Παρασκευή και μελέτη νέων ηλεκτροδίων για χρήση σε κυψελίδες καυσίμου άμεσης τροφοδοσίας με αιθανόλη

Abstract

Direct ethanol fuel cells (DEFCs) are considered as the most promising systems for the conversion of chemical energy to electricity due to a number of advantages of ethanol. Compared to hydrogen and methanol, which are the most widely investigated fuels for proton exchange membrane fuel cells (PEMFCs), ethanol presents a series of advantages as follows: it is easier than hydrogen to handle, transport, store and distribute, due to the fact that it is liquid, while with respect to methanol it has higher energy density and is non-toxic. Moreover, ethanol oxidation produces only the products that are required by nature to recompose ethanol molecules through the photosynthesis process. Therefore, the net carbon dioxide contribution to the environment is negligible. When ethanol is used as the fuel, its desired reaction in DEFCs is the complete oxidation to CO2 and water. However, this process involves 12 electrons transfer per ethanol molecule, leading to many adsorbed intermediates and byproducts during the ethanol oxidation process. The main challenge is the cleavage of C–C bond, which does not easily take place at lower temperatures. The second challenge in direct ethanol fuel cells is to improve the DEFC performance is the ability of ethanol to pass through the membrane, from the anode to the cathode side, which is widely known as ethanol crossover. This thesis focus on the development of new anode electrocatalysts for Direct Ethanol Fuel Cells and to define the main parameters affects the fuel cell efficiency. Specifically in the present thesis, the reaction of biomass-derived ethanol steam reforming for hydrogen rich gas streams production, over a commercial alumina supported palladium catalyst was investigated. In particular, the dependence of the catalytic activity and selectivity on reaction temperature, H2O/EtOH molar ratio and contact time was studied. In order to evaluate the catalytic stability long-term experiments were also performed. The results of this study are published in Applied Catalysis B: Enviromental, 2004. 49;p.135. Moreover, several carbon supported PtSn and PtSnRu catalysts were prepared with different atomic ratios and tested in direct ethanol fuel cells (DEFC) operated at lower temperature (T=90οC). XRD and TEM results indicate that all of these catalysts consist of uniform nano-sized particles of narrow distribution and the average particle sizes are always less than 3.0 nm. As the content of Sn increases, the Pt lattice parameter becomes longer. Single direct ethanol fuel cell tests were used to evaluate the performance of carbon supported PtSn catalysts for ethanol electro-oxidation. It was found that the addition of Sn can enhance the activity towards ethanol electro-oxidation. It was also found that a single DEFC of Pt/Sn atomic ratio 2, Pt1Sn1/C, Pt3Sn2/C, and Pt2Sn1/C shows better performance than those with Pt3Sn1/C and Pt4Sn1/C. But even adopting the least active PtSn catalyst, Pt4Sn1/C, the DEFC also exhibits higher performance than that with the commercial Pt1Ru1/C, which is dominatingly used in PEMFC at present as anode catalyst for both methanol electro-oxidation and CO tolerance.