Fig. 2: The theoretical model of H2 storage and trend in intrinsic storage capacity and rate of X-G-Ca carbon-based materials.

a The proposed theoretical model of generalized design principle. The volcano trend of H2 storage capacity (b) and rate (c) versus the minimum Gibbs free energy change under various external conditions consisting of T = 300 K, T = 200 K, T = 100 K and T = 0 K at given pressure P = 1 bar. Here, black, red and green dots in (b) and (c) refer to predicted properties for G-Ca models. The blue circular shadows indicated by the purple arrows in (b) and (c) indicate the ideal X-G-Ca H2 storage structure. The black, red and blue arrows in (b) represent the trend of properties with the decrease of the Gibbs free energy change. The thick arrows in (b) and (c) indicate the trend of properties with the decrease of absolute temperature T.