New Anode Material for Hydrogen PEM Fuel Cell: Pt - Cerium Oxide Composite Thin Films Deposited on DWCNT

摘要

Cerium oxide is an attractive material widely used in nowadays technologies. The most important are its well-established catalytic, electronic and optical applications. Ceria- based catalysts are successfully used in commercially produced catalytic converters for CO oxidation and NO reduction, and show significant activity in hydrocarbon reforming. Proton exchange membrane fuel cells (PEMFC) are being developed as an efficient power source for portable systems applications. Recently, anode property of Pt- (Sn)CeO2/carbon has been investigated for development of methanol and hydrogen fed PEMFC. Pt doped CeO2 layers were deposited by rf-magnetron sputtering on double wall carbon nanotube (DWCNT) diffusion layers of a miniaturized polymer membrane fuel cell by using a composite CeO2 – Pt target. Hydrogen/air fuel cell activity measurements normalized to the amount of used Pt revealed specific power of up to 35 W/mg(Pt) relative to 0.1 W/mg(Pt) obtained for a standard PtRu anode. The layer composition was investigated by the laboratory XPS, synchrotron radiation soft X-ray (SX PES at synchrotron Elettra in Italy) and SEM. Cerium 4f level occupation determines the properties of cerium oxide based catalysts in a significant way. The Ce 4f level of cerium oxide was efficiently investigated with the use of soft-X ray resonant photoelectron spectroscopy in the Ce 4d – 4f photoabsorption region. An interaction of ceria with Pt led to a partial Ce~(4+) → Ce~(3+) transition that was observed as a resonance enhancement of the Ce 4f photoemission intensity. Hard X ray photoemission spectroscopy (6 keV) had a high observation depth about 7 nm. Depth resolved measurements, by varying the emission angle or photon energy (SX PES and HX PES), gave information about depth resolved concentration of Pt0, Pt~(2+) and Pt4+ species, respectively. We showed a formation of cerium oxide with completely ionized species Pt~(2+,4+)embedded in the film. Small amount of Pt0 was present on the film surface only. Activity of this new cerium oxide based materials is explained by eminent activity of embedded Pt~(2+,4+) cations. High activity and low cost together with planar deposition techniques make this material particularly promising for fabrication of fuel cells to power mobile systems.