Cu对La-Y-Ni系A_(2)B_(7)型储氢合金微观结构及电化学性能的影响

Effects of Substituting Ni with Cu on Microstructure and Electrochemical Performance of A_(2)B_(7)-Type La-Y-Ni-Based Hydrogen Storage Alloy

采用真空感应熔炼法,浇铸于中频感应炉的旋转铜辊,快淬制备LaY_(2)Ni_(10-x)Mn_(0.5)Cu_(x)(x=0,0.2,0.4,0.6)储氢合金薄带,在Ar气氛下1248 K热处理32 h,研究Cu元素部分替代Ni对合金微观结构和电化学性能的影响。研究表明:LaY_(2)Ni_(10-x)Mn_(0.5)Cu_(x)由Ce_(2)Ni_(7)和PuNi_(3)相两相组成,其中x=0.6时,合金中Ce_(2)Ni_(7)相的相丰度最大为70.53%。由于电负性的影响,储氢合金LaY_(2)Ni_(10-x)Mn_(0.5)Cu_(x)的晶胞体积随着Cu的加入,呈现异常减小后开始逐渐增加的趋势。电化学结果表明,适量Cu元素的加入可以提高合金的电化学放电能力,x=0.2时,电化学容量最大容量0.2C_(max)=388.6 m Ah·g^(-1),x=0.4时,合金的倍率性能达到最大,高倍率放电性能HRD_(1500)=66.94%。动力学研究表明,合金LaY_(2)Ni_(10-x)Mn_(0.5)Cu_(x)的倍率性能由表面电荷转移速率和氢原子扩散速率共同决定。当x=0.6时,合金电化学最大容量0.2C_(max)=375.2 m Ah·g^(-1),循环性能S_(100)=79.68%,高倍率放电性能HRD_(1500)=65.84%,具有最佳电化学综合性能。

As the key material of hydrogen storage technology,La-Y-Ni-based hydrogen storage alloys showed great potential to act as novel negative electrode materials in Ni-MH batteries.Meanwhile,La-Y-Ni-based hydrogen storage alloys had superlattice structures where[AB_(5)]and[A_(2)B_(4)]subunits stack alternatively along c axis in different ratios forming AB_(3),A_(2)B_(7)or A_(5)B_(19)phases,respectively.Among them,the A_(2)B_(7)-type alloys were extensively studied owning to good comprehensive electrochemical performance.In order to improve the electrochemical properties of the A_(2)B_(7)-type La-Y-Ni-based hydrogen storage alloys,Mn and Al element partial substitution for Ni had been investigated in earlier study.However,the alloys were still facing the challenge of poor cycling stability.Copper exhibited high stretch ability and conductivity,which could be added to alloys to improve the electrochemical properties of hydrogen storage alloys.Therefore,the effects of partial substitution of Cu element for Ni on the microstructure and electrochemical properties of the A_(2)B_(7)-type La-Y-Ni-based hydrogen storage alloys were investigated.LaY_(2)Ni_(10-x)Mn_(0.5)Cu_(x)(x=0,0.2,0.4,0.6)hydrogen storage flakes alloys were prepared in a 0.05 MPa argon atmosphere using a vacuum induction quenching furnace with a rotating copper wheel.The quenching rate expressed by the linear velocity of the copper wheel was 4.33 m·s^(-1).The purities of all elements used in these alloys preparations were higher than 99%(mass fractoin).After that,the flakes were annealed at 1248 K for 32 h under argon atmosphere,and furnace cooled.The results showed that LaY_(2)Ni_(10-x)Mn_(0.5)Cu_(x)(x=0,0.2,0.4,0.6)alloys were composed of Ce_(2)Ni_(7)-and PuNi_(3)-type phases,when x=0.6,the abundance of Ce_(2)Ni_(7)-type phase reached the maximum value,i.e.70.53%(mass fraction).Due to the influence of electronegativity,the cell volume of LaY_(2)Ni_(10-x)Mn_(0.5)Cu_(x)(x=0,0.2,0.4,0.6)alloys decreased abnormally when the substitution amount was increased to 0.2 and then increased gradually with the increase of Cu element.The variation tendency of the cell volume of LaY_(2)Ni_(10-x)Mn_(0.5)Cu_(x)(x=0,0.2,0.4,0.6)alloys were in accordance with that of the abundance of Ce_(2)Ni_(7)-type,the results indicated that the cell volume increasing of alloys were conducive to the formation of Ce_(2)Ni_(7)-type phase.The electrochemical results showed that the addition of appropriate amount of Cu could improve the electrochemical discharge capacity,when x=0.2,the maximum discharge capacity(0.2 C_(max))was 388.6 mAh·g^(-1 ).Substantial works had demonstrated that the electrochemical performances were closely related to the crystal structure and phase constitution of the La-Y-Ni-based alloys,because the theoretical capacity of AB_(3)-type phase was higher than that of A_(2)B_(7)-type phase,the electrochemical discharge capacity of LaY_(2)Ni_(10-x)Mn_(0.5)Cu_(x)(x=0,0.2,0.4,0.6)alloys decreased with the increasing of the abundance of Ce_(2)Ni_(7)-type phases.On the contrary,higher Ce_(2)Ni_(7)-type phase abundance favored the cycling stability,consequently the capacity retention rate of alloys increased with increasing of the abundance of Ce_(2)Ni_(7)-type phases.The HRD(higher rate discharge ability)was mainly determined by the rate of charge transfer process at the electrode surface and the rate of hydrogen atoms diffusion process within the bulk alloy electrode.The results showed variation tendency of the value of exchange current density(I_(0))and hydrogen diffusion coefficient(D_(0))were the same as that of the HRD_(1500),which might indicate that both the rate of surface charge transfer and hydrogen diffusion rate in the bulk of the alloys were the reaction controlling step.When x=0.4,high rate discharge ability of the LaY_(2)Ni_(10-x)Mn_(0.5)Cu_(x)(x=0,0.2,0.4,0.6)alloys reached the maximum,HRD_(1500)=66.94%.When the substitution amount was increased to 0.6,the alloy had the good comprehensive electrochemical performance,0.2 C_(max)=375.2 mAh·g^(-1 ),S_(100)=79.68%,HRD_(1500)=65.84%.It suggested that the partial replacement of Cu for Ni had improved the electrochemical properties,such as improving electrochemical capacities and high rate discharge ability.However,the cyclic stability was still not satisfactory for practical application.In summary,new ideas and advanced research are needed to study La-Y-Ni-based hydrogen storage alloys in order to develop high electrochemical performance.For example,partial substitution of rare earth elements such as Ce,Sm,Pr,Nd,Gd for La or Y element,surface modification of hydrogen storage electrode and adjusting the phase structure and composition will be the next research work on La-Y-Ni-based hydrogen storage alloys.