Estimating Important Electrode Parameters of High Temperature PEM Fuel Cells By Fitting a Model to Polarisation Curves and Impedance Spectra

Abstract

A high temperature PEM (HTPEM) fuel cell model capable of simulating both steady state and dynamic operation is presented. The purpose is to enable extraction of unknown parameters from sets of impedance spectra and polarisation curves. The model is fitted to two polarisation curves and four impedance spectra measured on a Dapozol 77 MEA. The model is capable of achieving good agreement with the recorded curves. Except at OCV, where the voltage is overpredicted, the simulated polarisation curves deviate maximum 3.0% from the measurements. The impedance spectra deviate maximum 3.7%. The fitted parameter values are within the range reported in literature. The only exception is the catalyst layer acid content, which is an order of magnitude lower. This may derive from acid migration. The model is used to illustrate the effect of reactant dynamics on the impedance spectrum. The model can aid in the analysis of data from degradation tests. NomenclatureVariable [unit] Description Variable [unit] Description ai Activity of reactant i RH Relative humidity ‫ܣ‬ େ, [m 2 kg -1 ] Carbon surface area ܵ [mol m -3 s -1 ] Source term of species i ‫ܣ‬ ௧, [m ୲ ଶ m ିଷ ] ECSA per CL volume T [K] Absolute temperature ‫ܣ‬ ௦௨ [m 2 m -2 ] CL pore surface area TC [°C] Celsius temperature c [mol m -3 ] Total molar concentration tj [m] Thickness of MEA layer j ci [mol m -3 ] Molar concentration of i u [m s -1 ] Velocity vector ܿ , మ , ሾmol m ିଷ atm ିଵ ሿ Oxygen solubility in phase i V [V] Cell voltage ‫ܥ‬ ௗ [F m -2 ] Double layer capacitance ܸ [m 3 m -2 ] Area specific acid volume. dfilm [m] Acid film thickness ܸ [m 3 m -2 ] Specific pore volume. ‫ܦ‬ [m 2 s -1 ] Diffusion coefficient of i ‫ݓ‬ ୌ య ర H3PO4 mass fraction ‫ܦ‬ [m 2 s -1 ] Binary diffusion coefficient X H3PO4 per PBI repeat unit E [V] Open circuit potential ‫ݕ‬ Mole fraction F [C mol -1 ] Faraday's constant ߂ܵ [J mol -1 K -1 ] Reaction entropy i [A cm -2 ] Cell level current density ߳ Porosity ݅ ௧ [A m ୲ ିଶ ] Current density per ECSA ߳ ,() Volume fraction of i (in j) ݅ ሾA m ୲ ିଶ ሿ Exchange current density Η [V] Overpotential ݆ [A m -3 ] Volumetric current density ߢ [S m -1 ] Ionic conductivity ݇ [m 2 ] Permeability Μ [kg m -1 s -1 ] Dynamic viscosity ‫ܮ‬ ୧ [kg m -2 ] Loading of i in CL Ρ [kg m -3 ] Density M [kg mol -1 ] Molar mass ߶ [V] Electronic potential Rcell [Ω cm 2 ] Cell contact resistance ߶ [V] Ionic potential