Zehl, G. ; Bogdanoff, P. ; Dorbandt, I. ; Fiechter, S. ; Wippermann, K. ; Hartnig, C.: Carbon supported Ru-Se as methanol tolerant catalysts for DMFC cathodes. Part I: preparation and characterization of catalysts. Journal of Applied Electrochemistry 37 (2007), p. 1475-1484
DOI version (server of publisher)
Carbon supported RuxSeyOz catalyts have been prepared from RuCl3¿xH2O as ruthenium precursor and H2SeO3 and SeCl4 , respectively, as selenium sources. Highly active catalysts were obtained via a multi-step procedure consisting of a CO2 activation prior to the preparation of a highly disperse ruthenium particles catalyst powder that are subsequently modified by selenium precursors. Ultimately, an excess of selenium was removed during a final thermal annealing step at 800°C under forming gas atmosphere. These catalysts are under development for the selective oxygen reduction reaction (ORR) in direct methanol fuel cells (DMFC). The morphology of the catalysts was analyzed by transmission electron microscopy (TEM) and X-ray diffraction (XRD), which shows that the catalysts consist of crystalline ruthenium particles with sizes ranging from 2 to 4 nm exhibiting a good dispersion over the carbonaceous support. The corresponding catalytic activity for oxygen reduction (ORR) was analyzed by cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements The nature of the carbon support used for preparation of RuSe catalysts is of significant importance for the electrocatalytic activity of the final materials. On CO2 activated Black Pearls 2000 supported catalysts gave the highest ORR activity. Selenium stabilizes the ruthenium particles against bulk oxidation and actively contributes to the catalytic activity because after removal of selenium by a heat treatment under nitrogen or hydrogen above 800°C the catalytic activity decreases dramatically. An exceptional property of the carbon supported selenium modified ruthenium particles are not to coalesce up to temperatures of 800°C under inert or reductive conditions. Additional effects of selenium modification are the enhanced stability towards electrochemical oxidation of ruthenium and a lowering of the H2O2 formation in the ORR.