三明治结构graphene-2Li-graphene的储氢性能

本文使用密度泛函理论中的广义梯度近似对扩展三明治结构graphene-2Li-graphene的几何结构、电子性质和储氢性能进行计算研究.计算得知:位于单层石墨烯中六元环面心位上方的单个Li原子与基底之间的结合能最大(1.19 eV),但小于固体Li的实验内聚能(1.63 eV),然而,在双层石墨烯之间的单个Li原子与基底的结合能增加到3.41 eV,远大于固体Li的实验内聚能,因此位于双层石墨烯之间的多个Li原子不会成簇,有利于进一步储氢.扩展三明治结构graphene-2Li-graphene中每个Li原子最多可以吸附3个H_2分子,储氢密度高达10.20 wt.%,超过美国能源部制定的5.5 wt.%的目标.该体系对1—3个H_2分子的平均吸附能分别为0.37,0.17和0.12 eV,介于物理吸附和化学吸附(0.1—0.8 eV)之间,因此该体系可以实现常温常压下对H_2的可逆吸附.通过对态密度分析可知,每个Li原子主要通过电场极化作用吸附多个H_2分子.动力学和巨配分函数计算表明graphene-2Li-graphene结构对H_2分子具有良好的可逆吸附性能.该研究可以为开发良好的储氢材料提供一个好的研究思路,为实验工作提供理论依据.

Hydrogen storage capacity of expanded sandwich structure graphene-2Li-graphene

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.