(Plenary) Substrate Effects on the Activity and Stability of Nanoparticulated Electrocatalysts for the H2/O2 Fuel Cell Reactions

Among the hydrogen fuelled systems, the proton exchange membrane (PEMFC) and the alkaline (AFC) fuel cells appear as most adequate candidates for applications on automotive, portable or stationary devices. However, significant efforts still have to be done regarding the limitations introduced by the slow oxygen reduction reaction (ORR) kinetics and the poisoning of the anode by CO or other contaminants, particularly when the hydrogen is produced by the reforming or dehydrogenation of other fuels. In this work, evaluations of the performance and stability of platinum-based catalysts supported on carbon with or without tungsten or molybdenum carbides/oxides were investigated for the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR) in the presence of CO in acid and alkaline electrolytes and in PEM single cells.

Materials have been prepared by several techniques and characterized by high resolution transmission electron microscopy, X-ray diffraction and in situ X-ray absorption near edge structure. Electrochemical investigations for the ORR have been made by cyclic voltammetry and steady state polarization measurements, which have been carried out using a rotating disk electrode setup, with the disk working electrode containing a thin layer of the dispersed catalysts. Investigations of the CO tolerance comprise polymer electrolyte single cell polarization measurements, with the catalysts forming gas diffusion electrodes and the system supplied with pure oxygen in the cathode and hydrogen, with and without CO, in the anode, while OLEMS (on line electrochemical mass spectrometry) is used to follow the formation of CH₄ and/or CO₂ at the anode outlet.

Relevant findings of these investigations are: (i) Pt catalysts supported on metal oxides (Pt-MO_x/C, M = Mo or W) show higher activity for the ORR in acid media, as compared to those of Pt supported in the corresponding metal carbides (Pt-M_xC/C) and that of the carbon supported material (Pt/C). However, electrode cycling experiments evidence that anchoring the Pt on the MO_x/C does not improve the catalyst stability, which is somewhat smaller than that of Pt/C. In alkaline media, results show that W_xC/C-dispersed Ag nanoparticles present reasonable activity for the ORR but in this case there is a mixed participation of carbon, WC and Ag in the catalysis of the reaction that leads to the involvement of 3e^- per O₂ molecule, in contrast to the 4e^- observed for single phase Ag and all Pt-based catalysts considered here; (ii) regarding the HOR, the WC/C-supported Pt catalysts showed a stable electrocatalytic activity for the hydrogen oxidation in the presence of both pure hydrogen and hydrogen containing 100 ppm of carbon monoxide and improved resistance to CO poisoning relative to Pt/C. In the case of a Mo₂C/C-supported Pt, enhanced CO tolerance and stability are observed as compared to PtMo supported on both, Mo₂C/C and carbon, although some migration of Mo species from the anode toward cathode takes place during the cell operation; (iii) a quite satisfactory performance, very close to that obtained with pure hydrogen, was obtained for a PEM single cell with Pt/C electrodes fed with the CO free hydrogen gas effluent coming from an ethanol dehydrogenation catalytic unit (Cu/ZrO₂catalyst bed), after passing through a simple cold condenser. Liquid chromatography showed that the trapped liquid effluent is formed essentially by un-reacted ethanol, ethyl acetate, acetaldehyde and crotonaldehyde, which may contaminate the hydrogen gas effluent causing the observed reduction in the activity of the fuel cell anode.

Acknowledgments: This work has been supported by Fundação de Amparo a Pesquisa do Estado de São Paulo and Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brazil.