Abstract
The crystal structure, pressure composition isotherms, and electrochemical properties of Zr_(0.2)Ti_(0.2)Ni_(0.2+x)Cr_(0.2)Mn_(0.2) (x = 0, 0.025, 0.05, 0.075, and 0.1) high-entropy alloys (HEAs) were investigated. The crystal structures of all the HEAs consisted of two phases: a primary phase with a C14-type (Zr_(0.5)Ti_(0.5))Mn_2 hexagonal structure and a secondary phase with a B2-type Ti_(0.6)Zr_(0.4)Ni cubic structure. Rietveld analysis revealed that the secondary phase increased with an increase in the x value. The hydrogen storage capacity of the HEAs was lower than that of the alloy with x = 0 because of an increase in the B2-type Ti_(0.6)Zr_(0.4)Ni phase, whereas the change in enthalpy of hydride formation (|ΔH|) decreased with increasing x, leading to the instability of hydrides. The alkaline treatment was performed by immersing HEA powders or electrode in a 6 M KOH aqueous solution at 378 K for 2 h. The Zr_(0.2)Ti_(0.2)Ni_(0.2)Cr_(0.2)Mn_(0.2) alloy surface changed to a porous and rough structure, and a ZrO_2 passive thin layer on their surface, which is replaced by NiO or Ni(OH)_2 after the alkaline treatment. The charge-discharge tests conducted using the HEA negative electrodes for Ni-MH batteries depict that the discharge capacity increased with an increase in x, and the highest discharge capacity was 368 mAh g~(-1) at x = 0.075 after the alkaline treatment. On increasing the x value, the high-rate dischargeability and cycle performance were also enhanced.
NSTL
Elsevier