Product distributions and efficiencies for ethanol oxidation in proton exchange membrane electrolysis cells

Abstract

Ethanol is an attractive fuel for direct alcohol fuel cells (DAFCs). In comparison with other organic fuels, ethanol has a high energy density. Therefore, direct ethanol fuel cells (DEFCs) are considered to be highly attractive power sources for electronic devices and vehicles. In addition, ethanol can be oxidized in an ethanol electrolysis cell (EEC) to produce hydrogen for use in fuel cells. Although ethanol has a high energy density and DEFCs have a high theoretical efficiency (98%), these are based on complete oxidation of ethanol to CO₂, while the main products from DEFCs and EECs are acetic acid and acetaldehyde. A good understanding of what happens during ethanol oxidation in fuel cell hardware is therefore a crucial step in the evolution of these technologies. It is particularly important in the development of new catalysts to improve cell efficiencies and performances by facilitating the complete oxidation of ethanol. The methods reported here provide information on the efficiency and product distribution for ethanol oxidation in a DEFC or EEC. They are based on polymer electrolyte membrane (PEM) fuel cell technology. In comparison with those reported in the literature, our methodologies are shown to have advantages over them by detecting the fuel itself and reaction products from both the anode and cathode exhausts. The amounts of ethanol consumed and acetic acid and acetaldehyde produced were determined by proton NMR spectroscopy while CO₂ was measured with a non-dispersive infrared CO₂ monitor. The efficiencies of these cells are dependent on the cell potential, crossover of ethanol, and stoichiometry of the ethanol oxidation reaction (i.e. the average number of electrons transferred per ethanol molecule). The stoichiometry of the EOR (ethanol oxidation reaction) was determined by using different methods in this work: an electrochemical method, analysis of the amount of ethanol consumed (ΔC) and from the product distribution (faradaic and chemical). It was found that the results from these methods were in a good agreement. In addition, the effects of fuel and product crossover were closely examined. It was shown that analysis of only the anode exhaust solution leads to an underestimation of ethanol and products due to crossover through the membrane to the cathode. To obtain accurate product distributions, the anode and cathode exhausts were combined. In addition, the chemical reaction between ethanol and oxygen that occurs in a DEFC was avoided by making measurement in an EEC with N₂ gas at the cathode. The stoichiometry, efficiency, and product distribution for ethanol electrolysis in fuel cell hardware has been determined at 80°C for various anodes prepared with commercial Pt/C, PtRu/C, and PtSn/C catalysts. Also, synergetic effects between these catalysts were studied by using mixed and bilayer electrodes. It was found that bilayer electrodes increased the overall efficiency of the cell by increasing the faradaic efficiency while maintaining high potential efficiency. An octahedral PtNi catalyst was prepared by using a literature method and tested in our system. In comparison to a Pt, this catalyst was shown to increase selectivity towards complete oxidation (to carbon dioxide) at low potentials and thereby increase efficiency. These results are contrary to those reported in the literature for this catalyst in a conventional electrochemical cell, and demonstrate the importance of the new methodologies in the evaluation and study of new catalysts for ethanol oxidation.