Electrical energy storage systems such as supercapacitors have steadily grown in importance to store the energy generated from renewable energy resources such as the sun, wind and tide. Supercapacitors have high power density but their energy density is typically an order magnitude lower than that of most batteries. The development of improved supercapacitor devices is a key technical challenge if the higher energy demand requirements of future large scale energy storage applications are to be met. Carbons with high surface area and pore structures that can be tailored to electrolyte ion sizes are promising electrode materials for supercapacitorss. Moreover the presence of functional surface groups such as nitrogen surface groups may enhance the overall capacitance of an electrode material by providing a pseudocapacitance effect. The optimization of both porosity and surface chemistry of carbon electrode materials offers a potential avenue to increase the performance of supercapacitors. However, to achieve an optimization of the supercapacitor devices a better fundamental understanding of the charge storage mechanism is required. This thesis focuses on the synthesis and characterization and mechanism energy storage of nitrogen-rich porous carbon materials as carbon electrodes for supercapacitors.
To study the optimal porosity of electrode materials, the effect of microporosity and particle size on double-layer capacitance mechanism was investigated using silicon carbide-derived carbons (Si-CDC) as a model of electrode materials without nitrogen surface functional groups. The Si-CDCs were synthesized by the chlorination at 1000 °C of silicon carbide powders with two different particle sizes (6 μm and 50 nm). One of the Si-CDCs produced using the 50 nm particle sizes was heated at 1000 °C for 24 h to study the effect of prolonged heat treatment on the graphitic degree and its subsequent influence on capacitance. The electrochemical performances of SiC-DC electrodes in sulphuric acid (H2SO4) and tetraethyl ammonium tetrafluoroborate (TEABF4)/acetonitrile (AN) demonstrated that the presence of micropores close to the size of the solvated electrolyte ions provided a large number of adsorption sites for high capacitance at low current densities. In addition, the short pore diffusion lengths in the 50 nm SiC-DC particles and the presences of some mesopore channels significantly increased ion mobility in the carbon electrode, and this property in turn resulted in higher capacitance at fast charge rates.
The synergistic effects of porosity and surface chemistry on the capacitive performance were investigated by using nitrogen-rich porous carbons synthesised from two different melamine precursors, a melamine resin (MR) and a melamine network (MN). Large volumes of both micropores and mesopores in the melamine-based carbons improved the supercapacitors performances. The highest specific capacitance measured at a current load of 0.025 A g-1 in a 1 M H2SO4 cell was 145 F g-1 for the MR-based carbon carbonized at 800 °C. This carbon had both a high nitrogen concentration in the porous carbon structure (10.5 at.%) and a well-developed pore structure that provided access of the electrolyte ions to the micropores. The highest capacitance retention at a current load of 5 A g-1 was 79 % with the MR was carbonized at 1000 °C and this result was due to a large volume of small mesopores (2-4 nm) in this sample. In contrast to the MR-based carbons, nitrogen functional groups in MN-based carbons created structural defects such as blockages of the narrow micropores, which subsequently decreased the ions adsorption for the double-layer formation. The presence of the pyridine/pyrrolic and quaternary nitrogen groups and phenolic oxygen functional groups within the MR-based carbon matrix and their accessibility to electrolyte ions had a positive effect on the electron-donor properties of carbons, and thus on the capacitance. The study on the carbon surface chemistry after two-electrode charge-discharge cycles showed reduction-oxidation reactions involving COOH groups to quinone type groups. The redox reactions upon electrochemical test for the nitrogen surface functional groups suggested reversible reactions and showed no significant changes.
To further develop the porosity in melamine-based carbons, poly(vinylidene fluoride) (PVDF) was used as a porogen. The resulting PVDF/melamine-based carbons had larger surface areas, mesopores volumes and micropore volumes than the MR-based carbons, but the PVDF diluted the surface nitrogen concentrations from 2.4 to 5.6 at.%. The presence of mesopores in the PVDF/MR carbons facilitated better ion transport and access to micropores in both, H2SO4 and TEABF4/AN electrolytes. Therefore, the enhanced pore development of the PVDF/melamine-based carbons improved the electrochemical performances at fast charge-discharge rates compared with MR-based carbons, with better electrochemical stability. However the capacitances obtained for PVDF/melamine-based carbons (132 F g-1) were lower than MR-based carbons suggesting the important pseudocapacitance contribution from surface nitrogen groups to the overall capacitance. Thus carbon electrodes with microporosity and mesoporosity along with high nitrogen surface concentration may be good materials to achieve higher capacitance.
In the last study reported in this thesis, two nickel catalysts - a nickel nitrate and a nickel hydroxide added by different methods during MR carbon synthesis - were used during pyrolysis of melamine resin. The aim was to induce graphitization hence improve the electrical conductivity of the carbons. The presence of nickel particles during the carbonization of the melamine resin led to the formation of bamboo-like nanocarbons with some graphitic degree. In addition, the synthesis of melamine resin with 20 wt.% of Ni(OH)2 (MRNi20) resulted in the formation of mesopores at pore widths (4-5 nm) with a micropores surface area Smic of 85 m2 g-1 and surface nitrogen content of 1.6 at.%. Owing to the mesopores of MRNi20 the capacitance retention was 78 % at 5 A g-1. However, the Smic and the concentration of nitrogen surface functional groups were lower in the MRNi carbons than in the MR-based carbons carbonized at 800 °C, and this decrease resulted in lower specific capacitances of 23 to 49 F g-1 measured with the MRNi carbons.