Finding regularities in the hydrogen occupation of voids in metallic hydrides from ab-initio calculations

Identyfikator grantu: PT01117

Kierownik projektu: Ihor Oshchapovskyy

Instytut Fizyki Jądrowej im. Henryka Niewodniczańskiego PAN w Krakowie


Data otwarcia: 2024-02-01

Streszczenie projektu

Hydrogen is considered as clean energy storage medium. Metallic hydrogen storage materials appear to be one of the most promising reversible and safe storage media. Liquid, compressed or absorbed hydrogen requires additional energy for compressing, liquifying or keeping temperatures low.
None of storage systems have reached all of the ultimate goals of DOE for the onboard storage: reversible material with >=6.5 wt.% H, able to release hydrogen at -40-60 oC and 5-12 bar. Metallic hydrides, fulfilling all these requirements, except much lower hydrogen capacity (1-2.5 wt.% H), need an improvement.
The main idea of project is to establish, on basis of theoretical calculations, the main regularities of behavior of hydrogen atoms in the metal matrix.
The main task of the project itself is theoretical investigation of interactions between hydrogen atoms at the occupied sites of hydride of metals. We focus on hydrides of metals as the main class of reversible hydrogen storage materials.
It is known, that the occupation of the adjacent sites by hydrogen atoms with distances <2 Å is energetically unfavourable due to “proton repulsion”. There are only a few exceptions from this rule so far. Hydrides of metals are often disordered, having several partially occupied hydrogen positions with large multiplicities. A few examples: LaNi5Dx, Mg2NiH4, Mg2CoH5 and Ti2Ni -type derivatives (Ti4Fe2O0.25D4.9). A particular example is Ti2Ni -type derivatives, having eight positions, suitable for hydrogen insertion, with multiplicities from 8 to 192, 2-3 of which are partially occupied. This results in huge numbers of possible ordered models, one should generate to model the disordered hydride. While DFT calculations of energy of single ordered model, having 50-100 atoms, are trivial, the DFT calculations of all possible models quickly becomes unfeasible.
Taking proton repulsion into the account, the occupation of hydrogen atoms is never completely random, but has constrained disorder, which precludes usage of the most random model with the highest entropy. A problem of the choice of the right ordered model emerges, which is addressed by our project.
The approaches, presented in the literature, are — random sampling of large number of models, cluster expansion, evolutional methods of crystal structure prediction. The existing programs can solve the problem of optimization of occupation of sites in the disordered structures.
Our approach is beyond of this and is specialized towards metallic hydrides. It is a bit similar to force fields, namely to model H-H interactions as pair-wise and obtain distance dependence of energy of interactions between hydrogens. The goal is to find not only configuration of hydrogen sublattice of one single hydride, but try to find explicit regularities between related hydrides.
The ultimate goal is to direct experiment by the results of theoretical investigations. It is suggested that this approach may be useful for investigation of both bulk and 2d materials, or other objects with constrained disorder.
Such an approach can facilitate choice of the best models of hydrides for DFT calculations and be helpful for:
• computational determination of the structure of hydrogen sublattice,
• complementing/substituting expensive neutron diffraction studies;
• development of hydrogen storage materials;
• comparison between hydrogen interactions in related hydrides.
It can help directed synthesis and experimental investigations of high-capacity hydrogen storage

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