Electrochemical synthesis of carbon nanotubes from CO2 in molten salts: fundamentals, mechanisms, and regulation

Abstract

Electrochemical CO₂ conversion in high-temperature molten salts offers a promising route for coupling carbon utilization with the direct synthesis of value-added carbon nanotubes (CNTs). Molten salts provide high CO₂ uptake, wide electrochemical windows, fast ion transport, continuous reaction conditions, and tunable electrolyte chemistry, which are favorable for controlled carbon formation. This review summarizes recent progress in CNT synthesis from CO₂ electroreduction in molten carbonates and chloride-carbonate hybrid melts. The thermodynamic and kinetic basis of carbonate reduction is first discussed, including selectivity between carbon and CO formation, oxide-ion transport, and catalyst-related interfacial processes. The roles of anodes and cathodes are then examined, with emphasis on oxygen evolution, metal dissolution, catalyst supply, cathodic activation, CNT growth pathways, interfacial reconstruction, ion migration, redeposition, and electrode stability. These analyses indicate that CNT formation in molten salts is not a simple carbon electrodeposition process, but a dynamic interfacial growth process governed by electrolyte chemistry, electrode behavior, catalyst evolution, and mass transport. Separation and purification strategies are also reviewed. Finally, key challenges are highlighted, including unclear intermediates, insufficient understanding of CO₂ absorption-electrolysis coupling, limited evaluation criteria, and the need for integrated reactor operation, product purification, and electrolyte recycling.