  Hydrogen is playing a crucial role in the green energy transition. Yet, its
tendency to react with and diffuse into surrounding materials poses a
challenge. Therefore, it is critical to develop coatings that protect
hydrogen-sensitive system components in reactive-hydrogen environments. In this
work, we report group IV-V transition metal carbide (TMC) thin films as
potential candidates for hydrogen-protective coatings in hydrogen radical (H*)
environments at elevated temperatures. We identify three classes of TMCs based
on the reduction of carbides and surface oxides (TMOx). HfC, ZrC, TiC, TaC,
NbC, and VC (class A) are found to have a stable carbidic-C (TM-C) content,
with a further sub-division into partial (class A1: HfC, ZrC, and TiC) and
strong (class A2: TaC, NbC, and VC) surface TMOx reduction. In contrast to
class A, a strong carbide reduction is observed in Co2C (class B), along with a
strong surface TMOx reduction. The H*-TMC/TMOx interaction is hypothesized to
entail three processes: (i) hydrogenation of surface C/O-atoms, (ii) formation
of CHx/OHx species, and (iii) subsurface C/O-atoms diffusion to the surface
vacancies. The number of adsorbed H-atoms required to form CHx/OHx species (i),
and the corresponding energy barriers (ii) are estimated based on the change in
the Gibbs free energy (DeltaG) for the reduction reactions of TMCs and TMOx.
Hydrogenation of surface carbidic-C-atoms is proposed to limit the reduction of
TMCs, whereas the reduction of surface TMOx is governed by the thermodynamic
barrier for forming H2O.
