【作者】 刘光；

【导师】 袁华堂；

【作者基本信息】 南开大学 ， 材料物理与化学， 2013， 博士

【摘要】 氢的廉价制取、安全储运以及高效应用是目前氢能研究领域的重点,而安全、高效的氢的储运是实现氢能规模化应用的技术关键,因此新型高容量储氢材料的研发具有重要的学术意义和应用价值。氢化镁的理论储氢量为7.6wt%,体积储氢密度为110kg/m3,是一种非常有潜力的储氢材料。但是其较高的放氢温度、较差的动力学性能,阻碍了其实际应用。本论文以MgH2储氢体系为研究对象,在全面综述MgH2储氢体系研究进展的基础上,系统研究了以下几种Mg基复合储氢体系的储氢性能、微观结构及吸放氢反应机理：(1)本文首先研究了高温热裂解制备的石墨烯纳米片(GNS)添加球磨制备的MgH2-GNS复合储氢体系的吸放氢性能、微观结构以及吸放氢反应机理。研究结果表明不同GNS添加量、不同球磨时间制备的MgH2-GNS复合储氢材料都具有较好的吸放氢动力学性能,其中添加5wt%GNS球磨20h的MgH2-5wt%GNS-20h材料具有最优异的储氢性能。300℃时,1min之内即可吸氢6.6wt%,40min之内放氢6.1wt%,并且6周吸放氢循环后再次吸放氢量基本没有变化；当温度降为150℃时,180min之内还可吸氢6.0wt%。其Johnson-Mehl-Avrami(JMA)动力学方程拟合结果显示：对于吸氢反应,在实验温度范围内,动力学速度控制步骤为等速率界面反应控制；对于放氢反应,在350,320℃的放氢过程遵循二维成核与长大机制,而300℃的放氢过程则遵循界面反应等速率控制。微观结构分析表明,GNS添加球磨制备的MgH2-5wt%GNS-20h材料具有更小的晶粒尺寸,吸放氢循环前后GNS均以无序、不规则的形态均匀分布在储氢材料中,并能够提供大量的缺陷空位和活性位点,材料的纳米尺寸化以及高缺陷、多活性位点的GNS添加是促进复合储氢材料储氢性能的主要原因。(2)对乙二醇法制备的Ni@GNS添加制备的MgH2-5wt%Ni@GNS体系的研究表明,Ni@GNS对MgH2的吸放氢性能具有协同催化作用,Ni@GNS中Ni对MgH2-5wt%Ni@GNS的动力学性能具有促进作用,而Ni@GNS中的GNS对复合储氢材料的可逆吸放氢量具有明显的提升作用。MgH2-5wt%Ni@GNS在300℃时,10min之内即可放氢6.0wt%,而且在9周吸放氢循环过后放氢速率和放氢量没有明显衰减。该复合储氢材料在230℃时,120min之内还可放出5.07wt%的H2;同时MgH2-5wt%Ni@GNS具有优异的低温吸氢性能,在150℃时,25min之内可吸氢5.3wt%。对MgH2-5wt%Ni@GNS的吸放氢数据进行JMA方程拟合结果显示,对于吸放氢过程均符合Mampel模型,即吸放氢过程均为随机成核长大过程控制。对复合储氢体系的微观结构分析表明,Ni@GNS复合催化剂在MgH2-5wt%Ni@GNS体系的吸放氢过程中起着重要的作用。Ni@GNS具有的特殊孔状结构和较大比表面积,能够阻止储氢材料在吸放氢过程中的颗粒进一步增大和团聚,另外还能降低氢的解离能,为氢的扩散提供更多的通道,从而有利于吸放氢动力学性能和循环稳定性能。(3)对于球磨制备的不同NiB添加量的MgH2-xwt%NiB(x=5、10、15)体系,MgH2-10wt%NiB具有最优的放氢性能,其最大TPD放氢峰值温度为248.4℃,放氢量为6.12wt%。恒温放氢动力学性能测试表明,MgH2-10wt%NiB在300℃和5kPa氢压下,10min之内可放氢6.0wt%的氢气,温度低至230℃时,120min仍可放出4.79wt%的H2,而且HP-DSC结果显示其具有较好的吸放氢循环可逆性。动力学JMA拟合结果显示MgH2-10wt%NiB材料的放氢反应可用二维相界面迁移动力学方程来描述,得到该材料的放氢活化能为59.7kJ/mol H2,远远低于纯MgH2的放氢活化能。微观结构分析结果表明,NiB催化剂在MgH2分解放氢过程中起着重要的作用,NiB在放氢过程中与MgH2反应生成Mg2Ni和MgB2,Mg2Ni和MgB2在吸放氢过程中成为新相的成核点,促进材料中H的解离与扩散,从而促进材料的放氢性能。(4)对TiB2、TiB2/GNS添加球磨MgH2的放氢性能研究结果显示,TiB2/GNS具有更优异的催化氢化镁分解放氢性能。MgH2-5wt%TiB2/GNS起始放氢温度为215℃,300℃和5kPa氢压下,10min之内即可放氢6.5wt%,即使温度降至240℃,120min之内仍可放出5.8wt%的H2。放氢动力学数据的JMA拟合结果显示,其放氢过程符合二维成核与长大机制。上述吸放氢性能测试表明,TiB2/GNS对MgH2-5wt%TiB2/GNS体系的放氢性能具有协同催化作用。TiB2对吸放氢动力学速率具有促进作用,GNS对吸放氢容量具有提升作用。更多还原

【Abstract】 Hydrogen production, security storage as well as efficient applications are the focus in the field of hydrogen energy research. Among them, the safe and efficient storage of hydrogen is the most difficult obstacle to realize the scale application of hydrogen energy. Therefore, the research and development of new high-capacity hydrogen storage materials has important theoretical significance and application value. Recently, considerable attentions have been paid on MgH2,0which is one of the most promising hydrogen storage materials due to its high gravimetric (7.6wt%H2) density and volumetric (110kg m-3) density. However, the practical application of MgH2is limited by high thermal stability and sluggish sorption kinetics. Based on the overview of the process in MgH2as the hydrogen storage medium, several Mg-based hydrogen storage composites were systematically investigated and discussed.(1) Highly crumpled graphene nanosheets (GNS) were fabricated by a thermal exfoliation method, and then a systematic investigation was performed on the hydrogen sorption properties of MgH2-GNS nanocomposites acquired by ball-milling. It was found that the as-synthesized GNS exhibited a superior catalytic effect on dehydrogenation/hydrogenation of MgH2. It was found that both hydrogen sorption capacity and dehydrogenation/hydrogenation kinetics of the composites improved with increasing milling time. The composites MgH2-GNS milled for20h can absorb6.6wt%H2within1min and release6.1wt%H2at300℃, and there was no difference of hydrogen storage capacity after6th hydrogenation/dehydrogenation cycling. It was also demonstrated that MgH2-GNS-20h could absorb6.0wt%H2within180min even at150℃. The fitting results of JMA model revealed that the absorption process in the experimental temperature range and desorption process at300℃were controlled by one-dimensional growth with constant interface velocity, while at350and320℃, the desorption process was controlled by two-dimensional nucleation and growth. In addition, microstructure measurements revealed that the grain size of thus-prepared MgH2-GNS nanocomposites decreased with increasing milling time and the graphene layers were broken into smaller graphene nanosheets. Furthermore, the graphene nanosheets dispersed disorderly and irregularly among the MgH2particles during the hydrogenation/dehydrogenation cycling. It was confirmed that these smaller graphene nanosheets on the composite surface, providing more edge sites and hydrogen diffusion channels, prevented the nanograins sintering and agglomeration, thus, leading to promote the de/hydrogenation kinetics and cycling stability of MgH2.(2) Porous Ni@GNS nanocomposite was successfully prepared by ethylene glycol method followed by an annealing process. MgH2-5wt%Ni@GNS composite acquired by ball milling exhibited improved faster sorption kinetics and relatively lower sorption temperature than pure MgH2. The MgH2-5wt%Ni@GNS composite could release6.0wt.%H2within10min at300℃even after nine cycles, it can also desorb5.07wt%H2within120min at230℃. In addition, the composite had good hydrogen absorption kinetics and it can absorb5.3wt%H2within25min at150℃. The kinetic analysis revealed that the best fit for both hydrogenation and dehydrogenation based on the Mampel model formulated through the random nucleation approach. The activation energy (Ea) decreased significantly compared to pure MgH2and the presence of few layer graphene nanosheets on the MgH2surface prevented the nanograins sintering and agglomeration during cycling, which enhanced the MgH2decomposition and cycling stability. It was confirmed that the porous Ni@GNS composite has a synergetic effect on the MgH2hydrogen sorption properties.(3) The effect of NiB on hydrogen desorption properties of MgH2was investigated. Measurements using temperature-programmed desorption system (TPD) and volumetric pressure-composition isotherm (PCI) revealed that both the desorption temperature and desorption kinetics have been improved by adding amorphous NiB and the10wt%NiB addition had the best hydrogen desorption performance. For example, the MgH2-10wt%NiB mixture started to release hydrogen at180℃and a hydrogen desorption capacity of6.0wt%was reached within10min at300℃, while the desorption temperature lowerd to230℃, the mixture can also release4.79wt%H2within120min. Further cyclic kinetics investigation using high-pressure differential scanning calorimetry technique (HP-DSC) indicated that the composite had good cycle stability. An activation energy of59.7kJ/mol for the MgH2/NiB composite had been obtained from the desorption data, the enhanced kinetics possibly due to the formed Mg2Ni and MgB2during desorption process, which can reduce the barrier and lowered the driving forces for nucleation, thus, improving the desorption kinetics and cycling stability.(4) The catalytic effects of TiB2and TiB2/GNS on the hydrogen desorption of MgH2were investigated. It was found that TiB2/GNS exhibited more excellent catalytic effect on the dehydrogenation of MgH2. The MgH2-5wt%TiB2/GNS composite started to release hydrogen at215℃and a hydrogen desorption capacity of6.5wt%was reached within10min at300℃, while the desorption temperature lowerd to240℃, the mixture can also release5.8wt%H2within120min. The kinetic analysis based on JMA model assumed a two-dimensional nucleation and growth of MgH2decomposition. Above hydrogen desorption kinetics as well as microstructure analysis confirmed that TiB2/GNS had a synergetic effect on the MgH2hydrogen sorption properties. It was suggested that TiB2enhanced the hydrogen desorption kinetics and GNS had positive effect on the hydrogen storage capacity of MgH2-5wt%TiB2/GNS composite.更多还原