Dynamic Modeling for the Simulation of AC Spectra for HT-PEMFC

High Temperature Polymer Electrolyte Membrane Fuel Cells, (HT-PEMFCs), offer significant advantages over low temperature operation, such as simplified water management,  increased tolerance to CO poisoning and impurities and simple balance of plant [1]. Even though the operating principles of an HT-PEMFC are understood, the overall system’s behavior is determined by a number of strongly coupled processes, each proceeding at a different rate. Thus, to optimize the performance of the fuel cell, an in depth understanding of the various processes and their interactions is necessary.

One of the most powerful electrochemical characterization methods is the Electrochemical Impedance Spectroscopy technique, (EIS). It is a very powerful and sensitive technique that has already become a primary diagnostic tool in PEM fuel cell research, [2, 3]. EIS can provide valuable information on the various physicochemical and electrochemical processes taking place in the Membrane Electrode Assembly, (MEA), of  an HT-PEMFC.

Unfortunately the behavior of many electrochemical processes, such as Hydrogen and Oxygen adsorption relaxation phenomena, (Fig. 1), and the effect they have on the performance of the fuel cell is very difficult to deduce from experimental spectra.  

Fig. 1 Simulated impedance spectra for H₂/O₂ HT-PEMFC, , (I_cell = 200mA/cm2),  depicting the low frequency capacitive impedance caused by the relaxation of H₂ adsorption, (black) when O₂ adsorption dynamics are absent for both spectra, frequency range 10mHz – 50kHz.

In this study a 2-D dynamic mathematical model has been developed  on the cross section of the membrane electrode assembly in order to simulate AC impedance spectra under various operational conditions. The model includes a detailed macroscopic description of the diffusion processes within the electrode, the electro-kinetic model of the chemical and electrochemical reactions within the catalytic layers as well as the charge transfer across the various parts of the MEA.

The aim of the 2-D cross section simulation of the MEA is to carry out a parametric and sensitivity analysis of the various parameters that govern the various processes across the MEA.

By decoupling the various processes and simulating an AC impedance spectrum, information will be extracted on the various kinetic and equilibrium constants related with the interaction of the reacting gases in the catalytic layer and the electrochemical interface.  

References

[1]           J. Zhang, Z. Xie, J. Zhang, Y. Tang, C. Song, T. Navessin, Z. Shi, D. Song, H. Wang, D. P. Wilkinson, Z.-S. Liu, and S. Holdcroft, “High temperature PEM fuel cells,” Journal of Power Sources, vol. 160, no. 2, pp. 872–891, Oct. 2006.

[2]           P. M. Gomadam and J. W. Weidner. 2005. International Journal of Energy Research, vol: 29, 1133–1151.

[3]           X. Yuan, H. Wang, J. Colinsun and J. Zhang. 2007. International Journal of Hydrogen Energy, vol: 32,  4365–4380.

 Acknowledgements

 This work has been conducted and supported within the framework of DeMStack, (DEMSTACK 325368, FCH-JU-2012-1).