(Georgia Institute of Technology, 2023-04-26)
Allen, Julia Madeline
In the design for these electrochemical double layer capacitors, the electrodes are made with vertically-aligned carbon nanotube forests. This forest is functionalized with a coating of titania in an attempt to increase the energy density of the supercapacitor by adding pseudocapacitive redox reactions between the coating and the electrolyte. Functional alumina coatings are also investigated to improve the ability to understand and control the electrode-electrolyte interactions. The electrode materials investigated will be shown to form different morphologies depending on the presence of alumina as a base layer and the vertical location within the forest. In samples where the alumina is present, the coating forms a conformal shell around the individual carbon nanotubes. However, the alumina coating only forms near the top of the forest. In regions where the alumina is not present, either because the deposition process was not able to penetrate that far into the forest, or the sample did not have an alumina base layer applied at all, titania coatings form a non-continuous coating of discrete titania nanoparticles attached to the nanotube walls. There is no change to these coatings after 1,000 charging and discharging cycles observed.
In addition to fabrication of these devices and samples, a set of novel ionic liquids are synthesized with a methyl-carbonate(trifluoromethylsulfonyl)imide anion, an asymmetric anion. Asymmetric anions are theorized to have superior properties to symmetric anions. The structure of these ionic liquids is characterized for confirmation of successful synthesis, and the melting and degradation temperatures are determined experimentally. Many properties of ionic liquids impact device performance, although, not all of the effects have been well characterized. Some of the most important properties for energy storage, are the electrochemical stability window, conductivity, and melting temperature. There is not a database containing electrochemical stability window data, but electrical conductivity and normal melting temperature data have been compiled. Therefore, a machine learning algorithm is developed and used to create predictive models for the electrical conductivity. Models like these can be used to enhance the selection and testing process. They also offer potential to predict completely new ionic liquids with optimal combinations of several properties for use in specific applications.
Devices are made using these electrode materials and room temperature ionic liquid electrolytes and characterized using a variety of electrochemical techniques to evaluate the capacitance, series resistance, specific energy, specific power, cycling stability, and pseudocapacitance. Supercapacitors utilizing carbon nanotube forests with pseudocapacitive coatings are confirmed to exhibit signs of pseudocapacitive reactions occurring. Cyclic voltammetry results indicate that these pseudocapacitive reactions are occurring through surface redox reactions. Supercapacitors using a base layer of alumina beneath a layer of titania demonstrate improved performance (1.01 mF) over supercapacitors fabricated without the alumina layer (0.82 mF). The supercapacitors with no coatings added to the carbon nanotubes have an average capacitance of 0.67 mF. Galvanostatic charge/discharge testing results also indicate that the supercapacitors with alumina and titania coatings exhibit pseudocapacitance, while those without coatings do not. However, the shape for the discharge curve indicates that there are intercalation or intercalation with partial redox occurring as well. The supercapacitors with alumina and titania coatings have the lowest resistance and highest capacitance on average. A study of supercapacitor performance at different scan rates is performed to gain a better understanding of the reactions occurring within the devices. This method is used to separate the current into non-faradic and faradaic components. Faradaic reactions are observed to contribute to the device capacitance in the case of supercapacitors fabricated with pseudocapacitive coatings. The intercalation processes, identified with galvanostatic charge/discharge testing are responsible for the faradaic current measured in this technique.