Highly efficient and environmental-friendly separation and purification of carbon nanotubes from molten salt via ultrasound-assisted carbonation

Carbon dioxide (CO2) released by the carbon-intensive energy industry has caused heightened ecological and environment concerns globally. The electrochemical conversion of CO2 into high-valued carbon nanotubes (CNTs) in molten salt media offers a promising pathway to mitigate climate change and advance carbon neutrality goals. Pilot scale has been implemented, whereas, this cutting-edge technology is being impeded by separation and purification of the CNTs. The conventional method is to dissolve the crude cathode products with hydrochloric (HCl) acid, then rinse and dry the filtered carbon powder. Heavy dependence on HCl, undoubtedly, on the one hand increases the operating cost, on the other hand causes environment pollutions. From the perspective of industrial application, a green and sustainable purification method is urgently desired. Here, a simple but effective carbon product separation method, i.e. ultrasound-assisted carbonation, was firstly proposed and validated in this study, where CNTs and lithium carbonate (Li2CO3) can be thoroughly separated from each other with high efficiency and low resource consumption. Meanwhile, this study constructed a closed substance loop: with the CNTs separated, Li2CO3 can be employed to continue the next round of electrolysis, and water and CO2 that were used as feedstocks in the separation process could also be circulated. In this way, the work paved the way for CNTs separation from molten salts in a more sustainable and economic manner, thereby alleviating the environmental concerns and creating more economic benefits simultaneously.