【作者】 黄存可；

【导师】 朱敏；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2009， 博士

【摘要】 在没有催化剂情况下,配位氢化物的吸放氢动力学性能通常很差。原因有两种:一是热力学上可实现但动力学势垒过高;二是需要的热力学驱动力过高。前者可通过添加合适的催化剂实现可逆吸放氢;而对后者则无法通过降低动力学势垒实现。对配位氢化物的吸放氢热力学性能,如吸放氢可逆性和去稳定问题、微观吸放氢机理和催化机理的研究,是新型高容量配位氢化物储氢材料研究的几个焦点问题。环境对氢化物的稳定性和吸放氢行为有影响。化学势可考察环境变化对氢化物自由能的影响,并能结合实验平衡氢压分析吸放氢热力学性质,它还是一个能贯穿宏观和微观的物理量。虽然容量还不够高,NaAlH₄无疑是研究高容量配位氢化物的最好样板材料,本文以NaAlH₄为对象,建立和采用化学势平衡相图描述吸放氢热力学性能。从该相图可知氢化物稳定存在的化学势范围;可直接读出氢化学势最低的吸放氢临界点;可从吸放氢临界点的相平衡关系导出吸放氢反应方程;从吸放氢点的氢化学势值还可以直接得到氢化物和环境交换氢分子需耗费的能量,进而得到吸放氢反应的反应焓。另外,通过它还可为结合化学环境研究氢化物的各种缺陷形成焓打下基础。本文还提出以某种氢化物的实验范霍夫线为基准,通过比较0K下吸放氢临界点推导另外未知氢化物平衡氢压的方法,它可以结合第一性原理方法和实验方法的优势。以NaAlH₄的实验范霍夫线为基准得到LiAlH₄平衡氢压的准确度比直接用第一性原理方法情况提高。用这种方法可以快速而较为准确的估计得到新型配位氢化物的平衡氢压,这对在热力学上初步筛选出具有吸放氢可逆性的新型高容量配位氢化物储氢材料有重要意义。本文首次采用化学势平衡相图研究添加过渡金属（TM）对NaAlH₄的去稳定问题。研究发现,通过形成Al化学势低的TM-Al合金,降低吸放氢临界点的Al化学势,可间接提高氢的化学势。通过这种方法可以方便比较不同过渡金属对NaAlH₄的去稳定能力,还可讨论在不同TM化学势下的吸放氢热力学性质,这是一般传统方法所不具备的。Ti等催化剂催化NaAlH₄的微观机理与NaAlH₄的微观吸放氢机理有关,但这些都尚未达成共识。我们认为NaAlH₄分解的第一步是某种含H空位的形成,可通过系统计算NaAlH₄本征缺陷进行研究。由于NaAlH₄具有宽禁带,我们对缺陷计算既考虑了缺陷形成的原子化学环境也考虑了电子的化学环境。研究发现,在导致NaAlH₄分解的空位中,AlH₃空位无论在体内还是在表面能最低的（001）表面都有最高的形成概率;AlH₃空位是“AlH₅₊空位”结构,而AlH₅被认为是Na3AlH6形成的前驱体;脱出的AlH₃分子在吸放氢临界点下很容易失稳分解。因此NaAlH₄分解的AlH₃中间态机制是很有可能存在的。本文还系统的计算了在吸放氢临界点化学环境下,在NaAlH₄的体内和（001）面不同深度的Ti单缺陷。我们发现Ti_Al(2^nd)和Tii（Al rich）是两种最容易形成Ti单缺陷,它们分别形成TiAl₄H₂₀和TiAl₃H₁₂团簇。我们还把Ti的缺陷形成焓进行分解,并结合Ti的局域结构进行剖析。通过我们全面而深入的分析,人们可以更清晰的了解Ti缺陷的形成机理。为了对Ti缺陷对NaAlH₄分解作用有更细致的认识,我们较为全面的研究了最容易形成的Ti缺陷团簇内外的四种空位形成焓[H_f（V_H）, H_f(V_H-H), H_f(V_AlH3)和H_f(V_Na)]。结果一致的发现,Ti缺陷能有效降低NaAlH₄分解的各种空位形成焓,但Ti对NaAlH₄分解的促进作用有区域性。更多还原

【Abstract】 The hydriding/dehydriding performance of complex hydrides are often poor, which may be resulted from high kinetic barriers, or intrinsically resulted from high requirements in thermodynamic driving force. If the reaction process is hindered merely by the kinetic barrier, it can be improved by suitable catalysts. However, if it is intrinsically due to the latter reason, any further search for suitable catalysts may be futile and the material is impractical for reversible hydrogen storage.There are several hot issues on researches of high-capacity hydrogen storage materials of complex hydrides: hydriding/dehydriding thermodynamic properties （such as hydriding/dehydriding thermodynamic reversibility and destabilization）; micro mechanism of hydriding/dehydriding reaction and catalyst mechanism.The stability and hydriding/dehydriding performance of hydrides are often influenced by their environment. Chemical potential can be utilized to describe the interaction of hydride and its environment. It can also connect equilibrium hydrogen pressure and thermodynamic characteristics of hydridind/dehydriding, thus, chemical potential can be a link between theoretical and experimental studies. It is as well as a link between the related macro and micro phenomena. Although its capacity does not meet the practical criterion, NaAlH₄ is undoubtedly a prototype for the study of other high-capacity hydrogen storage materials. In this dissertation, we have established and used equilibrium phase diagram characterized with chemical potentials to study the thermodynamic properties of hydrogen absorption and desorption with NaAlH₄ as researched object. We find that the chemical potential range for a stable hydride phase can be illustrated directily from the phase diagram. The hydridind/dehydriding critical point with the lowest chemical potential of H can be obtained, which is directly related with equilibrium hydrogen pressure. The hydridind/dehydriding equation can be expressed directily from the phase equilibrirum relation at critical point. The energy required for the exchange of a H2 molecular between hydride and environment can be read from the coordination of critical point, and the reaction enthalpy can be derived based on it. In addition, we can calculate formation energy of various defects in hydrides with consideration of the practical experiment environment.We propose an approch to estimate the equilibrium hydrogen pressure of a target hydride from a reference hydride, which takes advantage of both first-principles calculations and experiments. That is, by taking experimental Van’t Hoff’s line of a hydride as reference, we may obtain the equilibrium hydrogen pressure of the target hydride through comparing the critical points of the target and reference hydrides. As an example, we have obtained the equilibrium hydrogen pressure of LiAlH₄ at different temperature from the experimental Van’t Hoff’s line of NaAlH₄ and the theretical relation between the critical points of NaAlH₄ and LiAlH₄. The obtained result is more accurate than that directly calculated from first-principles method. With this new approach, the equilibrium hydrogen pressure of a new complex hydride can be obtained rather quickly to a more accurate degree. This provides an efficient way to screen new potential high-capacity hydrogen storage materials of complex hydrides with good hydriding/dehydriding thermodynamic reversibility.For the first time, we use equilibrium phase diagram characterized with chemical potentials to study the destabilization of transition metals （TMs） to NaAlH₄. It is found that the chemical potential of H at critical point increases as TM-Al alloys are formed with lower chemical potential of Al. This is a destabilization mechanism of TMs to NaAlH₄. In this way, the destabilizing ability of TM to NaAlH₄ can be easily distinguished. It can also be adopted to study the hydridind/dehydriding thermodynamic properties under different chemical potentials of TM, which is generally not available in conventional methods.It is known that the catalytic mechanism of Ti-contained additives to NaAlH₄ is related to the micro hydriding/dehydriding mechanism of NaAlH₄, yet none of the mechanisms are clear. We believe that the first step of NaAlH₄ decomposition is the formation of some H-contained vacancy, which can be studied through calculating the intrinsic defects of NaAlH₄ systematically. Both atomic and electronic chemical potentials are considered when intrinsic defects are calculated since NaAlH₄ is an ionic crystal with a wide band gap. It is found that among the defects which lead to decomposition of NaAlH₄, the formation possibility of AlH₃ complex vacancy, V（AlH₃）, is the highest in both bulk and （001） surface of NaAlH₄. The structure of V（AlH₃） is“vacancy+AlH5”pair, and AlH5 is considered as a precursor of Na3AlH6. AlH₃ molecule is not stable under the chemical environment at critical point, thus it is easy to decompose after desorption from NaAlH₄. Overall, we believe that it is likely to have an“AlH₃ intermediate state mechanism”during decomposition.We also systematically calculate single Ti defects in bulk and （001） slab with different depth under the chemical environment of the critical point. Ti_Al(2^nd) and Ti_i（Al rich） are the two single Ti defects with high formation possibility accompanied by TiAl+3H（12） and TiAl₄H₂₀ complexes respectively. It is found that a deeper insight on the formation mechanism of single Ti defect can be obtained when the defect deformation enthalpies are divided into three terms which are directly related to the local structures of Ti defects. In addition, for the first time, we adopt the formation enthalpy of four vacancies [H_f（V_H）, H_f(V_H-H), H_f(V_AlH3) and H_f(V_Na)] within and around the two Ti-Al-H complexs to investigate the impact of Ti defects on the decomposition of NaAlH₄. It is found consistently that the two single Ti defects can effectively reduce all the vacancy formation enthalpies of NaAlH₄, but mainly at the regions inside the TiAl4H20 complex or outside TiAl3H12 complex.更多还原