Inaccuracy of Density Functional Theory Calculations for Dihydrogen Binding Energetics onto Ca Cation Centers

Abstract

We investigate the mechanism of dihydrogen adsorption onto Ca cation centers, which has been the significant focus of recent research for hydrogen storage. We particularly concentrate on reliability of commonly used density-functional theories, in comparison with correlated wave function theories. It is shown that, irrespective of the chosen exchange-correlation potentials, density-functional theories result in unphysical binding of H 2 molecules onto Ca 1þ system. This suggests that several previous publications could contain a serious overestimation of storage capacity at least in part of their results. DOI: 10.1103/PhysRevLett.103.216102 PACS numbers: 68.43.Bc, 84.60.Ve One of the greatest challenges of scientific communities worldwide is a pollution-free and renewable energy source. Hydrogen storage with high enough volumetric and gravimetric density is particularly important as an energy carrier for a mobile system [1,2]. It has been discussed that, in order for the storage and discharge to be cycled near room temperature, hydrogen adsorbents need to have a binding affinity with the hydrogen molecule of a few tens of kJ/mol [3,4]. Through a very particular chemistry between the open d shell of metal atoms and H 2 molecular orbitals, an optimal strength of hydrogen adsorption can be achieved [5,6]. In order to realize such a chemistry in the form of a practical hydrogen storage system, numerous previous theory articles investigated nanostructures with dispersed transition metal (TM) atoms [3,[7][8][9]. However, experimental trials to synthesize an open-TM-based hydrogen storage system have been unsuccessful for various reasons. The most prominent barrier is the strong tendency of aggregation of TM atoms which renders the suggested models of dispersed TMs rather hyphothetical [10]. As an alternative, numerous research groups are now focused toward the alkaline-earth metals (AEMs) which have less tendency of aggregation and are believed to have similar binding affinity with dihydrogen adsorbates as TMs [11][12][13][14][15]. In particular, the systems of dispersed Ca atoms have been suggested as possessing the most salient properties.In the series of computational searches for hydrogen storage materials, the density-functional theory (DFT) has been used most widely, mainly because of its practicality [16]. However, common implementations of DFT involve approximations in the exchange-correlation potential, and thus the accuracy of DFTs could not be perfectly trusted. In this regard, it is very pertinent to investigate the capability of common forms of DFT for hydrogen adsorption onto AEMs. We show that DFTs deviate significantly from the correlated wave function theories in the description of the dihydrogen adsorption onto the Ca 1þ system. We discuss that the valence configuration of the Ca cation can be sharply switched between 4s and 3d upon hydrogen adsorption [17]. This sort of interaction is not widely noticed, and the reliability of DFTs for that is intriguing in the context of hydrogen storage as wel...

Cha, Choi, and Park Reply:

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Cha, Choi, and Park Reply: We suggested that flavors of density functional theory (DFT) lead to unrealistic stabilization of the bonded state of four hydrogen molecules onto the Ca cation center, in contrast to the results of correlated wave function theories [1]. Ohk et al [2] commented pertinently that a larger basis set should have been used to obtain the converged results of correlated theories.

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The role of density functional theory methods in the prediction of nanostructured gas-adsorbent materials

“…In two subsequent works, the authors of article [217] reported further details on the anomalous performance of customary DFT methods in describing the fixation of H 2 molecules on Ca centers via the Kubas interaction [219,220]. They showed that when a sharp orbital transition occurs during adsorption of the gas molecule on the metal center, the incompleteness of the electronic exchange, present in most DFT functionals, acts critically by providing erroneous overstabilization of the final complex.…”

Applications of Al Modified Graphene on Gas Sensors and Hydrogen Storage

“…In these systems, the adsorbed Ca atoms become positively charged and the semimetallic graphene changes into a metallic state, while the hydrogen storage capacity (HSC) can be up to 8.4 wt %. However, a recent report claimed that DFT calculations overestimated significantly the binding energy between the H 2 molecules and the Ca +1 cation centers (Cha et al, 2009). On the other hand, Al-doped graphene where one Al atom replaces one C atom of a graphene layer was reported as a promising hydrogen storage material at room temperature with HSC of 5.13 wt % (Ao et al, 2009a).…”

“…24. Very recently, the mechanism of H 2 adsorption onto Ca cation centers was investigated using both DFT and wave function approaches (Cha et al, 2009). It was found that DFT calculations overestimated the binding energy between the H 2 molecules and the Ca 1+ cation centers significantly.…”