Nanostructured Model Electrodes for Studies of Fuel Cell Reactions
Abstract: There is currently a general need for alternative and sustainable energy systems. As a part in such a system, the polymer electrolyte fuel cell (PEMFC) offers a possibility for efficient and emission free energy conversion. Although fuel cells have several advantages over conventional technologies, several issues are currently preventing a large scale practical and commercial break through. At a system level, the hurdles are connected with high cost and limited life-time. For the system components, a major part of the problems is related to the electrodes. PEMFC electrodes are porous structures that consist of nanometer sized platinum particles supported on carbon structures, which are mixed with an ionomer. In order to optimize the fuel cell performance, it is essential to understand the processes that occur on the electrode surfaces. The structural complexity of real electrodes renders, however, fundamental studies of its function difficult. One possibility to overcome this issue is to use well-defined nanostructured model electrodes. In this thesis, a series of different model systems have been designed, fabricated, characterized, and evaluated. The model electrodes range from two dimensional structures, manufactured by lithography techniques, to more realistic systems prepared on conventional fuel cell materials. Several new methods for the preparation of controlled model electrodes were developed and demonstrated. The performance of these nanofabricated catalysts were evaluated in half-cell setups or in single cell fuel cells. The different model systems enable specific and selected aspects of the real system to be analyzed. Questions that were addressed include reduction and optimization of platinum use, mechanistic investigations of specific reactions, and electrode degradation. Single cell fuel cell experiments were employed to characterize the activity and stability of platinum upon introduction of a second material. Whith the use of model electrodes it was possible to determine the mechanism and kinetic parameters for the hydrogen oxidation reaction, which has been difficult to deduce with traditional methods. Arrays of platinum nanodisks measured in half-cell were used to illustrate and characterize the importance of mass transport and reactant intermediates for several fuel cell relevant reactions. It was, for example, proven that the oxygen reduction reaction proceeds via a serial pathway with hydrogen peroxide as an intermediate species. Electrode degradation was analyzed with thin-film model electrodes using electrochemical quartz crystal microbalance. This methodology enabled direct measurements of mass changes caused by platinum dissolution and carbon oxidation.
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