Modeling and Experimental Studies of Miniature Lithium Air Batteries and Alkaline Direct Ethanol Fuel Cells

Batteries and fuel cells directly convert chemical energy to electricity through controlled electrochemical reactions. Batteries also serve as energy storage devices, while fuel cells rely on a continuous supply of fuel to maintain power output. In this dissertation, modeling and experimental studies on lithium air batteries and alkaline direct ethanol fuel cells are presented. Both technologies can be designed as small-scale electrochemical devices that are suitable for miniature electronics and energy systems. Innovative concepts are presented regarding miniaturization of both technologies, including detailed physical simulation. The lithium air (Li-air) battery is considered a promising candidate for next generation secondary battery technology because of its extremely high theoretical energy density. Its application, however, has been impeded by issues including electrode clogging, electrolyte degradation, low cycling efficiency, and safety concerns. A unique Li-air battery concept is proposed to enhance oxygen supply and alleviate electrode clogging. The proposed flow cell has a specific capacity of 15.5 times higher than that of a conventional Li-air cell. Based on the physical modeling, a multi-layer electrode structure is also proposed which helps to increase cell capacity by 105%. A comprehensive 2D physical model of the battery is developed at the cell-level. Through the deformed mesh technique, the change of electrolyte level in a Li-air coin cell during discharge is tracked. It is found that without considering this effect, a battery model may underestimate cell capacity by up to 22%. The model also includes an air chamber in the computation domain to account for solvent evaporation. For highly volatile solvent-based cells, the chamber size may affect the experimental results significantly. These findings provide direction for further enhancement of battery performance and better design of experiments. Alkaline direct ethanol fuel cells (ADEFC) are considered as a replacement of direct methanol fuel cells. The alkaline environment improves reaction kinetics while ethanol is well regarded for wide availability and low toxicity. Through detailed modeling and experimental studies, it is shown that the costly anion exchange membrane in a conventional ADEFC can be replaced by a much less expensive porous separator without lowering overall cell performance.