Electronic Structure, Phonon Dynamical Properties, and CO2 Capture Capability of Na2-x MxZr O3 (M=Li,K): Density-Functional Calculations and Experimental Validations

The electronic structural and phonon properties of Na2-αMαZrO3 (M=Li,K, α=0.0,0.5,1.0,1.5,2.0) are investigated by first-principles density-functional theory and phonon dynamics. The thermodynamic properties of CO2 absorption and desorption in these materials are also analyzed. With increasing doping level α, the binding energies of Na2-αLiαZrO3 are increased while the binding energies of Na2-αKαZrO3 are decreased to destabilize the structures. The calculated band structures and density of states also show that, at the same doping level, the doping sites play a significant role in the electronic properties. The phonon dispersion results show that few soft modes are found in several doped configurations, which indicates that these structures are less stable than other configurations with different doping levels. From the calculated relationships among the chemical-potential change, the CO2 pressure, and the temperature of the CO2 capture reactions by Na2-αMαZrO3, and from thermogravimetric-analysis experimental measurements, the Li- and K-doped mixtures Na2-αMαZrO3 have lower turnover temperatures (Tt) and higher CO2 capture capacities, compared to pure Na2ZrO3. The Li-doped systems have a larger Tt decrease than the K-doped systems. When increasing the Li-doping level α, the Tt of the corresponding mixture Na2-αLiαZrO3 decreases further to a low-temperature range. However, in the case of K-doped systems Na2-αKαZrO3, although doping K into Na2ZrO3 initially shifts its Tt to lower temperatures, further increases of the K-doping level α causes Tt to increase. Therefore, doping Li into Na2ZrO3 has a larger influence on its CO2 capture performance than the K-doped Na2ZrO3. Compared with pure solids M2ZrO3, after doping with other elements, these doped systems' CO2 capture performances are improved.