Electrolytic synthesis of carbon from the captured CO^sub 2^ in molten LiCl–KCl–CaCO^sub 3^: Critical roles of electrode potential and temperature for hollow structure and lithium storage performance

In the present study, CaCO3-containing molten LiCl–KCl under CO2 atmosphere was investigated as the electrolyte for the preparation of electrolytic carbon. Varying the electrode potential and electrolysis temperature can manipulate the kinetics of the electro-deposition reaction of carbon, thereby allowing the formation of the carbon products with different morphologies. At 450 °C, high yields of micron-sized hollow carbon spheres (HCSs) and ultrathin sheets were obtained under 2.8 and 3.5 V cell voltages, respectively. At higher temperatures (550 and 650 °C) and cell voltages, the content of carbon sheets decreased, and various carbon materials, such as quasi–spherical particles, coral–like carbon and carbon nanofibers, could be observed. Micron-sized HCSs were only obtained with a moderate reaction rate (2.8 V and 450 °C), which allowed the occurrence of microbubble effect of the as-formed CO gas (one of the intermediates of electro-reduction of carbonate ions). The expansion of the CO gas shaped the plastic ultrathin carbon sheets, resulting in the formation of well-developed HCSs. Among the carbon products, the one deposited at 4.5 V and 650 °C exhibited the highest specific surface area of more than 400 m2 g−1, while the HCSs showed the best performance for lithium storage. The present study highlights the importance of manipulating electrode potential and working temperature to the preparation of value-added electrolytic carbon from the captured CO2 in molten salts.