Pulsed Current for Diameter-Controlled Carbon Nanotubes and Hybrid Carbon Nanostructures in Electrolysis of Captured Carbon Dioxide

Here, we demonstrate how temporally controlled pulses of current can control the physical properties of multiwalled carbon nanotubes (MWCNTs) synthesized from the electrolysis of air-captured carbon dioxide. Our findings demonstrate that a transient 1 min 7.5-fold rate increase of current or carbonate reduction during nucleation of MWCNTs leads to 2.5 times smaller average MWCNT diameters, a higher degree of graphitization in the walls, and an overall 10% lower energy consumption over the full growth duration. Conversely, when identical transient current pulses are applied after MWCNT nucleation in the middle of CNT growth, our findings indicate the deposition of noncatalytic onionlike carbons on the surface of the MWCNTs to form hybrid nanostructured materials, but no changes are observed to MWCNT diameters, energy consumption, or wall graphitization of the MWCNTs. A detailed study of this system by three-electrode cyclic voltammetry, imaging, X-ray diffraction (XRD), and Raman spectroscopy supports the mechanistic role of current pulses in nucleation to facilitate rapid catalyst reduction and minimize coarsening to sustain catalysts with high activity. This work demonstrates how temporally controlled electrochemical current density, and hence carbon flux, in molten carbonate electrolysis is a powerful tool to engineer the production of carbon nanostructures with tailored physical properties and a total energy consumption footprint.