Chemically Stable Group IV-V Transition Metal Carbide Thin Films in Hydrogen Radical Environments.

Hydrogen is a crucial element in the green energy transition. However, its tendency to react with and diffuse into surrounding materials poses a significant challenge. Therefore, developing coatings to protect system components in hydrogen environments (molecular, radicals (H*), and plasma) is essential. In this work, we report group IV-V transition metal carbide (TMC) thin films as potential candidates for protective coatings in H* environments at elevated temperatures. We expose TiC, ZrC, HfC, VC, NbC, TaC, and CoC thin films, with native surface oxycarbides/oxides (TMO C /TMO ), to H* at elevated temperatures. Based on X-ray photoelectron spectroscopy performed on the samples before and after H*-exposure, we identify three classes of TMCs. HfC, ZrC, TiC, TaC, NbC, and VC (class A) are found to have a stable carbidic-C (TM-C) content, with a further subdivision into partial (class A1: HfC, ZrC, and TiC) and strong (class A2: TaC, NbC, and VC) surface deoxidation. In contrast to class A, a strong carbide reduction is observed in CoC (class B), along with a strong surface deoxidation. The H* interaction with TMC/TMO C /TMO is hypothesized to entail three processes: (i) hydrogenation of surface C/O atoms, (ii) formation of CH /OH species, and (iii) subsurface C/O atom diffusion to the surface vacancies. The number of adsorbed H atoms required to form CH /OH species (i) and the corresponding thermodynamic energy barriers (ii) are estimated based on the change in the Gibbs free energy (Δ) for the reduction reactions of TMCs and TMO . Hydrogenation of surface carbidic-C atoms is proposed to limit the reduction of TMCs, whereas the deoxidation of TMC surfaces is governed by the thermodynamic energy barrier for forming HO.