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 that Kubas 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-storage 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.

Carbon‐Based Sorbents for Hydrogen Storage: Challenges and Sustainability at Operating Conditions for Renewable Energy

“…This rule is for calculating the number of hydrogen in Kubas‐type storage. Generally, transition metals agglomerate due to their vast cohesive energy of around 4 eV, resulting in reducing hydrogen storage capacity [55a,56] . It is reported that calcium atoms can also form complexes with hydrogen molecules with lesser cluster forming capacity due to low cohesive energy (1.83 eV).…”

“…It is reported that calcium atoms can also form complexes with hydrogen molecules with lesser cluster forming capacity due to low cohesive energy (1.83 eV). Therefore, Ca atoms are considered an excellent option for hydrogen storage via Kubas interaction [55a] . Many experimental studies have been done based on theoretical predictions and found that Kubas‐type interactions are stronger than Van der Waals interactions but weaker than chemisorption bonds [55a] …”

“…Therefore, Ca atoms are considered an excellent option for hydrogen storage via Kubas interaction [55a] . Many experimental studies have been done based on theoretical predictions and found that Kubas‐type interactions are stronger than Van der Waals interactions but weaker than chemisorption bonds [55a] …”

Cooperative physisorption and chemisorption of hydrogen on vanadium-decorated benzene

“…In chemisorption, H 2 molecules dissociate into double H atoms and even migrate to the material forming chemical bonds, which are strong. 6 Kubas-type orbital interaction 13,14 is similar to physisorption, in which the H-H bond in an H 2 molecule is elongated but not broken. It improves the hydrogen adsorption energy of transition metals (TMs)decorated carbon composites (0.10-0.80 eV) 6 and makes most of the hydrogen gravimetric storage capacities satisfy the target of the U.S. Department of Energy ($6.5 wt%).…”

Observation and Analysis of Large Dynamic Frequency Shifts in the 1H NMR Signals of H–D in Deuterium-Substituted Dihydrogen Complexes

“…One area of inorganic chemistry where NMR spectroscopy has been especially useful is in the study of dihydrogen complexes, M­(H 2 ), in which an essentially intact dihydrogen ligand binds “side-on” to a metal center, indicated by a short H–H distance, as low as ∼0.85 Å. − Dihydrogen complexes are fundamentally important because of this mode of binding, which makes them prototypical σ complexes . Dihydrogen complexes also play a significant role in catalysis and bond activation − and can potentially act as hydrogen storage materials when integrated into nanostructures …”