Figure 2.

Hydrogen purification techniques

Membrane method that selectively separates ${\text{CO}}_2$ molecules as the mixture passes through pores or small gaps in the molecular arrangement of a continuous structure (A), adapted form (Ji and Zhao, 2017). Absorption method by which ${\text{CO}}_2$ from the mixture is taken into the liquid phase so the species is separated from the mixture (B), adsorption method that involves separation of ${\text{CO}}_2$ from the mixture through its accumulation or concentration on a surface (C), carbon dioxide hydrate formation method that operates based on trapping the gaseous ${\text{CO}}_2$ molecule within a lattice cage created by the water molecules (D), adapted with permission from (Zheng et al., 2017).