The ethanol oxidation reaction and product distributions in a direct ethanol fuel cell

摘要

A novel online system for quantifying product distributions of ethanol oxidation in direct ethanol fuel cells was developed. This system was shown to have many advantages over previously reported methodologies such as: ease of operation, quick response time, economical, and real time analysis. This system consisted of a non-dispersive infrared CO₂ detector coupled with a flow-through conductivity detector for CO₂ and acetic acid measurements, respectively. The third ethanol oxidation product, acetaldehyde, was shown to be accurately calculated using a Faradaic charge balance. -- The effects of fuel and product crossover were closely examined. It was shown that the use of oxygen at the cathode can lead to an overestimation of ethanol oxidation products, mainly acetic acid, due to ethanol crossing through the membrane and reacting chemically with oxygen at the cathode. Furthermore, it was shown that significant amounts of acetaldehyde produced during ethanol oxidation were lost due to crossover, leading to an underestimation of its yield. To obtain more accurate product distributions, the fuel cell was operated using N₂/H₂ gas at the cathode (which eliminated the chemically oxidized ethanol reaction). To further improve the accuracy of product yields, a “crossover mode” approach to operating a fuel cell was examined. It was found that this method increased selectivity towards complete oxidation (carbon dioxide) by reducing poisoning of the electrode. -- A kinetic and mechanistic study on the ethanol oxidation reaction was also carried out using an electrode stripping technique. It was found that applying a potential in the range where CO is oxidized, followed by allowing the cell to return to open circuit and then re-applying the potential led to significant increases in CO₂ yields. It was found that the CO₂ yield was dependent on the length of the pulse applied, the shorter the pulsing interval, the higher the yield. This suggests that the majority of the CO₂ produced was attributed to the oxidation of CO adsorbates. The time interval between pulses was also examined. It was found that the CO₂ yield was independent of the resting time, suggesting a rapid dissociation of ethanol on the electrode, which supports previous Iiterature findings. -- A Pt-RuSnO₂/C anode catalyst was developed and tested in our system. This catalyst was found to increase both the performance and the selectivity towards CO₂ in comparison to a Pt electrode, which is rarely reported in the literature.