Tafel Slope Estimation of Electronic Resistance Laden Catalyst Layer in PEMFC Using Polarization Curve Correction Methodology
Authors: Sethi Yadul , Li Yan-Sheng , Suthar BharatKumar 
Published: December 2024

The higher cost of PEMFC systems proves to be a major bottleneck in their adoption. Platinum being a costly material (used as a catalyst in PEMFC) contributes significantly (42%) to the overall cost of the PEMFC stack.1 The majority of the Platinum catalyst is loaded on the cathode side due to the slow nature of the oxygen reduction reaction (10-8 A/cm2 for ORR). Hence, alternative options for the cathode catalysts are being persued as a potential alternative for platinum metal group-free (PGM-free) catalyst to improve the economics of such systems.2 The sluggish kinetics of PGM-free catalysts requires much thicker CCL (compared to the Pt catalyst) to have comparable fuel cell performance at low currents.3 Thicker CCLs will lead to poorer protonic and electronic conduction.
Voltage breakdown analysis from the polarization curve (Cell voltage vs Current density) is quite common in literature and is used to gather selective insights into the working of PEMFC. Work by Neyerlin et al. mathematically quantified the effect of proton conduction on the polarization curve.4 In their work, they also proposed a systematic correction methodology to retrieve the Tafel Slope by correcting the polarization curve for proton conduction and other ohmic resistances (membrane and external resistances). It is to be noted that the correction approach presented by Neyerlin et al. works well for systems with neglible electronic resistance (since it was designed for Platinum based CCLs which are thin and highly electronically conducting). However, for PGM-free catalysts which are thicker and poorer electronic conductors than Pt-based CCLs, contribution from both electronic and protonic conduction will affect the polarization curve.
We propose a method to correct the polarization curve in PEMFC for effective total transport loss (both electronic and protonic) to get back the Tafel slope of the oxygen reduction reaction on the cathode side. Further, we demonstrate the approach for two different CCLs made with commercially available catalysts named PMF and in-house developed catalysts named HM (presented in Li et al)5. Note that this work assumes that the mass transport losses are not present (which is reasonable assumption in H2/O2 system) and assumes that gas cross-over effects (due to the crossing of gases through the membrane to the other side of the PEMFC) are not current dependent during fuel cell operations.
Figure 1 shows the correction steps for hypothetical data generated from bvp-5C simulations (for an electronic resistance laden catalyst layer) which is treated as an actual polarization curve (orange curve represented as Ecell). In this work, we develop a semi-analytical expression for effective transport loss (represented by cyan shaded in Figure 1) which can be used to correct the (corrected for membrane proton conduction and other external electronic resistance referred to as Rmem+ext) to obtain a curve which, when fitted with the Tafel kinetics equation gives the value the Tafel slope of that CCL.

References

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K. C. Neyerlin, W. Gu, J. Jorne, A. Clark, and H. A. Gasteiger, J. Electrochem. Soc., 154, B279 (2007).

Y.-S. Li, D. Menga, H. A. Gasteiger, and B. Suthar, J. Electrochem. Soc., 170, 094503 (2023).

Figure 1