Department of Physics and Astronomy, University of Missouri, Columbia, Missouri 65211, USA.
J Am Chem Soc. 2012 Sep 12;134(36):15130-7. doi: 10.1021/ja306726u. Epub 2012 Aug 30.
A class of high-surface-area carbon hypothetical structures has been investigated that goes beyond the traditional model of parallel graphene sheets hosting layers of physisorbed hydrogen in slit-shaped pores of variable width. The investigation focuses on structures with locally planar units (unbounded or bounded fragments of graphene sheets), and variable ratios of in-plane to edge atoms. Adsorption of molecular hydrogen on these structures was studied by performing grand canonical Monte Carlo simulations with appropriately chosen adsorbent-adsorbate interaction potentials. The interaction models were tested by comparing simulated adsorption isotherms with experimental isotherms on a high-performance activated carbon with well-defined pore structure (approximately bimodal pore-size distribution), and remarkable agreement between computed and experimental isotherms was obtained, both for gravimetric excess adsorption and for gravimetric storage capacity. From this analysis and the simulations performed on the new structures, a rich spectrum of relationships between structural characteristics of carbons and ensuing hydrogen adsorption (structure-function relationships) emerges: (i) Storage capacities higher than in slit-shaped pores can be obtained by fragmentation/truncation of graphene sheets, which creates surface areas exceeding of 2600 m(2)/g, the maximum surface area for infinite graphene sheets, carried mainly by edge sites; we call the resulting structures open carbon frameworks (OCF). (ii) For OCFs with a ratio of in-plane to edge sites ≈1 and surface areas 3800-6500 m(2)/g, we found record maximum excess adsorption of 75-85 g of H(2)/kg of C at 77 K and record storage capacity of 100-260 g of H(2)/kg of C at 77 K and 100 bar. (iii) The adsorption in structures having large specific surface area built from small polycyclic aromatic hydrocarbons cannot be further increased because their energy of adsorption is low. (iv) Additional increase of hydrogen uptake could potentially be achieved by chemical substitution and/or intercalation of OCF structures, in order to increase the energy of adsorption. We conclude that OCF structures, if synthesized, will give hydrogen uptake at the level required for mobile applications. The conclusions define the physical limits of hydrogen adsorption in carbon-based porous structures.
一类具有高比表面积的假想碳结构已被研究，这些结构超出了传统的平行石墨烯片模型，其中石墨烯片层中物理吸附的氢层位于变宽的狭缝状孔中。研究的重点是具有局部平面单元（无边界或有边界的石墨烯片片段）和平面与边缘原子比例可变的结构。通过使用适当选择的吸附剂-吸附质相互作用势能进行巨正则蒙特卡罗模拟，研究了这些结构对分子氢的吸附。通过将模拟吸附等温线与具有明确定义的孔结构（近似双峰孔径分布）的高性能活性炭的实验等温线进行比较，测试了相互作用模型，并且计算出的和实验得出的等温线之间存在显著的一致性，无论是重量过剩吸附还是重量存储容量。通过这种分析和对新结构的模拟，碳的结构特性与随之而来的氢气吸附之间出现了丰富的关系（结构-功能关系）：（i）通过石墨烯片的碎裂/截断可以获得超过狭缝孔的存储容量，这会产生超过 2600 m²/g 的表面积，这是无限石墨烯片的最大表面积，主要由边缘位携带；我们将由此产生的结构称为开放碳骨架（OCF）。（ii）对于具有≈1 的平面与边缘位比例和 3800-6500 m²/g 的表面积的 OCF，我们发现 77 K 和 100 bar 下的最大过剩吸附量为 75-85 g H₂/kg C，最大存储容量为 100-260 g H₂/kg C。（iii）由小的多环芳烃构建的具有大比表面积的结构的吸附不能进一步增加，因为它们的吸附能低。（iv）通过 OCF 结构的化学取代和/或嵌入，可能会进一步增加氢的吸收量，以增加吸附能。我们得出的结论是，如果合成 OCF 结构，将能够在移动应用所需的水平上实现氢的吸收。这些结论确定了基于碳的多孔结构中氢吸附的物理极限。
