Fund Project:Project supported by the Fundamental Research Fund for the Central Universities,China (Grant Nos. 2016B01914,2018B19414), the Water Science Innovation Project of Jiangsu Province, China (Grant No. 2015087), the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20161501), and the Six Talent Peaks Project in Jiangsu Province, China (Grant No. 2015-XCL-010).
Received Date:07 August 2018
Accepted Date:28 December 2018
Available Online:01 March 2019
Published Online:05 March 2019
Abstract:The growth of population and the limited supply of fossil fuels have forced the world to seek for new kinds of alternative energy sources which are abundant, renewable, efficient, secure and pollution-free. In this regard, hydrogen is generally considered as a potential candidate. However, it is a great challenge to find hydrogen storage materials with large hydrogen gravimetric density under ambient thermodynamic conditions. The most effective way to improve the hydrogen storage capacity is to decorate the pure nanomaterials with transition metals, alkaline metals, and alkaline earth metals. The generalized gradient approximation based on density functional theory is used to study the hydrogen storage capacity of the expanded sandwich structure graphene-2Li-graphene. It is calculated that the structure with the Li atom located above the face site of the hexagonal ring of the graphene has the maximum binding energy (1.19 eV), which is less than the experimental cohesive energy of bulk Li (1.63 eV). However, the calculated binding energy values of the Li atom to the upper and lower graphene layer are both 3.43 eV, which is much larger than the experimental cohesive energy value of bulk Li, so it can prevent the Li atoms from clustering between graphene layers. Each Li atom in the graphene-2Li-graphene structure can adsorb 3 H2 molecules at most. Thus, the hydrogen gravimetric density of graphene-2(Li-3H2)-graphene is 10.20 wt.%, which had far exceeded the gravimetric density of the target value of 5.5 wt.% by the year 2017 specified by the US Department of Energy. The average adsorption energy values of H2 adsorbed per Li are 0.37, 0.17, and 0.12 eV respectively for 1?3 H2 molecules, which are between the physical adsorption and chemical adsorption(0.1?0.8 eV), therefore, it can realize the reversible adsorption of hydrogen. Each Li atom can adsorb 3 H2 molecules at most by the electronic polarization interaction. The dynamic calculations and GFRF calculations show that the interlayer Li atom doped double-layer graphene has good reversible adsorption performance for hydrogen. This research can provide a good research idea for developing good hydrogen storage materials and theoretical basis for experimental worker. These findings can suggest a way to design hydrogen storage materials under the near-ambient conditions. Keywords:graphene/ Li/ electronic properties/ hydrogen storage/ density functional theory
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3.结果与讨论首先对单个Li原子在C6H6上方的最稳定位置进行研究. 分析可知, 苯环上方有3种不等价位置, 分别为: 1) C原子顶位上方; 2) C—C键桥位上方; 3) 六元环面心位上方, 如图1(a)所示. 计算得知, 三种同分异构体的优化结构中, Li原子都稳定于面心位, 与苯环中心距离为1.78 ?, 如图1(b)所示. Li原子与基底的结合能binding energy (Eb)定义如下[14,31]: 图 1 (a)苯环中3种不等价位置; (b) Li原子位于苯环面心位上方的优化结构; (c) C6H6-Li-C6H6势能面扫描曲线和最稳定的C6H6-Li-C6H6三明治结构 Figure1. (a) Three unequal positions in benzene ring; (b) the optimized structure of the benzene ring with the Li atom located above the face site of the hexagonal ring; (c) potential energy surface scanning curve of C6H6-Li-C6H6 and the most stable sandwich structure of C6H6-Li-C6H6.
表1扩展三明治结构graphene-2(Li-nH2)-graphene[G-2(Li-nH2)-G)](n = 1—4)中的H2分子的Ead, Er, Li和H的平均bader电荷(QLi和QH), 双层石墨烯的层间距(DG-G) Table1.The Ead and Er of H2 molecules average bader charge of Li and H (QLi and QH), interlayer distance of double-layer graphene (DG-G) in the expanded sandwich structure graphene-2(Li-nH2)-graphene[G-2(Li-nH2)-G)](n = 1—4).
因为储氢材料必须具有一定的动力学稳定性, 我们进一步对图2(h)结构(graphene-2Li-graphene)进行了300 K温度下的动力学模拟. 本文使用Nose-Hoover Chain thermostats进行有限的温度调节, 研究体系的动力学稳定性. 在300 K, 1 fs步长, 5 ps后的动力学结构与图2(h)结构基本上没有改变, 因此, graphene-2(Li-3H2)-graphene结构具有较高的热力学和动力学稳定性. 对于已实验合成的二茂铁(C5H5-Fe-C5H5)[14,15]结构, Fe原子的外层电子排布为[Ar]3d64s2, 每个C5H5单元具有5个${\text{π}}$电子, Fe原子的最外层3d轨道有8个电子, 因此, 该体系一共具有18个电子, 形成类似惰性气体的稳定电子态, 因此以二茂铁结构为代表的过渡金属掺杂的三明治结构的稳定性可以使用18电子规则[32]进行解释. 那么, 如C6H6-Li-C6H6等碱金属掺杂的三明治结构如何能稳定下来呢?我们认为在该结构中吸附H2分子是一个很好的途径, 从该思路出发, 碱金属掺杂的三明治结构显然可以成为良好的储氢体系. 接下来就对扩展三明治结构graphene-2Li-graphene的(2 × 3)单元的储氢性能进行深入研究. 图3(a)—图3(d)分别给出了graphene-2Li-graphene体系的(2 × 3)单元中每个Li原子吸附1—4个H2分子后的优化结构. 表1给出了体系对H2的平均吸附能Ead、连续吸附能(consecutive adsorption energy, Ec)、和Li原子的Bader电荷(QLi和QH)、双层石墨烯的层间距DG-G. 图 3 graphene-2Li-graphene 的2 × 3晶胞中每个Li原子分别吸附1—4个H2分子的结构图 Figure3. The structural of the 2 × 3 unit cell of graphene-2Li-graphene with each Li atom adsorbed by 1?4 H2 molecules.