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

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 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 (H_i⁰ 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 α-Al₂O₃ from DFT: 5.05 × 10⁻³ exp(-0.81 eV/k_B/T) cm²/s and reported experiment: 9.7 ×10⁻⁴ exp(-0.83 eV/k_B/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 H_i⁺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 H_i⁺¹ in Al₂O₃. Overall, these results show promise for the application of alumina polymorphs as hydrogen permeation barriers.