First-principles studies on transition metal and carbon based materials for hydrogen storage

Abstract

Hydrogen storage is an integral part of hydrogen economy. First-principles studies on transition metal and carbon (TM − C) based materials for hydrogen storage were conducted with a strong emphasis on Kubas interaction. The coupling between empty dxz/dyz orbital of TM and sσ orbital of H$_2$ were shown to be important for Kubas interaction to arise. This study shows that single atomic TM adsorption on graphene divacancy site (TM = Sc to Zn), forming the TM − C$_4$ (or TM$_2$ − C$_4$) active site, is mostly stable against clustering. Compared to other active sites, TM$_2$ − C$_4$ is shown to have the best hydrogen storage property, where each Ti center can bind up to three H$_2$ molecules through Kubas interaction. Some materials are also proposed with Ti$_2$C$_6$ shows a storage capacity as high as 6.67 wt%, close to the 7.5 wt% target set by DOE. Furthermore, this study also shows that N doping can significantly lower the formation energy of TM − C$_4$ active sites, suggesting a way to overcome possible synthesis difficulty.