Effect of Ti and Zr additions on the characteristics of AB 5-type hydride electrode for Ni–MH secondary battery

Abstract

AB 5-type intermetallic compounds were prepared by arc-melting in an argon atmosphere. The composition of a stoichiometric compound ( LMNi 3.6 Al 0.4 Co 0.7 Mn 0.3) M y ( LM= La -rich mischmetal , M=Ti, Zr; y=0.0, 0.02, 0.05, 0.1, 0.2, 0.3 ) with a hexagonal CaCu 5 structure was varied by stoichiometric and nonstoichiometric addition of Ti or Zr. The alloy surface was analyzed by energy-dispersive spectroscopy and scanning electron microscopy and the crystal structure was characterized by X-ray diffraction. To determine hydrogen storage capacity, the pressure composition isotherms (PCT curves) were utilized. Metal hydride electrode was characterized by galvanostatic cycling test. It was found that the addition of Ti ( y=0.05) or Zr ( y=0.02) improves the activation, discharge capacity and cycle life property. But, the alloy with Ti addition ( y⩾0.2), containing a Zr addition ( y⩾0.2), is found to have bad discharge capacity and cycle life. The kinetic properties are additionally increased by the formation of a Ti- or Zr-rich second phase. The improvement in discharge capacity caused by increasing the Ti or Zr content (0.1⩾ y) in the alloy is attributed to active sites for the electrochemical reaction. Also, they seem to be strongly related with the shapes and amounts of the second phases.