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Energy generation and storage: Non-aqueous electrochemical synthesis of Cu(2-x)S and reverse bias removal of Al2O3

  • Author(s): Paw U, Latisha
  • Advisor(s): Stacy, Angelica
  • et al.
Abstract

Two investigations into energy generation and energy storage are presented in this dissertation. First the electrodeposition of copper sulfide, Cu2-xS in non-aqueous solvent is considered for photovoltaic applications. Second the reverse-bias removal of porous aluminum oxide is investigated for lithium rechargeable batteries.

To determine the electrochemical conditions for the deposition of copper sulfide the electrochemistry is investigated in dimethyl-sulfoxide, a non-aqueous solvent. Cyclic voltammetry was used to investigate the copper source, copper sulfate, and sulfur source, elemental sulfur. Comparisons to previous aqueous studies and published non-aqueous studies are made. Copper sulfate behaves as expected in DMSO, with copper(I) ions stabilized in solution. The electrochemistry of elemental sulfur is in agreement with published non-aqueous investigations. Studies with both sources offer simplified electrochemical reactions when compared to aqueous solutions. The reduction in possible reactions is promising for deposition of single composition films.

Once the electrochemsitry of the copper/sulfur solutions was completed, depositions were performed. Crystalline films of Cu1.8S and Cu1.96 S were deposited with minor impurities. Films were annealed via rapid thermal annealing to increase the crystallinity. Post-annealing films had the composition of Cu1.96S and Cu2S respectively. Electrodepositing crystalline films of copper sulfide with only small amounts of impurities from non-aqueous solutions is a significant success from previous investigations.

From energy generation to energy storage, porous aluminum oxide(PAO) is a promising material for rechargeable lithium battery separators. Previous studies showed that reverse-bias removal of aluminum oxide from aluminum generates a flexible membrane. The removal process was not well understood. A mechanism is proposed based on investigations of the removal conditions. Initially there are two simultaneous reactions, hydrogen reduction and the reduction of aluminum oxide to aluminum. The increased stress from the volume contraction as the reduction occurs leads to film delamination. Some areas of the membrane are still pinned to the aluminum backing and are etched away by the acid over time. Flexibility of the films are unchanged by the solution conditions. This supports the previous conclusions that the films are naturally flexible, and impurities in chemical film removal processes lead to brittleness. With flexibility agnostic of the synthetic conditions, they can be chosen based on other factors, such as availability and cost.

The viability of porous aluminum oxide for use in the battery industry is dependent on not only the performance, but also on the cost. The rechargeable lithium battery market was explored and a basic cost analysis for the porous aluminum oxide membrane was calculated to determine PAO's viability. Rechargeable lithium batteries are part of a large and growing market that is extremely cost sensitive. Even recent concerns of lithium-ion battery safety do not completely off-set the importance of overall battery cost. If industrial grade aluminum is used, a price of \$86/m2 is calculated. At only two orders of magnitude greater that current polypropylene separators the PAO membrane is a viable separator option.

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