Evaluation of electrochemical impedance spectra of metal-air batteries (Li-air/Zn-air) for aqueous electrolytes

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