Fig. 8: Invert-volcano relationship govern by intrinsic descriptor Φ and differential charge density distribution and Bader charge transfer on optimal dual-doped-graphene-supported Ca single atoms (X-Y-G-Ca).
a The invert-volcano relationship related to the minimum Gibbs free energy change \(\varDelta {G}_{{H}_{2}^{\ast }}^{\min }\) before and after H2 adsorption and descriptor Φ for X-Y-G-Ca. b, c The predicted H2 storage capacity and rate against the descriptor Φ for X-Y-G-Ca. Here, the red dashed arrows in (a)–(c) indicate the trend of H2 storage properties for X-Y-G-Ca models. And the red circle in (a) and the shadows in (b) and (c) the optimal X-Y-G-Ca model with the best H2 storage properties. d Bader charge transferred from Ca to H2 for each optimal X-Y-G-Ca(-H2) as the function of intrinsic descriptor Φ. e The volcano relationship between DOS2 and descriptor Φ for each optimal X-Y-G-Ca(-H2). f Comparasion between predicted and experimental H2 storage capacity of doped-graphene-supported Ca single atoms in this work and current other H2 storage carbon materials in references (Supplementary Notes 2 and 3). Here, the arrow shows the trend of normalized H2 storage properties improvement. g Atomic structure and charge distribution of the optimal S-Si-codoped-graphene-supported Ca single atoms (S-Si-G-Ca). h Atomic structure and charge distribution of the optimal S-Si-G-Ca with H2 molecule (Black arrow) adsorbed on optimal site (S-Si-G-Ca-H2). For the corresponding differential charge density and Bader charge transfer in (g) and (h)., blue color indicates positive charge and yellow color indicates negative values of electrons quantities. The isosurface value is set to 0.0002 e/Bohr3. Here, red arrows in (g) and (h). indicate the direction of charge transfer and the black arrow in (h) refer to the H2 molecule.
