Improving and Understanding Direct Methanol Fuel Cell (DMFC) Performance Public
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Direct methanol fuel cell (DMFC) is considered as a highly promising power source. It is based on polymer electrolytes membrane (PEM) fuel cell technology. It posses a number of advantages such as a liquid fuel, quick refueling, low cost of methanol and the compact cell, design making it suitable for various potential applications including stationary and portable applications. DMFCs are also environmentally friendly. Although carbon dioxide is produced, there is no production of sulfur or nitrogen oxides. The development of commercial DMFCs has nevertheless been hindered by some important issues. The most important are the low power density caused by the slow electrochemical methanol oxidation at the anode and methanol crossover through PEM, which is responsible for inhibiting the activity of the cathode catalyst as well. With the eventual goal of improving the overall performance of the DMFC, this study has been concerned with an investigation of the issues and effect of various parameters on its performance. First of all, the electrode preparation methodology and the effect of the catalyst were investigated. The most efficient membrane electrode assembly (MEA) was prepared with Pt/Ru black at anode and Pt black cathode on either side of a Nafion 117 membrane. Performance was however limited by current oscillations observed at low cell voltage and high current density attributed to carbon dioxide removal. Consequently, the effect of flow rate was investigated. Higher flow rates eliminated these oscillations. Then attention was focused on the management of the two-phase flow that occurs in the diffusion layer of the electrode as well as in the anode bipolar plate flow channels. Removal of carbon dioxide formed during methanol oxidation was thus found to be an important issue in DMFC. There is a competition between methanol diffusion to the catalyst layer and CO2 removal in the opposite direction. The two fluxes needed to be balanced in order to optimize performance. To accomplish this, the ratio of hydrophilic and hydrophobic pores respectively formed in the catalytic layer by Nafion and PTFE (Teflon) was altered. It also had an effect on crossover. The effect of a barrier layer was investigated to reduce crossover. Finally, zirconia and silica nano-composite membranes were tested instead of Nafion and found to reduce crossover. Developing a good understanding of what happens on the catalyst surface is important to develop a strategy on how improve DMFC performance. Thus is why a dynamic model based on a simplified mechanism for methanol electro-oxidation reaction was developed. It shows, amongst other insights, how the intermediate species coverage evolves with time. The mechanism was however too simple to provide an idea of which poisoning species are formed on the catalyst surface. A more exhaustive mechanism is thus being developed using Reaction Route analysis.
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