Recent progress on Kubas-type hydrogen-storage nanomaterials: from theories to experiments

Abstract

Transition-metal (TM) atoms are known to form TM-H2 complexes, which are collectively called Kubas dihydrogen complexes. The TM-H2 complexes are formed through the hybridization of the TM d orbitals with the H2 σ and σ* orbitals. The adsorption energy of H2 molecules in the TM-H2 complexes is usually within the range of energy required for reversible H2 storage at room temperature and ambient pressure (−0.4 ~ −0.2 eV/H2). Thus, TM-H2 complexes have been investigated as potential Kubas-type hydrogen-storage materials. Recently, TM-decorated nanomaterials have attracted much attention because of their promising high capacity and reversibility as Kubas-type hydrogen-storage materials. The hydrogen storage capacity of TM-decorated nanomaterials is expected to be as large as ~9 wt%, which is suitable for certain vehicular applications. However, in the TM-decorated nanostructures, the TM atoms prefer to form clusters because of the large cohesive energy (approximately 4 eV), which leads to a significant reduction in the hydrogen-storage capacity. On the other hand, Ca atoms can form complexes with H2 molecules via Kubas-like interactions. Ca atoms attached to nanomaterials have been reported to be able to adsorb as many H2 molecules as TM atoms. Ca atoms tend to cluster less because of the small cohesive energy of bulk Ca (1.83 eV), which is much smaller than those of bulk TMs. These observations suggest thatKubas interactions can occur in d orbital-free elements, thereby making Ca a more suitable element for attracting H2 in hydrogen-storage materials. Recently, Kubas-type TM-based, hydrogen- stor ge materials were experimentally synthesized, and the Kubas-type interactions were measured to be stronger than the van der Waals interactions. In this review, the recent progress of Kubas-type hydrogen- storage materials will be discussed from both theoretical and experimental viewpoints.