EIS at High Temperatures

1 High PEM Fuel Cell Advantage and Characteristics of its AC Impedance PEM fuel cells operated at high temperatures ( 100°C) have several advantages over those operated at lower temperatures (1) faster electrochemical kinetics, (2) improved and simplified water management, (3) effective thermal management, 

Assuming negligible anode polarization, the steady-state polarization curves can be described by a semi-empirical equation  

In Equation 6.18, Rt is a function of current density and temperature, Rm is less dependent on the current density but is strongly dependent on the temperature, and Rmt is the mass transfer impedance, which is strongly dependent on current density and temperature. Note that this equation is only accurate at an AC impedance frequency at or near zero. 

The percentage of the cell voltage drop (AVcel/%) caused by an individual resistance (A, = Rt, Rm, or Rmt) at a constant current density can be expressed as 

The results also suggest that through AC impedance measurements, the performance drops caused by individual processes such as electrode kinetic resistance, membrane resistance, and mass transfer resistance can be correlated to either reduction or improvement in cell performance. If individual impedances are known, the contribution to the change in performance can be identified, which is very important in the design and optimization of high-temperature MEA catalyst layer components, structure down-selection, and MEA architecture. 

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This chapter has examined a variety of EIS applications in PEMFCs, including optimization of MEA structure, ionic conductivity studies of the catalyst layer, fuel cell contamination, fuel cell stacks, localized impedance, and EIS at high temperatures, and in DMFCs, including ex situ methanol oxidation, and in situ anode and cathode reactions. These materials therefore cover most aspects of PEMFCs and DMFCs. It is hoped that this chapter will provide a fundamental understanding of EIS applications in PEMFC and DMFC research, and will help fuel cell researchers to further understand PEMFC and DMFC processes. 

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