A Simple Analytical Approach for Fitting Steady-State Polarization Behavior of Polymer Electrolyte Fuel Cells Using Tafel Slope Component Analysis (TSCA)
Authors: Tryk Donald A. , Kakinuma Katsuyoshi , Uchida Makoto , Iiyama Akihiro 
Published: December 2020

There is a need for methods with which to characterize the current-voltage (I-V) or polarization behavior of fuel cells with minimal computational resources. One candidate is the direct analytical approach.1 In our work, in contrast to most other approaches, we have used the potential (approximated by the voltage) as the governing variable to both characterize catalysts and ionomers with rotating disk electrode measurements2 and characterize the performance of fuel cell membrane-electrode assemblies.3-5 This approach allows us to use a single, multi-component equation to calculate the current density from the potential. The various components are based on (1) an intrinsic kinetic current density with a relatively low Tafel slope (TS), based on an electron transfer coefficient α, which is a variable, (2) a double TS current density, corresponding to α/2, (3) a quadrupled TS current density, corresponding to α/4, and (4) a limiting current density, with α = 0. Essentially, the equation takes the simple form of a summation of series resistances, where each term refers to a TS component. Each resistance is then converted to the reciprocal of a characteristic current density, similar to an exchange current density. The doubling can be due to a change in mechanism, the onset of pore-based mass transport, or to coupled ionic conduction. Quadrupling can occur when two of these operate in tandem. Only five parameters are needed to characterize the polarization behavior, the intrinsic α value, the characteristic current densities for α
,
α/2 and α/4, and the limiting current density.

This approach has no intrinsic physical basis but can be used to compare various operating conditions and electrode compositions (Fig. 1). One application we have been interested in is the effectiveness of the catalyst.3,4 Another is determining the extent to which the polarization behavior is time-dependent. In ongoing work, we are (1) attempting to elucidate the intrinsic α values with specific mechanisms for the oxygen reduction reaction, (2) examining the relationship of hierarchical pore structures on TS quadrupling, and (3) developing a comprehensive database linking the fitted parameters with physically meaningful characteristics.

Acknowledgements

This project was partly supported by NEDO Japan through funds for the “Superlative, Stable and Sustainable Fuel Cells” project.

References

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A. Tryk, M. Lee, M. Uchida, H. Uchida, and M. Watanabe, ECS Trans., 35, 13-23 (2011).

Figure 1. Steady-state polarization behavior for MEAs with Pt/C catalyst layers with varying Nafion/carbon weight ratios: (left) log (Tafel) plots; (right) linear plots. Cell was operated at 65 °C with pure O2 feed (1 atm) at 100% RH (data from Ref. 3).

Figure 1