Effect of CO Poisoning of PEM Fuel Cell Anode on Impedance Spectra-Simulations
In this work, we simulate chrono-potentiometric data and EIS spectra of the hydrogen oxidation reaction occurring in the half cell, under galvanostatic mode. Two anode kinetic models were chosen from literature [3, 4]. The two models differ in the adsorption mechanism of CO on the anode (linear and bridge). Using the kinetic model proposed by Springer and his coworkers, the steady state polarization curves, transient potential curves and galvanostatic EIS at different concentrations of CO in the feed gas are generated by simulations. The linear adsorption model was employed in the first set of results. The steady state polarization curves predict that the poisoning effect is prominent only for CO concentrations of greater than 10 ppm in the feed gas. At 200 mA/cm2current density, the anode potential increases due to CO poisoning and at 100 ppm CO concentration it reaches a steady state value of 50 mV. At higher CO concentrations the steady state anode potentials are higher. The transient potentials (normalized) at 0, 30, 50 and 100 ppm concentration of CO in the inlet stream are shown in Fig. a, and the results compare well with experimental results in literature. It is seen that the potential stabilizes faster at higher concentration of CO.
The EIS obtained after stabilization of the potential corresponds to that when the poisoning of the electrode is under steady state conditions. The spectra simulated on the anode fed with pure hydrogen gas (0 ppm CO) and under steady state poisoned condition, for 50 and 100ppm concentration of CO is shown in Fig. b. These are obtained under galvanostatic conditions with 200 mA/cm2current density. The inset shows the complete spectrum at 100 ppm CO. The value of impedance increases from a few mΩ to the order of Ω due to the CO poisoning of the anode. The patterns observed are at variance with the experimental results reported. This indicates that either the kinetic parameters are not representative or the model itself may have to be changed.
Normally, EIS data is analyzed with electrical equivalent circuits, implicitly assuming that the system has been linear, causal and stable. However, during the transient state of poisoning, when most of the data is acquired experimentally, the stability assumption is violated. Using numerical methods, we simulate the spectra duringthe progression of the poisoning process, i.e. under transient conditions, for various CO concentrations and current densities. The change in dc potential with time results in instabilities in the spectra, which are accounted for in numerically simulations. The spectra obtained using the two anode kinetic models in literature are also compared.
 N. Wagner, M. Schulze, Electrochimica Acta 48, 3899-3907 (2003)
 N. Wagner, E. Gulzow, Journal of Power Sources 127, 341-347 (2004)
 T.E Springer, T. Rockward, T.A. Zawodzinski, S. Gottesfeld, Journal of the Electrochemical Society, 148, A11-A23 (2001)