Li₃NaSiO₄ as reversible CO₂ absorbent: elucidation of performance-governing factors and regeneration mechanism

In situ reversible CO₂ capture by thermally stable materials such as oxides has drawn much attention because of the high operation temperatures of the main CO₂ sources, namely thermal power plants and internal combustion engines. In particular, Li₄SiO₄ features a high CO₂ absorption capacity but suffers from slow CO₂ absorption at practical CO₂ partial pressures (P(CO₂)), which can be mitigated through the incorporation of Na to afford Li₃NaSiO₄. Given that the Li₃NaSiO₄ regeneration kinetics and the possibility of the corresponding absorption/desorption cycling based on temperature or P(CO₂) control remain underexplored, we herein characterized Li₃NaSiO₄ as a material for reversible CO₂ absorption at high temperatures in response to CO₂/N₂ gas switching, achieving excellent cycling performance at 750 °C. CO₂ desorption (i.e., Li₃NaSiO₄ regeneration) was concluded to occur at the interface between solid Li₂SiO₃ and molten LiNaCO₃, generating a Li₃NaSiO₄ layer that prevented further desorption at a thickness of > 1 μm and thus resulted in the saturation of the CO₂ desorption reaction in the middle. The lack of such saturation during cycling was attributed to the formation of needle-like (short side length ≈ 2 μm) Li₂SiO₃ particles upon the absorption of CO₂ by Li₃NaSiO₄. Thus, this work paves the way for the development of industrially applicable high-temperature CO₂ capture/release materials and thus contributes to the establishment of a greener society.