Effect of varying Ni content on hydrogen absorption–desorption and electrochemical properties of Zr-Ti-Ni-Cr-Mn high-entropy alloys

Abstract

The crystal structure, pressure composition isotherms, and electrochemical properties of Zr0.2Ti0.2Ni0.2+xCr0.2Mn0.2 (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 (Zr0.5Ti0.5)Mn2 hexagonal structure and a secondary phase with a B2-type Ti0.6Zr0.4Ni 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 Ti0.6Zr0.4Ni 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 Zr0.2Ti0.2Ni0.2Cr0.2Mn0.2 alloy surface changed to a porous and rough structure, and a ZrO2 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.