Faradaic efficiencies for methanol oxidation in proton-exchange membrane electrolysis and fuel cells with various anode catalysts

Abstract

The energy efficiency of a direct methanol fuel cell (DMFC) is dependent on the cell voltage, crossover of oxygen and/or methanol through the polymer electrolyte membrane, and stoichiometry of the methanol oxidation reaction (i.e., the average number of electrons transferred per methanol molecule (nav). The stoichiometry is determined by the product distribution (carbon dioxide: formic acid: formaldehyde) and the amount of methanol consumed. The influence of crossover is investigated by using air and N2 at the cathode. Accurate determination of nav requires analysis of the methanol and products from both the anode and cathode and quantitative determination of the consumption of methanol. In this work, nav values obtained from the analysis of the cell exhausts by proton NMR and infrared spectrometry are compared with values obtained from a simple electrochemical method based on the dependence of the current on the flow rate of the methanol solution. The methodology presented here provides a comprehensive evaluation of methanol oxidation, along with a full determination of the methanol consumption and hence the fuel efficiency of the cell. The methods are suitable for rapid evaluation of catalysts in fuel cell hardware with low-cost online sensors for determination of product distributions.