Influence of the boron additive on the structure, thermodynamics and electrochemical properties of the MmNi3.55Co0.75Mn0.4Al0.3 hydrogen storage alloy

Over-stoichiometric double-phase MmNi(3.55)Co(0.75)Mn(0.4)Al(0.3)B(x) compound has been prepared based on the commercial MmNi(3.55)Co(0.75)Mn(0.4)Al(0.3) hydrogen storage alloy by adding boron element. The over-stoichiometric compounds containing baron show CaCu5-type main-phase and a small amount of CeCo4B-type phases. The abundance of secondary phase increases with the increase of boron content. The addition of boron displays a small influence on thermodynamic characteristics of compounds, whereas it reduces its hydrogen storage capacity. The electrochemical experiments show that the addition of boron modifies the activation performance and significantly improves the high-rate capability. Micro-polarization and constant potential discharge measurement reveal that the appearance of the secondary phase enhances the electrochemical activity of the hydride electrodes' surface and facilitates hydrogen diffusion in the bulk of the compound. (C) 2000 Elsevier Science B.V. All rights reserved.