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

Abstract

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 Hi+1 interstitial is the most stable defect species for most values of the electronic bandgap in both α 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 ∼340 times faster and scalable to larger systems. Comparisons with diffusivity values for other interstitial charge states (Hi0 and Hi−1) and published experimental literature indicate that Hi+1 diffusion is the likely mechanism of hydrogen transport. A good agreement is obtained between Hi+1 diffusivity calculated in α-Al2O3 from DFT: 5.05 × 10−3 exp(-0.81 eV/kB/T) cm2/s and reported experiment: 9.7 ×10−4 exp(-0.83 eV/kB/T) cm2/s. Computationally and experimentally calculated energy barriers (0.81 and 0.83 eV respectively) only differ by 2.5%. Similarly, the pre-exponential diffusion coefficients only differ by 0.5 orders of magnitude. Moreover, the diffusivity of Hi+1 in amorphous Al2O3 in the 1000–2000 K range is calculated to be 2.53 × 10−2 exp(-0.89 eV/k B/T), just one order of magnitude higher than the corresponding value in α-Al2O3. This suggests that local structural disorder does not significantly affect the energy landscape and diffusion behavior of Hi+1 in Al2O3. Overall, these results show promise for the application of alumina polymorphs as hydrogen permeation barriers.