多孔石墨片作为氢气提纯和存储材料的理论研究

发布时间：2018-06-03 13:56

本文选题：第一性原理 + 密度泛函理论　；参考：《南京师范大学》2017年硕士论文

【摘要】：近年来,氢气作为可再生能源引起了极大的关注。由于它具有丰富的含量、较高的能量密度、以及高清洁度等特点,有望成为未来的重要能量来源之一。同时,它也具有重要的工业应用价值,尤其在石化、冶金、食品加工、精密电子工业合成等领域已经得到了广泛应用。然而,在实际应用中,氢气的高效分离、氢气的存储等问题还面临着巨大的挑战。最近,石墨烯纳米片C_(222)被成功合成,该纳米片一经合成便引起科学家们强烈的兴趣。已有的研究发现石墨片上的孔状缺陷对其光学和电学性质都有很大影响。因此本文通过第一性原理研究了三种不同形状的缺陷对C_(222)石墨烯纳米片的电、光学性质的影响。研究结果表明中心挖去一个苯环的C_(216)相比C_(222)能隙增加了0.39 eV,使其半导体特性明显。相反(C221和(C220分别是由(C_(222)中心挖去一个和两个碳原子形成的单空位缺陷和双空位缺陷结构,这两个缺陷的存在降低了 C_(222)的能隙,使其导体特性明显。同时能隙大小的改变也使得UV-vis吸收光谱中的最大吸收峰发生了蓝移(C_(216))或红移(C221, C220)。另外,计算结果显示随着缺陷密度的增加,HOMO-LUMO能隙也增加,最高至4.28 eV。吸收光谱也从400-800 nm波段移至300-500 nm波段。由于石墨烯纳米片C_(216)是孔状材料,符合分离膜材料的几何特征,因此我们计算了它对于常见气体(H2, O2, N2, NO, NO2,H2O, CO和C02)的选择性和过渡能垒,研究发现它对于氢气的扩散能垒明显低于其它气体,在室温下该石墨片对氢气相对于其他气体的选择率高达1048,明显优于多孔石墨烯、石墨炔以及传统氢气分离膜,有望成为一种理想的氢气分离膜材料。同时,本文设计了一种Li掺杂的多孔石墨片C180,并研究了该材料作为作为储氢材料的可能。计算结果表明Li原子在该石墨片表面不容易形成团簇。而且氢气在Li掺杂的多孔石墨片上的平均吸附能为-0.17 eV,其绝对值明显高于纯多孔石墨片或Li掺杂的石墨烯。该吸附能的大小说明Li掺杂的多孔石墨片能够实现在温和条件下对氢气的自由吸附和解离。当增加Li原子的吸附浓度时,该多孔石墨片的质量储氢密度高达4.76 wt%,明显优于Li原子掺杂的石墨烯和碳纳米管等材料。因此多孔石墨片C180有望成为高效的储氢材料。
[Abstract]:In recent years, hydrogen as a renewable energy has attracted great attention. Because of its rich content, high energy density and high cleanliness, it is expected to be one of the important energy sources in the future. At the same time, it also has important industrial application value, especially in petrochemical, metallurgy, food processing, precision electronic industrial synthesis and other fields have been widely used. However, in practical application, the high efficiency separation of hydrogen and the storage of hydrogen are still facing great challenges. Recently, graphene nanochip C _ s _ 2 _ 2) was successfully synthesized, and once synthesized, it has aroused great interest from scientists. It has been found that the porous defects on the graphite sheet have great influence on the optical and electrical properties. Therefore, the effects of three different defects on the electrical and optical properties of C _ S _ (222) graphene nanocrystals have been investigated by first-principles. The results show that the gap of C _ S _ (216) with a benzene ring is increased by 0.39 EV compared with that of C _ S _ (222), which makes the semiconductor characteristic obvious. On the contrary, C221 and C220 are the structures of single vacancy defect and double vacancy defect formed by one and two carbon atoms, respectively. The existence of these two defects reduces the energy gap of CStue 222) and makes its conductor characteristic obvious. At the same time, the change of energy gap also makes the maximum absorption peak in UV-vis absorption spectrum blue shift C216C) or red shift C221, C220. In addition, the calculated results show that the HOMO-LUMO gap also increases with the increase of defect density, reaching to 4.28 EV. The absorption spectra were also shifted from 400-800 nm to 300-500 nm. Because the graphene nanochip Che 216) is a porous material which conforms to the geometric characteristics of the membrane material, we have calculated its selectivity and transition energy barrier for the common gases (H _ 2, O _ 2, N _ 2, no _ 2, O _ 2H _ 2O, CO and C _ 02). It is found that the diffusion barrier for hydrogen is obviously lower than that for other gases, and the selectivity of the graphite sheet to hydrogen relative to other gases is as high as 1048 at room temperature, which is obviously superior to that of porous graphene, graphite acetylene and traditional hydrogen separation membranes. It is expected to be an ideal membrane material for hydrogen separation. At the same time, a Li-doped porous graphite sheet C180 has been designed, and the possibility of using this material as hydrogen storage material has been studied. The results show that the Li atoms are not easy to form clusters on the surface of the graphite sheet. The average adsorption energy of hydrogen on Li-doped porous graphite is -0.17 EV, which is obviously higher than that of pure porous graphite or Li doped graphene. The adsorption energy shows that Li doped porous graphite can achieve the free adsorption and dissociation of hydrogen under mild conditions. When the adsorption concentration of Li atom is increased, the mass hydrogen storage density of the porous graphite wafer is up to 4.76 wt, which is obviously superior to that of graphene doped with Li atom and carbon nanotubes and so on. Therefore, porous graphite sheet C 180 is expected to be an efficient hydrogen storage material.
【学位授予单位】：南京师范大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TQ127.11;TB383.4

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