Study of hydrogen diffusivity in equiatomic TiVZrNbHf high entropy alloy for hydrogen storage applications
K. Jarc (TU Delft - Mechanical Engineering)
M.H.F. Sluiter – Mentor (TU Delft - Mechanical Engineering)
A.J. Bottger – Mentor (TU Delft - Mechanical Engineering)
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Abstract
High entropy alloys (HEAs) are new potential materials for hydrogen storage applications which could help with the transition towards sustainable energy sources. The rate of hydrogen kinetics is one of the material properties that is important for the storage and other hydrogen related applications. One of the limiting factors on the rate of hydrogen kinetics is hydrogen diffusivity. Thus far, there has been no reports on hydrogen diffusivity for HEAs in relation to hydrogenation kinetics.
Therefore, this project investigates hydrogen diffusivity in equimolar TiVZrHfNb and the influence of hydrogen concentration on hydrogen diffusivity to gain better understanding of hydrogenation kinetics. The selected HEA has been found to absorb the highest amount of hydrogen (2.5 H/M) among other HEAs.
The investigation was done by a computational approach using ab initio molecular dynamics. BCC and face-centered cubic (FCC) supercells with different hydrogen concentrations (H/M = 0.2, 0.8, 1.4, 2, 2.4) were simulated at a temperature range of 773 – 973 K. At the same time, experimental electrochemical hydrogen charging using chronoamperometry and cyclic voltammetry was performed in order to compare computational and experimental values of hydrogen diffusivity.
The electrochemical hydrogen charging did not result in hydrogen absorption, most probably due to the passivation of the sample surface.
From the simulation results, the values of activation energy and pre-exponential factor were estimated to be in the range of 0.26 – 0.48 eV and 0.73 – 2.95 x 10-7 m2/s, respectively. Hydrogen diffusivity was found to be higher in BCC than in FCC. In BCC the hydrogen diffusivity slowly decreases linearly with increasing H/M. In the case of FCC, the hydrogen diffusivity was found to be the highest at 2.4 H/M while at 2 H/M the diffusivity was the lowest. The analysis of hydrogen occupation at 2 H/M shows that most of the hydrogen atoms are trapped inside tetrahedral sites. It is possible that the hydrogen occupation in the tetrahedral sites results in the optimum hydrogen distribution where the repulsive interaction between hydrogen atoms is the lowest. At concentrations above 2 H/M, an additional repulsive force between hydrogen atoms seems to contribute to the increase of hydrogen diffusivity.