(Invited) Kinetic Insight into the Degradation Mechanism of PGM-Free ORR Catalysts
Authors: Yin Xi , Martinez Ulises , Komini Babu Siddharth , Chung Hoon T , Purdy Geraldine M , Zelenay Piotr 
Published: February 2020

Platinum group metal-free (PGM-free) catalysts, based on earth-abundant elements, such as carbon, nitrogen, and transition metals, have by now shown promising oxygen reduction reaction (ORR) activity in the fuel cell cathode (1-3). However, any possible implementation of the PGM-free catalysts in practical fuel cells is being challenged by their poor durability, lower than that of PGM-based catalysts currently used in polymer electrolyte fuel cells (4, 5). Several different degradation mechanisms have been discussed in the literature to account for the performance loss of PGM-free catalysts, in particular, micropore flooding (6, 7), carbon corrosion/oxidation (8), demetalation (8, 9), and oxidation of the catalyst by various oxygen-rich radicals (10). However, the leading cause of the PGM-free catalyst performance loss remains unclear, calling for further studies, including modeling of the catalyst degradation kinetics, which can help reveal the underlying mechanism of the performance loss.

In this presentation, we will summarize the results of a kinetic study of two possible degradation mechanisms of PGM-free ORR catalysts at a high fuel cell voltage, greater than 0.8 V. The models accounting for different plausible degradation paths, fitted to the experimental data, will be presented and discussed. The models yield different relationships between the kinetic current density and reaction time giving rise to either exponential or logistic decay plots. The experimental results are better represented by the logistic decay, suggesting an autocatalytic degradation mechanism. This in turn implies that, in parallel to the four-electron ORR, the intermediates formed in the side reactions are likely responsible for the degradation of PGM-free catalysts. The measurements of the CO2 and fluoride amounts in the cathode effluent, as well as the active-site quantification by molecular probes will be also presented to verify the proposed catalyst degradation mechanism.

Acknowledgements

Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged.

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