The activity and durability of both platinum group metal (PGM)-free and low-PGM catalysts rely heavily on the interaction of the active species with the support structure [1]. These carbon-based supports can be tailored in a number of ways to impact the morphology and performance of the electrocatalyst. Visualizing these interactions by scanning transmission electron microscopy (STEM) in two and/or three dimensions can provide valuable multi-scale insight, which can be leveraged towards maximizing catalyst utilization and durability. In this work, quantitative analytical STEM characterization is performed at high spatial resolution on a number of Pt-based and PGM-free catalyst and support systems to elucidate the nature of these critical interactions.

The interplay between carbon graphitization and Pt loading was examined in three dimensions by quantitative electron tomography. The initial Pt dispersion and subsequent agglomeration following accelerated stress tests were quantified for high surface area carbon, Vulcan, and low surface-area carbons, with Pt catalyst loadings ranging from 5 to 40 wt.%. Higher degrees of graphitization in the carbon support led to a poor initial Pt dispersion, which in turn resulted in increased agglomeration during cycling. These results showed that in order to improve the mass activity and durability of the corrosion-resistance graphitized carbons, steps must be taken to improve initial Pt dispersion. To this end, a series of nitrogen-doped graphitized carbon supports were synthesized to study possible interactions between the nitrogen-doped surfaces and Pt nanoparticles. Low-voltage, aberration-corrected STEM coupled with electron energy loss spectroscopy (EELS) was used to probe these interactions in an effort to improve Pt dispersion and stability on corrosion-resistance supports.

Such catalyst-support interactions are perhaps best epitomized in the atomic-level interplay found in heat-treated metal-nitrogen-carbon (M-N-C) catalysts in fuel cells with PGM-free cathodes [2]. The resulting atomic-level structures derived from zeolitic imidazolate frameworks (ZIFs) and hybrid cyanamide-polyaniline precursors were visualized at the atomic level by the low-voltage, aberration-corrected STEM. The composition of catalytic oxygen reduction reaction (ORR) active sites and other structures explored by STEM-EELS complement electrochemical measurements to further guide synthesis protocols as efforts continue to produce high-performance and durable PGM-free based fuel cells.

 References

1. L. Du, Y. Shao, J. Sun, G. Yin, J. Liu, Y. Wang, Nano Energy 29 (2016) 314.
2. G. Wu, K. L. More, C. M. Johnston, P. Zelenay, Science 332 (2011) 443.

 Acknowledgements

Research sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE), and through a user project supported by ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility. A portion of this research was performed using instrumentation provided by the U.S. DOE Office of Nuclear Energy, Fuel Cycle R&D Program, and the Nuclear Science User Facilities.