Theoretical Study of Sp2-sp3 Hybrid Carbon Networks for Energy Storage
Authors: Wen Yanwei , Liu Xiao , Shan Bin , Chen Rong 
Published: February 2020

Carbon nanostructures have attracted a lot of attentions in hydrogen storage for its light weight and high surfaces area. However, carbon nanotubes, graphene expanded structures suffer from the weak H2 physisorptions for practical use.[1] Light metal doping is thought to be a feasiable way to improve the H2 binding and those structures are predicted to have ultra-high hydrogen storage capacity.[2] Nevertheless, no experiment has yielded comparable capacity to the theoretical expectations. Sigal et al. [3] doubt about the practicability of hydrogen storage on metal-doped two-dimensional (2D) systems since even slight oxygen content in H2 atmosphere can block the metal sites which suppresses the H2 adsorption. The metal-decorated porous structures could be more practical by hindering the oxygen access to the metal sites. Three-dimensional (3D) porous structures such as carbide-derived carbons,[4] metal organic framework (MOF)[5] and covalent organic framework (COF) [6] are designed and exhibit promising capacity close to the DOE target. However, none of nanoporous structures could meet the requirement of hydrogen storage in both weight percentage and volumetric density up to now.

We present here that the bundles of carbon nanotubes could undergo phase transition to a type of 3D sp2-sp3 hybrid carbon networks (HCN) by changing intertube van der Waals forces to strong sp3 covalent bonding. With well-aligned and size-tunable 1-D nanopores, we show high Li capacity, stable Li-ion intercalation voltage profile, low Li diffusion barriers, slight volumetric change upon Li-doping in HCN, making it a potential Li-ion battery anode material. [7] Moreover, the unique nanoporous strcuture is beneficial for the achoring Li atoms and induces improved H2 binding energy due to the polarized interaction between H2 and Li ion. A maximum hydrogen capacity of 6.28 wt % is reached with average binding energy of -0.19 eV in Li-doped HCN. Combined with an ultrahigh volumetric density of hydrogen (102 g/L), it is a promising candidate for the hydrogen storage medium.[8]

References:

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[2] Yoon, M.; Yang, S.; Hicke, C.; Wang, E.; Geohegan, D.; Zhang, Z. Phys. Rev. Lett. 2008, 100, 206806; Gao, Y.; Zhao, N.; Li, J.; Liu, E.; He, C.; Shi, C. Int. J. Hydrogen Energy 2012, 37, 11835.

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[7] Wen, Y. W.; Liu, X.; Duan, K.B.; Cho, K.; Chen, R.; Shan, B. J. Phys. Chem. C 2013, 117, 4951−4956.

[8] Wen, Y. W.; Liu, X.; Duan, K.B.; Cho, K.; Chen, R.; Shan, B. J. Phys. Chem. C 2015, 119, 15831−15838.