【作者】 蒋卫卿；

【导师】 郭进；

【作者基本信息】 广西大学 ， 化学工艺， 2010， 博士

【摘要】 本文在全面综述PuNi3结构稀土系贮氢合金国内外研究进展的基础上,以AB2C9型La-Ti-Mg-Ni系合金为研究对象。通过XRD、PCT、吸放氢动力学、SEM、正电子湮没符合多普勒展宽和正电子寿命的测量,以及恒流充放电、循环伏安的电化学测试,系统考察元素替代以及热处理对La-Ti-Mg-Ni系合金的微观结构、气态贮氢和电化学性能的影响规律及作用机制。对La2-xTixMgNi9 (x=0.1,0.2,0.3,0.4)合金的相结构及贮氢性能的研究结果表明,具有六方CaCu5型结构的LaNi5相(空间群P6/mmm)和具有斜六体PuNi3型结构的LaNi3相(空间群R-3m)、LaMg2Ni9相(空间群R3m)为合金的主相。随x的增加,LaNi5、LaNi3相的晶胞参数基本没有变化,但当x>0.3时,LaMg2Ni9相晶胞体积开始减小,主要原因是Ti的原子半径(1.45A)小于La的原子半径(1.88A)所致。合金气态贮氢测试表明,随Ti含量的增加,合金的最大贮氢量从1.51wt.%(x=0.1)逐渐减小至1.22wt.%(x=0.4)；而吸放氢平台压则先减小后增大,以La1.8Tio.2MgNi9合金吸放氢平台压最低,这表明,适当的Ti取代La可以降低La2-xTixMgNi9 (x=0.1,0.2,0.3,0.4)合金的平台压。在La2-xTixMgNi9 (x=0.1,0.2,0.3,0.4)合金中,La1.9Tio.1MgNi9合金氢化物的稳定性相对较高,导致其放氢过程放氢动力学性能较差。此外,电化学性能测试显示,当Ti替代量为x=0.2时,合金电极具有较好的综合电化学性能,其最大放电容量为333.2mAh/g, 1100mA/g放电电流密度下的高倍率放电能力HRD1100为83.7%,但50次充放电循环后放电容量仅有203.7mAh/g,尚有待于进一步改进研究。为改进上述合金的综合贮氢性能,对La2-xTixMgNi9 (x=0.1,0.2,0.3)合金进行退火处理,研究退火条件对这三种合金相结构、气态贮氢和电化学性能的影响。结果表明,在所有合金组成相中(LaNi5、LaNi3、LaMg2Ni9和Ti2Ni相),Ti2Ni相在900℃时出现,而LaMg2Ni9相则在900℃的Lai.9Ti0.1MgNi9合金中消失。热处理增强合金相衍射峰的强度,使合金成分和组织结构更均匀,从而提高合金的最大贮氢量、有效吸氢量和吸放氢动力学性能,降低合金氢化物的稳定性以及吸放氢平台压。退火后,合金的吸放氢滞后增大,主要原因是退火处理降低合金的晶格应变和缺陷,改善合金成分和组织均匀性,从而减少合金的吸放氢通道。退火温度对三种合金最大贮氢量、有效吸氢量以及吸放氢动力学性能影响的效果不相同,对La1.9Ti0.1MgNi9合金,这三个特征量在800℃时达到最大,而对La1.8Ti0.2MgNi9和La1.7Ti0.3MgNi9合金,则在900℃时为最高,主要原因是由于900℃时,在La1.9Ti0.1MgNi9合金中,具有温和吸放氢条件的LaMg2Ni9相消失,而吸放氢条件苛刻的Ti2Ni相出现所致。电化学测试表明,La2-xTixMgNi9 (x=0.1,0.2,0.3)合金经退火处理,电化学性能(最大放电容量、循环稳定性、高倍率放电能力)得到显著改善,而且以900℃的La1.8Tio.2MgNi9合金电极的性能为最佳,其最大放电容量为365.7mAh/g, 1100mA/g放电电流密度下的高倍率放电能力HRD1100达到85.1%。而且,该合金电极在放电容量衰减至最大放电容量60%时需要经过177次充放电循环,远高于其铸态下的52次循环,主要原因是退火合金成分组织均匀化,抗粉化能力提高；合金中具有吸氢作用与催化作用的LaNi5相对电催化活性差的Ti2Ni相起到了催化作用,使电化学容量较高Ti2Ni相实现了可逆吸放氢。为提高La1.8Tio.2MgNi9合金的综合性能,采用Co部分取代合金B侧Ni,系统研究La1.8Tio.2MgNi9-xCox (x=0,0.1,0.2,0.3,0.4,0.5)合金(分别标记为Co0、Co1、Co2、Co3、Co4、Co5合金)的微观结构、气态及电化学贮氢性能,结果发现,上述合金的主相为LaNi5相和LaMg2Ni9相。Co的力口入基本没有改变合金中LaNi5相的晶胞参数,却使LaNi3相消失。随Co含量的增加,合金中LaMg2Ni9相晶胞体积的大小变化顺序为：Co0>Co5>Co3>Co1>Co4>Co2。与Co2和Co4合金相比,具有较大晶胞体积的Co0、Co1、Co3和Co5合金吸氢容量较高。所有合金中,Co4合金的吸氢容量和吸放氢滞后最小,而放氢平台压最高。对Co2和Co5合金吸氢前和反复吸放氢10次后的正电子淹没寿命谱与符合多普勒展宽谱的研究表明,Co2和Co5合金吸放氢后,正电子平均寿命增大、高动量电子浓度降低,这主要与合金多次吸放氢后缺陷增多有关。随Co含量的增加,合金电极的最大放电容量从333.2mAh/g (Co0)增大到365.2mAh/g (Co1)、364.9mAh/g (Co2)、350.2mAh/g (Co3)、353.5mAh/g (Co4)和364.7mAh/g(Co5),这表明Co的加入有利于合金电极最大放电容量的提高。在电化学循环测试中,Co的加入可以改善合金电极的循环稳定性,而且以Co5合金电极为最佳,但是Co5合金电极每次充放中放电容量衰减量高达1.83mAh/g·cycle,经80次循环后放电容量仅有218.7mAh/g,因此,其循环稳定性仍有待提高。对La1.8Tio.2MgNi9-xAlx(x=0,0.1,0.2,0.3,0.4,0.5)合金的相结构及电化学性能的研究表明,所有合金均包含LaMg2Ni9相。掺A1后,由于A1的固溶,合金中LaNi5相转变为La(Ni, Al)5固溶相,同时,合金相的峰强度增大,合金成分和组织更均匀。随A1含量的增加,合金相的晶胞体积先减小后增大,当x=0.3时达到最小,而x=0.4或0.5时则为最大；x>0.2时,合金中LaNi3相消失,LaNi2相出现。所有合金在2-3次充放电中就可达到其最大放电容量,表现出良好的活化性能。它们的最大放电容量随x的增加先增加后减小,从333.2mAh/g (x=0)增大到357.7mAh/g (x=0.1)后逐渐降低至319.8mAh/g (x=0.5)。La1.8Tio.2MgNi8.9Al0.1合金电极活化过程放电容量最大,主要与该合金具有比La1.8Tio.2MgNi9合金更均匀的组织结构以及当A1含量X≥0.2时,LaNi3相消失、LaNi2相出现有关。A1部分取代Ni,有利于合金电极循环稳定性的提高,其中以La1.8Tio.2MgNi8.7Alo3合金电极循环寿命最长,这可能归因于其适量的A1含量以及较小的晶胞体积所致。比较分析可知,所有合金电极中,La1.8Tio.2MgNi8.7Al0.3合金电极综合电化学性能较好,其最大放电容量为340.0mAh/g,经100次充放电循环后放电容量保持率为60%。更多还原

【Abstract】 Based on the review of the research and development of the rare earth-based hydrogen storage alloys with PuNi3 structure, the AB2C9-type La-Ti-Mg-Ni system hydrogen storage alloys were selected as the study object of this work. The effect of element substitution and annealing treatment on the microstructure, the hydrogen storage and electrochemical properties of La-Ti-Mg-Ni system alloys were investigated systematically by means of XRD, PCT, hydrogen absorption/desoprion kineti, SEM, positron annihilation lifetime (PAL) and coincidence Doppler broadening (CDB) measurements and electrochemical analysis including galvanostatic charge-discharge and cyclic voltammetries.For the La2-xTixMgNi9 (x=0.1,0.2,0.3,0.4) alloys, LaNi5 phase (space group P6/mmm) with the hexagonal CaCu5-type structure, LaNi3 phase (space group R-3m) and LaMg2Ni9 phase (space group R3m) with rhombohedral PuNi3-type structure are the main phase. With x increasing, the lattice parameters of LaNi5 and LaNi3 phase remain almost unchanged, and the cell volume of LaMg2Ni9 phase becomes smaller as x is above 0.2 because the radius of Ti of 1.45A is smaller than that of La of 1.88A. As Ti content increases, the maximum hydrogen storage capacities decrease gradually from 1.51wt.%(x=0.1) to 1.22wt.%(x=0.4), the hydrogen absorption/desorption plateau pressures first decrease and then increase, and the La1.8MgTi0.2Ni9 alloy has the lowest plateau pressure, which indicates that substituting La by suitable Ti content can lower the plateau pressure of La2-xTixMgNi9 (x=0.1,0.2,0.3,0.4) alloys. Among all the studied hydrides, the most stable La1.9Ti0.1MgNi9 hydride shows the slowest hydrogen desorption rate. Electrochemical studies reveal that the overall electrochemical properties of the alloy electrode with x=0.2 are better, e.g., its maximum discharge capacity is 333.2mAh/g, and the HRD1100 (high rate dischargeability at 1100mA/g discharging current density) reaches 83.7%, but its discharge capacity reduces to only 203.7mAh/g after 50 charge/discharge cycles, which needs also to be further improved.In order to improve the overall hydrogen storage properties of the alloys, La2-xTixMgNi9(x=0.1,0.2,0.3) alloys were prepared by annealing treatment, and the effect of heat treatment on the phase structure, hydrogen storage and electrochemical properties of La2-xTixMgNi9 (x=0.1,0.2,0.3) alloys were investigated in detail. The results indicate that for LaNi5, LaNi3, LaMg2Ni9 and Ti2Ni phases, Ti2Ni phase appears at 900℃, while LaMg2Ni9 phase disappears in the La1.9Ti0.1MgNi9 alloy annealed at 900℃. The diffraction peaks of the phases become narrowed or sharper by thermal treatment meaning higher composition homogeneity, which favors not only the improvement of the maximum/effective hydrogen storage capacity and hydrogen absorption/desorption kinetic, but also the decrease of the stability of hydrides and hydrogen absorption/desorption plateau pressure. The hysteresis factor increases after heat treatment because the higher composition homogeneity of annealed alloys decreases the grain boundaries and lattice defects, and increases the obstruction in the process of hydriding and dehydriding. For the maximum/effective hydrogen storage capacities and hydrogen absorption/desorption rate, they reach the optimization at 800℃for La1.9Ti0.1MgNi9 alloy, while at 900℃for La1.8Ti0.2MgNi9 and La1.7Ti0.3MgNi9 alloys. The reason is that LaMg2Ni9 phase disappears and Ti2Ni appears for the La1.9Ti0.1MgNi9 alloy annealed at 900℃. To summarize the results obtained by electrochemical measurements, the electrochemical properties including the maximum discharge capacity, the cycling stability and the high rate dischargeability (HRD) have been markedly improved after annealing treatment, and the optimum alloy is found to be La1.8Ti0.2MgNi9 alloy annealed at 900℃. For the alloy electrode, the maximum discharge capacity and HRD at the discharge current density 1100mA/g(HRD1100) are 365.7mAh/g and 85.1%, respectively. And it undergoes 177 charge/discharge cycles when the discharge capacity reduces to 60% maximum discharge capacity, which is higher than that of its as-cast alloy electrode of 52 cycles. The improvement of cycling stability can be associated with two facts. One is the higher composition homogeneity of annealed alloys, which reduces the particle pulverization, and improves anti-oxidation ability. The other is the LaNi5 phase with catalysis activity, which makes the stable Ti2Ni phase with high discharge capacity absorb/desorb hydrogen reversibly. For further improving the overall properties of La1.8Tio.2MgNi9 alloy, Ni is substituted by Co, and the La1.8Tio.2MgNi9-xCox (x=0,0.1,0.2,0.3,0.4,0.5) alloys defined as Col, Col,Co2, Co3, Co4, Co5 alloys, respectively, were prepared. The microstructure and the hydrogen storage and electrochemical properties of the alloys were systematically studied. It is found that LaNi5 phase and LaMg2Ni9 phase are the main phase. Co addition has almost not change the lattice parameters of LaNi5 phase, but leads to the disappearance of LaNi3 phase. The order of the cell volume of LaMg2Ni9 phase with increasing x is CoO>Co5>Co3>Col>Co4>Co2. The higher hydrogen storage capacity of CoO, Col, Co3 and Co5 alloys is due to their larger cell volume when compared to Co2 and Co4 alloys. Among all the La1.8Tio.2MgNi9-xCox (x=0,0.1,0.2,0.3,0.4, 0.5) alloys, Co4 alloy shows the smallest hydrogen storage capacity and the lowest hysteresis factor, but the highest hydrogen desorption plateau pressure. For Co2 and Co5 alloy, the positron annihilation lifetime spectroscopy and coincidence Doppler broadening spetra show that the mean positron lifetimes increases and the number of high momentum electode decreases after the alloys absorb and desorb hydrogen repeatedly (10 cycles) comparing with those of the alloys without hydrogenation, which is mainly attributed to the increase of defects. The maximum discharge capacity has been markedly improved by partial substitution of Co for Ni, and increases from 333.2mAh/g (CoO) to 365.2mAh/g (Co1),364.9mAh/g (Co2),350.2mAh/g (Co3),353.5mAh/g (Co4) and 364.7mAh/g (Co5). Moreover, the addition of Co slows down the capacity degradation and prolongs the cycle life, and the optimum Co content is x=0.5. However, the discharge capacity of Co5 alloy electrode decreases to 218.7mAh/g after 80 charge/discharge cycles with the decay of discharge capacity in each cycle of 1.83mAh/g·cycle, so the cycling stability should be further improved.For La1.8Tio.2MgNi9-xAlx (x=0,0.1,0.2,0.3,0.4,0.5) alloys, the results obtained by XRD analyses and electrochemical measurements shows that LaMg2Ni9 phase appears in all alloys. The presence of Al leads to the replacement of La(Ni,Al)5 phase for LaNi5 phase. LaNi2 phase appears and LaNi3 phase disappears with further increasing x (x>0.2). With the increasing of Al content, the cell volume of the alloys first decreases to the smallest as x=0.3 and then increases to the largest when x is 0.4 and 0.5. All alloy electrodes are easily activated to the maximum discharge capacity within 2-3 cycles. With x increasing, the maximum discharge capacity first increases from 333.2mAh/g (x=0) to 357.7mAh/g (x=0.1) and then decreases gradually to 319.8mAh/g (x=0.5). La1.8Tio.2MgNi8.9Alo.1 alloy electrode exhibits relatively higher discharge capacity in activation may be due to its better crystallization compared with Lai.8Tio.2MgNi9 alloy electrode and the disappearance of LaNi3 and appearance of LaNi2 phase as x>0.2. The addition of Al leads to a noticeable improvement of cycling stability, and the cycle life of La1.8Tio.2MgNi8.7Alo.3 alloy electrode is longest, which may be associated with its suitable Al content and the smallest cell volume. Among the alloys studied, the La1.8Tio.2MgNi8.7Al0.3 alloy electrode show a relatively good overall properties with the maximum discharge capacity of 340.0mAh/g and the retention of discharge capacity of 60% after 100 charge/discharge cycles.更多还原