Carbon onions, exhibit unique zero-dimensional spherical or concentric shell structures with small (<10nm) diameters. They often go by other names such as carbon nano-onions (CNOs) or onion-like carbon (OLC). We synthesize carbon onions by annealing (usually in vacuum) nanodiamond particles. Because of their structure, high conductivities, and dispersion properties, they are often used as conductive additives in electrochemical energy storage devices, such as battery and supercapacitor electrodes, where an increase in the rate performance (power) of the device is necessary.¹

However, carbon onions can be more than just an additive. When investigated as the main active material in microsupercapacitors, devices containing carbon onion electrodes showed stack capacitances 4 orders of magnitude higher than that of electrolytic capacitors, an impressive performance in the field of ultracapacitors.² Additionally, carbon onions have also been explored with ionic liquid electrolytes, widening the operating voltage range and enabling use in low-temperature regimes.³ Carbon onions have also been coated with redox active molecules or functionalized with electrochemically active functional groups, improving the overall electrochemical performance.⁴ Because of each of these qualities and performance ranges, carbon onions could be expanded into many other applications, such as textile supercapacitors, antennas, biomedical devices, sensors, and much more.

References:

1. K. McDonough, Y. Gogotsi,Carbon Onions: Synthesis and Electrochemical Applications, Interface, Fall 2013, 61-66 (2013)
2. Pech, M. Brunet, H. Durou, P. Huang, V. Mochalin, Y. Gogotsi, P.-L. Taberna, P. Simon,Ultrahigh power electrochemical micro-capacitors based on onion-like carbon, Nature Nanotechnology 5, 651-654 (2010)
3. Lin, PL. Taberna, S. Fantini, V. Presser, CR. Perez, F. Malbosc, NL. Rupesinghe, KBK. Teo, Y. Gogotsi, P. Simon,Capacitive Energy Storage from -50 to 100 degrees C Using an Ionic Liquid Electrolyte, Journal of Physical Chemistry Letters, 2, 19, (2011), 2396-2401
4. M. Anjos, J. K. McDonough, E. Perre, G. M. Brown, S. H. Overbury, Y. Gogotsi, V. Presser,Pseudocapacitance and Performance Stability of Quinone-coated Carbon Onions, Nano Energy, 2 (5), 702-712 (2013)