•Performance and degradation of three large-scale SOECs were compared.•Different single cells were assembled in the same stack.•The large-scale SOEC was divided equally into four subparts for post-mortem analysis.•Delamination occurred in the steam and air inlet areas of the LSM and LSC cells.•Honeycomb Ni was observed in the steam and air inlet areas of the LSC cell.Three hydrogen electrode-supported, large-scale solid oxide electrolysis cells (SOECs) with different composite air electrodes, namely LSM–YSZ, LSC–GDC, and LSCF–GDC (LSM, LSC, and LSCF cells, respectively), were compared for performance and degradation in the same three cell stack with an H2O/H2 ratio of 90/10. The initial SOEC performance increased in the following order: LSM, LSCF, and LSC. The three cells were operated at a constant current density of 0.50 A cm−2 for 640 h during hydrogen production at 750 °C. After the operation, the 10 cm × 10 cm large-scale single cell was divided equally into four subparts from the two diagonal parts with the steam or air inlet area and steam or air outlet area for post-mortem analysis. Delamination mainly occurred in the steam and air inlet areas of the LSM and LSC cells, with more severe delamination in the steam inlet area than that in the air inlet area. Agglomeration of Ni in the Ni–YSZ hydrogen electrode was observed in the LSM and LSCF cells, whereas honeycomb Ni was observed in the steam and air inlet areas of the LSC cell.

•SOEC stacks of 1-cell, 2-cell and 30-cell were tested for electrolysis up to 500 h.•The 1-cell, 2-cell and 30-cell stacks obtained efficiencies of 16.1%, 27.2% and 52.7%.•Multi-cell stack obtained higher system efficiency for electrolysis.Steam electrolysis in solid oxide electrolysis cells (SOECs) is considered as an effective method to achieve high-efficiency hydrogen production. In the present investigation, samples of 1-cell, 2-cell and 30-cell SOEC stacks were tested under electrolysis of steam to give a practical evaluation of the SOEC system efficiency of hydrogen production. The samples were tested at 800 °C under various operating conditions up to 500 h without significant degradation, and obtained steam conversion rates of 12.4%, 23% and 82.2% for the 1-cell, 2-cell and 30-cell stacks, respectively. System efficiencies of hydrogen production were calculated for the samples based on their real performance. A maximum efficiency value of 52.7% was achieved in the 30-cell stack.

This study investigates the 0.2% hydrogen sulfide poisoning of Ni/YSZ anode-supported solid oxide fuel cells (SOFCs). The deterioration degrees and recovery extents of the cell current density, cell voltage and operation temperature are monitored. The results of impedance spectroscopy analysis show that hydrogen sulfide poisoning behavior may affect oxygen ion migration and gas diffusion and conversion on the anode side. Microstructural inspection reveals sulfur or sulfide formed on the anode-active area, which accounts for the immediate and severe cell power drop upon the injection of H2S. The nickel sulfide in the anodic functional layer cannot be completely removed after long-term regeneration and thus may be a key factor in the permanent degradation of the cell.

This paper presents a systematical evaluation of the effects of CO2, H2O, CO, N2 and CH4 in the coal syngas on the properties of typical Ni/YSZ anode-supported solid oxide fuel cells (SOFCs). The results show that CO2, H2O, CO, N2 and CH4 have complicated effects on the cell performance and the electrochemical impedance spectra (EIS) analysis reveals the addition of these gases influences electrode processes such as the oxygen ion exchange from YSZ to anode TPBs, the charge transfer at the anode TPBs, gas diffusion and conversion at the anode. Two kinds of mixture gases with different compositions are thus constituted and used as fuel for aging test on two cells at 750 °C. No degradation or carbon deposition is observed for the cell fueled with 40% H2–20% CO–20% H2O–20% CO2 for 360 h while the cell fueled with 50% H2–30% CO–10% H2O–10% CO2 exhibits an abrupt degradation after 50 h due to the severe carbon deposition.