The Fate of Phosphine in SOFC Fuels

Phosphine (PH₃) is present in part-per-million levels of coal syngas, a potential fuel for SOFCs.  Previous studies have shown that the SOFC loses power and ultimately fails during prolonged exposure to phosphine in wet and dry fuels.  The presence of nickel phosphide phases on the anode suggests that phosphine survives the operating conditions of the SOFC fuel.  In this study, a mass spectrometer was used to analyze fuel gases after passing the gases through a heated tube or after passing the fuel gases across the anode of an operating SOFC.  Phosphine appears to react with residual oxygen at intermediate temperatures of 400-600ºC in dry (P(H₂O) < 1 torr) hydrogen, but does not react above 600ºC and consequently reaches the anode.  At higher water pressures (5-20 torr), phosphine reacts at temperatures above 400ºC.  Phosphine is detected in the exhaust gas of a SOFC using dry hydrogen while at open circuit, and disappears from the exhaust gas during current flow.  The inverse correlation between water content of the exhaust gas and the phosphine concentration suggests that phosphine is reacting with electrochemically generated water.  No new compounds containing phosphorus were detected in the m/z range of 45-100, eliminating HPO, HPO₂ and HPO₃ as possible reaction products.  There is evidence for the presence of nonvolatile products of phosphine in the supporting tubes after prolonged exposure to phosphine.  These products can generate measurable amounts of phosphine at 800ºC in dry hydrogen initially free of phosphine.