In state-of-the-art catalysts, the platinum or platinum alloy nanoparticles are supported on high-surface-area carbon powders, such as Vulcan XC72R, Ketjen black, or graphitized carbon blacks. These inexpensive supports fulfill the necessary functions of electron conduction and dispersing and anchoring the catalyst particles. However, they are prone to oxidation and have weak interaction with the Pt nanoparticles. These shortcomings allow the nanoparticles to migrate and coalesce during fuel cell operation and to become disconnected from the support due to oxidation, leading to ECA loss, with support oxidation also causing loss of catalyst layer porosity.7 There have been extensive alternative support development efforts aimed at addressing these issues while maintaining the electrical conductivity of carbon (~2 S/cm).8 A wide array of alternative supports have been developed, including doped oxides, carbides,9,10 and nitrides, to name a few, but most lack the requisite high surface area and/or high electrical conductivity.8 While a improved materials are the ultimate solution to the carbon support instability issues, automotive fuel cell system developers have designed system-level mitigation strategies to allow the existing carbon supports to achieve the lifetime targets. A more recently-studied support issue is the impact of support structure on catalyst utilization.11 While the higher surface area carbon supports are desirable in terms of their ability to disperse the catalyst nanoparticles, they contain internal porosity. A significant portion of the catalyst nanoparticles can be buried in the pores.12 Limited proton conductivity in the pores leads to under-utilization of the buried catalyst particles at high current densities and, especially, under low humidity conditions.11 To overcome this limitation, recent efforts focus on catalyst deposition methods or carbon supports that can limit catalyst deposition to the external surface of the support.11,13
This presentation will give an overview of the current understanding of catalyst and catalyst support performance and performance stability and the interactions between catalyst, support, and ionomer that define these properties. The impact of catalyst and support degradation on the ionomer properties will also be discussed.
Acknowledgements
This work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the auspices of the Fuel Cell Performance and Durability (FC-PAD) Consortium. Argonne National Laboratory is managed for the U.S. DOE by the University of Chicago Argonne, LLC, under contract DE-AC-02-06CH11357.
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