Carbon nano-onions for supercapacitor electrodes: recent developments and applications

This critical review covers the use of carbon nano-onion (CNO) based materials for electrochemical capacitor (EC) electrodes. The performance of CNO capacitors is reviewed in detail and compared to the results obtained using different carbon nanostructures. The physico-chemical vs. electrochemical properties of CNOs are discussed. Their advantages, challenges and applications, especially in EC electrodes, are discussed in detail through an extensive analysis of the literature. Exohedral carbon nano-onions are much less limited by mass transfer kinetics; they show low resistance and can operate over a wide voltage and temperature window, at very high charge-discharge rates with almost unlimited cyclabilities. Although carbon nano-onions make excellent supercapacitor electrode materials, many challenges still remain, particularly to characterize and understand the physical, chemical, and electrochemical interactions within multicomponent systems that contain CNOs. The full potential of carbon nano-onions as electrochemical power sources has not been realized yet, but as outlined in this review, the future looks very promising.