The effect of SO₂ on the Ni-YSZ electrode of a solid oxide electrolyzer cell operated in co-electrolysis

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 SO₂ were performed at OCV and under polarization (0.25{Acm}⁻²). 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 H₂S. Electrochemical impedance spectroscopy and the analysis of the distribution of relaxation times showed that both the catalytic and electrochemical reactions were affected by SO₂. After extended periods in SO₂-free reactant, only a partial recovery of the performances was observed even when exposure amounted to only 0.5 ppmv of SO₂ independently on the current density. A durability test at a constant polarization of 0.5{Acm}⁻² showed a voltage ‘runaway’ behavior during successive exposures to 1 ppmv and 2 ppmv of SO₂. This behavior originated from a drastic increase of the serial resistance, which almost completely recovered when the SO₂ supply was cut. This behavior was not observed during exposure to 0.5 ppmv of SO₂, 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 SO₂ and a low frequency pseudo-inductive arc was observed in the impedance response during and after the second exposure to SO₂.