Characterization of amorphous hydrocarbon CD_x films (x ∼ 0.5) for energy storage applications

Smooth hydrocarbon films formed on the vacuum chamber walls in tokamaks under the erosion of graphite elements during deuterium plasma discharges are the main accumulators of hydrogen isotopes in carbon matrices. Therefore, they can be considered as a promising material for hydrogen storage applications. In the present work, such hydrocarbon CDx films (x ∼ 0.5) produced in T-10 tokamak (NRC Kurchatov Institute, Russia) were studied using scanning electron microscopy (SEM), NEXAFS and EXAFS spectroscopies together with thermal desorption (TD) for the films’ characterization and estimation of their hydrogen storage capacity. The C 1s X-ray absorption spectra of the CDx films were recorded for the first time using the BESSY II storage ring facility (Germany). The obtained NEXAFS spectra were found typical for CK-spectra of sp3 + sp2 hydrocarbon systems with a high H/C ratio. The role of Fe impurities from the tokamak chamber walls was elucidated as a catalytic effect “facilitating” the thermal desorption of hydrogen (deuterium) from CDx films. The Fe K-edge spectra allowed to confirm a fractal (self-affined) structure of CDx films, with a minimal sp2 fractal aggregate ∼2–3 nm, and formation of 3D carbon sp3 + sp2 network, accumulated a large number of H-isotopes and CxHy hydrocarbons. It was found that the H storage and thermal desorption properties of the CDx films can be improved by their inherent Fe impurities, capable to reduce the barrier of TD and to increase the H/C storage capacity by several times even at room temperature.