Upscaling activated carbon production using a Rotary Kiln

Activated carbon is a valuable carbon material with tuneable porosity and surface area that can be employed in many applications, such as electrode materials for energy storage devices, water and gas purification, catalysts, gas adsorption and storage, air filters, etc. The increasing demand for clean energy and efficient storage options has heightened interest and focus on activated carbon for use as supercapacitors and Lithium-ion capacitor (LiC) cathodes, especially in high power-fast charge/discharge applications. Activated carbon has been selected due to its tuneable surface area and porosity for storing charges, ease of availability and manufacture from precursors such as wood waste, coconut shells, waste tires and virtually any carbon-containing material. To produce activated carbon, two activation means can be utilised: physical and/or chemical activation. The required properties, such as high surface area, sufficient pore volumes and optimised pore size distributions, can be efficiently achieved using chemical activation, which involves heat treating a mixture of the carbon precursor with a suitable chemical activating agent. Using this technique, surface areas of about 3000 m2/g have been obtained in laboratory small-size batch reactors producing in grams. However, for use as electrode materials, larger-scale continuous production in Kg is required. In the first aspect, this project explored the use of a Rotary kiln, a larger reactor consisting of a rotating tube with variable inclination, continuous feeder, and temperature controller, which can activate the carbon precursor continuously on a pre-industrial scale while producing in kg. To obtain high-quality activated carbon and upscale production, factors such as the rotation speed of the tube, activation temperature and inclination degree were optimised using carefully designed experiments. High-quality activated carbon was synthesised, and the process data obtained served as the background for further expansion into industrial-scale production. As a second objective, different product optimisation strategies were investigated with the aim of understanding the activated carbon electrode/Li-ion electrolyte interactions to improve the performance and ensure the long-term stability of the LiC incorporating the activated carbon electrode. Electrolyte decomposition is a crucial cause of cell failure. It is accelerated upon contact with the activated carbon's high surface area and surface oxygen-rich active sites, especially at high voltages. The failure mechanisms of the activated carbon electrode in contact with Li-ion electrolytes were therefore investigated using a combination of electrochemical and material characterisation techniques, which examined the activated carbon electrode/electrolyte interface after different durations of cycling. This enabled a holistic time-dependent revelation of the nature of decomposition products at the surface, when they were formed, and the induced transformations of the activated carbon electrode with respect to these decomposition products. The instability of the PF6- anion was identified as a significant cause of the electrolyte degradation, and the corresponding capacity fade process was elucidated. In the concluding aspects, different methods were investigated to mitigate the electrolyte decomposition on the activated carbon surface and improve the cycle life of LiC incorporating the activated carbon electrode. Optimising the electrolyte dielectric was revealed as a method of enhancing the electrochemical stability of the LiC through improved anion oxidative stability. Electrolyte solutions with high dielectric afforded a strengthened solvation shell around the PF6 anion. This mitigated the anion degradation when exposed to high polarisation potentials in the inner Helmholtz plane (located at the activated carbon electrode double layer). Therefore, the cycle life of LiC and other energy storage devices containing the activated carbon electrode can be extended by increasing the dielectric of the electrolyte.

A method of producing activated carbon on a large scale using a rotary kiln was developed. Carefully designed experiments were conducted to optimise the process operating parameters, yielding high-quality activated carbon for use as electrode materials in Beyonder’s Lithium-ion capacitors. In addition, detailed studies were conducted on the interactions between the activated carbon electrode and Li-ion electrolyte to understand the failure mechanisms and causes of its capacity fade during the operation of Lithium-ion capacitors. The failure mechanisms were identified, and the degradation-induced transformations were reported. Furthermore, a method for extending the cycle life by delaying the electrolyte degradation at the activated carbon electrode surface was discovered. The knowledge of these degradation mechanisms and methods for mitigating such will aid the research community in developing high-performance and long cycle-life Lithium-ion capacitors incorporating the activated carbon cathode.

The purpose of the PhD project will be to upscale the production of activated carbon using a rotary kiln. The use of activated carbon spans across a broad range of fields. Water purification, gas adsorption/storage, electrode fabrication in energy storage devices are some of the most common applications. With the rising quest for clean energy, it has become necessary to provide adequate energy storage mediums to cater for the erratic nature of renewable energy storage devices especially in periods of intermittency. Activated carbon electrodes have been widely used due to their large surface area in energy storage devices, most notably super-capacitors to satisfy high power, long cycle life and high rate capability requirements. At present, most of the activated carbon R&D work is typically carried out on a batch scale, utilizing small sized reactors with an activated carbon yield in grams after each run. The rotary kiln is a larger reactor which would upscale the production and reduce the need to purchase activated carbon. Laboratory batch reactor experiments have proven that saw dust can be converted into valuable activated carbon with very high surface areas up to 3000 m2/g. However, the use of the rotary kiln for large scale production of activated carbon has not been potentially exploited. Challenges related to heat and mass transfer occurring during the reaction, in addition to control of mean residence time for optimum activation of the precursor have been encountered. Interaction between process parameters such as mass flow rate of precursor, rotating speed, activation agent mass/mass flow rate, inclination angles of rotary kiln has made process of modelling and simulation cumbersome with simulated results not in close fit to actual experiment outcomes at certain experimental conditions. Experiments would be efficiently designed in this PhD project to upscale, study and optimize process parameters affecting activated carbon production.