The Effect of Operating Temperature on Galvanostatic Operation of Solid Oxide Electrolysis Cells

Abstract

The purpose of this work is to study the degradation of Nickel Yttrium Stabilized Zirconia (Ni-YSZ) supported Solid Oxide Electrolysis Cells at different temperatures during galvanostatic steam electrolysis.Four long-term experiments (>=700 h) electrolysis tests are carried out in galvanostatic mode (i.e. constant current), at a current density of 1 A/cm2 and testing temperatures of 925, 850, 800 and 750 oC. The cells’ performance and degradation are comprehensively analyzed via operando Electrochemical Impedance Spectroscopy (EIS). To quantify the degradation, EIS data from Nyquist, Bode and Distribution of Relaxation Times (DRT) plots are utilized in conjunction with complex-non-linear-least-square (CNLS) fitting on suitable equivalent circuit models (ECM).The durability tests show that increasing the operational temperature has significantly beneficial effect on the long-term performance of the cells. More specifically, the cells operated at 850 oC and 925 oC not only did not reach 1.3 V during operation but they also have considerable low voltage degradation rates.To highlight the beneficial effect of increasing the operational temperature, one more durability test at 925oC and 1.25 A/cm2 is conducted. It is showed that even at higher current density the cell shows superior performance compared to lower temperatures and lower current densities.The results obtained from the CNLS fitting of the EIS data revealed that the observed degradation is attributed mainly to the increase of fuel electrode’s resistance and the corresponding increase of fuel electrode’s overpotential. At higher temperatures, the increase of fuel electrode’s resistance and overpotential is significantly lower during operation. The data from the CNLS fitting are used in order to approximate the time evolution of the different overpotentials of the cell.In addition, the first and last EIS measurements under current of each of the durability tests are used for fitting to an ECM which contains Transmission Line Model (TLM) elements. The physical parameters obtained by the TLM fitting are compared with the results of the microstructure analysis. More specifically, post-mortem analysis of the cells using Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS) provide extra info about the microstructure changes in the Ni-YSZ and the depleted layer of Ni close to the electrode/electrolyte interface.The findings of this work highlight the significance of preserving a low fuel electrode overpotential during SOEC operation by increasing the operational temperature and therefore, by improving the reaction kinetics. It is suggested that Ni coarsening does not have a detrimental effect on long-term performance and that the overpotential-driven degradation mechanisms, such as Ni migration, mainly determine the life time of a SOEC.