Physical and electrochemical characterization of catalysts for oxygen reduction in fuel cells

Abstract

The cathode catalysts in low temperature fuel cells are associated with major cell efficiency losses, because of kinetic limitations of the oxygen reduction reaction. Additionally, methanol oxidation at the cathode leads to significant lowering of the efficiency in direct methanol fuel cells, which can be alleviated by use of methanoltolerant catalysts. In this work, alternative carbon-supported platinum-alloy catalysts were investigated by physical methods. Second, methanol-tolerant rutheniumselenide catalysts were characterized by physical and electrochemical methods. Besides V-i characteristics and electrochemical impedance spectroscopy as electrochemical methods, physical methods such as X-ray photoelectron spectroscopy, nitrogen adsorption, porosimetry by mercury intrusion and temperature programmed reduction are used to characterize the catalysts. The electrochemical characterization yields information about properties and behavior of the catalyst. In contrast to platinum a significantly different hydrophobic behavior of the RuSe/C catalysts is found. Low open circuit voltage values measured for RuSe/ C indicate an effect on both electrodes. The anode reaction was also influenced by the different cathode catalysts. As a result of the formation of H 2 O 2 at the cathode, which passes through the membrane from cathode to anode side, a mixed anode potential is formed. By comparing RuSe/C catalysts before and after electrochemical stressing, changes of the catalysts are determined. Postmortem surface analysis (by X-ray photoelectron spectroscopy) revealed that catalyst composition and MEA structure changed during electrochemical stressing. During fuel cell operation selenium oxide is removed from the surface of the catalysts to a large extent. Additionally, a segregation effect of selenium in RuSe to the surface is identified.

Se-modified Ru nanoparticles as ORR catalysts

“…Although the difference was not as large as expected, the Se/Ru(aq) catalyst was indeed shown to be superior to Ru black in DMFC testing. The current densities achieved by this Se/Ru catalyst in DMFC testing are higher than those in any other reports of Se/Ru-type catalysts tested under similar conditions, to the best of our knowledge [6,14,[18][19][20].…”

Aqueous-based synthesis of ruthenium–selenium catalyst for oxygen reduction reaction

“…These results are consistent with those reported for Se/Ru(Se)/C catalysts but still lower than those reported for Pt. 15,38,39 Wipperman et al have compared the polarization curves obtained in DMFC using Se/Ru(Se)/C catalyst with different carbon support. 16 They reported 53 and 45 mA cm À2 at 0.40 V vs. RHE for Vulcan XC72 and BP2000, respectively.…”

Se-modified Ru nanoparticles as ORR catalysts – Part 1: Synthesis and analysis by RRDE and in PEFCs

“…By first examining the performance of the Se/Ru catalyst for the ORR in the absence of methanol in part 1, we can better separate the effect of the catalyst's ORR activity from the effect of its methanol tolerance on the fuel cell performance in part 2. Although there have been a few reports that have examined the performance of other chalcogenides at the hydrogen-air cathode [30,31] and at the DMFC cathode [32][33][34][35][36], none have specifically demonstrated the conditions under which such catalysts perform better than pure Ru or, in the case of DMFCs, pure Pt catalysts. The former comparison to Ru is important for fundamental understanding, and the latter comparison to Pt is necessary to define the conditions of technological relevance for Ru-based catalysts.…”