【作者】 吴广新；

【导师】 周国治； 张捷宇；

【作者基本信息】 上海大学 ， 钢铁冶金， 2009， 博士

【摘要】 镁基合金是最有可能实用化的储氢材料之一,但同时它也存在动力学性能差、放氢温度高等缺点。针对这些缺点,国内外进行了大量的实验和理论研究,如元素取代、材料纳米化/复合化、添加催化剂、热处理等,不同程度地改善了材料的吸放氢热力学、动力学、电化学性能和循环寿命。但到目前为止,有关储氢金属与氢原子之间的相互作用以及具体的吸放氢的热力学和动力学过程(基于微观视角)仍非常不清楚。因此急需要解决储氢合金吸放氢反应的机制问题,以及在此机制上建立理论模型。本论文从镁基储氢材料微观结构出发,主要研究了H2分子在镁基金属、氢化物和氧化物表面解离吸附的完整过程和H原子在上述体系表面的最优吸附位置、电荷转移、体内扩散情况等过程,并采用机械合金实验和Chou动力学模型来验证计算结果的可靠性;随后从统计力学角度出发推导出压力-组分-温度热力学模型;最后,在Chou模型基础上,进一步考虑了混合控速机理,推导出了混合控速动力学模型。主要得到如下创新性结论:(1)本文采用密度泛函理论和Nudged Elastic Band算法,对洁净及掺杂Fe3O4和La催化剂的Mg/MgH2表面的吸放氢过程进行了考察。结果显示:吸氢时,对于洁净Mg/MgH2表面,氢的化学吸附环节是控速步骤;添加催化剂Fe3O4和La后,Fe的掺杂对H2的解离影响最大,其次是La掺杂和空位,但Fe、La和空位的引入均对H的扩散起反作用,最终导致控速环节变为扩散。放氢时,对于洁净Mg/MgH2表面,表面的解氢反应为控速步骤;掺杂催化剂后,Fe、La分别对近邻和次紧邻Mg周围的H的脱附的催化作用较为明显,而Mg空位对解氢起反作用,同时Fe、La掺杂和Mg空位模型对H的扩散均表现出较好的催化作用,最终此过程控速环节没有出现变化。根据上述微观计算构型条件,采用机械合金法制备了MgH2、MgH2+1.9mol%Fe3O4和MgH2+1.9mol%La三个体系的镁基储氢合金,并采用PCT设备台考察了体系的热力学和动力学性能,再利用修正的Chou模型对动力学数据进行了拟合,将获得的活化能与微观计算数据做了定性的对接。(2)当表面存在钝化层MgO时,H2和H在MgO表面的解离势垒和扩散势垒明显变大,并通过Arrhenius公式结合他人实验数据对本文中H的扩散系数进行了验证。(3)利用统计热力学中的Bragg-Williams点阵模型,得到储氢合金体系的PCT热力学表达式:并采用新模型对洁净及掺杂催化剂的MgH2体系、Mg2Ni体系和La2Mg17体系进行了定量分析和计算,得到了不同体系的焓变、熵变与浓度的变化关系。(4)以Chou模型为基础,推导出表征吸氢反应不同混合控速环节的动力学新模型,并求解出其解析解。它们将反应时间t表达为一个关于反应分数ξ,温度T以及颗粒大小R0的函数。表达式为:随后将模型应用不同体系中,结果表明本模型使用灵活方便,可以对单步和混合控速的反应机理进行定性和定量描述。总之,本论文对镁基储氢合金吸放氢过程的反应机理进行了考察,从微观结构出发,论证了微观结构与热力学模型和动力学模型的相互交融、密不可分的关系,建立了较为完整的储氢合金吸放氢反应的理论体系。更多还原

【Abstract】 Magnesium alloy is one of potential hydrogen storage materials in the hydrogen energy field. However, due to its kinetic limitations and unmanageable desorption temperatures, pure magnesium is unsuitable as a hydrogen storage material. To overcome these disadvantages, tremendous studies in both experiments and theories, such as element substitute and heat treatment, have been done to improve its thermodynamics, kinetics, electrochemistry and circle life to some extent. However, the interaction between metal and hydrogen and the mechanism of thermodynamics and kinetics from the microcosmic point view were still not clear. Therefore, a better understanding of its reaction mechanism is very urgent and also provides the base for further building a theoretical model.In this thesis, firstly, the dissociation/diffusion processes of H2/H on the Mg, MgH2 and MgO surfaces and the charge transfer were investigated. Secondly, the experiment data of ball milling and Chou model were used to validate our calculation results. Then, the pressure-composition-temperature model was deduced with the statistical mechanics. Finally, the mechanism of mixing rate controlling was analyzed by Chou model and the related models were derived.(1) The hydriding/dehydriding processes of pure and Fe3O4/La doped Mg/MgH2 surfaces were investigated with the Density Functional Theory and Nudged Elastic Band methods. Our results show:For the hydrogenation process, the dissociation of H2 was the rate-limiting step for pure MgH2 system. When the MgH2 mixed with catalyst Fe3O4 and La was considered, it was found that the doping of Fe atom has higher catalytic activities than the doping of La atom and vacancy for the H2 dissociation. However, the catalyst has the negative effect on the H diffusion and could eventually make the diffusion become the rate-limiting step. For the dehydrogenation process, the decomposition of H2 was the rate-limiting step for pure MgH2 system. When the MgH2 mixed with catalysts Fe3O4 and La was considered, it was found that Fe atom has higher catalytic effect on the desorption of H atom nearby the Mg atom. Meanwhile, the doping of La has a positive effect on the H atom of next-nearby Mg while the vacancy on the Mg site has a negative effect on the H desorption. Doping of Fe and La together with vacancy could improve the kinetics of H diffusion in the bulk and did not change the rate-limiting step until the end.According to the model calculation, MgH2, MgH2+1.9mol%Fe3O4 and MgH2+1.9mol%La were prepared by mechanical alloy method. The modified Chou model was then used to analyze the experimental results. The fitted results show that the experimental results could be well described by Chou model.(2) The dissociation/diffusion processes of H2/H on the MgO(001) surface were investigated. Our results show shat the activation energies of dissociation and diffusion for H2/H on the MgO(001) surface were much larger than that of H2/H on the Mg(0001) surface. Thus, MgO layer could prevent the hydrogen molecules from penetrating into the material.The diffusion coefficient was then obtained according to the Arrhenius equation and generally agreed with the experiments result.(3) Using the Bragg-Williams model in the statistical mechanics, we got the pressure-composition-temperature model of hydrogen storage alloys:In this new model, compressibility factorαwas used to correct the high pressure and improve the fitting of data. Then, the thermodynamic model was used to describe MgH2, catalyst-MgH2, Mg2Ni and La2Mg17 systems and the enthalpy and entropy as a function of composition were obtained.(4) On the basis of Chou model, a new model for predicting the reaction time has been expressed as an analytic formula using the reaction fraction of hydrogen in hydrogen storage materials, temperature and the granule radii: This model has been used in different systems, which showed that it was easy and flexible to be applied. Meanwhile, the model can describe reaction mechanism of the single and mixed rate-controlling steps.In conclusion, the reaction mechanism of hydriding/dehydriding of Mg-based hydrogen storage alloys was investigated. The close relationships among microstructure, thermodynamics and kinetics were analyzed and a theoretical system has been built for studying the reaction mechanism.更多还原