Pt free metal and metal oxides electrocatalysts for the Oxygen Reduction Reaction in alkaline DAFC

Direct alcohol fuel cells (DAFCs) represent a promising energy conversion device for mobile and stationary applications. In comparison with H2-based fuel cells, the alcohol cross-over may represent a limiting factor for their technological success since the alcohol can permeate the polymeric electrolyte and be directly oxidized at the cathode. This short-circuit implies not only an energy loss but may also result in the poisoning of the ORR electrocatalyst by CO and other products of alcohol oxidation. In parallel the recent improvement of anion membranes, that allow for alkaline operating conditions with predictable advantages in terms of energy yield, makes the development of alkaline DAFCs competitive with the acid ones. In this work we studied metal (Ag/C) synthetized by the wet method and metal oxides synthetized by the sol-gel technique (IrO2-SnO2), as possible cathode materials in alkaline DAFCs. The investigation is carried out using potentiodynamic techniques and the Rotating Ring Disk Electrode (RRDE) as support. The electrocatalytic powders are glued to the support with thin layer of Tokuyama anion exchange membrane. The influence of alcohol cross-over on the oxygen reduction reaction (ORR) kinetics was also considered. The use of RRDE allow to obtain important information about the reaction mechanism since it is possible quantify inter alia the amount of hydrogen peroxide produced in a broad range of potentials. A comparison study of different electrode materials in term of kinetic current, amount of hydrogen peroxide produced and the potential of the beginning the ORR was carried out. A interesting dependence of morphology and size of the Ag nanoparticles and of the iridium content (15-100%mol) on the ORR activity is observed. The influence of the alcohol cross-over on the oxygen reduction reaction (ORR) kinetic is also presented and discussed. Acknowledgements: The financial contributions of PRIN 2008 – “Pt-free electrocatalysts for direct alcohol fuel cells.” 2008N7CYL5 and PUR-Università degli Studi di Milano (2009 – 2010) funds are gratefully acknowledged.