Electrochemical performance of carbon onions, nanodiamonds, carbon black and multiwalled nanotubes in electrical double layer capacitors

This paper describes the electrochemical performance of carbon onions, nanodiamonds, carbon black and multiwalled nanotubes as electrodes in electrical double layer capacitors with organic electrolyte. Onions were formed by vacuum annealing of 5 nm nanodiamond (ND) powder at 1200–2000 °C with the goal to investigate the effect of carbon microstructure on specific capacitance and ion transport. In contrast to micro- or mesoporous activated carbons, the outer surface of carbon onions is fully accessible to electrolyte ions and the size of pores between carbon onions or nanotubes does not depend on the annealing temperature. Charge-discharge measurements revealed a two times decrease in the specific capacitance of onions and nanotubes upon graphitization and formation of polyhedral particles after annealing at 1800 °C and above. However, the capacitance became less current dependant. The carbon onion cells are able to deliver the stored energy under a high current density with a capacitance twice than the one obtained with MWCNT. Electrical measurements and impedance spectroscopy showed about two orders of magnitude increase in conductivity of electrodes and twofold decrease in the equivalent series resistance of the assembled cells after heat treatments of ND. The time constant extracted from the impedance data is around 10 times smaller for ND annealed at above 1800 °C than for activated carbons and is closely approaching the one for MWCNT. This shows that the open structure of carbon onions leads to an increased ability to quickly deliver the stored energy.