Capture and electrochemical conversion of CO 2 in molten alkali metal borate-carbonate blends.

A family of blended compositions of molten mixed lithium and sodium borate (Li 1.5 Na 1.5 BO 3 ) and eutectic lithium-potassium carbonate (Li 1.24 K 0.76 CO 3 ) salts has been introduced as reversible carbon dioxide absorbents and as media for CO 2 electrolysis for carbon conversion. Material properties, temperature effects and kinetics of CO 2 uptake were examined. Li, Na borate can absorb up to 7.3 mmol g -1 CO 2 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 CO 2 pressure swings without significant deterioration of their CO 2 uptake capabilities. Addition of eutectic carbonate into mixed Li, Na borate salts lowers overall CO 2 uptake due to the lower solubility of CO 2 in carbonate. However, addition of the eutectic lowers the temperature of the pressure swing operation and dramatically accelerates the CO 2 uptake during the initial stage of the absorption, potentially enabling a faster cycling. Electroreduction of CO 2 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 CO 2 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.