Analysis of Transmission Line Models for PEMFC Impedance Modelling

Abstract

The behaviour of Polymer Electrolyte Membrane Fuel Cells (PEMFC) shows a complex, non-linear dependence on material properties and operation conditions. Therefore, modelling and model parameterization by electrochemical characterization of PEMFCs is challenging. To deconvolute the different loss mechanisms in PEMFCs, impedance spectra are recorded and modelled by equivalent circuit models. Transmission Line Models (TLMs) are a well-established, physicochemical approach for modelling the impedance of PEMFC electrodes. TLMs consider the processes occurring in an electrode and the model parameters are directly related to material and microstructural characteristics. PEM electrodes have inhomogeneous distributions of the material, the current density, stored water, etc. Assuming a homogeneous electrode with homogeneous current distribution, the basic TLM shown in [1] turned during impedance fitting out to be inaccurate in operation conditions where these assumptions do not hold.To increase the fit accuracy, literature proposes a number of adapted TLMs. These TLMs have additional components and therefore degrees of freedom. This enables these models to take additional processes and the microstructure into consideration.In this contribution an incremental 1 cm² PEMFC ensuring zero gradient operation is characterized by means of impedance spectroscopy. Different operating conditions are varied within wide ranges and additional characterization methods, like H2/N2 operation were applied.Different TLMs proposed in literature are evaluated by fitting them to the measured spectra with and without considering the information gained from additional measurements. The DRT was applied as additional criteria in the fitting.In general, the accuracy of a fit increased with the number of free fitting parameters. Thus more complex TLMs accounting for additional physicochemical processes provide a lower error in the fit. Nevertheless the physicochemical meaningfulness of the determined model parameters is sometimes uncertain. It will be shown that for a meaningful parameterization by CNLS-fitting such TLMs require additional tests and characterization methods.[1] Heinzmann Marcel; Weber André; Ivers-Tiffée Ellen. „Impedance modelling of porous electorde structures in polymer electrolyte membrane fuel cells“. In: Journal of Power Sources (2019).