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.
A planar solid oxide fuel cell (SOFC) operated with hydrogen at T=1123 K was equipped with an optically transparent anode flow field to apply species concentration measurements by 1D laser Raman scattering. The flow channels had a cross section of 3 mm × 4 mm and a length of 40 mm. The beam from a pulsed high-power frequency-doubled Nd:YAG laser (λ=532 nm) was directed through one channel and the Raman scattered light from different molecular species was imaged onto an intensified CCD camera. The main goal of the study was an assessment of the potential of this experimental configuration for a quantitative determination of local gas concentrations. The paper describes the configuration of the optically accessible SOFC, the laser system and optical setup for 1D Raman spectroscopy as well as the challenges associated with the measurements. Important aspects like laser pulse shaping, signal background and signal quality are addressed. Examples of measured species concentration profiles are presented.
This contribution presents exemplary investigations by surface science methods for the investigation of electrodes and catalyst systems in fuel cells. The aim is to illustrate the information gain which is obtained with the combination of physical analysis even of complex system. Particularly, systems for DMFC application were selected. The examples range from commercially available catalysts and their electrodes to model systems with novel experimental composition. For platinum alloy catalysts on the anode side XPS analysis shows that the non-noble alloy metals are dissolved. On the cathode an agglomeration of the platinum can be observed by electron microscopies, whereby the agglomeration results in a loss of active surface area. The investigated RuSe catalysts are significantly changed due to electrochemical operation, whereby the non active species is dissolved. Investigations of model systems are presented that show that a core-shell catalyst structures help to stabilize nonnoble components.
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