Researchers at George Washington University led by Prof. Stuart Licht (earlier post) report a process for the high-yield, low-energy synthesis of carbon nano-onions (CNOs) by electrolysis of CO₂ in molten carbonate. A paper on their work is published in the journal Advanced Sustainable Systems.

High yield electrolytic synthesis of carbon nano-onions from CO₂, either directly from the air or from smoke stack CO₂, in molten carbonate. Liu et al.

Carbon nano-onions are a recently recognized, less-studied morphology of carbon nanomaterials consisting of nested concentric carbon spheroids (concentric nested buckyballs). The first observation of the structures appears to have been in 1980, and the first synthesis—using intense electron-beam irradiation of soot—in 1992.

Carbon nano-onions have a range of remarkable applications, but these applications have been largely ignored due to their high synthesis cost. Closely related morphologies of molybdenum and tungsten diselenide nested fullerenes and their lubrication properties have driven sales of thousands of tons per year of these alternative inorganic fullerenes. Carbon nano-onion applications often focus on the confined, high surface area of chargeable surfaces or symmetry of the CNO morphology.

Examples include nano-onion ultrahigh power super capacitors with unusually high charge storage due to the ease with which ions can access the active material, maximum anodic capacity lithium batteries, increased capacity gas, and energy storage materials with a high 984.3 m² g⁻¹ specific area, increased activity in heterogeneous catalysis such as for styrene oxidative dehydrogenation of ethylbenzene to styrene, and CNOs were employed as the support for Pt in direct methanol fuel cells outperforming Pt/Vulcan XC-72 in the electro-oxidation of methanol.

—Liu et al.

CNOs are currently valued at more than $1 million per ton, according to the researchers.

Previously, the Licht Group achieved a high yield growth of carbon nanotubes (CNT) by CO₂ electrolysis in molten carbonate through transition metal nucleation points on the electrolysis cathode.

Now, the researchers instead achieve effective low energy CNO synthesis from CO₂ by excluding those nucleating agents from the molten carbonate growth medium. This exclusion of transition metal nucleation agents inhibits CNT growth but results in a profusion of uniform CNOs, with an increasing diameter correlated to increasing growth time.

The source of CO₂ to produce CNOs can be industrial flue gas, or direct air carbon capture.

The researchers estimate the upper-bound cost of the new chemistry of CNO production by molten carbonate electrolysis—excluding anode costs to be determined—to be $1,000 per ton. This is several orders of magnitude less than alternatives.

The team said that future studies will explore inexpensive anodes for use in the CO₂ electrolytic growth of CNOs, and also explore application-level material testing of the charge storage, electrochemical, and tribological properties of CNOs from CO₂.

Rather than carbon taxes or extended debates on the necessary international path to combat global warming, transforming the greenhouse carbon dioxide to a valuable product incentivizes its “mining” and removal, either from industrial flue gas or directly from the air.

—Prof. Licht

Resources

• X. Liu, J. Ren, G. Licht, X. Wang, S. Licht (2019) “Carbon Nano-Onions Made Directly from CO₂ by Molten Electrolysis for Greenhouse Gas Mitigation” Advanced Sustainable Systems doi: 10.1002/adsu.201900056