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Electrochemical impedance spectroscopy (EIS) is a powerful tool for investigating electrochemical systems such as fuel-cells and batteries. But because of the overlapping of processes due to similar time constants, it is necessary to understand the system well to set up a correct equivalent circuit for analysis of the measured spectra. Distribution of relaxation times (DRT) offers a model-free approach for impedance analysis. In this work DRT was used to analyze a Ni/Co3O4 gas diffusion electrode (GDE) for metal-air batteries. To identify the corresponding process for the identified peaks temperature, current density, gas composition, electrolyte concentration and electrode material composition were varied. In total five processes could be identified. Four were observed both during oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). A fifth could only be recognized during OER. In particular the charge transfer coupled with an adsorption process and the porous structure could be identified. Furthermore, it could be shown that having a bimodal pore size distribution, consisting of two different materials, the correct calculation of the DRT spectra is inhibited, resulting in two peaks instead of the characteristic number of peaks that are getting smaller towards smaller time constants.

To optimize lithium-ion batteries it is important to understand the formation of the solid electrolyte interphase (SEI) occurring at the graphitic anode during the first cycles. In this study we measured electrochemical impedance spectra at equidistant voltage intervals during first and second lithiation (charging process). The distribution of relaxation times (DRT) was calculated from the measured impedance spectra. Based on the DRT calculations an equivalent circuit was set up to evaluate the different impedance contributions. In addition to measurements at room temperature we also studied SEI behavior at elevated and lowered temperatures. Above 55 °C we monitored an unwanted thermally induced electrolyte decomposition. Below −10 °C an incomplete SEI formation was observed resulting in an increased impedance during the second lithiation. The activation energies of the different processes were calculated assuming Arrhenius behavior.

The electrochemical behavior of lithium–sulfur (Li–S) batteries was investigated by means of electrochemical impedance spectroscopy (EIS). Measurements were performed in equidistant charge intervals at different depths of discharge and charge during the first cycle. Additionally, the degradation of the cells was analyzed for up to 50 cycles. An equivalent electrical circuit is proposed to simulate the electrochemical processes and to quantify the impedance contributions of Li–S batteries. EIS as a function of the cycle number shows an increased capacity for fading correlated with a decrease in the charge transfer resistance of the cathode. Atomic force microscopy (AFM) was also used to provide information about changes in the electrical conductivity of the cathode surface as they are related to the building of isolating films.

Solid oxide fuel cells (SOFC) for mobile applications are developed and investigated at the German Aerospace Center (DLR) in Stuttgart. Therefore a light-weight stack design was developed in cooperation with the automotive industry (BMW/Munich, Elring-Klinger/Dettingen, ThyssenKrupp/Essen) and the Research Center Julich (FZJ). This concept is based on the application of stamped metal sheet bipolar plates, into which the SOFC cells are integrated by brazing technology. For the development and the investigation of the SOFC cells and short stacks, the electrochemical impedance spectroscopy (EIS) is an important and useful characterization method. The paper concentrates on the investigation and on the electrochemical testing of the SOFC short stacks with sintered anode-supported cells (ASC). The short stacks were electrochemically characterized mainly by electrochemical impedance spectroscopy, by current–voltage measurements and by long-term measurements. The cells and stacks were operated at different temperatures, varying fuel gas compositions, different fuel gas flow rates and at different electrical current loads. The influence of these operating conditions on the electrochemical performance of the short stacks is outlined. The nature of losses, e.g. ohmic and the polarization resistances of the electrodes were examined and determined by fitting of the impedance spectra to an equivalent circuit.