Date of Award

8-2011

Document Type

Campus Access Thesis

Degree Name

Master of Science (MS)

Department

Chemistry

First Advisor

Deyang Qu

Second Advisor

Bela Torok

Third Advisor

Jonathan Rochford

Abstract

The carbon gas-diffusion-electrodes were studied in a unique home-made electrochemical cell. The major obstacles for the development of a feasible Li-air system were discussed with a focus on the development of a functional gas-diffusion-electrode in non-aqueous electrolytes and ways of avoiding passivation of the electrodes caused by the deposition of the reduction products (Li2O2, Li2O and Li2CO3). The importance of establishing the 3-phase electrochemical interface in non-aqueous electrolyte is demonstrated by creating air-diffusion paths and an air saturated portion for an aircathode. A mechanism of electrode passivation by the reaction products was proposed. Lithium oxides formed during O2 reduction tend to block small pores, preventing them from further utilization in the electrochemical reaction. On the other hand, Solid reduction products would accumulate inside the large pores during the reduction until the density of oxides becomes high enough to choke-off the mass transfer. Carbon materials with a large percentage of surface area associated with larger pores should be selected to make the gas-diffusion-electrode (GDE) for Li-air battery. For the first time, a near linear relationship between the capacity of GDE in a non-aqueous electrolyte and the average pore diameter was demonstrated, which could be used to estimate the capacity of the GDE quantitatively.

To combat the accumulation of Li oxides on the carbon GDE, the impact of the surface modification of carbon electrode for the discharge capacity has been investigated. More than three times discharge capacity increase was demonstrated through modification of the carbon surface with long-chain hydrophobic molecules. The capacity loss of the Li-air activated carbon cathode was found to be caused by the formation of compact Li oxide layers directly on the surface of the carbon during the oxygen reduction. The layers of Li oxide blocks electrochemical reaction increase the impedance and drops the discharge voltage rapidly. This investigation reveals that the capacity for the (GDE) can be substantially increased, if the activated carbon is modified by attaching long-chain hydrophobic molecules onto the surface. The carbon surface modification significantly delays the formation of the dense lithium oxide layers thereby increasing discharge capacity substantially.

Comments

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