Open Access

Marco Zeiger, Nicolas Jäckel, Vadym N. Mochalin, Volker Presser

• Carbon onions are a relatively new member of the carbon nanomaterials family. They consist of multiple concentric fullerene-like carbon shells which are highly defective and disordered. Due to their small size of typically below 10 nm, the moderate surface area, and high conductivity they were used for supercapacitor applications. As electrode material, carbon onions provide fast charge/discharge rates resulting in high specific power but present comparable low specific energy. They improve the performance of activated carbon electrodes as conductive additive and show suitable properties as substrate for redox-active materials. This review provides a critical discussion of the electrochemical properties of different types of carbon onions as electrode material. It also compares general advantages and disadvantages of different carbon onion synthesis methods. The structure, physical and chemical properties of carbon onions, in particular nanodiamond-derived carbon onions, are described with emphasis on those parameters especially important for electrochemical energy storage systems, including among others structure, conductivity, and porosity. Although the primary focus of current research is on electrode materials for supercapacitors, the use of carbon onions as conductive additive for activated carbon and electro-active polymers, as well as substrate for redox-active species is also discussed.