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

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 NOx 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.