Catalysts for the Oxygen Reduction Reaction. Analysis of their Tolerance to Alcohol Crossover.

A.M. CASTRO LUNA; M. ASTEAZARAN; S. BENGIÓ; M.S. MORENO; W.E. TRIACA

Congreso; 65th Meeting of the Internat. Society of Electrochemistry; 2014

Direct alcohol fuel cells (DAFCs) represent an interesting alternative to obtain electricity in a clean an efficient way, potentially valuable to substitute traditional environmentally harmful technologies.Both, methanol and ethanol fuels have high specific energy densities and their thermodynamic oxidation potentials to CO2 are comparable to the equilibrium potential of hydrogen. Moreover, these fuels are easy to handle, store and transport.Portable power sources are one of the most promising applications of passive DAFCs. One requirement in passive DAFCs is to feed the anode compartment with alcohol in high concentration. Unfortunately, the alcohol permeation across the polymer electrolyte membrane (alcohol crossover) causes a considerable loss of the fuel cell efficiency. It has been concluded that the cell voltage loss occurs mostly in the cathode because the oxygen reduction reaction (ORR) and the alcohol oxidation reaction compete for the catalytic sites producing a mixed potential.In order to develop alcohol tolerant cathode catalysts with suitable activity at low temperature, Pt based catalysts have been proposed and tested. Thus, different synthesized binary PtM and ternary PtMRu catalysts, with M = Co or Fe were prepared either via poliol reduction (EG catalysts) or incipient wetness (IW catalysts), the latter followed by a thermal treatment in a reducing atmosphere (N2/H2). All cathode-catalysts were studied to determine the role of the components in enhancing the ORR and, simultaneously, to discourage the alcohol oxidation reaction.The physical characterization of the synthesized materials was accomplished by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS). XPS spectra showed that according to the synthesis methodology the amount of metal oxide (MO) in the catalyst varies. TEM images show well distributed particles on the carbon support with a size between 2-5 nm for EG catalysts. For the electrochemical characterization linear voltammetry (LV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) were employed.By employing electrochemical techniques it has been observed that the IW catalysts have better catalytic activity for the ORR. However, the enhanced activity of IW catalysts is completely lost when the ORR is accomplished in presence of alcohol. It appears that binary or ternary EG catalysts are more alcohol tolerant. The role of MO in improving the tolerance is discussed.