No abstract

The performance and degradation of Solid Oxide Fuel Cells (SOFC) were studied under severe operating conditions. The cells studied were manufactured in a small series by ECN, in the framework of the EU funded CORE‐SOFC project. The cells were of the anode‐supported type with a double layer LSM cathode. They were operated at 750 °C or 850 °C in hydrogen with 5% or 50% water at current densities ranging from 0.25 A cm–2 to 1 A cm–2 for periods of 300 hours or more. The area specific cell resistance, corrected for fuel utilisation, ranged between 0.20 Ω cm2 and 0.34 Ω cm2 at 850 °C and 520 mV, and between 0.51 Ω cm2 and 0.92 Ω cm2 at 750 °C and 520 mV.The degradation of cell performance was found to be low (ranging from 0 to 8%/1,000 hours) at regular operating conditions. Voltage degradation rates of 20 to 40%/1,000 hours were observed under severe operating conditions, depending on the test conditions.Data analysis revealed a critical cell voltage of ca 750 mV, above which the degradation rates were trivial, but below which they were significant.Some cells were also tested using a different procedure to that usually applied at Risø. This gave a different aging behaviour, indicating that the detailed test circumstances may be decisive to the outcome.

The performance of anode‐supported cells with a composite LSM‐YSZ cathode and an LSM current collector was investigated. Over the first 48 hours, after the application of a constant current, the cell voltage was observed to increase by up to 20%. When the current was switched off, the cell resistance increased significantly over the next four days at open circuit conditions. Apparently, at OCV conditions cell passivation occurs. The cell gradually reactivates, once the current is switched on again. Part of this activation / passivation process is fast enough to influence the resistance of the cell during i–V measurements (over less than 1 hour) and a considerable hysteresis is observed in the cell voltage during these measurements.Impedance spectroscopy was used to investigate the activation / passivation process. It was found that the series resistance and the part of the polarisation impedance above approximately 100 Hz were not influenced by the activation / passivation process. The part of the polarisation impedance between 1 and 100 Hz was highly influenced by the activation / passivation process and during cell polarisation this part of the polarisation impedance was up to 40% lower than at open circuit conditions. This frequency range of the spectrum was also sensitive to the oxygen partial pressure at the cathode side, indicating that it is the cathode that activates and passivates.

Long-term strong cathodic polarization experiments of down to-2.4 V vs. E o (O 2) of the Ni-YSZ interface were performed at 900 °C in 97 % H 2 /3 % H 2 O on model electrodes. The Ni-YSZ interface underwent extensive changes and a large affected volume with a complex microstructure and phase distribution resulted. Impedance spectroscopy shows initial decrease but later increase in the series resistance and polarization resistance during the 140-160 h of polarization, and significant inductive behavior. An intermetallic Ni-Zr phase that formed during polarization was preserved when the polarization was kept during cooling, and was identified post-mortem by transmission electron microscopy as Ni 7 Zr 2. ZrO 2 nanoparticles were formed on the Ni-gas surface next to the Ni-YSZ-gas triple phase boundary. Explanations of the observed features are offered based on electron microscopy and impedance spectroscopy.

High temperature electrolysis based on solid oxide electrolysis cells (SOECs) is a promising technology for energy storage and synthetic fuel production. In recent years extensive efforts have been devoted to improving performance and durability of SOEC cells and stacks. Due to historical reasons and the convenience of doing constant current tests, (almost) all the reported SOEC tests have been galvanostatic. In this work, we report test results on two types of SOEC cells operated for electrolysis of steam in potentiostatic mode at 1.29 V. Both cells are Ni/YSZ fuel electrode supported type with different oxygen electrodes. The two cells exhibited different initial performance and different long-term degradation behavior. Detailed impedance analysis indicates that degradation happened mainly at the Ni/YSZ electrode for both cells. Large overpotential on the Ni/YSZ electrode was identified as the main cause of the degradation. Operation strategies were further proposed for electrolysis operation in potentiostatic mode.