Golsharifi, Nima; (2018) Carbon nano onions for the development of supercapacitors. Doctoral thesis (Ph.D), UCL (University College London).

Abstract

The integration of carbon nano onions (CNO) has opened new horizons in the use of nanoscale energy storage for applications for which conventional electrolytic capacitors are not sufficient. The use of CNO in the development of supercapacitors now seems to be a promising venture. Ultra-capacitors or supercapacitors are electrochemical systems that store energy within their double-layered structure consisting of oppositely 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 supercapacitors. Results of chapter 5 describes a novel approach to a high-pressure, high-temperature technique for the synthesis of CNOs. Here, it has been demonstrated that functionalised CNOs which were prepared by a reaction between copper dichloride hydrate
(CuCl2 3 2H2O) and calcium carbide (CaC2) with particle size of 6.6 nm and interlayer separation distance of 0.354 nm and surface terminated with different element groups including oxygen, hydrogen and nitrogen led to the enhancement of ion storage capacity of carbon cathodes. Nitrogen terminated samples presented the highest surface area of 566 m^2/g. Alternatively, integration of these nanoparticles in supercapacitors with a high surface-to-volume ratio, without the use of organic binders and polymer separators, improved performance because of the ease with which ions can access the active material, increasing the energy density and discharge rates of supercapacitors. Alongside we have demonstrated the specific capacitance of 59.14 F.g-1 (78.2% improvement of specific capacitance by addition of a layer of graphene/PANi composite to CNO electrodes over supercapacitors) compared to electrodes made of CNOs with no functionalisation. Chapter 6 describes that CNOs can be produced by annealing nanodiamond (ND) in a vacuum furnace or an inert atmosphere. It has been shown that increase in annealing temperature has led to CNOs with a larger diameter and crystallite size, i.e. NDCNO@2500 with d-spacing of 0.350 nm and crystallite size of 5.04 nm. Here, we functionalised CNOs produced as a result of graphitising pre-purified NDs at high temperature; with N2, NH3 and O3 to improve their wettability for use in supercapacitor electrodes and did characterise the resulting structures with different techniques. Raman results suggested the introduction of a higher level of imperfection in the graphene planes for the NDCNO@1780 N2 Plasma and NDCNO@1780 NH3 UVA. The effect of functionalised samples presented open pores distribution size with a higher level of mesoporosity and slightly lower microporosity where they represent micropores less than 1.3 nm with the highest BET surface area value of 395 m2/g that belongs to NDCNO@1780 NH3 UVA. The functionalisation using NH3 UVA and Ozone has led to the introduction of a slightly higher level of mesoporosity in the range of 2.7 – 4.9 nm and slightly lower microporosity at around 0.8 nm compared to pristine CNO. Reduction in the initial decomposition rate compared to that of pristine CNO been observed using TGA; where NDCNO@1780 N2 Plasma presented stability in the air up to the temperature of 706 °C and then it starts burning out within temperature range of 719 – 880 °C with decomposition rate of 6.98 g/m. High concentration of sp2 carbon confirmed with the π-π* at high binding energy and with increasing values of sp2 area while the value of π-π* area was also increasing conversely after functionalisation. The results have demonstrated the strong effect of utilised activation methods in the creation of porosity by creating more open mesopores and development of the higher surface area. Chapter 7 describes the effect of chemical and physical activation over the CNOs. The results have demonstrated the strong effect of such activation methods in the creation of porosity by creating more open mesopores and development of the higher surface area. 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. 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 the 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.

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