2019 Fiscal Year Research-status Report
Quantum paradigms in hydrogen storage in nanostructures
| Project/Area Number |
19K15397
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| Research Institution | The University of Tokyo |
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Principal Investigator |
ARGUELLES ELVIS 東京大学, 大学院工学系研究科(工学部), 特任研究員 (50816072)
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| Project Period (FY) |
2019-04-01 – 2023-03-31
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| Keywords | quantum effects / hydrogen storage / nuclear spin / 2D nanostructure / graphene / transition metal |
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| Outline of Annual Research Achievements |
In this research, the quantum effects on hydrogen storage is being investigated by means of first principles simulations based on the density functional theory (DFT) and model calculations. The study of quantum effects is twofold: (1) the rotational and vibrational motion of H2 molecule during adsorption and (2) the nuclear spin conversion of H2.
This year, the research is focused on the structural stabilities of possible hydrogen storage materials and the development of the theory of efficient ortho-para H2 conversion. As representative candidate hydrogen storage materials, transition metal (TM) functionalized graphene and graphitic carbon nitride (g-C3N4) are considered. It was found that TMs are stable in graphene. For Pd atom on graphene, DFT structural optimization finds the stable site is on top of a C atom. Co functionalized graphene on the other hand, exhibit a totally different bonding configuration. Due to symmetry, Co bonding site is located in the middle of the hexagon formed by six carbon atoms of graphene.
The ground state geometry of g-C3N4 is a wavy-like structure with low symmetry. The resulting formation energies of Pt, Pd, Ag, Au and Ni TMs are indeed lower as compared to that of their respective bulk energies. On the other hand, the theory of ortho-para conversion developed for metal surfaces is currently being extended for the TM-nanostructures. On metals, the relevant parameters which could be extracted from DFT have been identified. The method of treating the surface-molecule hopping integral based on DFT has also been clarified.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
The tuning of adsorption energy of H2 on TM functionalized nanostructure strongly relies on the stability of TM ions in the host matrix. In g-C3N4, TM metals were introduced in the void and the formation energies were found to be less stable than their corresponding bulk states. This is due to the lack of symmetry and large size of the void in g-C3N4. This instability of the TMs in g-C3N4 imposed problems in H2 adsorption. For Pt-g-C3N4 H2 adsorption results in Pt detachment from the void. This problem is addressed by considering other candidate materials such as silicene or porphyrins. TM functionalized graphene on the other hand does not suffer the above mentioned issues. The construction of 6-dimensional (hyper) potential energy surface of H2 adsorption is currently underway for different TM decorated graphene sheets. Several H2 adsorption profiles including dissociation and hydrogen storage capacity on TM-graphene are being investigated. Further, from the information of electronic structures of TM-nanostructructures, the relevant parameters for the evaluation of o-p conversion are being extracted. The extraction of surface wave functions and hopping matrix elements from first principles calculations is not always straightforward and can be challenging. Furthermore, since it is expected that H2 adsorption will occur on TM atoms, the role of local surface properties on the o-p conversion rates are currently being investigated.
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| Strategy for Future Research Activity |
After construction of the potential energy surfaces (PES), the Schrodinger equation for the nuclear motion of H2 will be numerically solved in the spirit of variational method. In the calculations, the trial wave functions will be expanded in terms of Gaussian and spherical harmonics basis functions. If the PES exhibit angular potential anisotropy, the degeneracies of rotational levels of H2 are expected to be lifted and level splitting would be observed. This would also result in different adsorption energies of ortho and para hydrogen molecules which can be exploited to separate the two H2 species. A direct and potential application is on the liquified hydrogen storage systems which require only para species. The resulting Eigenvalues and Eigenfunctions provide information on the rotational and vibrational states of the H2 on different TM-nanostructures. These will serve as corrections to the hydrogen adsorption energies obtained from first principles calculations. Subsequently, the adsorption energies will be plotted for different H chemical potential and partial pressures to deduce the hydrogen storage capacities of TM-nanostructures. Parallel to this, the theoretical description of o-p conversion in TM-nanostructures will be further refined and tested for validity.
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Research Products
(3 results)
