First-principles studies of hydrogen interaction with ultrathin Mg and Mg-based alloy films

Abstract

The search for technologically and economically viable storage solutions for hydrogen fuel would greatly benefit from research strategies that involve systematic property tuning of potential storage materials via atomic-level modification. Here, we use first-principles density functional theory (DFT) to theoretically investigate the structural and electronic properties of ultrathin Mg films and Mg-based alloy films and their interaction with atomic hydrogen.Additional delocalized charges are distributed over the Mg films upon alloying them with 11.1% of Al or Na atoms. These extra charges contribute to enhance the hydrogen binding strength to the films. We calculated the chemical potential of hydrogen in Mg films for different dopant species and film thickness, and included the vibrational degrees of freedom. By comparing the chemical potential with that of free hydrogen gas at finite temperature (T) and pressure (P), we construct a hydrogenation phase diagram and identify the conditions for hydrogen absorption/desorption. The formation enthalpies of metal hydrides are greatly increased in thin films, and in stark contrast to its bulk phase, the hydride state can only be stabilized at high P and T (where chemical potential of free H 2 is very high). Metal-doping increases the thermodynamic stabilities of the hydride films and thus significantly helps to reduce the required pressure condition for hydrogen absorption from H 2 gas. In particular, with Na alloying hydrogen can be absorbed/desorbed at experimentally accessible T and P conditions.

First-principles study on the stability and electronic structure of Mg/ZrB2 interfaces

“…Mg has a hexagonal crystal structure of space group P63/ mmc (No. 194 [22,23] and the experimental results in [24]. For the bulk ZrB 2 , the calculated lattice constants are a=3.168 Å and c=3.536 Å, which are in good agreement with other theoretical results in [25,26] and the experimental results in [27].…”

CO Oxidation on Supported Single Pt Atoms: Experimental and ab Initio Density Functional Studies of CO Interaction with Pt Atom on θ-Al2O3(010) Surface

“…The k -point sampling was done with a 4 × 2 × 4 number of mesh points for bulk and adopted large vacuum size (>30 Å) in the supercell configuration. We evaluated the full vibrational spectra of the films and calculated the vibrational free energy ( F vib ) using the harmonic approximation as F vib = ∫ normald ω g ( ω ) { ℏ ω / 2 + k B T ln false[ 1 − exp ( − ℏ ω / k B T ) false] } where k B is the Boltzmann constant and g ( ω ) is the phonon density of states depending on vibrational frequencies (ω). Here, we take into account all vibrational modes contributing to the atomic degree of freedom.…”

Assessing the Predictive Power of Density Functional Theory in Finite-Temperature Hydrogen Adsorption/Desorption Thermodynamics

“…, obtained from tabulated experimental values, is the chemical potential of the hydrogen molecule at the reference state (P = P 0 = 1 atm). The Helmholtz free energy can be calculated by 15,23,24…”

“…The adsorption/desorption properties of hydrogen at a given temperature ( T ) and pressure ( P ) condition are determined by the chemical potential difference, Δμ­( T , P ), between H 2 adsorbed on the surface and free H 2 by the following relationship where k B is the Boltzmann constant, Δ E is the binding energy, Δ F is the change in Helmholtz free energy related to vibrational modes of the hydrogen molecule, and , obtained from tabulated experimental values, is the chemical potential of the hydrogen molecule at the reference state ( P = P 0 = 1 atm). The Helmholtz free energy can be calculated by ,, where ω i and ℏ are the frequency of the vibrational normal mode i and the Planck constant, respectively. Here, we assumed that the vibrational coupling of adsorbed H 2 and MPIG is weak and that atoms of MPIG are fixed during the vibrational potential calculation.…”