Onion-like Carbon for Electrochemical Capacitors

This thesis is designated to the study of onion-like carbon (OLC) for electrochemical energy storage. Its structure has been extensively studied and correlated with electrochemical characterization of the material for electrochemical capacitor applications. Synthesis of OLC at low temperatures (500--1100 °C) gave new insight into the transformation mechanism and kinetics, showing an intermediate step involving a disordered carbon phase as the nanodiamond is transformed into graphitic OLC. Analysis of OLC synthesized at higher temperatures (1300--1800 °C) was the first in depth study relating extensive physical characterization with electrochemical characterization. Electrochemical dilatometry investigated the expansion behavior of OLC electrodes upon charging and discharging. From this, a model is proposed detailing the structural behavior of OLC particles and clusters in a supercapacitor electrode. OLC was tested in aqueous, organic, and ionic liquid (IL) electrolytes. Strategies for increasing capacitance, such as altering the structure via chemical activation or by combining with a redox material, have been investigated. Chemical activation with KOH saw a 3-fold increase in capacitance, and maintained its high rate (high power) performance. The structure of activated OLC, and subsequent electrochemical performance, can be tuned by varying the activation temperature between 700--800 °C. Three different quinones were adsorbed on the surface of OLC that subsequently increased capacitance 10-fold. The desorption activation energy, derived from thermal analysis, saw a linear correlation with the quinone molecular size. The exohedral, non-porous structure of OLC makes it an ideal candidate for electrolyte diffusion studies. Three IL electrolytes of varying size were investigated with OLC electrodes. The use of impedance spectroscopy as a tool to measure ion diffusion of the IL electrolytes was confirmed. Micro-supercapacitor devices were tested using a eutectic mixture of ionic liquids and operated efficiently in a wide temperature range, -50 to 100 °C. OLC was used as an additive to conventional supercapacitor materials and showed improved performance when compared to carbon black as an additive, which is the industry standard.