Differentiating PGM Ostwald Ripening from Catalyst Degradation Using Identical Location TEM and Controlled Potential Cycling
Authors: Zhu Guangqi , Jeon Sungho , Ortiz Erika , Stach Eric A. , Li Chenzhao , Behzadnia Mohammad , Zhang Hannah , Yu Haoran , Cullen David A. , Marina Oana C. , Li Zheng-Hua , Xie Jian 
Published: December 2024

Nanoscale Platinum group metals (PGMs) over conductive supports with large specific area have been widely used as highly efficient catalysts for various reactions, including oxygen reduction reaction (ORR) (for fuel cells) and oxygen evolution reactions (OER) (for water electrolyers). However, these PGM nanoparticles (NPs) usually suffer from significant performance degradation over prolonged operation, primarily attributed to catalyst degradation, which remains a long-standing challenge. Two principal mechanisms, Ostwald ripening and particle migration, contribute to this degradation. Ostwald ripening is a process involves the first dissolution of smaller PGM NPs into PGM ions, followed by their diffusion and redeposition onto the surfaces of larger NP, where the driven force is surface energy. Particle migration, on the other hand, is a process of movement of PGM NPs leading to the formation of large agglomerates due to the weak interactions between NPs and supports. In general, both processes occurred simultaneously during catalyst degradation in the fuel cell/electrolyzer operation. However, addressing the PGM catalyst degradations caused by these mechanisms requires completely different strategies: enhancing intrinsic catalyst stability for overcoming Ostwald ripening and improving interactions between PGM NPs and supports for reducing particle migration. Hence, it is critically required to quantitatively differentiate between these two mechanisms during catalyst degradation. Here, we propose an innovative approach to distinguish the two mechanisms utilizing Identical Location Transmission Electron Microscope (IL-TEM) coupled with carefully designed experiments. Specifically, the IL-TEM grids with dispersed PGM catalysts, such as TKK 46%Pt/Kejten Black EC-300, would be imaged to characterized the particle geometry, size and interparticle distance of PGM NPs on before and after accelerated stress test (AST) for various cycles (i.e. 0, 5000, 10,000, 30,000 cycles, etc.) within an electrochemical cell. These PGM catalysts will undergo AST cycling among two potential regions: (1) 0.2 ~ 0.6V (vs. standard hydrogen electrode (SHE)) and (2) 0.6 ~ 0.95V (vs. SHE). PGM particles cycled within the potential region (1) should exhibit only particle migration, since potential below 0.6 V do not cause any oxidation of PGM, while those cycled within potential region (2) would experience both Ostwald ripening and particle migration. By comparing changes in particle geometry, size and interparticle distance for PGM NPs subjected to cycling within different potential regions, we could quantitively determine the contributive effect of each mechanism on the catalyst degradation. This study will shed the light on differentiation of these two different mechanisms and provide guidance for the design of highly active and stable PGM catalysts.