Optimization Of Microstructure And Electrochemical Properties Of Co-free AB₅ Hydrogen Storage Alloys

In recent years,the Co-free AB₅-type alloys has been commercially applied as the negative electrode of the nickel-metal hydride battery?Ni/MH?because of their high excellent overall electrochemical performances and low cost.The commercial Co-free AB5-type alloys are prepared by substituting Co element with cheap metal elements.However,the overall electrochemical properties of alloy electrode are reduced.Therefore,it is important to further reduce the cost and enhance the overall electrochemical properties of Co-free AB₅-type alloys for improving the market competitiveness of Ni/MH batteries.Herein,the cheaper WFe and MoFe alloys are used to substitute Ni or Mn in the Co-free AB₅-type alloys to improve the overall electrochemical performance and reduce the alloy cost.The La0.75Ce0.25Ni3.85-xMn0.35Al0.15Cu0.65?W0.42Fe0.58?x,La0.7Ce0.3Ni4.2Mn0.9-xCu0.3?Mo0.46Fe0.54?x and La0.75Ce0.25Ni3.85-xMn0.35Al0.15Cu0.65?Mo0.46Fe0.54?x?x=0,0.05,0.10,0.15 and 0.20?alloys were prepared by vacuum arc melting in a water-cooled copper hearth under highly pure argon atmosphere.The cast ingots were annealed by using vacuum heat-treating furnace.The samples were heated from room temperature to 1223K and then kept in 1223 K for 6 h followed by furnace cooling.The alloys were characterized by XRD,BSE,EDS to investigate the microstructures,and the electrochemistry impedance spectroscopy?EIS?,linear polarization,potentialstatic step discharge etc were measured to investigate electrochemical characteristics of alloy electrodes.The microstructures and electrochemical characteristics of La0.75Ce0.25Ni3.85-xMn0.35Al0.15Cu0.65?W0.42Fe0.58?x?x=0-0.20?hydrogen storage alloys are systematically investigated.The pristine alloy is single LaNi₅ phase with a hexagonal CaCu₅-type structure,while the alloys containing W_0.42Fe_0.58 consist of LaNi₅matrix phase and a trace of W segregate phase.The lattice parameters a,c,c/a and V of LaNi₅ phase increase with increasing x value.The electrochemical performance of alloy electrodes is significantly improved with increasing the content of W_0.46Fe_0.54 alloy.The high-rate dischargeability the alloy electrodes at the discharge current density of 1200mA/g first increases with increasing x from 0 to 0.15 and then decreases when x increases to 0.20.The best maximum discharge capacity of alloy electrodes is 333.6mAh/g when x value is 0.10.The cycling stability of alloy electrodes is increase with increasing x value and the best performance is obtained when x value is 0.20.It is proved that the alloys with W_0.42Fe_0.58 alloy substitution for Ni contributed to improve the overall electrochemical performance and reduce the alloy cost.The microstructures and electrochemical characteristics of La_0.7Ce_0.3Ni_4.2Mn_0.9-xCu_0.3(Mo_0.46Fe_0.54)_x?x=0-0.20?hydrogen storage alloys are systematically investigated.X-ray diffraction and backscattered electron results indicate that all the alloys are LaNi₅ phase with a hexagonal CaCu₅-type structure.The lattice parameters a,c,c/a and V of LaNi₅ phase decrease after the addition of Mo_0.46Fe_0.54alloy.As x increase from 0 to 0.20,maximum discharge capacity of alloy electrodes monotonically decreases from 332.5 mAh/g?x=0?to 310.2 mAh/g?x=0.20?.The high-rate dischargeability the alloy electrodes at the discharge current density of 1200mA/g first increases from 61.6%?x=0?to 73.6%?x=0.10?,while further increasing the value of x jeopardizes HRD₁₂₀₀.This is consistent with the variation tendency of hydrogen diffusion coefficient,which indicated that the electrochemical kinetic property is dominated by hydrogen atoms diffusion process in the bulk of alloy.The cycling capacity retention rate at the 100^th charge/discharge cycle is decrease monotonously from 79.3%?x=0?to 62.6%?x=0.20?with increasing the content of Mo_0.46Fe_0.54alloy,which should be ascribed to the deterioration of corrosion resistance of alloy electrode in the charging/discharging cycle.The microstructures and electrochemical characteristics of La_0.75Ce_0.25Ni_3.85-xMn_0.35Al_0.15Cu_0.65(Mo_0.46Fe_0.54)_x?x=0-0.20?hydrogen storage alloys are systematically investigated.X-ray diffraction and backscattered electron results indicate that the pristine alloy is LaNi₅ phase with a hexagonal CaCu₅-type structure,while the alloys containing Mo_0.46Fe_0.54 consist of LaNi₅ matrix phase and a trace of Mo segregate phase.The lattice parameters a,c,c/a and cell volume V of LaNi₅ phase gradually decrease with increasing x value.As x increases from 0 to 0.20,maximum discharge capacity of alloy electrodes monotonically decreases from 328.4 mAh/g?x=0?to 309.6 mAh/g?x=0.20?.The high-rate dischargeability of the alloy electrodes is improved dramatically and the best performance is obtained when x value is 0.10.The cycling stability of alloy electrodes is significantly improved with increasing x value from 0 to 0.20,which should be ascribed to the improvement in the anti-pulverization of the alloy electrode in the charging/discharging cycle.