Preparation And Electrochemical Properties Of Single-phase Nd-Mg-Ni-based Hydrogen Storage Alloys

Nd-Mg-Ni-based hydrogen storage alloys with better cycling stability,high rate dischargeability and lower self-discharge rate have been investigated extensively.However,relationships between phase structures and electrochemical properties remain unclear enough.Therefore,single-phase PuNi₃,Gd₂Co₇ and Pr₅Co₁₉-type Nd-Mg-Ni-based alloys are prepared in this work,and the electrochemical hydrogen storage characteristics are studied systematically;then a single-phase A₂B₇-type Nd-Mg-Ni-based alloy with good overall performance is obtained and formation mechanism toghter with electrochemical properties of the alloy is studied.Single-phase Nd_0.8Mg_0.2Ni_n（n=3.0,3.5,3.8）alloys are prepared by powder sintering methods.Electrochemical tests show that with the increasing[NdNi₅]/[NdMgNi₄]subunit ratio,maximum discharge capacity of the three alloy electrodes decreases,cycling stability,high rate dischargeability and self-discharge rates increase.Then the self-discharge mechanism of the alloy electrodes is studied emphatically.Results show that with the increasing[NdNi₅]/[NdMgNi₄]subunit ratio,the hydrogen desorption plateau pressures sharply elevate,leading to less stability of the corresponding hydrides and more reversible self-discharge of the alloys;whereas,the oxidation/corrosion degree alleviates and less irreversible self-discharge with the higher[NdNi₅]/[NdMgNi₄]ratio.The results of calculation on the proportion of reversible and irreversible self-discharge illustrate that reversible self-discharge is the dominate factor in self-discharge of Nd-Mg-Ni-based alloys.A single-phase Ce₂Ni₇-type Nd_0.80Mg_0.20Ni_3.58 alloy is prepared through induction melting followed by zoning annealing methods and formation mechanism and electrochemical properties of the alloy are studied.The as-cast alloy contains Ce₂Ni₇,Gd₂Co₇,Ce₅Co₁₉,CaCu₅ and MgCu₄Sn phases.CaCu₅ and MgCu₄Sn phases convert to A₂B₇ phase by peritectic reaction while Ce₅Co₁₉9 phase has no reaction via annealing treatment at 950℃ for 24 h.Rising annealing temperature to 985℃,Ce₅Co₁₉ decomposes into A₂B₇ phase.At the range of 985℃ to 1020℃,Ce₂Ni₇ transforms into Gd₂Co₇ phase,while Gd₂Co₇continually transforms into Ce₂Ni₇ phase after 1025℃ and totally forms a single-phase Ce₂Ni₇-type alloy at 1100℃.The single-phase Nd_0.80Mg_0.20Ni_3.58 alloy delivers a superior cycling stability with the capacity retention rate of 64%after 700 electrochemical cycles.Besides,capacity attenuation of the single-phase alloy is mainly attributed to the oxidation of Nd and Mg elements,and Nd（OH）₃ grows from needles to sticks which cover the surface of alloy suppressing the further oxidation/corrosion of alloy.