Molecular engineering and structural design of electrochemically active organic and organometallic materials for energy storage devices

Date

2018-08-10

Authors

Ding, Yu, Ph. D.

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Abstract

In modern society energy and environmental issues are regarded as two 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 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. In this dissertation, two families of materials, metallocene-based organometallics and quinone-based organic compounds are investigated comprehensively to build the high-performance redox flow batteries (RFB) for large-scale energy storage. Despite that metallocenes are also based on the redox reaction of metal centers, the cyclopentadienyl ring permits great flexibility to tune the electrochemical and physical properties through molecular engineering. Thanks to the fast reaction kinetics, the ferrocene-based membrane-free liquid battery delivers a superior power capability. Moreover, the vast family of metallocenes provides an opportunity to build an all-metallocene-based RFB. The prototype device exploits ferrocene and cobaltocene as the redox-active cathode and anode, respectively. In light of the Hammett equation, the output voltage can be finely tuned by introduction of methyl groups on the ligand rings of cobaltocene. We further investigated fundamental electrochemistry of quinones to enable heavy-metal-free, low-cost, environmental friendly energy storage devices. A bio-inspired, heavy-metal-free liquid battery has been built by directly using hydroquinone solution as catholyte and graphite as anode. The electrochemistry of hydroquinone is fundamentally studied in a broad pH range. By leveraging the knowledge of solubility enhancement techniques in pharmaceutical research, the solubility of hydroquinone in water is improved in the presence of urea as hydrotropic agent. 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. Last but not least, the application of quinone family for energy systems is extended to non-aqueous electrolytes. 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. Theoretical modeling is conducted to examine Li-binding characteristics, electronic properties, and structural stabilities of organic redox species that govern electrochemical performance of those novel energy systems. By integrating the function-oriented organic synthesis, detailed chemical characterizations, and advanced molecular dynamics simulations, we aim to provide a useful platform to design the next-generation of sustainable energy storage systems for grid-scale applications

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