Capture and electrochemical conversion of CO2 in molten alkali metal borate–carbonate blends

Abstract

A family of blended compositions of molten mixed lithium and sodium borate (Li_1.5Na_1.5BO₃) and eutectic lithium–potassium carbonate (Li_1.24K_0.76CO₃) salts has been introduced as reversible carbon dioxide absorbents and as media for CO₂ electrolysis for carbon conversion. Material properties, temperature effects and kinetics of CO₂ uptake were examined. Li, Na borate can absorb up to 7.3 mmol g⁻¹ CO₂ at 600 °C. The blended borate–carbonate compositions are molten in the 550–600 °C temperature range, with viscosity adjustable to within a 10–1000 Pa s window depending on the borate/carbonate ratio. The blends can withstand cyclic temperature and CO₂ pressure swings without significant deterioration of their CO₂ uptake capabilities. Addition of eutectic carbonate into mixed Li, Na borate salts lowers overall CO₂ uptake due to the lower solubility of CO₂ in carbonate. However, addition of the eutectic lowers the temperature of the pressure swing operation and dramatically accelerates the CO₂ uptake during the initial stage of the absorption, potentially enabling a faster cycling. Electroreduction of CO₂ and carbon deposition on a galvanized steel cathode was more effective with increasing carbonate fraction in the molten alkali borate/carbonate blend. Blended borate/carbonate compositions with 50–60% borate content possessed sufficiently high loading capacity for CO₂ and simultaneously enabled maximum carbon product yield and Coulombic efficiency. Most of the recovered carbon product was shown to be in the form of multiwalled carbon nanotube.