Cellulose-derived porous carbon electrodes for electrochemical capacitors

Abstract

Electrochemical capacitors (ECs), also normally called as supercapacitors are important energy storage devices with characteristics such as long cycling life, high power density and safety. Currently, activated carbon is the most popular electrode for fabricating ECs because of its high specific surface area, good electrical conductivity and low cost. To improve the electrochemical performance of ECs, especially their energy density, the past decade has witnessed a great deal of research interest in searching for advanced electrode materials, which are expected to be sustainable, cost-effective, stable against cycling and of high performance.Biomass is a carbon-rich, earth-abundant, renewable and low-cost resource for making carbon materials. Because of the complexity of biomass in terms of chemical composition, source and physical state, it has been challenging to establish structure-property-performance correlations, which are important for optimising biomass-derived carbon electrode materials. Biomass consists mainly of cellulose, lignin and hemicellulose. Cellulose, with a formula of (C6H10O5)n, is a polysaccharide consisting of a linear chain of glucose units. Being a natural biopolymer, cellulose is almost inexhaustible. It exists in various forms from micrometric cellulose fibres to nanocellulose and water/solvent soluble cellulose derivatives, among which microcrystalline cellulose (MC) is commercially available. Therefore, this PhD project aims to investigate the feasibility of transforming MC into high value-added carbon electrode materials for new-generation ECs.Results obtained in this project have shown that MC-derived carbons display very promising electrocapacitive properties. It was found that heteroatom-doped porous carbon from MC exhibits excellent electrochemical performance. Both heteroatom doping and porous structure were found to play important roles in charge storage. Further improvement on electrocapacitive properties of the heteroatom-doped carbon was conducted by manipulating the N/O doping level and introducing intrinsic defects. In 1 M H2SO4 electrolyte, the improved carbon sample displays a specific capacitance as high as 426 F/g at a current density of 0.25 A/g or 177 F/g at 100 A/g measured using a three-electrode system. About 90 % of its original capacitance was retained after 60,000 cycles at 5 A/g as measured in a symmetric cell. In addition, the electrode with a high mass loading of 12 mg/cm2 displays high areal capacitances of 2,518 and 1,128 mF/cm2 at current densities of 0.5 and 50 mA/cm2, respectively, along with a good cycling stability, making the sample a promising candidate for practical EC application.The focus of the thesis was then laid on the porous structure. Hierarchical porous carbons (HPCs) with macropores, mesopores and micropores are believed to be ideal electrode materials for ECs. To understand the role of pore hierarchy of carbon electrode materials in capacitive charge storage, HPCs were prepared using the zeolite-template method with ethylene as the carbon precursor. Results showed that the carbon templated with calcium-containing nano Beta zeolite displays the best electrocapacitive performance amongst the HPCs studied. The appropriate amount of micropores provided enough active sites for electric double layer formation and the rich mesopores enabled the active ions in the micropores to have nanometer transport distances. And the ordered straight hierarchy templated from the zeolite further reduced the charge transfer and electrolyte diffusion resistance, which is better for both rate capability and cycling stability. A symmetric capacitor fabricated with this carbon as both electrodes exhibits a capacitance of 246 F/g at a current density of 1 A/g after 17, 000 times cycling.The electrocapacitive properties of the MC-derived carbons were further improved by using graphene to prepare composite electrode materials, which were evaluated using different electrolytes with different cell configurations, including symmetric EC cells and sodium-ion capacitor (NIC) cells. Microporous and thin graphene oxide layers further interconnected and covered along the mesoporous carbon network, which provides more electrocapacitive sites and is beneficial for both the electronic and ionic transport. The NIC built with the obtained sample as positive electrode exhibits an energy density of 48.1 Wh/Kg at 250 W/Kg and remains 22.0 Wh/Kg at 18, 080 W/Kg.In summary, this thesis demonstrates the MC-derived porous carbons for EC electrode application. The effect of heteroatom doping level was studied, and a relatively low N-doping level combined with intrinsic defects is recommended for porous carbon materials of good electrocapacitive performance. Besides, the importance of a hierarchical porous structure for EC electrodes was further confirmed. In this thesis, MC-derived porous carbons were for the first time reported as the positive electrode for NICs, which, as a new generation of ECs, bridge the gap between conventional ECs and ion batteries. As a cost-effective and renewable carbon source, MC is promising for the sustainable development of ECs. More research focused on exploring MC-derived carbons of better performance is suggested in future, especially for NIC positive electrodes.