Clustering of Ti on a C₆₀ Surface and Its Effect on Hydrogen Storage

TLDR

Hydrogen storage in carbon nanostructures has been pursued, but pure carbon materials bind hydrogen weakly; coating with isolated transition metals such as Sc or Ti was predicted to enhance binding and reach high capacities, generating excitement, though this relies on the assumption that metal atoms remain isolated. First‑principles DFT calculations reveal that Ti atoms tend to cluster on C60, altering hydrogen bonding and impacting storage capacity, thermodynamics, and kinetics. Coating C60 with isolated Ti atoms was predicted to raise hydrogen binding energy to ~8 wt % (1.6× DOE target), but our DFT study shows Ti clustering changes hydrogen bonding, reducing storage capacity and altering thermodynamics and kinetics.

Abstract

Recent efforts in finding materials suitable for storing hydrogen with large gravimetric density have focused attention on carbon-based nanostructures. Unfortunately, pure carbon nanotubes and fullerenes are unsuitable as hydrogen storage materials because of the weak bonding of the hydrogen molecules to the carbon frame. It has been shown very recently that coating of carbon nanostructures with isolated transition metal atoms such as Sc and Ti can increase the binding energy of hydrogen and lead to high storage capacity (up to 8 wt % hydrogen, which is 1.6 times the U.S. Department of Energy target set for 2005). This prediction has led to a great deal of excitement in the fuel cell community [see The Fuel Cell Review, http://fcr.iop.org/articles/features/2/7/4]. However, this prediction depends on the assumption that the metal atoms coated on the fullerene surface will remain isolated. Using first-principles calculations based on density functional theory, we show that Ti atoms would prefer to cluster on the C60 surface, which can significantly alter the nature of hydrogen bonding, thus affecting not only the amount of stored hydrogen but also their thermodynamics and kinetics.