Dissertation Defense: Yu Ding

Molecular Engineering and Structural Design of Electrochemically Active Organic and Organometallic Materials for Energy Storage Devices

Yu Ding
Supervised by: Dr. Guihua Yu

In modern society energy and environmental issues are regarded as two main grand challenges for human beings. Researchers are trying to utilize sustainable energy more efficiently without squandering resources or polluting the environment. However, the widespread application of conventional energy storage devices, such as Li ion batteries and redox flow batteries, is limited by the uncompetitive performance, as well as the high cost and environmental concerns associated with the use of metal-based inorganic redox species. In consideration of advantageous features such as potentially low cost, vast molecular diversity, and highly tailorable properties, organic and organometallic molecules emerge as promising alternative electroactive species for building the next-generation of sustainable energy systems. My thesis is mainly focused on two families of materials, metallocene-based organometallics and quinone-based organic compounds.

Despite that metallocenes are also based on the redox reaction of metal centers, the cyclopentadienyl ring permits more flexibility to tune the electrochemical and physical properties through molecular engineering. The prototype of all-metallocene-based redox flow battery exploits ferrocene and cobaltocene as the redox-active cathode and anode, respectively. In light of the Hammett equation, the output voltage can be improved from 1.7 V to 2.1 V by introduction of methyl groups on the ligand rings of cobaltocene.

Moreover, the application of quinones can further enable heavy-metal-free, low-cost, environmental friendly energy storage devices. By rational screening of different solvents and functionalization of electrochemically active molecules, the redox potential, solubility and molecular mobility of the redox species can be tuned systematically. Detailed chemical characterizations are carried out to comprehensively understand the battery chemistry of quinones. Theoretical modeling is conducted to examine Li-binding characteristics, electronic properties, and structural stabilities of organic redox species, which govern electrochemical performance of those novel energy systems.

Last but not least, by leveraging the knowledge of solubility enhancement techniques in pharmaceutical research, hydrotrope-enhanced quinone solution is developed as the novel electrolyte. Compared with arduous chemical functionalizations to improve the solubility of organic redox species, the hydrotropic solubilization method represents a sustainable and cost-effective approach to the design of grid-scale energy storage systems. In addition, a comprehensive study unifying chemical characterizations and molecular dynamics simulations is conducted to elucidate the nature and mechanisms of hydrotropic solubilization processes.