Poisoning effects of chlorine on a solid oxide cell operated in co-electrolysis

Although the effects of impurities on the Ni-YSZ electrode of a solid oxide cell in fuel cell operation have been studied, reports on their effects during electrolysis operation are limited. Here, short-term experiments at various current densities and a durability test (1400 h) are performed to investigate the effects of HCl on the Ni-YSZ electrode of a solid oxide electrolysis cell (SOEC) operated in co-electrolysis mode. Without current bias, exposure to 10 ppmv of HCl was tolerated whereas significant degradation was observed when the SOEC was exposed to 5 ppmv of HCl under polarization, mostly related to an increase of the resistance associated with the charge-transfer processes. Prolonged exposure to HCl revealed an initial steady degradation before an increase by a factor of >10 caused by polarization resistance increases, thereby emphasizing the importance of prolonged poisoning tests. Stopping the HCl supply and operating the SOEC in an HCl-free atmosphere allowed for voltage stabilization but no performance recovery was observed. An increased serial resistance indicates that the desorption of Cl can also be detrimental. A low-frequency pseudo-inductive hook appeared in the electrochemical impedance spectra, possibly related to the increase of the charge-transfer processes' resistance and indicating modification to the electrochemical pathways.

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

Stefan Diethelm, Guillaume Jeanmonod, Jan Van Herle

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 A cm−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 A cm−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.

2020

Steam and Co-Electrolysis Sensitivity Analysis on Ni-YSZ Supported Cells

Stefan Diethelm, Giorgio Rinaldi, Jan Van Herle

The present paper investigates the variation in performances in steam electrolysis and co-electrolysis of steam/CO2 on SOEC single cells.In electrolysis mode, a sensitivity analysis was performed looking in particular at the effects of temperature (750°C and 800°C) and inlet gas flows (from 100 to 200 Nml/min) on the performances. I-V characterization and EIS measurements at different current densities were carried out for each sequence. As expected, the increase of temperature has a positive impact on the performances, while the change of fuel flow slightly affects them. In co-electrolysis mode CO2 and H2O content was varied, keeping constant the total molar flow and the hydrogen fraction. Resulting plots demonstrate different ongoing processes favored at high current densities and high inlet CO2 molar fraction. In particular, reverse water-gas-shift reaction plays an important role on the overall balance of the gas species, and it directly depends on temperature and inlet fuel composition.

2015

Stability and performance of tape cast anode supported electrolyte (ASE) cells.

Raphaël Ihringer, Jan Van Herle

Recent progress in our development of anode supported electrolyte (ASE) SOFcells is reported. The cells are fabricated by cocasting of aqueous slurries of NiO/YSZ and 8YSZ, followed by cofiring. A gradient cathode is deposited by screenprinting successive layers of LSM/YSZ composite and lanthanum cobaltite, followed by a 2nd firing step. Full cells show a final thickness of ? 0.25 mm, high flexibility and reasonable warpage. Proper anode current collection is crucial and obtained by cofiring an additional outer layer together with the substrate. Power density can reach 0.2 W/cm2 at 600°C with active cathodes (LSC, short-lived) and >1 W/cm2 at 810°C with the LSM cathode. Stability was tested during 2900 h, giving an average degradation of 3%/1000 h at 0.5 A/cm2 current density. Degradation can be delayed by short-time polarisation at higher current density: e.g. “reactivation” at 0.85 A/cm2 (30 min.) delayed degradation by 300 h.

European Solid Oxide Fuel Cell Forum2002