Date on Master's Thesis/Doctoral Dissertation

5-2015

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Degree Program

Chemical Engineering, PhD

Committee Chair

Amos, Delaina A.

Committee Co-Chair (if applicable)

Sunkara, Mahendra

Committee Member

Zamborini, Francis Patrick

Committee Member

Starr, Thomas L.

Committee Member

McNamara, Shamus

Subject

Electrodes--Design and construction; Energy storage; Photoelectrochemistry--Equipment and supplies; Nanoelectronics

Abstract

Nanoscale electrode architectures are at the heart of photoelectrochemical devices converting sunlight into electricity and electrochemical devices that store energy. Advanced nanomaterials encompassing 1-D architectures such as single crystalline metal oxide nanowires, 2-D architectures such graphene nanosheets and nanoparticle-nanowire hybrids have garnered significant interest as promising materials for fabricating thicker electrodes due their unique electronic, phase transformation and strain relaxation properties. However, electrode architectures incorporating these advanced nanomaterials will have a transformational impact on large scale electrode manufacturing only if the challenges associated with the electrode durability are addressed. At a fundamental level, the reactions happening at the electrode-electrolyte interface need to be studied in detail in order to address the durability issues. The overall objective of this dissertation is to develop electrode architectures with high durability for solar energy conversion and storage by employing engineered materials with excellent charge transport and charge separation properties. Specifically, electrodes architectures that enable high durability for dye-sensitized solar cells using alternate redox electrolytes and moisture resistant perovskite solar cells, and lithium manganese oxide cathodes for Li ion batteries have been developed. In dye-sensitized solar cells, there is immense interest in replacing the conventionally used, highly corrosive iodide redox electrolyte with non-corrosive one-electron redox electrolytes that can result in higher cell stability. However, a major shortcoming of one-electron redox electrolytes is their fast electron recombination kinetics when compared to the iodide electrolyte. To address this issue, tin oxide nanowires are investigated as an alternative for conventionally used titania nanoparticles in dye sensitized solar cells. Further, hybrid architectures comprising of titania nanoparticles-tin oxide nanowires are found to be effective in overcoming the limitations of low dye adsorption and low open-circuit voltage, posed by nanowire electrodes. To gain deeper insight into the electron transport and recombination kinetics of different electrode architectures in conjunction with different redox electrolytes, fundamental studies are performed using electrochemical techniques. In addition, the application of alternate absorbers in solar cells is crucial in attempting to address the challenges posed by the state-of-the art dyes. Of particular interest are the metal-organic hybrid perovskites that have high absorption coefficient and excellent hole conductivity. Though perovskites have resulted in breakthrough performance for solar cells, degradation of perovskites due to moisture is a huge road block limiting the progress of perovskite solar cells. The moisture instability of perovskites is addressed using a novel concept of a thick, highly conductive graphene-conductive polymer composite for the encapsulation of the perovskite nanocrystals. Electrode architectures for highly durable cathode materials for application in Li ion batteries have also been developed in this dissertation. Most of the widely investigated anodes have higher capacity when compared to the cathodes. In order to balance the capacity of both electrodes, the cathodes need to be made thicker. Nanoparticle electrodes are not ideal for making thicker electrodes as the poor charge transport characteristics result in increased series in thicker nanoparticle electrodes and subsequently lead to a loss in cell voltage. Further, electrode materials for Li ion batteries require the structural integrity of the material to be maintained through several charge-discharge cycles. Layered transition metal oxides and their alloys have been widely investigated as cathode materials in Li ion batteries due to their potential for achieving high capacity. Poor surface stability and increased strain on the lattice associated with the phase transformation occurring during battery charging and discharging are the major reasons for poor cycle stability of the commonly used layered transition metal cathodes. In this work, single crystalline nanowires of a layered transition metal oxide, namely Li2MnO3 are investigated as cathode materials in Li ion batteries in order to address the limitations posed by layered transition metal cathodes. A detailed investigation of the cycled electrodes showed that single crystalline nanowires allow for the facile phase transformation of the Li2MnO3 to a zero strain spinel phase of LiMn2O4. The phase transformation on single crystalline Li2MnO3 NWs result in the formation of conformal and thicker (20-30 nm) spinel LiMn2O4 shell that is very effective in improving the surface stability of the electrodes and hence prevents the capacity loss during cycling.

Download

Share

COinS