The integration of carbon nano-onions has opened new horizons in the use of nano-scale energy storage for applications for which conventional electrolytic capacitors are not sufficient. The use of carbon nano-onion (CNO) in the development of super capacitors now seems to be a promising venture. Ultra-capacitors or supercapacitors are electrochemical systems that store energy within their double-layered structure consisting of opposite charged materials. Supercapacitors by offering fast charging and discharging rates, and the ability to sustain millions of cycles are bridging the gap between batteries, which offer high energy densities but are slow, and conventional electrolytic capacitors, which are fast but have low energy densities. Electrodes made from activated or porous carbon are used in the production of the highest rated super capacitors. Here we describe that CNOs can be produced by annealing nanodiamond (ND) in a vacuum furnace or an inert atmosphere.
Here, we functionalised CNOs produced as a result of graphitising pre-purified NDs at high temperature; with Nitrogen, NH4 and O3 to improve their wettability for use in supercapacitor electrodes and did characterise the resulting structures with different techniques. We also studied the effect of chemical and physical activation over the CNOs.
The effect of functionalised samples presented open pores distribution size with higher level of mesoporosity and slightly lower microporosity, alongside reduction in the initial decomposition rate compared to that of pristine CNO.
High concentration of sp2 carbon confirmed with the p-p* at high binding energy and with increasing values of sp2 area while the value of p-p* area was also increasing conversely after functionalisation.
The results have demonstrated strong effect of used activation methods in creation of porosity by creating more open mesopores and development of higher surface area. The TEM results suggested the increase of interlayer spacing of up to 0.72 nm for ACNO KOH-7M N2 sample compared to that of Pristine CNO (0.35 nm) which is also confirmed by XRD measurements. XRD measurements confirmed reduction of crystallite size upon activation procedure. Although it has been deduced that the ultimate performance of activation with plasma is strongly dependent on the selection of chemical reagent in contrast to activation by Ultra-Violet, concluding that proper selection of chemical reagent in conjunction with a posterior physical activation method, gives desirable results for CNOs surface activation.
Finally, it has been shown that that K2CO3 chemically activated CNO presents the highest surface area of 834 m2/g with presence of narrow mesopores (4.724 nm) and larger micropore (1.822 nm), the smallest crystallite size of 1.06 nm and stability temperature of 710 °C in air, makes it perfect candidate for electrolyte accessible pores and pseudocapacitive behaviour with shorten ion diffusion pathways and reduced charge-transfer resistance.