Chini Clusters on Oxygen Reduction Reaction (ORR) Catalysts
Authors: Heitkamp Ryan , Ivanov Sergei , Dolgopolova Ekaterina , Wang Xiaojing , Wu Gang , Spendelow Jacob S. 
Published: December 2024

State-of-the-art oxygen reduction reaction (ORR) catalysts for fuel cells commonly use Pt which heavily increases cost of manufacturing and is a concern for long term sustainability due to the reliance on a precious metal. Therefore, it is clearly desirable to reduce the amount of Pt used in fuel cells. A realistic route to achieve this goal is to continue using this precious metal but minimize the size of Pt particles. In doing so, the surface-to-volume ratio of Pt in the catalyst can be maximized so that the necessary mass of Pt in the fuel cell may be reduced without compromising catalyst performance. To produce small particles (nanoparticles) of Pt with controlled, variable sizes, we developed a method of depositing platinum carbonyl clusters, commonly referred to as Chini clusters, on a carbon support.
Pt Chini clusters, [Pt3n(CO)6n]2-, ranging from n = 2–8, have been reported in literature [1-3]. These clusters are synthesized and mixed with different types of carbon supports to perform incipient wetness impregnation (IWI), depositing the Chini clusters onto the support surface. For comparison, an alternative two-step polyol synthesis method is performed to grow Pt nanoparticles to a uniform size and deposit these nanoparticles onto a carbon support surface. Different carbon supports, including Vulcan XC-72R and KB EC300J, are used in synthesis to investigate if Pt Chini clusters of varying sizes and nanoparticles from the two-step polyol method can infiltrate pores and how the nature of a carbon support affects fuel cell testing performance. The as-synthesized Pt/C catalysts are systematically characterized using various techniques including, but not limited to, X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Membrane electrode assemblies are fabricated using the spray coating method and tested in a single 5 cm2 differential cell. Fuel cell performance from these catalysts is compared and electrochemical characterization techniques such as carbon monoxide stripping are employed to measure electrochemically active surface area and Pt accessibility. Our work aims to establish a means of synthesizing ORR catalysts using Pt Chini clusters and characterizing their stability under ambient and fuel cell testing conditions as well as overall performance in fuel cell testing. If the small Pt Chini clusters can be used effectively in ORR Pt/C catalysts, then this may be an effective route to improving fuel cell cost-efficiency and accessibility.

Acknowledgement:

This research 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) consortium, technology managers G. Kleen and D. Papageorgopoulos.

References:

Ceriotti, A. et al. “Bis(Tetrabutylammonium) Hexa-µ-carbonyl-hexacarbonylhexaplatinate (2 - ), [N(C4H9)4]2[Pt6(CO)6(µ-CO)6].” Inorganic Synthesis Volume 26 (1989): 316-319.

D’ Aniello, Michael J. et al. “Preparation of the Carbonyl Platinum Anions, [Pt3(CO)6]n
2-, (n= 3-5).” Inorganic Synthesis Volume 26 (1989): 319-323.

Femoni, Christina et al. “Infinite Molecular {[Pt3n
(CO)6n
]2–}∞ Conductor Wires by Self-Assembly of [Pt3n
(CO)6n
]2– (n = 5–8) Cluster Dianions Formally Resembling CO-Sheathed Three-Platinum Cables.” European Journal of Inorganic Chemistry Volume 2007, Issue11 (2007): 1483-1486.

Figure 1