Surface processes of H2 on rare earth based hydrogen storage alloys with various surface modifications

This paper reviews typical and significant results of the recent studies for the surface processes in H-2 absorption by rare earth based hydrogen storage alloys (RE-HSA). The kinematics of the hydrogen uptake by RE-HSA in the Hz gas and electrochemical reactions can drastically be influenced by spontaneous and intentional modifications of the surface of RE-HSA. This paper deals with the following prototypical cases: The formation of the oxide layer on the surface of the alloys shifts the rate controlling step of the H-2 absorption from H-2 dissociation on the surface with thin oxide layers to the H permeation through the layer as the layer grows. Partial substitution of Ni with Al, Co or Mn for LaNi5-x(Al,Mn,Co)(x) results in the change in the surface H-2 processes, depending upon each substituent element. The penetration of alkaline atoms (Li, Na, K) into the surface of the alloys markedly enhances the initial activation in the H-2 gas and electrochemical reactions. The fluorination treatment of the alloy surfaces effectively protects the surface from the attack of contaminations in the H-2 gas. (C) 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.