While several decades of research have been dedicated to improving the understanding of the electrochemical kinetics on Pt-based oxygen reduction reaction (ORR) electrocatalysts,^1–3 similar experimental and modeling efforts for platinum group metal-free (PGM-free) electrocatalysts have been less prevalent.⁴ Limitations in PGM-free electrocatalyst/electrode stability⁵ and active-site quantification⁶ are perhaps the biggest barriers against extracting electrochemical kinetic data from operando studies at the MEA level. However, recent efforts in PGM-free catalyst synthesis at Los Alamos National Laboratory (LANL),⁷ have yielded a robust PGM-free electrocatalyst capable of producing repeatable polarization curves over several hours of testing. This electrocatalyst is derived from a pyrolyzed Fe-Zn metal organic framework containing “atomically dispersed” N-coordinated Fe groups, (AD)Fe-N-C, which may act as the ORR active sites.^7,8 Here, we present a systematic operando electrochemical kinetic examination of the LANL (AD)Fe-N-C catalyst in a membrane-electrode assembly in the polymer electrolyte fuel cell (PEFC). These experiments were performed using a 5 cm² differential cell (i.e., high oxygen flow rates and thus high stoichiometry).³ Based on the assessment of the cathode proton resistance, oxygen reaction order and ORR activation energy, we propose the inclusion of a potential-dependent descriptor for the availability of electrochemical active sites, as shown in Eq. 1 below:

i + i_s = i₀ (1 – θ) exp(αnFη/RT) [1]

Here, i, i_s and i₀ are, respectively, the measured, shorting, and ORR exchange current densities, (1 – θ) is the fraction of active sites available for ORR, and η_c is the ORR kinetic overpontential.⁹ As seen in Fig. 1a, the availability of active sites has a sigmoidal dependence on the cathode potential, with the largest increase in site availability occurring between 0.7 and 0.8 V. This finding will be discussed in the context of the potential window, over which the Fe³⁺/Fe²⁺ transition occurs.^4,10–12 When the potential dependence of the PGM-free active site availability is incorporated into the electrochemical model, it results in an excellent fit of the experimental data, depicted in Fig. 1b. The results from this experimental/model approach not only offer guidance for means to improve the performance of (AD)Fe-N-C catalysts, but also increase our understanding of the kinetic limitations of other PGM-free ORR electrocatalysts systems.

Figure 1. a) Potential-dependent availability of ORR active sites. b) Fitting of the modeling results with experimental data from polarization curves taken at different O₂ partial pressures for PEMFC fabricated with the LANL PGM-free catalysts at the cathode (80^oC, 100% RH, 300/300 sccm H₂/O₂, 3.5 mg_cat/cm²_elec).

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