Electrochemical impedance spectroscopy measurements of solid oxide cells: beyond open circuit voltage conditions

Solid oxide cells offer a variety of electrochemical conversion processes that occur with high efficiency and performance at elevated temperatures (above 400 °C), thus contributing to hydrogen energy initiatives, carbon emission issues, and the generation of high value-added products. These cells are extensively characterized via various electrochemical approaches, including electrochemical impedance spectroscopy (EIS), to gain insight into the nature of electrode and electrolyte materials either under direct cell operation or after long-term stability testing. Although the output parameters of solid oxide cells (e.g., power density and hydrogen production rate) are obtained under conditions far from the open circuit voltage (OCV), EIS analysis is often performed at OCV. Such an analysis may include the evolution of spectra depending on temperature, gas composition, or test time. However, matching the output cell parameters with the EIS data recorded under OCV conditions can be incorrect in most cases because of nonlinear volt–ampere curves within the studied current/voltage range. This nonlinearity can be attributed to the non-monotonic behavior of both the electrode and electrolyte components, which complicates the comparative analysis of data obtained for different electrochemical cells. This review presents details of traditional OCV measurements, highlights salient features of EIS analysis provided far from OCV, and highlights the merits of such unconventional characterization. This review is the first of its kind to address this significant subject, thereby establishing the foundation for a comprehensive electrochemical analysis of solid oxide cells despite their operating principles, design, and materials employed. We are sure that the presented data offer a valuable resource for researchers specializing in electrochemistry, energy conversion, solid-state ionics, and materials science.