**High temperature stability and transport characteristics of hydrogen in
alumina via multiscale computation**

A Sundar and JG Yu and L Qi and MN Cinbiz, INTERNATIONAL JOURNAL OF HYDROGEN ENERGY, 47, 32345-32357 (2022).

DOI: 10.1016/j.ijhydene.2022.07.123

The impact of hydrogen charge states on the stability and transport characteristics of hydrogen interstitials in alumina polymorphs is evaluated by multiscale computational methods including density functional theory (DFT), ab initio molecular dynamics (AIMD) and machine learned force fields. Thermodynamic calculations show that the protonic H-i(+1) interstitial is the most stable defect species for most values of the electronic bandgap in both a and amorphous alumina (Al2O3). Further, active learned Gaussian approximation potentials (GAP) were developed using AIMD data to study temperature dependent long time proton diffusion in alumina. Diffusivity calculations from GAP-MD simulations are found to be comparable with of the AIMD data, while being similar to 340 times faster and scalable to larger systems. Comparisons with diffusivity values for other interstitial charge states (H-i(0) and H-i(-1)) and published experimental literature indicate that H(i)(+1 )diffusion is the likely mechanism of hydrogen transport. A good agreement is obtained between H(i)(+1 )diffusivity calculated in alpha- Al2O3 from DFT: 5.05 x 10(-3) exp(-0.81 eV/k(B)/T) cm(2)/s and reported experiment: 9.7 x 10(-4) exp(-0.83 eV/k(B)/T) cm(2)/s. Computationally and experimentally calculated energy barriers (0.81 and 0.83 eV respectively) only differ by 2.5%. Similarly, the preexponential diffusion coefficients only differ by 0.5 orders of magnitude. Moreover, the diffusivity of in amorphous Al2O3 in the 1000-2000 K range is calculated to be 2.53 x 10(-2) exp(-0.89 eV/k(B)/T), just one order of magnitude higher than the corresponding value in alpha-Al2O3. This suggests that local structural disorder does not significantly affect the energy landscape and diffusion behavior of in Al2O3. Overall, these results show promise for the application of alumina polymorphs as hydrogen permeation barriers. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.

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