Catalyst layers contamination

This chapter has examined a variety of EIS applications in PEMFCs, including optimization of MEA structure, ionic conductivity studies of the catalyst layer, fuel cell contamination, fuel cell stacks, localized impedance, and EIS at high temperatures, and in DMFCs, including ex situ methanol oxidation, and in situ anode and cathode reactions. These materials therefore cover most aspects of PEMFCs and DMFCs. It is hoped that this chapter will provide a fundamental understanding of EIS applications in PEMFC and DMFC research, and will help fuel cell researchers to further understand PEMFC and DMFC processes. 

It must be appreciated, that surfaces of Nb, Ta, V and Zr will be immediately reoxidized or otherwise contaminated upon re-exposure to air, albeit with thitmer layers (or in the limit a monolayer) relative to thick native films often encountered before abrasion. As a minimum, monolayer adsorption of contaminants is almost certainly assured in all but the best ultra-high vacuum (< 1 x torr or <1 X 10 Pa) or in systems which simultaneously sputter substrate surfaces with argon or other inert ion during catalyst deposition. Some interfacial impurities between the substrate and the catalyst layer are tolerated in practice. However, the state of the substrate surface immediately prior to catalyst deposition is critical for wetting and adherence of the catalyst layers and for prevention of delamination. Theoretical flux maxima will not be achieved if thick impurity layers at the cata-lyst/substrate interface hinder hydrogen diffusion. 

Certain performance losses of fuel cells during steady-state operation can be fully or partially recovered by stopping and then restarting the life test. These recoverable losses are associated to reversible phenomena, such as cathode catalyst surface oxidation, cell dehydration or incomplete water removal from the catalyst or diffusion layers [85]. Other changes are irreversible and lead to unrecoverable performance losses, such as the decrease in the ECSA of catalysts, cathode contamination with ruthenium, membrane degradation, and delamination of the catalyst layers. 

Whatever preparation procedure is used to produce these non-noble metal catalysts, any excess metal ions, which cannot be coordinated to the carbon support, will aggregate and form metal and/or metal carbide particles that will eventually be surrounded with a carbonaceous envelope during the heat treatment at high temperature. These surrounded particles have no catalytic activity for ORR and do not seem to contaminate with metal ions the membrane and the ionomer in the catalyst layer during fuel cell stability tests. 

Structural changes in PEM and catalyst layers due to platinum oxidation or catalyst contamination under open-circuit conditions On/off cyclic operation under different humid conditions Effect of hygro-thermal cycle on membrane stresses Water uptake effect on cyclic stress and dimensional change, hydrogen crossover... 

Catalyst/catalyst layer Potential cycles acid washing elevated temperature fuel or oxidant contaminates... 

Based on the above general model, Shi et al. [33] has proposed a transient kinetic model to describe the contamination of the PEM anode catalyst layer by H2S present in the fuel feed stream. Figure 6.5 shows flie model-predicted cell voltages in comparison with experimental results. It can be seen that their model provides an excellent fit with the experimental results. 

Figure 6.11. Schematic of air contaminants adsorbing on the catalyst layer [34]. (Reprinted from Journal of Power Sources, 166(1), ling F, Hon M, Shi W, Fu J, Yu H, Ming P, et al.. The effect of ambient contamination on PEMFC performance, 172-6, 2007, with permission from Elsevier.)...

At the cathode catalyst layer, the common contaminants include SO, NO, H2S, NH3, VOCs, and ozone. A trace amount of SQc in air can cause a significant performance decrease. Increases in SO concentration accelerate the degradation. This degradation is due to the adsorbed sulfur on the Pt surface produced from SO reduction, which not only poisons the catalyst but also changes the ORR mechanism. The fuel cell performance is only partly recovered after SO contamination. contamination of the cathode catalyst is also concentration-... 

Electrocatalytic reactions occur on catalyst surfaces. The catalyst surface structure and chemically bonded or physically absorbed substances on the catalyst surface exert strong influences on catalyst activity and efficiency. X-ray photoelectron spectroscopy (XPS) (also known as electron spectroscopy for chemical analysis (ESCA), auger emission spectroscopy (AES), or auger analysis) is a failure analysis technique used to identify elements present on the surface of the sample. For instance, this can be used to identify Pt and carbon surface chemical species that may present histories of chemical reactions or contamination in the catalyst layer. AES and XPS can also provide depth profiles of element analysis. Wang et al. [41] studied XPS spectra of carbon and Pt before and after fuel cell operation. They observed a significant increase in O Is peak value for each oxidized carbon support, the result of a higher surface oxide content in the support surface due to electrochemical oxidation. However, sample preparation in AES and XPS analysis is critical because these methods are very sensitive to a trace amount of contaminants on sample surfaces, and detect as little as 2-10 atoms on the sample surface. 

Anode Catalyst Layer Degradation Caused by Contamination... 

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