Teruhiko Imoto

Development of an F-class refrigeration system using hydrogen-absorbing alloys

Abstract This paper presents a new refrigeration system to generate refrigeration heat below −20 °C and a coefficient of performance (COP) over 0.4 by using driving heat in the vicinity of 150 °C which can be supplied by solar collectors. The performance of the system was achieved mainly by development of a quaternary La-Y-Ni-Mn alloy with appropriate performance in the vicinity of −20 °C with regard to both equilibrium and kinetic characteristics for generating refrigeration heat.

The surface structure and the electrochemical properties of hydrogen-absorbing alloys treated with an HCl aqueous solution

The surface structure and the electrochemical properties of hydrogen-absorbing alloys treated with an HCl aqueous solution were investigated. This process was observed to form a porous layer rich in nickel on the alloy surface because HCl partially dissolved some of the other elements from the alloy. Therefore, the discharge capacity increased by increasing the reaction surface area. However, the overvoltage of an excessively treated alloy increased along with the depth of discharge as the surface layer of the alloy oxidized and thickened. The effects of treatment on alloys were found to be determined by the hydrogen ion amounts in the acid solution per unit mass of the alloy, regardless of the concentration and volume of the solution. The optimal hydrogen ion amount for a treatment solution was found to be 0.1 mol/kg.

Influence of surface treatment by HCl aqueous solution on electrochemical characteristics of a Mm(Ni–Co–Al–Mn)4.76 alloy for nickel–metal hydride batteries

Abstract Surface treatment of a Mm(Ni0.64Co0.20Al0.04Mn0.12)4.76 alloy used as negative electrode material was examined to improve the performance of nickel–metal hydride secondary batteries. The surface treatment was conducted by dipping and stirring the alloy into an HCl aqueous solution of pH 1.0 at room temperature. Initial discharge characteristics and charge–discharge cycle performance of the surface-treated alloy were evaluated by an electrochemical half cell and a sealed test cell. The surface structure of the surface-treated alloy was analyzed by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) with an energy disperse X-ray spectrometer (EDX). In this paper, the influence of the surface treatment on the charge–discharge characteristics, particularly on the initial discharge characteristics, and surface structure of the alloy are discussed.

Poisoning by air of AB5 type rare-earth nickel hydrogen-absorbing alloys

Abstract Poisoning by air of Mm 0.9 Y 0.1 Ni 4.9 Mn 0.1 and LaNi 4.55 Al 0.45 alloys with CaCu 5 structure was studied to clarify their applicability to refrigeration systems. The decrease in hydrogen capacity on hydrogen absorption-desorption cycling with air contamination was limited to a certain degree under an extended air concentration up to 1.0 vol.%. The poisoned alloys recovered their hydrogen-absorbing capacity on reactivation treatment. These results and X-ray powder diffraction analysis suggest that the degradation was not caused by complete oxidation. Thus, we propose that the degradation is caused by adsorption of oxygen in air, and that the poisoned state is an intermediate state which is more stable than the adsorption equilibrium state but less stable than the oxidation state. Poisoning by adsorption was analyzed using Langmuirs adsorption theory to explain the relations between the degradation rate and concentration of air.

Microstructure and electrochemical characteristics of surface-treated Mm(Ni-Co-Al-Mn)4.76 alloys for nickel–metal hydride batteries

Abstract The microstructure and electrochemical characteristics of surface-treated Mm(Ni 0.64 Co 0.20 Al 0.04 Mn 0.12 ) 4.76 alloys, (a) induction-melted and subsequently annealed alloy and (b) rapidly quenched and subsequently annealed alloy were examined to clarify the influence of the microstructure on the surface treatment effects. The surface treatment was conducted by dipping and stirring the alloys into an HCl aqueous solution of pH 1.0 at room temperature. Their initial discharge characteristics and charge–discharge cycle performance were evaluated by an electrochemical half cell and a sealed test cell. The surface structure was analyzed by using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) with an energy disperse X-ray spectrometer (EDX). It was found that a more homogeneous microstructure present before the surface treatment leads to an increase of the specific surface area and to an enrichment of Ni and Co metal on the surface layer of the surface-treated alloy. This leads to an enhancement of the initial electrochemical activity.