Performance, Interactions, and Degradation of Polymer Electrolyte Fuel Cell Cathode Catalysts, Supports, and Ionomers

Abstract

Over the past decade, the intrinsic oxygen reduction reaction (ORR) activity of platinum-based nanoparticle catalysts for polymer electrolyte fuel cells (PEFCs) has been improved by over an order of magnitude, with numerous catalysts now exceeding the catalytic activity target (>0.44 A/mg-Pt) established for the automotive propulsion power application.1-3 However, the majority of catalysts meeting the activity targets are unable to achieve the current densities at low catalyst loadings (cathode loading: 0.1 mg-Pt/cm²) necessary to meet the automotive performance and cost targets simultaneously.2 Many of the high activity catalysts also rapidly lose activity during voltage cycling in the PEFC environment.2,4 Research focus in recent years has broadened beyond catalyst activity improvement to the understanding the sources and mechanisms of activity and performance loss and using that knowledge to mitigate the losses. For example, the major source of electrochemically-active surface area (ECA) loss for platinum nanoparticles is dissolution of particles of <~4 nm, leading to the development of catalysts with larger as-prepared mean diameters .5,6 It has also been determined that base metal is rapidly lost from platinum-base alloy nanoparticles during electrode preparation and during voltage cycling in the fuel cell environment, leading to development of catalysts with lower initial base metal content and “pre-leaching” of base metal.4 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. AcknowledgementsThis 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. References U.S. DOE FCTO Multi-Year Research, Development, and Demonstration Plan, 2016.A. Kongkanand et al., J. Phys. Chem. Lett., 7 (2016) 1127−1137.M. Debe, Nature, 486 (2012) 43–51.R. Mukundan, et al., 232nd Electrochemical Society Meeting, National Harbor, MD, Oct., 2017, 1681.J.A. Gilbert, et al., Electrochimica Acta, 173 (2015) 223-234.K. Yu, et al., Chemistry of Materials, 2014.L. Castanheira, et al., ACS Catal. 5 (2015) 2184–2194.Y. Shao, et al., J. Mater. Chem. 19 (2009) 46–59.M. Nie, et al., J. Power Sources, 162 (2006) 173-176.H. Chhina, et al, J. Power Sources, 179 (2008) 50-59.K. Shinozaki, et al., J. Electrochem. Soc., 158 (2011) B467-B475.K.L. More, “FC136 – FC-PAD: Components and Characterization”, 2017 DOE-FCTO Annual Merit Review and Peer Evaluation, June 6, 2017. https://www.hydrogen.energy.gov/pdfs/review17/fc136_more_2017_o.pdfA. Kongkanand, “FC144 – Highly-Accessible Catalysts for Durable High-Power Performance”, 2017 DOE-FCTO Annual Merit Review and Peer Evaluation, June 7, 2017. https://www.hydrogen.energy.gov/pdfs/review17/fc144_kongkanand_2017_o.pdf