CO2 Capture Characteristics of Electrochemically Generated Li2O in Molten Eutectic LiNO3–KNO3 at Moderate Temperature

Abstract

Metal oxides (MO) have emerged as attractive candidates for carbon dioxide (CO₂) capture due to their excellent thermal stability and high theoretical CO₂ uptake capacity, with particular emphasis on magnesium oxide (MgO) or calcium oxide (CaO) that requires high operating temperatures (300–900 °C). Here, we present CO₂ capture characteristics of a MO system of growing interest: lithium oxide (Li₂O) in molten eutectic lithium nitrate (LiNO₃) and potassium nitrate (KNO₃), which is attractive due to its high theoretical capacities and a moderate operating temperature of only 150 °C. Our results reveal that CO₂ capture kinetics and capacities are markedly improved when Li₂O is blended with the eutectic molten salt. Especially at low Li₂O concentrations (<0.7 wt %, below the solubility limit of lithium carbonate (Li₂CO₃) in the salt), a complete conversion to Li₂CO₃ was achieved within 40 h. Next, we generated Li₂O electrochemically in the same medium and applied it for CO₂ capture, finding that maintaining Li₂O content below its solubility limit offers kinetic advantages, achieving complete conversion of produced Li₂O to Li₂CO₃ within 10 h. Clear evidence of reactivity of nitrite ions (NO₂^–) with CO₂ was also observed. Although NO₂^– was previously described as functioning as a CO₂ uptake promoter for MgO in molten salts, X-ray diffraction and gas sensing analysis herein revealed an irreversible side reaction between NO₂^– and CO₂ in Li₂O systems, resulting in undesired NO_x formation. These findings strongly caution against the specific use of salts producing nitrite ions in CO₂ capture applications, yet underscore the potential for developing improved salts that circumvent this issue and enable electrochemical reversibility in future work.