The desire of increasing the use of renewable energy has made energy storage and conversion one of the biggest challenges in today?s society. More and more applications require energy to be stored efficiently and with a high energy density. In order to satisfy future requirements, significant enhancements in the energy density of supercapacitors must be achieved. Supercapacitors are energy storage devices that possess a higher specific energy than conventional capacitors. Supercapacitors can provide a higher specific power compared to batteries, but their specific energy is lower. In addition, compared to batteries, supercapacitors have a longer cycling life, which means they can be charged and discharged millions of times without damage. These differences between supercapacitors and batteries arise from their fundamental way of storing electrical charges. While batteries store charges through redox reactions, supercapacitors store charges electrostatically.
The one of the goals of this project is to design and fabricate supercapacitors with specific energy greater than 80 Wh/kg. This would be a significant enhancement considering that conventional supercapacitors provide a specific energy of about 5 Wh/kg. Activated carbon nanospheres are used as electrode material and ionic liquids are used as electrolyte. Activated carbon materials are considered one of the most promising electrode materials, due to their high specific surface area, good electrical conductivity and chemical stability. In order to produce activated carbon spheres with these properties, two activation processes are used, namely KOH and CO2-steam activation. KOH is a more efficient activating agent, which yields larger surface areas and pore volumes. On the other hand, CO2-steam activation is a more environmentally friendly method to produce activated carbon.
Specific capacitance is a widely used measure of the ability to store electrical charges. It is defined as the amount of stored charges in coulomb divided by the potential difference in voltage between the electrodes. Generally, the capacitance of supercapacitors increases with increasing surface area of the electrodes. This is because more surface is available to store electrical charges. The highest specific capacitance achieved using CO2-steam activated carbon was 220 F/g at 0.1 A/g. By using higher surface area carbon, namely the potassium activated carbon, the specific capacitance reached a maximum at 260 F/g at 0.1 A/g. Owing to the hierarchical pore structure, both materials showed a good rate capability of about 75 % retention in the capacitance at 7 A/g.
In order to increase the energy density of supercapacitors, the operating voltage window needs to be expanded. Currently, supercapacitors using ionic liquid can achieve a voltage window of 0 ? 4 V. However, at this wide operating window the coulombic efficiency and cycling life are insufficient. This is due to carbon-electrolyte interactions occurring at high potential, which ultimately leads to decomposition of the electrolyte. Present on the surface of activated carbon are different functional groups consisting of oxygen and nitrogen, which will react with the electrolyte at high voltage and lead to decomposition. In this work, it was shown that by modifying the carbon surface through high temperature treatment in both hydrogen and ammonia atmosphere, the coulombic efficiency decreases from 63.9% to 28.2% at a potential window of 0 - 4 V. This is due to surface restructuring at high temperature via the presence of oxygen functional groups and the creation of defects, which enhances the carbon-electrolyte interactions.
Oxygen functional groups present on the carbon surface can enhance the energy density of supercapacitors by introducing battery-like faradaic processes to the charging/discharging mechanism. This work has demonstrated that by tuning the amount and type of oxygen functional groups on the carbon surface, the relative capacitance increased by 53.1 %. This was realized with an overall oxygen content of 0.0038 %/m2, which proved to enhance the adsorption of ions while introducing faradaic processes.
There are several benefits of using ionic liquids based supercapacitors, including high operating potential windows and high ion density. However, the drawbacks are high costs and insufficient performance at lower temperatures due to their high melting points, which reduces the ion mobility. To improve the performance at lower temperatures, several ionic liquids with various properties has been mixed and tested at different temperatures in order to optimize the electrolyte composition. The mixing of EMIMBF4, which is a high performance ionic liquid but with a high melting point of 15°C, with BMIMPF6 or EMIMTFSI (melting points of 6 and -16°C respectively) could significantly improve the performance at low temperatures.
The strategy to enhance both energy and power density as well as extend the operating temperature window has been established. It will be applied to design new supercapacitors with ultra-high energy.
The results achieved in this project will complement the understanding of charge and discharge mechanisms at a molecular level.
The principle of tune the operating voltage window and lifetime by manipulating the carbon surface properties is of great significance for selecting carbon electrode materials and improve the performance of supercapacitors in the industry. It is anticipated the principle can be applied to metal ion batteries and anode materials in Li-ion and other metal-ion batteries.
The project, SuperEnergy, aims to ground-breaking research for developing ultrahigh energy supercapacitors (SCs) with similar specific energy as batteries, but remarkable improvements on safety, power density, stability and operating temperature window compared to batteries. The project is based on our recent progress of the fundamental study of carbon-ionic liquid (IL) SCs, where ultrahigh capacitance (>600 F/g) at 4V is predicted theoretically. The new principle will be applied to achieve the ambitious goal by means of maximize the ion packing density in the nanopores. This project will develop a new approach with highly integrated multiscale hierarchical modelling and experimental investigation to optimize carbon nanomaterials-IL hybrids. The multiscale hierarchical modelling involves molecular dynamic modelling at molecular level and ion-packing model at electrode level. With the new approach, novel IL mixtures confined in nanopores will be rationally designed and screened, to significantly extend the operating temperature window by lowering the crystallization temperature, at the same time, to increase the capacitance by decreasing the effective ion size of the IL mixture. A new strategy for synthesis of mesoporous carbon sponge will be explored, aiming to gaining a better control of porosity, pore size and distribution. Project will develop new interconnected graphene like mesoporous carbon sponge to achieve narrow pore size distribution in a mesoporous rang and high pore volume. The project has a great potential to change the current energy storage and energy-use system. It could potentially help Norwegian society to use renewable energy in public transportation sector.