Disclosed herein are methods of making a plurality of carbon nanotubes, the methods comprising applying a current across a catalytically passive anode and a catalytic cathode; wherein the catalytic cathode comprises a catalyst and the catalyst comprises Fe, Co, Mo, Cr, Cu, or a combination thereof; wherein the catalytically passive anode and the catalytic cathode are in electrochemical contact with a molten carbonate electrolyte and a source of CO2; thereby forming a plurality of carbon nanotubes on the catalytic cathode.