Dr. Jud Ready, GTRI (advisor)
Prof. Matthew McDowell, MSE/ME
Prof. Rampi Ramprasad, MSE
Prof. Paul Kohl, ChBE
Mr. Curtis Hill, NASA MSFC
Dr. Eric Fox, NASA MSFC
Understanding Electrode-Electrolyte Interactions for Increased Energy Density in Supercapacitors for Aerospace Applications
In space, energy storage is extremely important because energy must either be brought along or harvested using technologies such as solar panels. The cost of failure in the power system is incredibly high - often failure of the mission. In addition, the limiting factor on many probes, landers, rovers, and satellites is the degradation of their power systems. However, energy harvesting and storage has also become a global concern. Lithium ion batteries are a common solution but are not a sustainable solution due to the mining and production processes required. This work aims to further our understanding of alternative energy storage processes so that a more sustainable solution may be implemented in the future.
In the design for the electrochemical double layer capacitors fabricated in this work, the electrodes are made with vertically-aligned carbon nanotube forests. The effects of adding different coatings using atomic layer deposition are explored. One coating investigated is a 6.03 nm titania coating which improves the energy density of the supercapacitor by adding pseudocapacitive redox reactions between the coating and the electrolyte. The addition of a thin, 4.8 nm alumina coating prior to growing the titania coating is also investigated to determine whether increase devices performance and cycling stability are observed. The electrode materials investigated are 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, 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, indicating that there is little electrode degradation and that the supercapacitors have excellent cycle life. In addition to fabrication of these devices and samples, a set of novel ionic liquids are synthesized with a methylcarbonate(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.
Devices are assembled 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 pseudocapacitance. 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 also intercalation or intercalation with partial redox reactions occurring. 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.