The effect of SO<sub>2</sub> on the Ni-YSZ electrode of a solid oxide electrolyzer cell operated in co-electrolysis
Guillaume Jeanmonod, Stefan Diethelm, Jan Van herle
Abstract
Abstract In this work, the effect of sulphur poisoning of the Ni-YSZ electrode of an SOEC operated in co-electrolysis mode was investigated. Short-term tests with exposure up to 5 ppmv of SO 2 were performed at OCV and under polarization ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>0.25</mml:mn> <mml:mtext> </mml:mtext> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">A</mml:mi> <mml:mtext> </mml:mtext> <mml:mi mathvariant="normal">c</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> ). The two-stages degradation pattern observed consisted of an initial fast voltage increase followed by a slower voltage increase similar to that of an SOFC exposed to H 2 S. Electrochemical impedance spectroscopy and the analysis of the distribution of relaxation times showed that both the catalytic and electrochemical reactions were affected by SO 2 . After extended periods in SO 2 -free reactant, only a partial recovery of the performances was observed even when exposure amounted to only 0.5 ppmv of SO 2 independently on the current density. A durability test at a constant polarization of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>0.5</mml:mn> <mml:mtext> </mml:mtext> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">A</mml:mi> <mml:mtext> </mml:mtext> <mml:mi mathvariant="normal">c</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> showed a voltage ‘runaway’ behavior during successive exposures to 1 ppmv and 2 ppmv of SO 2 . This behavior originated from a drastic increase of the serial resistance, which almost completely recovered when the SO 2 supply was cut. This behavior was not observed during exposure to 0.5 ppmv of SO 2 , suggesting that, in these test conditions, the voltage ‘runaway’ behavior could be avoided at a sub-ppmv level. Successive exposure-recovery cycles were found to weaken the SOEC tolerance to SO 2 and a low frequency pseudo-inductive arc was observed in the impedance response during and after the second exposure to SO 2 .