(Invited, Digital Presentation) Impedance Analysis of Porous Electrodes in Solid Oxide and Polymer Electrolyte Fuel Cells

Abstract

Technical electrodes in fuel cells rely on complex multiphase microstructures that facilitate electronic, ionic and gas transport to active reaction sites distributed in the electrode volume. In such electrodes, material and microstructural properties of the porous electrode likewise determine the cell performance and different losses related to gas diffusion, electronic / ionic transport or the charge transfer at the active reaction sites might dominate the electrode resistance.Impedance spectroscopy is a powerful tool to evaluate and quantify the losses within a porous electrode. In this contribution it will be shown how impedance spectroscopy, an impedance data analysis by the distribution of relaxation times (DRT) (1,2) and a subsequent transmission line modeling (TLM) can provide physicochemically meaningful information about technical electrodes applied in solid oxide cells (SOC) (3-7) and polymer electrolyte fuel cells (PEFC) (4,5). Limitations in TLM will be discussed and it will be shown that the well-established straight-forward approach of CNLS-fitting might lead to physicochemically incorrect results. Possible countermeasures as a pre-identification of TLM-parameters by means of separate electrical measurements and microstructural analysis or an appropriate selection of operating parameters during impedance analysis will be presented.An example for the application of TLM in electrode development and microstructural design of the multiphase microstructure will be discussed and examples how the TLM-approach can be used to optimize an electrode in consideration of available materials and producible microstructures will be shown.(1) S. Dierickx, A. Weber and E. Ivers-Tiffée, Electrochim Acta, 355, p. 136764 (2020).(2) A. Weber, tm - Technisches Messen, 88 (1), p. 1 (2021).(3) C. Endler-Schuck, J. Joos, C. Niedrig, A. Weber and E. Ivers-Tiffée, Solid State Ionics, 269, p. 67 (2015).(4) M. Heinzmann, A. Weber and E. Ivers-Tiffée, J. Power Sources, 402, p. 24 (2018).(5) M. Heinzmann, A. Weber and E. Ivers-Tiffée, J. Power Sources, 444, p. 227279 (2019).(6) S. Dierickx, J. Joos, A. Weber and E. Ivers-Tiffée, Electrochim Acta, 265, p. 736 (2018).(7) S. Dierickx, T. Mundloch, A. Weber and E. Ivers-Tiffée, J. Power Sources, 415, p. 69 (2019).