Direct Ethanol Fuel Cells: Studies On Ethanol Crossover & MEA Preparation And Their Influence On Single DEFC's Performance

Direct ethanol fuel cells (DEFCs) have sparked more and more interest inrecent years due to the natural availability of bioethanol, its low toxicity andregeneration. However, the power density of DEFCs is far away from thecommercial requirements in comparison with that of direct methanol fuel cellsand H₂-O₂ proton exchange membrane fuel cells. For direct methanol fuel cells,methanol crossover has been obtained attention and much effort has been made toits research. On the other hand, there are few reports about ethanol crossover. Inthe present dissertation, the crossover rates of ethanol through Nafion?-115membranes at different temperatures and different concentrations have beenmeasured with respect to methanol. At the same time, the influence of alcoholcrossover on PtRu-based DAFC's performance and the effect of membraneelectrode assembly (MEA) preparation procedures on single DEFC's performancewere also investigated. Ethanol electro-oxidation mechanism over PtSn/C catalystwas studied by using the techniques of half-cell and single DEFC tests. Moreover,an exergy (available energy) analysis has been conducted on a typical protonexchange membrane fuel cell (PEMFC) using ethanol as the fuel.It was found that ethanol shows a lower crossover rate than methanol throughthe Nafion membrane. Furthermore, it was found that ethanol presents lessnegative effect on the cathode performance due to both its smaller permeabilitythrough Nafion membrane and its slower electrochemical oxidation kinetics overPt/C cathode. On the other hand, Nafion? membrane presents a more swellingdegree in ethanol aqueous solutions, leading to the attenuation and deactivation ofthe DEFC cell performance, which is one of the key technological problems forDEFC's development. MEA preparation procedures presented an obvious effect on ethanolcrossover, the open circuit voltage and the cell performance of single DEFC. Thedecal transfer electrode preparation method provides MEA with a better singleDEFC performance due to the thinner catalyst layer and the better contactbetween the catalyst layer and the electrolyte membrane with respect to theconventional method, even if, the multi-steps involved in this procedureinfluences the surface composition of PtRu anode and the particle size of Pt in thecathode. Based on the ethanol oxidation product distribution in single fuel cell andelectrochemical data, it can be found that tin can greatly enhance theelectrocatalytic activity of Pt to ethanol oxidation and it allows the formation ofacetic acid at lower potentials than over Pt alone. However, most of the ethanoloxidation products are C-C containing substance, from which it could beconcluded that the C-C cleavage is still the challenge for the complete ethanoloxidation. From these results, a mechanism of ethanol electro-oxidation overPtSn/C catalyst was proposed.