【作者】 王大辉；

【导师】 康龙；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2007， 博士

【摘要】 在综述国内外对La-Mg-Ni系贮氢合金的结构、贮氢行为及电化学性能研究现状的基础上,本文选择以La-Mg-Ni系La₂MgNi_7.5Co_1.5贮氢合金的结构和电化学性能作为研究对象。采用X射线衍射（XRD）、中子衍射（ND）和Rietveld全谱拟合方法研究了La₂MgNi_7.5CO_1.5合金及其氘化物的微结构特征,并探讨了改善La₂MgNi_7.5Co_1.5贮氢合金电化学性能的途径,即采用Al元素替代Ni优化合金成分;采用Al元素替代Mg优化合金成分;采用快速凝固方法制备贮氢合金;在贮氢合金电极表面电沉积金属Cu膜。采用XRD、EPMA、WDS、XPS、粒度分析和电化学性能测试等方法研究了合金及其氢化物的相结构、原子占位、相分布、合金颗粒和电极表面的形貌、合金表面氧化情况、合金颗粒粒径变化及综合电化学性能。La₂MgNi_7.5Co_1.5合金和La₂MgNi_7.5Co_1.5D_12.0氘化物微结构分析表明,在PuNi₃型结构中,Co原子占据在18h和6c位置上,Co原子分布在CaCu₅结构单元中及CaCu₅结构单元与Laves相结构单元的共格面上。合金充满氘后,氘原子在PuNi₃型结构中主要占据在CaCu₅结构单元中的36i₂、18h₅、6c₄位置上及Laves相结构单元中的6c₁、18h₃和18h₁位置上;充氘后,PuNi₃结构中Laves相结构单元各向异性膨胀大（c/a=0.88,δc/c=15.8%,δV/V=32.8%）,而在CaCu₅结构单元中各向异性膨胀小（c/a=0.77,δc/c=2.4%,δV/V=17.5%）,降低PuNi₃型结构贮氢合金的吸氢体积膨胀应以减弱PuNi₃型结构中Laves相结构单元的吸氢膨胀为主。采用部分的Al替代Ni优化La₂MgNi_7.5Co_1.5的成分,研究结果表明,少量的Al替代（x=0.1）,La₂MgNi_7.5-xCo_1.5Al_x贮氢合金的物相组成没有发生变化,合金仍然由LaNi₃相和αLa₂Ni₇相两相组成,但两相的相丰度发生了变化,αLa₂Ni₇相的丰度增加,LaNi₃相的丰度减少。Al含量的进一步增加导致合金中LaNi₃相消失并出现了新的物相LaNi₅相、La₅Ni₁₉相和LaMgNi₄相。LaNi₅相的丰度随合金中Al替代量的增加变化最明显,并成为La₂MgNi_7.0Co_1.5Al_0.5合金中的主相。Al替代Ni使LaNi₃相晶胞α轴增加、c轴减小、晶胞体积V增加,αLa₂Ni₇相、La₅Ni₁₉相和LaMgNi₄相的α轴、c轴及晶胞体积V均增加,但Al替代Ni对LaNi₅相的晶胞参数α轴、c轴和晶胞体积V的影响很小。Al元素主要分布在LaNi₅相和La₅Ni₁₉相,在LaMgNi₄相的含量则很少,Mg元素则主要分布在LaMgNi₄相、LaNi₃相和αLa₂Ni₇相中,而在LaNi₅相和La₅Ni₁₉相中的含量很少,Mg元素和Al元素在不同物相中的分布呈现矛盾的特点。Al替代Ni对该合金电极的最大电化学放电容量有负面影响,随着合金中Al含量的增加,最大电化学放电容量呈现不断下降的趋势。此外,Al替代Ni,能改善La₂MgNi_7.5Co_1.5合金电极的电化学循环稳定性。La₂MgNi_7.5Co_1.5高倍率放电性能随合金中Al含量的变化呈现先增加（x=0.1和0.3）后降低（x=0.5）的趋势。采用Al部分替代Mg优化La₂MgNi_7.5Co_1.5的成分,研究结果表明,少量的Al替代Mg（x=0.1）不改变La₂MgNi_7.5Co_1.5合金的物相组成,合金仍然由LaNi₃相和αLa₂Ni₇相组成,但合金中αLa₂Ni₇相的含量增加,LaNi₃相的含量减少,较多的Al替代（x=0.3和0.5）改变La₂MgNi_7.5Co_1.5贮氢合金的物相组成,合金中LaNi₃相消失并出现新的物相LaNi₅相,随合金中Al含量的增加,αLa₂Ni₇相的丰度降低,而LaNi₅相的丰度增加,在La₂Mg_0.5Al_0.5Ni_7.5Co_1.5合金中LaNi₅相是该合金中的主相。Al替代Mg形成La₂Mg_1-xAl_xNi_7.5Co_1.5合金,Al原子在合金结构中占据的是Ni原子的位置,而不是Mg原子的位置,Al替代Mg本质上讲是合金中Al含量增加的同时降低了合金中Mg的含量。Al替代Mg使LaNi₃相晶胞沿c轴膨胀,沿α轴收缩、晶胞体积V增加,αLa₂Ni₇相晶胞的α轴、c轴及晶胞体积V均增加,而LaNi₅相的晶胞参数受Al含量变化的影响很小。Al替代Mg对La₂MgNi_7.5Co_1.5贮氢合金的放氢平台压力影响很小,少量的Al替代Mg（x=0.1）也几乎不降低La₂MgNi_7.5Co_1.5贮氢合金的贮氢容量,但随La₂Mg_1-xAl_xNi_7.5Co_1.5合金中Al含量的增加（x=0.3和0.5）,合金电极电化学活化性能和最大电化学放电容量呈不断下降的趋势,Al替代Mg能够明显改善La₂MgNi_7.5Co_1.5贮氢合金电极的电化学循环稳定性,对提高该合金电极的高倍率放电性能也有利。La₂MgNi_7.5Co_1.5合金和La₂Mg_0.9Al_0.1Ni_7.5Co_1.5合金经过不同次数电化学充放电循环后,合金中的LaNi₃相和αLa₂Ni₇相仍然保持PuNi₃型结构和Ce₂Ni₇型结构,没有发生变化,但在这两个合金的XRD图谱中出现新的物相La（OH）₃、Mg（OH）₂和单质Ni的衍射峰。La₂MgNi_7.5Co_1.5合金和La₂Mg_0.9Al_0.1Ni_7.5Co_1.5合金所包含的LaNi₃相和αLa₂Ni₇相吸氢形成氢化物后,仍然保持PuNi₃型结构和Ce₂Ni₇型结构,La₂MgNi_7.5Co_1.5合金吸氢形成氢化物后,LaNi₃相和αLa₂Ni₇相晶胞均呈各向异性膨胀,但LaNi₃相吸氢后,其晶胞内部将承受比αLa₂Ni₇相更大的内应力,Al替代Mg能够明显抑制La₂Mg_0.9Al_0.1Ni_7.5Co_1.5合金中的LaNi₃相吸氢后晶胞体积的膨胀,但对αLa₂Ni₇相吸氢体积膨胀的抑制作用很小。少量的Al替代Mg（x=0.1）使合金中αLa₂Ni₇相的丰度增加并减少了LaNi₃相的丰度,而αLa₂Ni₇相本身具有优于LaNi₃相的吸氢抗粉化能力,且Al替代Mg明显抑制了合金中LaNi₃相的吸氢体积膨胀,使La₂Mg_0.9Al_0.1Ni_7.5Co_1.5合金相比La₂MgNi_7.5Co_1.5合金具有更好的抗粉化能力和抗氧化性能,这是少量Al替代Mg（x=0.1）改善La₂MgNi_7.5Co_1.5合金电极电化学循环稳定性的主要原因。采用快速凝固方法制备了La₂Mg_0.9Al_0.1Ni_7.5Co_1.5贮氢合金。XRD分析表明,随着冷却速率的增加,La₂Mg_0.9Al_0.1Ni_7.5Co_1.5合金的相组成发生了明显变化。0m/s冷却速率下,合金由αLa₂Ni₇主相（Ce₂Ni₇型结构）和LaNi₃相（PuNi₃型结构）组成。冷却速率从0m/s增加到20m/s,合金中出现新相βLa₂Ni₇相、LaNi₅相和LaMgNi₄相,新相LaNi5的相丰度随冷却速率的增加迅速增加,而合金中的αLa₂Ni₇相和LaNi₃相的丰度减少。快速凝固方法制备的La₂Mg_0.9Al_0.1Ni_7.5Co_1.5贮氢合金为柱状晶且晶粒细小。冷却速率对合金的活化性能影响不大,但随冷却速率的增加,合金的最大放电容量减少、高倍率放电性能下降,在较高的冷却速率下（10m/s和20m/s）,合金电极表现出好的循环稳定性。采用在6M KOH电解液中添加Cu（OH）₂在La₂MgNi_7.5Co_1.5贮氢合金电极的外表面电沉积金属Cu膜的研究结果表明,电极表面镀Cu膜的厚度和致密性随充放电循环次数的增加而增加。合金电极表面的铜膜,可以有效地阻止电极内部贮氢合金的氧化,但对电极内部贮氢合金颗粒抗粉化性能的改善作用不大。La₂MgNi_7.5Co_1.5合金电极表面电化学镀铜,降低了合金电极的活化性能和高倍率放电性能,对合金电极的最大电化学放电容量没有影响,这种方法能够改善该合金电极的电化学循环稳定性,经过120次充放电循环,La₂MgNi_7.5Co_1.5贮氢合金电极的循环稳定性从33.81%被提高到71.95%。更多还原

【Abstract】 In this paper,the present situation of research of structure characteristic,hydrogenation behavior,electrochemical properties of La-Mg-Ni system hydrogen storage alloy have been reviewed.On this basis,Crystal structure and electrochemical properties of La-Mg-Ni system La₂MgNi_7.5Co（1.5）hydrogen storage alloy electrode was selected as the subject of this study.The structural characterization of La₂MgNi_7.5Co_1.5alloy and its deuteride were investigated by means of X-ray diffraction（XRD）,neutron diffraction（ND）and Rietveld method.Moreover,the paper also put forward some new measures to improve electrochemical properties of La₂MgNi_7.5Co_1.5hydrogen storage alloy electrode,Namely, employing partial substitution of Al for Ni to optimize the alloy composition;employing partial Al element substitution for Mg to optimize the alloy composition;utilizing rapid solidified technique to prepare hydrogen storage alloy;depositing copper film on the alloy electrode surface using electrochemical method.The phase structure,microstructure,surface morphology and composition,size of alloy particle and electrochemical properties of hydrogen storage alloy and its hydride electrode have been studied systematically by XRD, EPMA,WDS,XPS,MasterSizer and electrochemical measurement technique.The microstructure of La₂MgNi_7.5Co_1.5alloy and its deuteride were investigated in detail.It can be found that,the Co atom occupies at 18h and 6c positions in PuNi₃ type structure.The distributing of Co atom mainly located at the Caeu₅ structure unit and the boundary between CaCu₅ and Laves phase structure blocks of PuNi₃ type structure.Rietveld analysis for(La₂MgNi_7.5Co_1.5)D₁₂deuteride indicate that the D atom occupy the 36i₂,18h₅, 6c₄ positions within the CaCu₅ structure unit and the 6c₁,18h₃,18h₁ positions within the Laves phase structure unit in PuNi₃ type structure.The uniaxial volume expansion（c/a=0.88,δc/c=15.8%,δV/V =32.8%）of Laves phase structure unit is bigger than that of（c/a=0.77,δic/c=2.4%,δV/V=17.5%）CaCu₅ structure unit in PuNi₃ type structure.It is the key step of the repression of volume expansion of PuNi₃ type structure alloy by decreasing the rate of anisotropic expansion of Laves phase structure unit.The influence of partial Al substitution for Ni on phase structure and electrochemical properties of La₂MgNi_7.5-xCo_1.5Al_x（x=0.0,0.1 and 0.5）hydrogen storage alloy have also been studied systematically in this paper.The result show that small quantity replacement Ni by Al（x=0.1）do not change phase composition of La₂MgNi_7.5Co_1.5alloy,the alloy is still constituted with the LaNi₃ phase andαLa₂Ni₇ phase,but the abundance of LaNi₃ phase decrease and theαLa₂Ni₇ increase in the La₂MgNi_7.4Co_1.5Al_0.1alloy.While the more Al substitution for Ni in La₂MgNi_7.5Co_1.5alloy changed phase composition of the alloy.LaNi₃ phase disappeared,and appear three new phases,LaNi₅ phase,La₅Ni₁₉phase and LaMgNi₄ phase in La₂Mg_1-xAl_xNi_7.5Co_1.5（x=0.3 and 0.5）alloy.The abundance of LaNi₅ phase increase, the abundance ofαLa₂Ni₇ phase decrease,the abundance of La₅Ni₁₉phase decrease slightly and the phase abundance of LaMgNi₄ increase with the increase of Al content.LaNi₅ phase is main phase in La₂MgNi₇Co_1.5Al_0.5alloy.The lattice parameter a and the unit-cell volume V of LaNi₃ phase increase,but the lattice parameter c decrease,the lattice parameters a,c and the unit-cell volumes V ofα_La₂Ni₇ phase,La₅Ni₁₉phase and LaMgNi₄ phase increase with the increase of Al content,but the influence of the change of Al content on cell parameters of LaNi₅ phase is very small.Al element was mainly dispersed in LaNi₅ phase and La₅Ni₁₉ phase of La₂MgNi_7.5-xCo_1.5Al_x alloy,Al element content in LaMgNi₄ phase is very small.Mg element was mainly dispersed in LaMgNi₄ phase,LaNi₃ phase andαLa₂Ni₇ phase,Mg element content in LaNi₅ phase and La₅Ni₁₉phase is very small.The distributing and content in different phase of Mg and Al element present the characteristic of contradict.With the increment of the Al content in the La₂MgNi_7.5-xCo_1.5Al_x alloy,the maximum discharge current capacity decreases continuously.Al substitution Ni is also beneficial to improve the cyclic stability of the alloy electrode.The high rate dischargeability of La₂MgNi_7.5-xCo_1.5Al_x alloy electrode increases firstly then decreases with the increment of Al content.The influence of partial Al substitution for Mg on phase structure and electrochemical properties of La₂Mg_1-xAl_xNi_7.5Co_1.5（x=0.0-0.5）hydrogen storage alloy have been studied systematically in this paper.The result showed that small quantity replacement Mg by Al （x=0.1）didn’t change phase composition of La₂MgNi_7.5Co_1.5alloy,The alloy was still constituted with the LaNi₃ phase andαLa₂Ni₇ phase,but the abundance of LaNi₃ phase decreased obviously and theαLa₂Ni₇ increased in the La₂Mg_0.9Al_0.1Ni_7.5Co_1.5alloy.While the more Al substitution in La₂Mg_1-xAl_xNi_7.5Co_1.5alloy changed phase composition of the alloy,LaNi₃ phase disappeared,αLa₂Ni₇ phase still existed,and appeared a new phase LaNi₅ in La₂Mg_1-xAl_xNi_7.5Co_1.5.（x=0.3 and 0.5）alloy.The abundance of LaNi₅ phase increased and the abundance ofαLa₂Ni₇ phase decreased with the increase of Al content.LaNi₅ phase was the main phase in La₂Mg_0.5Al_0.5Ni_7.5Co_1.5alloy.Moreover,the position that Al atom occupied at the position of the Ni atom,not the position of Mg atom in alloy structure.In fact,the Al substitution for Mg changed merely the Al and Mg content in the meantime in La₂Mg_1-xAl_xNi_7.5Co_1.5alloy.The influence of Al substitution for Mg on cell parameters of different phase of the La₂Mg_1-xAl_xNi_7.5Co_1.5alloy was different,the lattice parameter c and the unit-cell volume V of LaNi₃ phase increased,but the lattice parameter a decreased,the lattice parameter a,c and the unit-cell volumes V ofαLa₂Ni₇ phase increased with the increase of Al content,but the influence of the change of Al content on cell parameters of LaNi₅ phase was very small.The influence of Al substitution for Mg on the hydrogen desorption equilibrium pressure of La₂MgNi_7.5Co_1.5alloy was also Very small,in addition, little replacement of Mg by Al lowered scarcely the hydrogen storage capacity and the maximum electrochemical discharge capacity of the alloy,but with the increment of the Al content,the hydrogen storage capacity,the activation property and the electrochemical discharge capacity of the La₂Mg_1-xMg_xNi_7.5Co_1.5alloy decreased continuously.Al substitution for Mg can improve obviously the electrochemical cyclic stability of La₂MgNi_7.5Co_1.5alloy electrode,and is also beneficial to improve the high rate dischargeability of the alloy electrode.After some times electrochemical charge-discharge cycle,LaNi₃ andαLa₂Ni₇ phase in La₂MgNi_7.5Co_1.5and La₂MgNi_7.5Co_1.5alloys still remain PuNi₃-type structure and Ce₂Ni₇ type structure,however,small new phase La（OH）₃, Mg（OH）₂ and Ni appear in alloys.The structure of LaNi₃ phase hydride andαLa₂Ni₇ phase hydride also remain PuNi₃-type structure and Ce₂Ni₇ type structure.The expansion of LaNi₃ cell andαLa₂Ni₇ phase cell were anisotropic in La₂MgNi_7.5Co_1.5hydride,but the rate of anisotropic expansion and volume expansion of the former was larger than the latter.Compared with the La₂MgNi_7.5Co_1.5hydride,the repression of volume expansion of Al substitution for Mg onαLa₂Ni₇ phase in La₂Mg_0.9Al_0.1Ni_7.5Co_1.5alloy is very small,but the expansion of c axis and cell volume V of LaNi₃ phase lower obviously due to Al substitution. The abundance ofαLa₂Ni₇ phase increase,and that of LaNi₃ phase decrease in La₂Mg_0.9Al_0.1Ni_7.5Co_1.5alloy due to Al substitution for Mg,the anti- pulverization property ofαLa₂Ni₇ phase was superior than that of LaNi₃ phase,and the cell volume of LaNi₃ phase was suppressed,so the anti-pulverization and anti-oxidation property of La₂Mg_0.9Al_0.1Ni_7.5Co_1.5is superior to that of La₂MgNi_7.5Co_1.5alloy.That is main reason of improvement of electrochemical cyclic stability of La₂MgNi_7.5Co_1.5alloy due to small Al substitution for Mg（x=0.1）.The effect of melt spun on the phase structure and electrochemical properties of La₂Mg_0.9Al_0.1Ni_7.5Co_1.5hydrogen storage alloys was investigated systematically. La₂Mg_0.9Al_0.1Ni_7.5Co_1.5is composed ofαLa₂Ni₇ main phase with Ce₂Ni₇ type structure and a little LaNi₃ phases.With the increase of quenching rate,the content of LaNi₅ phase increase remarkably and the content ofαLa₂Ni₇ phase decrease gradually;a littleβLa₂Ni₇ phase and LaMgNi₄ phase appeared also in the melt spun alloys.The La₂Mg_0.9Ni_7.5Co_1.5Al_0.1alloys prepared by melt spun were fine columnar crystal structure,the melt spun technique has little effect on the activation property,the maximum discharge capacity C_maxdecrease with the increase of quenching rate,The alloy electrodes with a quenching rate 10m/s and 20m/s show a better cyclic stability.Influence of electrochemical depositing Cu film on the electrode surface on the electrochemical properties of La₂MgNi_7.5Co_1.5hydrogen storage alloy has also been studied systematically.The result showed that the thickness and compactness of coating Cu film increase with the increment of charge-discharge cycle.The copper film of the alloy electrode surface can keep alloy particle in the electrode interior from oxidizing availably,but can not improve the anti-pulverization ability of the alloy.Electrochemical coating Cu film on the alloy electrodes surface lower the activation property and high rate dischargeability of the La₂MgNi_7.5Co_1.5alloy,but has no negative effect on the maximum discharge capacity of the alloy electrodc.Moreover,it is effective to improve the cyclic stability of the alloy electrode utilizing electrodeposit Cu film on the electrode surface.After 120 charge-discharge cycles, the cyclic stability of the La₂MgNi_7.5Co_1.5alloy electrode was improved from 33.81%to 71.95%.更多还原