(269e) Chemical Vs. Electrochemical Lithium Oxide Capture of CO? in Molten Salts: Balancing Solubility and Reactivity

As anthropogenic carbon dioxide (CO₂) emission intensifies, the development of post-combustion carbon capture technologies is of increasing importance. Although mature, thermal swing processes with aqueous amines have significant limitations, including high energy requirements and sorbent degradation. Metal oxides have been developed as promising alternatives to amine-based systems due to their exceptional oxidation stability. However, their practical application remains challenging due to a high operation temperature (600-900⁰C), which stems from kinetic hindrance of the CO₂ capture reaction and thermodynamic barrier associated with the sorbent regeneration. Recent studies have reported that the impregnation of nitrate molten salt significantly enhances CO₂ capture kinetics by generating active dissolved metal oxide ions, thereby reducing operation temperature to 300-500 ⁰C.^1,2 Furthermore, reports on reversible electrochemical Li₂O production and decomposition in eutectic nitrate molten salt (LiNO₃/KNO₃) motivate its application to CO₂ capture, which may further decrease operation temperature by the use of electrified energy instead of thermal energy during sorbent regeneration.^3–5

Herein, we explored the potential for CO₂ capture with electrochemically produced Li₂O in eutectic LiNO₃/KNO₃ molten at a mild temperature (150 ⁰C). Primarily, we investigated the correlation between Li₂O (commercial and electrochemical) content in molten salt and CO₂ uptake via thermal gravimetric analysis and titration techniques. Low Li₂O content (<0.7 wt%, below Li₂CO₃ solubility in molten salt) achieved complete conversion to Li₂CO₃ (Li₂O + CO₂ → Li₂CO₃) within 40 hours, whereas higher Li₂O contents exhibited less than 50% conversion. The facilitation is attributed to preventing the permanent passivation of Li₂CO₃ on Li₂O through dissolution. Next, we explored the effect of nitrite ion (NO₂^-), previously identified as a CO₂ uptake promoter of magnesium oxide (MgO) in molten salt. While NO₂^- facilitated CO₂ uptake of Li₂O, X-ray diffraction, and gas sensing revealed an irreversible side reaction between NO₂^- and CO₂, leading to NO_x formation.

We propose a novel electrochemical approach to metal oxide-based CO₂ capture, highlighting the importance of metal carbonate solubility in system design. Furthermore, this work underscores the need to avoid adding nitrite for CO₂ capture to prevent NO_x release into the atmosphere. Further studies on electrochemical sorbent regeneration, along with the investigation on suitable metal oxide + molten salt combinations guided by our criteria, could provide a new strategy for CO₂ capture at moderate temperatures.

References

1. Dal Pozzo, A., Armutlulu, A., Rekhtina, M., Abdala, P. M. & Müller, C. R. CO2 Uptake and Cyclic Stability of MgO-Based CO2 Sorbents Promoted with Alkali Metal Nitrates and Their Eutectic Mixtures. ACS Appl Energy Mater 2, 1295–1307 (2019).
2. Harada, T., Simeon, F., Hamad, E. Z. & Hatton, T. A. Alkali Metal Nitrate-Promoted High-Capacity MgO Adsorbents for Regenerable CO2 Capture at Moderate Temperatures. Chemistry of Materials 27, 1943–1949 (2015).
3. Zhu, Y. G. et al. Nitrate-mediated four-electron oxygen reduction on metal oxides for lithium-oxygen batteries. Joule 6, 1887–1903 (2022).
4. Xia, C., Kwok, C. Y. & Nazar, L. F. A high-energy-density lithium-oxygen battery based on a reversible four-electron conversion to lithium oxide. Science (1979) 361, 777–781 (2018).
5. Giordani, V. et al. Rechargeable-battery chemistry based on lithium oxide growth through nitrate anion redox. Nat Chem 11, 1133–1138 (2019).