrodes for the Li ion battery. In this work, we focused on electrolyte decomposition which is one of the main effects related to film formation. We investigated the properties of the film and the process of Li + intercalation into the mesophase pitch-based carbon fiber (MPCF) in 1 M LiPF 6 -EC/DEC (1:1, v/v) electrolyte solution. The charge-discharge profile, cyclic voltammetry and impedance spectroscopy showed the electrochemical properties of the film which is formed on the MPCF at the voltage range of 2.4–0.5 V (vs. Li/Li + ) during the first charge. FTIR-ATR spectra confirmed the film composition, which consist of Li 2 CO 3 , ROCO 2 Li, ROLi (R=CH 3 or CH 2 CH 3 ). We discuss a systematic change of the structure of MPCF electrode–electrolyte interface at the first intercalation potentials.

Oxygen reduction at Co(II)2-disalophen modified carbon electrodes

Reduction of dioxygen in both aqueous and nonaqueous solutions has been receiving a great deal of attention due to its implications in practical applications such as fuel cells and batteries, as well as in biological reactions. Oxygen reduction at a dinuclear Co(II)-Schiff-base complex, Co(II){sub 2}-3,3{prime},4,4{prime}-tetra(salicylidene imino-1,1{prime}-biphenyl tetrahydrate) [Co(II){sub 2}-disalophen; Co(II){sub 2}-DSP {center_dot} 4H{sub 2}O], modified carbon electrodes has been studied and results thereof are reported. The absorbed complex is shown to have a large catalytic effect for oxygen reduction. The oxygen reduction at the Co(II){sub 2}-DSP {center_dot} 4H{sub 2}O modified carbon electrodes is reasonably reversible in alkaline solutions with an apparent number of electrons transferred of around one and an estimated exchange rate constant of about 0.04 cm/s. The strong catalytic activity is explained by the formation of a reversible adduct with oxygen followed by reduction of Co(III) in the adduct. The cyclic voltammetric and chronoamperometric experimental results suggest that the first electron transfer should be a rate limiting step.

Intercalation/deintercalation characteristics of electrodeposited and anodized nickel thin film on ITO electrode in aqueous and nonaqueous electrolytes

Abstract The intercalation/deintercalation characteristics of electrochemically formed Ni(OH) 2 /Ni/ITO electrodes in aqueous KOH and nonaqueous LiPF 6 +EC:DEC (1:1) electrolyte solutions were studied. The Ni(OH) 2 film was grown by Ni electrodeposition and a subsequent anodizing process. The metallic Ni nano-layer between Ni(OH) 2 and ITO was identified by X-ray photoelectron spectroscopy (XPS). To understand the characteristics of the Ni(OH) 2 film, the intercalation and deintercalation of OH − ion in KOH and Li + in LiPF 6 +EC:DEC (1:1) electrolyte were investigated using cyclic voltammetry and charge calculations and further confirmed with the structure, oxide states and interfacial properties of the Ni(OH) 2 layer using X-ray diffraction, atomic force microscopy, scanning electron microscopy, XPS, ac impedance spectroscopy, and in situ transmittance spectroscopy. The Ni(OH) 2 /Ni/ITO electrode showed improved interfacial properties due to the metallic Ni nano-layer when it was compared with directly electrodeposited Ni(OH) 2 /ITO.

Electrocatalytic Reduction of Dioxygen at Carbon Electrodes Modified with Co(II)-Schiff Base Complexes in Various pH Solutions

The electrocatalytic reduction of dioxygen has been studied by cyclic voltammetry and chronoamperometry at a glassy carbon electrode and a carbon microelectrode modified with Schiff base Co(II) complex in a dioxygen saturated 1 M KOH aqueous solution of various pH and temperature. The electrochemical reduction of dioxygen at the modified electrode establishes a pathway of two-electron reduction to form hydrogen peroxide. The reduction potential (EP) of dioxygen shows pH dependence. The electrochemical reduction of dioxygen is irreversible and diffusion controlled. The activation energy obtained from Arrhenius plots for the reduction of dioxygen was 3.64 Kcal/mol at the bare glassy carbon electrode, whereas it was 2.51 Kcal/mol at the modified glassy carbon electrode.

Electrocatalytic reduction of dioxygen by quadruply aza bridged closely interfaced cofacial bis(5,10,15,20-tetraphenylporphyrin)s in various pH solutions

Abstract The electrocatalytic reduction of dioxygen is investigated by cyclic voltammetry and chronoamperometry at a glassy carbon electrode and a carbon microelectrode adsorbed with cobalt porphyrins in dioxygen-saturated aqueous solutions of various pH. The reduction potential ( E p ) of dioxygen at chemically modified electrodes shows dependence on pH. Electrochemical reduction of dioxygen at monomeric Co-1 or dimeric Co 2 -2 modified electrodes establishes a pathway of 2e − reduction to form hydrogen peroxide, but it carries out 4e − reduction of dioxygen to water at the electrode adsorbed with dimeric Co 2 -3 or Co 2 -4 in all pH solutions. The electrocatalytic pathway of dioxygen reduction by cofacial biscobalt porphyrins is independent of pH, but the E p of dioxygen reduction is dependent on pH. Theelectrochemical reduction of dioxygen is irreversible and diffusion controlled.

Electrochemical Reduction of Thionyl Chloride Studied by Cyclic Voltammetry, Chronocoulometry, and Chronoamperometry

Electrochemical reduction of thionyl chloride has been studied at a molybdenum electrode employing cyclic voltammetric, chronocoulometric, and chronoamperometric techniques. The charge transfer process is affected strongly by the film formed on the electrode surface during the reduction. The exchange rate constants, k 0 , of the order of 10 -6 cm/s were observed at fresh electrodes, whereas much smaller constants of about 10 -10 cm/s were observed at film-covered electrodes

Studies on the effects of coated Li2CO3 on lithium electrode

Abstract Although a lithium metal anode has a high energy density compared with a carbon insertion anode, the poor rechargeability prevents the practical use of anode materials. A lithium electrode coated with Li 2 CO 3 was prepared as a negative electrode to enhance cycleability through the control of the solid electrolyte interface (SEI) layer formation in Li secondary batteries. The electrochemical characteristics of the SEI layer were examined using chronopotentiometry (CP) and impedance spectroscopy. The Li 2 CO 3 –SEI layer prevents electrolyte decomposition reaction and has low interface resistance. In addition, the lithium ion diffusion in the SEI layer of the uncoated and the Li 2 CO 3 -coated electrode was evaluated using chronoamperometry (CA).

Properties of passivation film on carbon electrode in LiPF6/EC+DEC electrolytes

Abstract The co-solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) was used to investigate the decomposition of electrolyte in Li-ion batteries. The electrolyte solutions were prepared by mixing in various volume ratios from pure DEC to 7:3 (EC:DEC). The potentials at which they are decomposed on the anodic electrode were examined using cyclic voltammetry. It was found that some kinds of reduction reactions proceeded and a film on the surface of the anode was formed. The film showed different properties, which were dependent on the mixing ratio of the solvents. From our results, we concluded that the best composition ratio of EC:DEC in 1 M LiPF 6 /(EC+DEC) system was approximately 4:6 (EC:DEC, volume ratio).