Stability and tolerance to impurities of the fluorinated surface of hydrogen-absorbing alloys

Abstract

The surface structure of a fluorinated LaNi 4.7Al 0.3 alloy that was ground by hydriding-dehydriding (HD) cycling was determined by means of scanning electron microscopy, electron probe microanalysis and X-ray photoelectron spectroscopy. The change and the stability of the fluorinated surface were studied after exposure to water and air for various periods, and after further HD cycles. It was found that an LaF 3 layer was formed on the surfaces of the particles and on the surfaces of the cracks that were formed during HD cycling, after treating the sample with an F −-containing aqueous solution. Excellent hydriding characteristics were observed for the fluorinated alloy. After exposure of the fluorinated sample to air and water for a long period, and after repeating the reaction cycles with pure hydrogen gas or H 2 gas containing CO, a stable LaF 3 layer still remained on the surface. Excellent reactivity was still maintained, correspondingly. The degradation of the hydriding reaction rate and of the hydrogen capacity of the fluorinated alloy were found to be very small, even when using a H 2 + 300 ppm CO gas mixture at lower temperatures. On the contrary, this gas mixture deactivates the untreated alloy severely under the same conditions. These observations indicate that the overlying fluoride layer can effectively protect the sublayer from impurities.