A Post-Treatment with Facile Thermal Control to Prepare Pt/C Catalysts with Accessible Carbon Supports

Abstract

Mass market adoption of PEM fuel cells (PEMFCs) for emission-free propulsion still hinges on catalyst materials that remain highly active at low platinum loadings without comprising durability of the PEMFC system. A promising path towards this goal is the development of novel carbon support morphologies, as research efforts throughout the past decade have shone light on their importance for determining overall cell performance and for mitigating catalyst degradation.[ 1-2] Briefly, porous carbon supports such as Ketjenblack accommodate Pt nanoparticles in internal pores, protecting the Pt surface from contact with the ionomers’ sulfonate endgroups at the expense of hindered oxygen diffusion into these pores. Conversely, platinum deposited onto solid carbons without internal mesoporosity (e.g., Vulcan) necessarily displays depressed oxygen reduction reaction kinetics due to ionomer poisoning, but is characterized by superior oxygen transport properties.‘Accessible’ porous carbons, a concept coined by Yarlagadda et al., get around this trade-off by protecting Pt nanoparticles in carbon pores that do not impede oxygen diffusion to the Pt active site by virtue of their pore size and absence of bottlenecking elements.[ 3] Such accessible carbon morphologies may be obtained by tailored carbon-templating techniques, as increasingly featured in industry research.[ 4-5] A more economic route is the oxidative post-treatment of commercially available Pt catalysts on porous carbon supports, which was first patented by General Motors, and subsequently studied by our group.[ 6-7] Essentially, this leverages the Pt-catalyzed oxidation of the carbon for targeted pore widening, producing catalysts that enable outstanding performance at both low and high current density. A crucial disadvantage of this process is, however, the high exothermicity of the carbon oxidation reaction, requiring careful thermal control and small batch sizes to selectively etch the carbon structures without uncontrolled combustion of the entire carbon support.In this work, we present a post-treatment that achieves Pt-catalyzed pore opening by an alternative route, allowing a more controlled etching of the carbon support structure. This enables >90% shorter reaction times and larger batch sizes, potentially paving the way to a larger-scale implementation. Changes to the thus-obtained materials are characterized by N2 physisorption and transmission electron microscopy. The modified catalysts are then fabricated into low-loaded membrane electrode assemblies (MEAs) and tested comprehensively in 5 cm2 single cells. We demonstrate substantially increased high current density performance in H2/air compared to the pristine catalyst (up to 60 mV at 2 A cm-2, Figure 1), which we ascribe to improved catalyst accessibility as evidenced by oxygen limiting current measurements. REFERENCES N. Ramaswamy, W. Gu, J. M. Ziegelbauer and S. Kumaraguru, J. Electrochem. Soc., 167, 6, 64515 (2020).E. Padgett, V. Yarlagadda, M. E. Holtz, M. Ko, B. D. A. Levin, R. S. Kukreja, J. M. Ziegelbauer, R. N. Andrews, J. Ilavsky, A. Kongkanand and D. A. Muller, J. Electrochem. Soc., 166, 4, F198-F207 (2019).V. Yarlagadda, M. K. Carpenter, T. E. Moylan, R. S. Kukreja, R. Koestner, W. Gu, L. Thompson and A. Kongkanand, ACS Energy Lett., 3, 3, 618–621 (2018).Y. Kamitaka, T. Takeshita and Y. Morimoto, Catalysts, 8, 6, 230 (2018).V. Yarlagadda, N. Ramaswamy, R. S. Kukreja and S. Kumaraguru, Journal of Power Sources, 532, 231349 (2022).A. Kongkanand and M. K. Carpenter, US 9,947,935 B1, filed Sep. 30, 2016, and published Apr. 17, 2018.T. Lazaridis and H. A. Gasteiger, J. Electrochem. Soc., 168, 11, 114517 (2021). Figure 1