Efficient CO2 capture by humidified polymer electrolyte membranes with tunable water state

Abstract

Polymer electrolyte membranes containing alkali or alkaline-earth metal salts were designed and utilized for CO₂ capture. These membranes showed higher CO₂ permeability than the un-doped control membrane due to the increase of water content, and CO₂/gas selectivity was simultaneously enhanced due to the “salting-out” effect, which was strongly dependent on the content of bound water. More specifically, water content, water state and separation performance of polymer electrolyte membranes were strongly dependent on the salt type: (1) membranes containing alkaline-earth metal salts displayed a higher amount of bound water than those containing alkali cations, because the hydration energy of the alkaline-earth cation is relatively larger than that of the alkali cation; (2) the salts (KCl and CaCl₂) that can efficiently interrupt chain packing by metal–polymer complexation facilitated the diffusion of water molecules into the polymer matrix and thus increased the total amount of absorbed water. As a consequence, CaCl₂-doped membranes showed the highest CO₂ permeability (2030 Barrer) and a high separation factor (108 for CO₂/N₂ and 31 for CO₂/CH₄) at 2 bar (gage pressure) and 298 K for fully humidified gas streams. The effects of annealing conditions and feed pressure were also explored to elucidate the relevant separation mechanism of the polymer electrolyte membrane.