Activity and stability of Pt and PtCo alloy catalysts supported on high surface area carbon have been evaluated in 5-cm² differential and 50-cm² integral cells. Both catalysts had nearly identical Pt loading (50-wt% Pt on carbon, 0.25 mg_Pt/cm²) and had undergone thermal treatment or annealing to grow the average particle size to 4-5 nm, referred to as a-Pt/C and d-PtCo/C. Both were subjected to standard accelerated stress tests (AST) consisting of 0.6-0.95 V square wave potentials, 3-s hold at upper and lower potential limits in H₂/N₂ at 1.5 atm, 80^oC and 100% RH.

Analysis of the polarization data for the two catalyst systems in differential cells at low current densities indicates no significant change in the parameters characterizing the dependence of ORR kinetics on oxygen partial pressure, relative humidity and temperature with catalyst aging up to 90k AST cycles, implying that there is no alteration in the underlying mechanism or the rate controlling steps.¹ However, after 90k cycles, the specific exchange current density (mA/cm²-Pt) for ORR decreased by 50% for a-Pt/C catalyst and by 75% for d-PtCo/C catalyst. The higher loss in the specific activity for the alloy catalyst may be attributed to Co dissolution resulting in the relaxation of the surface strain deemed responsible for its enhanced ORR activity in the fresh state. After 90k cycles, the two catalyst systems had nearly the same mass activity defined as current density in H₂/O₂ at 0.9 V IR-corrected potential, 80^oC, 1 atm O₂ partial pressure, and 100% RH.

We derived the O₂ transport resistance () from the limiting current density () corresponding to a set value of mass transfer overpotential that is large enough to not materially affect but can be determined from the available polarization data.¹ Knowing at different pressures, we evaluated the pressure-dependent and pressure-independent () portions of , and the local O₂ transport resistance, .³ We associate the pressure dependent resistance to O₂ transport in gas channel and diffusion media and the pressure independent resistance to O₂ transport electrode (). We observed that in a-Pt/C electrode decreases with aging, or Pt roughness (), from 30k to 90k cycles probably because of redistribution of Pt from carbon pores to carbon surface. In contrast, has been observed to increase as is decreased by reducing the Pt loading.² Compared to a-Pt/C, and are higher in d-PtCo/C electrode especially at low RH and after potential cycling probably because of ionomer poisoning caused by Co dissolution. This result is consistent with the results from other electrochemical impedance spectroscopy studies showing a reduction in O₂ permeability with Co poisoning of the ionomer.⁴

Consistent with the above conclusions concerning activity, stability and O₂ transport, the polarization data in Fig. 1 shows better performance for the fresh d-PtCo/C electrode than a-Pt/C electrode in the kinetic region but worse performance at higher current densities. Depending on the operating conditions, the polarization curves for a-Pt/C and d-PtCo/C electrodes intersect at about 0.8 – 1 A/cm². With aging on 0.6-0.95 V AST potential cycles, the d-PtCo/C electrode degrades faster than a-Pt/Co electrode and the point of intersection of the polarization curves continues to move to lower current densities.

Acknowledgement

This work was supported by the Hydrogen and Fuel Cell Technologies Office (HFTO), Office of Energy Efficiency and Renewable Energy, US Department of Energy (DOE) through the Million Mile Fuel Cell Truck (M2FCT) consortia, technology managers G. Kleen and D. Papageorgopoulos. While we do not endorse any materials presented in this study, we wish to thank Sascha Toelle and Umicore for the electrocatalysts and MEAs utilized in this study. Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, also under contract DE-AC-02-06CH11357.

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