Carbon Dioxide Absorption Behavior and Carbonate Ion Transport of Lithium Orthosilicate / Potassium Carbonate Coexistence System Prepared By Ball Milling

Tuesday, October 13, 2015
West Hall 1 (Phoenix Convention Center)
K. Kanki, H. Maki (Kobe University), and M. Mizuhata (Kobe University)
Lithium Orthosilicate (Li4SiO4), a type of ceramic CO2 absorbent has a large absorptive capacity (36.7 wt. %) and also has heat resistance. This material has some useful application as a separator in Molten Carbonate Fuel Cell and as a direct CO2 absorbent in large-scale emission sources, for example, thermal power stations; because the following equilibrium reaction at the temperature around 993 K; Li4SiO4 + CO⇄  Li2CO3 + Li2SiO3. In this study, we focused on the absorption behaviors of CO2 at lower temperature into Li4SiO4 in combination of these two methods and evaluated the physiochemical dependence of surface properties of CO2 absorption reaction.

Li4SiO4 was prepared from solid state reaction. SiO2 powder and Li2CO3 powder were mixed at the molar ratio Li/Si = 4.0. The mixture was calcined at 973 K for 20 h in air. The resulting powder (Li4SiO4) kept in an Argon atmosphere glove box and in order to confirm the effect of carbonate, was mixed with a given amount of K2CO3. Each mixture (2.5 g) was put in the grinding bowl with the grinding balls, it sealed up under the Ar atmosphere, and the grinding processing was done for 0-300 minutes in revolution speed 250 rpm using planetary ball mill.              Also, the grinding process was done under the condition of one cycle for 15 minutes of the grinding processing and two minutes of rest time to prevent the samples temperature from rising. Samples were characterized with X-ray diffraction (XRD), N2-isotherm, SEM, SEM-EDX, TG-DTA and Electro spin resonance (ESR).

It was found that samples whose surface was modified with ball milling method under Ar gas flow showed a tendency to increase the surface area estimated by BET method measured by N2-isotherm and increase of Si dangling bond measured by ESR. Also, it was found that there are two different absorption processes, one is a chemisorption at the surface (T<773 K) the other is a bulk diffusion (T>773 K) by the result of TG-DTA. As seen in isothermal analysis (Fig. 1), the CO2 absorb reaction rate improved by the coexistence of K2CO3 infiltrated in Li4SiO4 being promoted by not only the surface area was increased and activated by the ball milling but also absorbing CO2 of coexisting K2CO3. Furthermore, it was found that ΔG decreased from the result of the isothermal analysis at 873 K. We would like to consider controlling transportation of carbonate ion electrochemically according to the CO2 absorption reaction as a separator in Molten Carbonate Fuel Cell.