Additional Voltage Loss to Explain an Equilibration Process in Response to a Change in the Anode Gas Using Sm-Doped Ceria Electrolytes

Abstract

The electronic conductivity of Sm-doped ceria is very low in air but increases substantially in H2 gas. We measured the equilibration process using a very thick (6.6 mm) samarium-doped ceria electrolyte in response to a change in the anode gas. For this purpose, Weppner’s approach was used to analyze the drift current and the electron diffusion current throughout the entire electrolyte. The direction of the electron drift current was the same as that of the ionic current. The electron drift current was recycled as the electrons diffused throughout the electrolyte. The open-circuit voltage (OCV) gradually increased to an equilibrium voltage of 0.80 V within 5 min. The time constant was only 1 min. According to Weppner’s approach, the equilibrium time should be much longer than 5 min. It is impossible to calculate the OCV at equilibration in response to a change in the oxygen activity of the anode gas. Assuming a current-independent constant anode voltage loss (approximately 0.35 V), the calculation revealed a significantly improved explanation of the experimental results, both quantitatively and qualitatively. Consequently, both Weppner’s approach and Wagner’s equation can coexist with this method. This proposed voltage loss does not contradict the model for very thin films but is necessary to improve the analysis of the experimental results. A hypothesis was developed to explain this voltage loss in the SDC electrolyte. Furthermore, a current-independent constant anode voltage loss is a general topic in electrochemistry and important to explain physical phenomena not only in the area of solid oxide fuel cells.