Phosphorus Poisoning of Ni-Cermet Anodes in Solid Oxide Fuel Cells

Abstract

Chemical degradation of anodes in solid oxide fuel cells ͑SOFCs͒ induced by phosphorous impurities has been investigated by thermochemical calculations, electrochemical characterization, and microstructural analysis. Thermochemical calculations indicate that Ni phosphides ͑Ni m P n ͒ such as Ni 5 P 2 are readily formed under SOFC operational conditions even in the case where the concentration of phosphorous impurities is as low as ppb level. Phosphorous impurities in fuels resulted in the formation of the secondary phase Ni phosphides during operation especially at an operating temperature ϳ1000°C. The formation of Ni phosphides and change in anode microstructure were confirmed by using a field-emission-scanning electron microscope coupled with an energy dispersive X-ray analyzer, scanning transmission electron microscope, and X-ray diffraction, leading to the fatal cell performance degradation in the manner that anodic overpotential increased, current incorporation was obstructed, and internal fuel reforming reaction was deactivated.Solid oxide fuel cells ͑SOFCs͒ are considered to be promising power generation systems due to their potential advantages with respect to high energy conversion efficiency, environmental compatibility, and/or multifuel capability. In particular, one of the most appealing benefits over other types of fuel cells is fuel flexibility, that is, a wide variety of fuels such as natural gas, coal-derived syngas, and biogas can be applied as SOFC fuels with a simple prereforming process or even without prior reforming, thus enabling us to use the existing energy sources more effectively and to decrease operational costs by simplifying the system, e.g., omitting high grade purifying equipments. Nevertheless, various kinds of fuel impurities are contained in practical fuels and are supplied in SOFC systems, and hence, it is likely that these minor constituents could cause fatal degradation in cell performance or affect overall SOFC system durability. Furthermore, gaseous impurities can be volatilized from system components while exposed at high temperatures for a long period of time up to a decade. Such impurities may often be contained in low cost raw materials of SOFC components.Many research efforts have been currently directed toward studying the poisoning effect of H 2 S on SOFC anodes. Although various kinds of impurities are contained in practical fuels besides H 2 S, little attention has been paid to the poisoning effects by other impurities. Because several challenges including improved performance, durability, and lower cost have yet to be resolved toward future SOFC commercialization, a deeper understanding of degradation mechanisms and concentration threshold in SOFC fuels for each specific impurity is essential to achieve adequate system durability and reliability as well as cost reduction.Although cell performance degradation takes place upon introducing H 2 S in SOFC fuels, 1-5 it was verified that poisoning was a reversible process under a H 2 S concentration of ϳ5 ppm. Once H 2 ...

Mass Spectrometry of SOFC Fuel Mixtures Containing Phosphine

“…As in previous studies, [3][4][5][6][7][8][9][10][11][12][13] this coating yields XRD spectra consistent with nickel phosphide phases.…”

“…The experimental observations above disagree with the thermochemical calculations of Haga et al 12 In their calculations, an equilibrium mixture obtained from 20% hydrogen with 3% water (22 torr) and 5 ppm phosphorous species predicts that PH 3 is the dominant species from 400 to 800…”

“…Potential reaction products of PH 3 include P 2 (mass 62), HPO (mass 64), HPO 2 (mass 80) and HPO 3 (mass 96). 12,16 These products are not observed in the preceding experiments. Other possible products include phosphorus oxides (P 2 O 5 ) which, if volatile, are beyond the mass range of the mass spectrometer.…”

“…• C or 400 • C. 12 Attempts to remove solid contaminants from the alumina or quartz by water or alcohol washes reduce the mass 34 signal but do not eliminate it. The PH 3 signal is sensitive to the partial pressure of H 2 as well as the temperature.…”

“…• C, 12 to form various phosphorus oxides depending on the oxygen partial pressure, 15 and to form HPO 2 and HPO 3 in the presence of water. 16 Using an effusion beam source at low pressure, Bao et al determined that phosphine was stable at 995 K in dry argon and that HPO 2 and HPO 3 were detected in argon with 20% water.…”