Date of Award

5-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical and Biomolecular Engineering

Committee Member

Dr. Mark E. Roberts , Committee Chair

Committee Member

Dr. David A. Bruce

Committee Member

Dr. Stephen H. Foulger

Committee Member

Dr. Christopher L. Kitchens

Abstract

Advances in the development of sustainable, low-cost, and reliable energy storage technologies have become a high priority as the demand for high power, and high energy storage devices has risen with emerging technologies in electronics, transportation, and renewable energy systems. Supercapacitors, due to their relatively high energy density and power density, provide an attractive alternative to bridge the gap between conventional batteries and capacitors. Materials ranging from high surface area, inert carbons to Faradaic metal oxides and conducting polymers have been used to achieve a range of performance properties in supercapacitors. However, the development of new technologies faces many challenges, such as sustainability, charge efficiency, capacity, cycle stability and scalable manufacturing processes.

In this work, to overcome some of these challenges, we developed straightforward, low-cost approaches for the design of micro- and nano-structured electrodes with enhanced electrochemical performance. Two main pathways were taken (1) manipulation of the electrode composition through the incorporation of lignin, as a redox polymer, into the active electrode material, for enhanced energy density, and (2) modification of the electrode structure through changes in the synthesis process of the electrode materials to improve the electrochemical performance.

For the first approach, lignin polymers were incorporated into a conducting polymer during electrochemical polymerization, providing increased Faradaic charge storage from the phenolic lignin groups. Polypyrrole (PPy) electrodes were prepared with alkali lignin (AL) and sulfonated lignin (SLS), and the electrochemical performance was compared with pure PPy films. We demonstrated an increase in capacitance of 30% in PPy/AL compared to PPy/SLS and 56% to PPy. Subsequently, AL and SLS were combined with porous carbon, which is electrochemically inert and non-reactive with lignin to improve the electrode stability and study the electrochemical performance of lignin without possible chemical/physical interactions with PPy. We found that intermediate pore sizes (>40 nm) led to optimal redox activity as lignin cannot get inside small pores, and large pores do not adsorb significant amounts of polymer.

In the second approach, lignin was used as a precursor to make high surface area carbon fibers, in which the structure of conventional fibers (polyacrylonitrile) was manipulated to produce porous materials. Decreasing the fiber diameter (115 to 8.5 µm) led to an increase in capacitance from ~2 F/g to ~70 F/g and a chemical activation process resulted in capacitances of ~192 F/g. Under the same scope, high surface area resorcinol–formaldehyde carbon aerogels reinforced with a backbone material allowed the fabrication of free-standing electrodes, eliminating the need for a binder and current collector during supercapacitor assembly. Finally, we developed a template-free synthesis method for creating microstructured electrodes to improve ion transport within thick conducting polymer films (~16 µm) while maintaining high energy storage capacity. Electrodes comprising these materials validate low cost, high energy density and innovative ways to manipulate the chemical composition and physical structure of Faradaic and non-Faradaic materials.

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