Humidity effect on ion behaviors of moisture-driven CO₂ sorbents

Ion hydration is a fundamental process in many natural phenomena. This paper presents a quantitative analysis, based on atomistic modeling, of the behavior of ions and the impact of hydration in a novel CO₂ sorbent. We explore moisture-driven CO₂ sorbents focusing on diffusion of ions and the structure of ion hydration complexes forming inside water-laden resin structures. We show that the stability of the carbonate ion is reduced as the water content of the resin is lowered. As the hydration cloud of the carbonate ion shrinks, it becomes energetically favorable to split a remaining water molecule and form a bicarbonate ion plus a hydroxide ion. These two ions bind less water than a single, doubly charged carbonate ion. As a result, under relatively dry conditions, more OH^- ions are available to capture CO₂ than in the presence of high humidity. Local concentrations of dissolved inorganic carbon and water determine chemical equilibria. Reaction kinetics is then driven to a large extent by diffusion rates that allow water and anions to move through the resin structure. Understanding the basic mechanics of chemical equilibria and transport may help us to rationally design next-generation efficient moisture-driven CO₂ sorbents.