Degradation mechanisms of SOFC anodes in coal gas containing phosphorus

The interaction of phosphorus in synthetic coal gas with the nickel-based anode of solid oxide fuel cells has been investigated. Tests with both anode-supported and electrolyte-supported button cells were performed at 700 to 800 °C in synthetic coal gas containing 0.5 to 10 ppm phosphorus, which was...

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Bibliographic Details
Published in:Solid state ionics 2010-03, Vol.181 (8), p.430-440
Main Authors: Marina, O.A., Coyle, C.A., Thomsen, E.C., Edwards, D.J., Coffey, G.W., Pederson, L.R.
Format: Article
Language:English
Subjects:
01 COAL, LIGNITE, AND PEAT
30 DIRECT ENERGY CONVERSION
ADSORPTION
ANODES
COAL GAS
Contaminant
Degradation
DIFFUSION
Electrolytic cells
Environmental Molecular Sciences Laboratory
FRACTURES
FUEL CELLS
IMPEDANCE
Ni/YSZ anode
NICKEL
Nickel phosphide
NICKEL PHOSPHIDES
Nickel phosphorus interaction
Phosphides
PHOSPHORUS
SOFC
SOLID OXIDE FUEL CELLS
SPECTROSCOPY
Surface chemistry
THERMODYNAMICS
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Summary:The interaction of phosphorus in synthetic coal gas with the nickel-based anode of solid oxide fuel cells has been investigated. Tests with both anode-supported and electrolyte-supported button cells were performed at 700 to 800 °C in synthetic coal gas containing 0.5 to 10 ppm phosphorus, which was introduced as phosphine. Two primary modes of degradation were observed. The most obvious was the formation of a series of bulk nickel phosphide phases, of which Ni 3P, Ni 5P 2, Ni 12P 5 and Ni 2P were identified. Phosphorus was essentially completely captured by the anode, forming a sharp boundary between converted and unconverted anode portions. These products partially coalesced into large grains, which eventually affected electronic percolation through the anode support. From thermodynamic calculations, formation of the first binary nickel phosphide phase is possible at phosphorus concentrations < 1 ppb in coal gas at typical fuel cell operating temperatures. A second mode of degradation is attributed to surface diffusion of phosphorus to the active anode/electrolyte interface to form an adsorption layer. Direct evidence for the presence of such an adsorption layer on nickel was obtained by surface spectroscopies on fracture surfaces. Further, cell performance losses were observed well before the entire anode was converted to bulk nickel phosphide. Impedance spectroscopy revealed that these losses were primarily due to growth in electrodic resistance, whereas large ohmic increases were visible when the entire anode was converted to nickel phosphide phases. The rate of resistance growth for anode-supported cells showed a low dependence on phosphorus concentration, attributed to phosphorus activity control within the anode by bulk nickel phosphide products.
ISSN:0167-2738
1872-7689
DOI:10.1016/j.ssi.2010.01.018