| Direct ethanol fuel cells (DEFC), which promise to be a clean and efficient energy production technology, have recently attracted worldwide attention, primarily because ethanol is a carbon-neutral, sustainable fuel and possesses many unique physicochemical properties including high energy density and ease of transportation, storage as well as handling. However, conventional DEFCs that use acid proton exchange membranes (PEM) and precious metal catalysts result in rather low performance (the state-of-the-art peak power density of the PEM-DEFC is only 30 mW cm-2 at 60°C). The primary objective of this thesis work is to create a proof-of-concept DEFC that uses anion exchange membranes (AEM) as the ion conductor. It is demonstrated that the peak power density of the developed AEM-DEFC with no noble metal catalysts is 3 times higher than the PEM-DEFC at the same operating temperature. It is further shown that the alkaline DEFC with the AEM replaced by a cation exchange membrane (CEM) also yields similar performance, but the CEM-DEFC can be operated stably at higher temperatures (typically 90°C). To understand what the charge carriers in the alkaline DEFCs with different types of membrane are, three different types of membrane, including AEM, CEM, and alkali-doped PBI membrane (APM), are tested; it turns out that the main charge carrier of the alkaline DEFCs is OH- ions, suggesting that the AEM is the most promising membrane for the alkaline DEFC system. The work reported in this thesis further demonstrates that the alkaline fuel cell system can be operated with different liquid fuels, such as ethylene glycol and glucose, and results in much higher performance than the conventional fuel cell system with PEMs does. More significantly, an alkaline-acid hybrid DEFC (AA-DEFC) system that consists of an alkaline anode, a membrane, and an acid cathode is created; it is shown that this hybrid fuel cell yields an open circuit voltage (OCV) of 1.60 V and a peak power density of 240 mW cm -2 at 60°C, which is the highest performance reported in the literature. Further investigations into the effects of design parameters and operating conditions on the performance of the AA-DEFC lead to a tremendous boost in the cell performance: the peak power density is as high as 360 mW -2 at 60°C. Finally, a bi-functional electrode architecture is proposed for the AA-DEFC; it is demonstrated that this new electrode architecture not only alleviates the problem of H2O2 decomposition, but also enhances the mass transport of reactants/products in the fuel cell system.;Keywords: Fuel cell; Alkaline direct ethanol fuel cell; Ion exchange membrane; Mass transport; Electrode architecture. |
