Under the gravity field parallel to an electrode surface, a convection flow along the surface occurs by the difference of fluid density arising from electrode reactions. To examine this parallel gravity effect on electrochemical reactions, at first, the convective-diffusion equation of the reactant was solved. Then, to examine the gravity effect on the electron-transfer process in electrochemical reaction, a theoretical equation was derived which depicted the reaction in the mixed rate-controlling state of the diffusion- and the electron-transfer processes in the gravity fields parallel to the electrode surface. Then, by using a new type of electrode (called the gravity electrode) that utilized a gravity field generated by rotation, the electrolytic currents of the ferrocyanide oxidation and the ferricyanide reduction were measured under various gravity fields with various revolution rates. The Tafel lines obtained were compared with the results from the usual chronopotentiometry in the natural gravity field. Consequently, it was concluded that there is no change in the reaction process in the gravity field up to 170 g.
Dissolution process of Mn from LiMn2O4 cathode during the cyclic use of an electrochemical cell was examined by newly developed in situ TXRF (Total Reflection X-ray fluorescence) technique. Mn concentration in the electrolyte solution during electrochemical cycling was successfully determined for the first time without disassembling the cell using ten micro liters of the sample solution taken by inserting a fused silica capillary. The Mn concentration versus XRF intensity calibration curve showed a good linearity from 3 to 500 ppm. The dissolution of Mn in the cell increased with increasing the cycle number at 50°C. The Mn concentration is the highest at the center of the electrodes. It was also found that dissolution of Mn from the cathode material took place during the change process. The chemical state of Mn in the electrolyte solution was examined by using fluorescence X-ray absorption fine structure (XAFS) technique utilizing a 19-element solid state detector and synchrotron radiation. Mn K-XANES (X-ray absorption near-edge structure) spectra of the dissolved Mn in the electrolyte solution suggested that Mn existed as Mn2+ state regardless of the temperature. The unknown material deposited on the Li anode was estimated to be a divalent manganese compound with fluorine ligand (Mn-F distance = 2.16 Å) by XAFS analysis.
Deterioration of the gas-diffusion type oxygen cathodes loaded with Ag catalyst has been found during the long term operation under the practical conditions of chlor-alkali electrolysis. The mechanism of the deterioration has been investigated by analyzing the change of the gas-diffusion layer and the reaction layer. It is concluded that change of the carbon black in the gas-diffusion layer from hydrophobic to hydrophilic nature is the main reason for the deterioration although loss of the Ag catalyst from the reaction layer contributes to some extent to the deterioration of the cathode performance. The change of the carbon black, caused probably by hydrogen peroxide generated at the cathode, has led most likely to intrusion of caustic soda solution into the gas-diffusion layer, which is the direct reason of the deterioration of the cathode performance.
A dye-sensitized solar cell using a chlorine-e6 (Chl-e6) derived from chlorophyll-α, as visible and near-infrared sensitizer for nanocrystalline TiO2 films was developed. The short-circuit photocurrent density (ISC), the open-circuit photo-voltage (VOC), and the fill factor (FF) of solar cell using Chl-e6 adsorbed on nanocrystalline TiO2 film electrode were estimated to be 0.305 ± 0.012 mA cm−2, 426 ± 10 mV, and 45.0%, respectively. The overall photoenergy conversion efficiency (η) was 0.73%.
Crystalline olivine type LiFePO4 as cathode material for Li-ion battery was prepared by emulsion drying method. By employing this method, difficulty of conventional preparation, that is, repeated recalculations and subsequent regrindings, was solved. This method replaced successfully relatively expensive divalent iron to cheaper trivalent iron as a starting material by burning out the emulsion-dried precipitates with carbon, which act as a strong reducing agent. High temperature calcination of the emulsion-dried precursor powders in an Ar atmosphere resulted in high crystalline olivine type LiFePO4 powders which was confirmed and refined by Rietveld method of X-ray diffraction data. The prepared material delivered a capacity as high as 120 and 140 mAh (g-phosphate)−1 at 25 and 50°C, respectively.
The past decade has seen explosive growth in the synthesis and study of a wide range of nanostructured materials, including preparation and characterization of nanostructured surfaces (and materials) in contemporary electrochemistry. In this lecture, several examples from our recent results will be presented including investigations of nanoislands on single crystal electrode surfaces and on nanoparticle surfaces for a variety of fundamental science, electrocatalytic and fuel cell applications. Some opportunities and challenges in this exciting and growing area of electrochemistry and nanotechnology will be discussed.