Fig. 2

Compositional characterization and stability analysis of iodine–carbon composites. a The time-dependent profiles for the mass ratio of iodine adsorbed on various carbons (the mass ratio is normalized to the mass of carbon). b N₂ adsorption-desorption isotherms and c the corresponding pore size distribution curves of HPMC-NP with different mass loading of iodine. d High-resolution XPS spectra of N1s, P2p, and I3d for I₂-HPCM-NP samples. Thermogravimetric analysis curves of e pure iodine and iodine-carbon composites (iodine content, 40 wt%), and f I₂-HPCM-NP composite with different iodine content. g The contour plots of the difference in charge density for the optimized structures of iodine molecule adsorbed on graphene I, graphene doped with N (II), P (III), isolated N and P (IV), and coupled N and P (V). The differential charge density was calculated from: Δρ = ρ ₁₂−ρ ₁−ρ ₂, where ρ ₁, ρ ₁₂ and ρ ₂ are the chare density of iodine, doped graphene with and without iodine adsorbed on the surface, respectively. Yellow and green color indicate the charge depletion and accumulation, respectively. The adsorption energies for I₂ molecule on these fragments were obtained using: E _ad = −(E ₁ + E ₂−E ₁₂), where E _ad is the adsorption energy of the I₂ molecule on the corresponding surfaces, E ₁ is the total energy of the graphene (or heteroatom doped graphene), E ₂ is the energy of one isolated I₂ molecule, and E ₁₂ is the energy of the optimized structures for I₂ molecules adsorbed on the graphene planes. The bond lengths and heights of adsorbed I₂ molecule are also listed in the figure, along with the corresponding adsorption energies