Estimating Important Electrode Parameters of High Temperature PEM Fuel Cells By Fitting a Model to Polarisation Curves and Impedance Spectra
Authors: Vang Jakob Rabjerg , Zhou Fan , Andreasen Søren Juhl , Kær Soren Knudsen 
Published: February 2020

This work presents a high temperature PEM (HTPEM) fuel cell model capable of simulating both steady state and dynamic operation. The purpose of the model is to enable extraction of unknown parameters from sets of impedance spectra and polarisation curves.
The model is resolved in 1D through the electrode using the finite-volume method. Only the cathode is considered. The flow channel is resolved separately along the length to include dynamics which are important for the impedance spectra.
The model takes into account species transport, electrode kinetics, potential distribution, and effects of phosphoric acid (PA) water content on the electrochemical processes. The catalyst layer (CL) is modelled as a macrohomogeneous porous structure with platinum catalyst particles on the surface. The catalyst surface is covered in a thin film of PA that impedes reactant transport to the surface and provides ionic conductivity to the CL. The cathode gas phase is assumed to be in equilibrium with the PA in the CL and the membrane in terms of water content. The water content affects the reactant solubility and diffusivity, the exchange current density, and the ionic conductivity of the CL and the membrane.
The model is fitted to a data set consisting of two polarisation curves and four impedance spectra measured on a Dapozol 77 membrane-electrode-assembly (MEA) from Danish Power Systems. The MEA is 46 cm2 with a catalyst loading of 1.6 mg/cm2 for both anode and cathode. The reference data is recorded at 160°C with hydrogen stoichiometry of 1.2 and air stoichiometry (λ) of 2 and 4 respectively.
The fitting parameters for the present case are the charge transfer coefficient (α), the platinum electrochemical surface area (ESA), the surface area of the carbon phase in the catalyst layer (AC), which controls the magnitude of the Knudsen diffusion resistance, the PA loading in the catalyst layer (LPA), which determines the catalyst layer conductivity, the double layer capacitance (Cdl), which controls the dynamic response of the electrode kinetics, and the cell assembly contact resistance (Rcell), which represents the conduction losses at the interfaces between the individual fuel cell components. The fitting parameters can be easily changed depending on the available data on the tested MEA.
The fitted values are generally within range of the values quoted in the literature. The ESA agrees with the values measured by [1] and α is between the values used by [2] and [3]. Only the fitted value of LPAis an order of magnitude lower than values quoted in the literature [4,5]. This is most likely related to the calculation of the CL ionic conductivity. This will have to be investigated further.
The fit is shown in the figure. The agreement between the simulation and the data is within a few percent in most of the range. For the polarisation curves, the simulations only deviate significantly close to OCV, where the voltage is over predicted. The discrepancy at low current may be attributed to the model underestimating the effects of reactant cross over.
The impedance spectra are also faithfully reproduced, except for a tendency of the low frequency part of the simulated spectra to be smaller than in the measured spectra. This is most pronounced at low current. This discrepancy may to some extend be explained by the model not considering the effect of the anode or by the fact that the water content of the acid and the gas phases are assumed to be in equilibrium rather than considering the kinetics of water exchange between the PA and gas phase.
The model is considered suitable for analysing the origin of performance degradation in long term HTPEM fuel cell tests.
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Figure 1