Designer Electrocatalysts for the Oxygen Reduction Reaction with Controlled Platinum Nanoparticle Locality

To achieve global deployment of proton exchange membrane technologies, growing efforts in platinum group metals (PGM) recycling must be coupled with substantial improvements in the durability of PGM catalysts. This need for longevity is emphasized by the establishment of the Million Mile Fuel Cell Truck (M2FCT) Department of Energy consortium, devised with durability at its core. So far, improvements in the durability of Pt and Pt-alloy catalysts have focused on surface modifications by small organic molecules (such as melamine), formation of Pt skins (for alloys), and cell design [1–3]. However, the literature transpires a certain reluctance to exploring the interaction between the active material and the often not fully understood carbon support. A handful of studies directly address the effect of support morphology on durability, focusing on the ability of porous carbons to host nanoparticles inside (IN) and outside (OUT) of mesopores [4–7]. Still, the effect of IN vs. OUT is studied on different carbon supports—namely high surface area carbons (HSA) and solid carbons (such as Vulcan), respectively—thus coming short of decoupling nanoparticle locality from carbon support effects. Furthermore, the effects of nanoparticle locality on long-term durability remain unclear. To address these challenges, we present a new class of carbon supported Pt-based electrocatalysts with distinct nanoparticle localities. A methodology was developed to place nanoparticles at will on the inside or outside of carbon mesopores. Specialty carbon blacks from Cabot Corporation pre-production FCX series were used as supports. Synthesis protocols were tuned to ensure that nanoparticle size, crystallinity and loading were the same for IN and OUT catalysts; this way the effect of locality could be studied in isolation from all other parameters. Pt/C catalysts with nanoparticles IN or OUT of mesopores were prepared with Pt loadings of 20 and 40 wt% and shown to have ECSA and mass activity comparable to Pt/Vulcan (30 wt%) from Tanaka Kikinzoku (TKK). Pt-alloy catalysts with controlled nanoparticle locality were also prepared. We believe these materials can serve as model catalysts for long-term durability studies in membrane electrode assemblies. Specifically, given their carefully tailored structure, these catalysts can help address the question of whether placing nanoparticles in mesopores slows down their degradation. Especially for alloys, where degradation is more pronounced and complex, controlling parameters such as carbon structure and nanoparticle locality becomes even more important if we are to improve their durability. [1] H. Daimon, S.I. Yamazaki, M. Asahi, T. Ioroi, M. Inaba, A Strategy for Drastic Improvement in the Durability of Pt/C and PtCo/C Alloy Catalysts for the Oxygen Reduction Reaction by Melamine Surface Modification, ACS Catal. 12 (2022) 8976–8985. https://doi.org/10.1021/ACSCATAL.2C01942. [2] C. Wang, M. Chi, D. Li, D. Strmcnik, D. Van Der Vliet, G. Wang, V. Komanicky, K.C. Chang, A.P. Paulikas, D. Tripkovic, J. Pearson, K.L. More, N.M. Markovic, V.R. Stamenkovic, Design and synthesis of bimetallic electrocatalyst with multilayered Pt-skin surfaces, J Am Chem Soc. 133 (2011) 14396–14403. https://doi.org/10.1021/JA2047655/SUPPL_FILE/JA2047655_SI_001.PDF. [3] S. Liu, S. Hua, R. Lin, H. Wang, X. Cai, W. Ji, Improving the performance and durability of low Pt-loaded MEAs by adjusting the distribution positions of Pt particles in cathode catalyst layer, Energy. 253 (2022) 124201. https://doi.org/10.1016/J.ENERGY.2022.124201. [4] E. Padgett, N. Andrejevic, Z. Liu, A. Kongkanand, W. Gu, K. Moriyama, Y. Jiang, S. Kumaraguru, T.E. Moylan, R. Kukreja, D.A. Muller, Editors’ Choice—Connecting Fuel Cell Catalyst Nanostructure and Accessibility Using Quantitative Cryo-STEM Tomography, J Electrochem Soc. 165 (2018) F173. https://doi.org/10.1149/2.0541803JES. [5] 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, D.A. Muller, Mitigation of PEM Fuel Cell Catalyst Degradation with Porous Carbon Supports, J Electrochem Soc. 166 (2019) F198–F207. https://doi.org/10.1149/2.0371904JES/XML. [6] Y.-C. Park, H. Tokiwa, K. Kakinuma, M. Watanabe, M. Uchida, Effects of carbon supports on Pt distribution, ionomer coverage and cathode performance for polymer electrolyte fuel cells, (2016). https://doi.org/10.1016/j.jpowsour.2016.02.091. [7] X. Tuaev, S. Rudi, P. Strasser, The impact of the morphology of the carbon support on the activity and stability of nanoparticle fuel cell catalysts, Catal Sci Technol. 6 (2016) 8276–8288. https://doi.org/10.1039/C6CY01679K. Figure 1