Carbon supported Ru–Se as methanol tolerant catalysts for DMFC cathodes. Part I: preparation and characterization of catalysts

Abstract

Carbon supported Ru x Se y O z catalysts were prepared from Ru 3 (CO) 12 and RuCl 3 Á xH 2 O as ruthenium precursors and H 2 SeO 3 and SeCl 4 as the selenium sources. Highly active catalysts for the oxygen reduction reaction (ORR) in direct methanol fuel cells (DMFC) were obtained via a multi-step preparation procedure consisting of a CO 2 -activation of the carbon support prior to the preparation of a highly disperse Ru particles catalyst powder that is subsequently modified by Se. Ultimately, an excess of Se was removed during a final thermal annealing step at 800°C under forming gas atmosphere. 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 Ru-particles with sizes ranging from 2 to 4 nm exhibiting a good dispersion over the carbonaceous support. The corresponding catalytic activity in the process of oxygen reduction was analyzed by cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements. The nature of the carbon support used for the preparation of RuSe cathode catalysts is of significant importance for the activity of the final materials. Catalysts supported on CO 2 -activated Black Pearls 2000 gave the highest ORR-activity. Se stabilizes the Ru-particles against bulk oxidation and actively contributes to the catalytic activity. An exceptional property of the carbon supported Ru-particles modified with Se is their resistance to coalescence up to temperatures of 800°C under inert or reducing conditions. Additional effects of Se-modification are the enhanced stability towards electrochemical oxidation of Ru and a lowering of the H 2 O 2 formation in the ORR.

Relevance of the synthesis route of Se-modified Ru/C as methanol tolerant electrocatalysts for the oxygen reduction reaction

“…The Se concentration was found to increase with the increasing acid concentration; recording a maximum value of Se (Ru:Se 1:0.27) for 1.25 × 10 −1 M H 2 SeO 3 as shown in Table 1. This behavior has been observed by other authors [25]. Fig.…”

“…In agreement with Zehl et al [25], the performance of Se-Ru in the ORR is related to the existence of an optimum concentration of Se on the Ru surface. At such a concentration value, a particular chemical structure favourable for the ORR is generated.…”

Reductive annealing for generating Se doped 20 wt% Ru/C cathode catalysts for the oxygen reduction reaction

“…5c). This corroborates with the observed substantial reduction in the oxidation of ruthenium nano-particles modified with higher amounts of selenium [19,20], which coincides with the HRTEM results, revealing a complete transformation of the Ru-surface into a crystalline Ru x Se y compound. The kinetic current density of this catalyst was found to be nearly the same as for the sample without selenium modification (Table 2).…”

“…Thus, the current result once more points towards the existence of an optimum amount of selenium to modify the surface of the ruthenium nanoparticles. Investigations towards the optimization of the Ru:Se ratio, already reported [19]. The signal ratios of the Si L a -and Ru L a -lines were evaluated …”

Methanol‐Tolerant Cathode Catalysts for DMFC

“…The intermediate cluster compound Ru 4 Se 2 (CO) 11 changes continuously with the reaction time to the novel cluster-like material Ru x Se y . Following alternative reaction processes other workers have succeeded to obtain a fairly good distribution of chalcogenide-ruthenium nanoparticles on carbon support [119,120]. Following alternative reaction processes other workers have succeeded to obtain a fairly good distribution of chalcogenide-ruthenium nanoparticles on carbon support [119,120].…”