Outgassing remains a pertinent issue in high-power systems as it can lead to effects such as breakdown, surface flashover, and pulse shortening and is typically the first stage of deleterious plasma formation. In this context, experimental reports suggest that carbon fibers (CFs) may likely be a superior cathode material for low outgassing. Here, model-based assessments of outgassing from CFs are performed based on molecular dynamics simulations. Carbon fibers were generated based on interconnection of an array of graphene sheets resembling ladder-like structures. Our results of temperature-dependent diffusion coefficients for hydrogen in CFs are shown to exhibit Arrhenius behavior and have values smaller than copper by factors of 15.5 and 86.8 at 400 K and 1000 K, respectively. This points to even stronger improvements for operation at high temperatures, with the asymptotic diffusion constant ratio predicted to be ∼187. With reduced outgassing, higher temperature operation, and durability, our results support CF cathodes as an excellent choice for cathode material in high-power microwave and pulsed power systems.