Synthesis of a Novel and Highly HOR/HER Selective Ru@Pt/TiOx/C Catalyst Exhibiting Strong Metal-Support Interactions (SMSI) for PEMFC Applications

Abstract

One main goal in Proton Exchange Membrane Fuel Cell (PEMFC) research is to make Fuel Cell Electric Vehicles (FCEV) viable for long-haul travel and increase their durability. Besides many degradation modes which target different parts of a PEMFC such as the catalyst layer, the membrane, bipolar plates, and other components, one major concern is the start-up/shut-down (SUSD) of the cell, inducing degradation of the catalyst support on the cathode. Addressing this, two mitigation strategies are possible: a) the system approach, e.g., reducing number of damaging SUSD events; b) the catalyst approach, e.g., suppressing the Oxygen Reduction Reaction (ORR) activity of the anode.The latter is feasible by utilizing a highly selective HOR/HER (Hydrogen Oxidation/Evolution Reaction) catalyst on the anode side that catalyzes the HOR but has a poor ORR activity, a property that was reported for a Pt/TiOx/C catalyst by Stühmeier et al.[1,2] The high selectivity towards the HOR/HER is explained by the Strong-Metal-Support-Interaction (SMSI) formed between the Pt nanoparticles and the TiOx-support, where the support creeps over the particle during a reductive heat treatment, as observed in HR-TEM.[1] In general, the SMSI effect between a metal and a suitable metal support is found in various heterogeneous catalytic applications, altering the catalyst’s activity for various reactions.[3] In the particular case of Pt/TiOx/C, the SMSI layer forming on the Pt nanoparticle is said to be permeable for protons but impermeable for any other oxygen-containing species, explaining a still rather high HOR/HER activity compared to a Pt/C catalyst (≈ 30% vs Pt/C) and an ORR activity that is decreased by two magnitudes (≈ 2% vs Pt/C).[1] Such interaction was also observed by Stühmeier et al. between Ru and a TiOx-support, where similar trends were determined. However, since the intrinsic HOR/HER activity of a Ru catalyst is quite low compared to Pt, a Ru/TiOx/C catalyst is not feasible for PEMFC applications.[4] Ru@Pt core-shell particles, as shown by Schwämmlein et al., exhibit a 3-5x higher HOR/HER activity compared to a Pt/C catalyst, making a Ru@Pt/C catalyst viable even for alkaline fuel cell applications.[5] Due to the already successfully explored interaction of Pt and Ru with a TiOx-support, we will examine the HOR and ORR activity of Ru@Pt/TiOx/C catalysts..In this work we successfully synthesized a novel catalyst system by forming an SMSI between Ru@Pt core-shell particles and a TiOx-support. Via RDE (Rotating Disk Electrode) characterization, we elucidate if the Ru@Pt/TiOx/C catalyst gains all the benefits from the single parts of the catalyst, namely: i) a high HOR/HER activity from the Ru@Pt core-shell particles; ii) a low ORR activity due to the SMSI effect. Figure 1 shows the HOR/HER performance catalysts based on Ru@Pt particles supported on Vulcan carbon (black line) or on TiO2 nanoparticles, whereby for the latter support the catalysts were either tested as-synthesized (red line) or after a reductive heat-treatment under 5% H2/Ar to induce SMSI effects (green line). Apparently, the SMSI layer on the Ru@Pt core-shell particles enable a sustained HOR/HER activity at > 1.0 VRHE, in contrast to the other two catalysts which show the typical decrease of the HOR/HER activity at high potentials. Similar observations had been made by Stühmeier et al. for a Pt/TiOx/C and Ru/TiOx/C catalyst.[1,2,4] The findings in this particular study with the TiOx-support, as an example, should serve as proof of principle to possibly translate the gained knowledge on different types of metal oxide supports, since the stability of TiOx is known to be limited under PEMFC operation.[2,6] References [1] B. M. Stühmeier, S. Selve, M. U. M. Patel, T. N. Geppert, A. Hubert, H. A. Gasteiger, H. A. El-Sayed, ACS Appl. Energy Mater. 2019, 1–8.[2] B. M. Stühmeier, A. M. Damjanović, K. Rodewald, H. A. Gasteiger, J. Power Sources 2023, 558, DOI 10.1016/j.jpowsour.2022.232572.[3] C. J. Pan, M. C. Tsai, W. N. Su, J. Rick, N. G. Akalework, A. K. Agegnehu, S. Y. Cheng, B. J. Hwang, J. Taiwan Inst. Chem. Eng. 2017, 74, 154–186.[4] B. M. Stühmeier, R. J. Schuster, L. Hartmann, S. Selve, H. A. El-Sayed, H. A. Gasteiger, J. Electrochem. Soc. 2022, 169, 034519.[5] J. N. Schwämmlein, H. A. El-Sayed, B. M. Stühmeier, K. F. Wagenbauer, H. Dietz, H. A. Gasteiger, ECS Trans. 2016, 75, 971–982.[6] J. Zhang, F. Coms, S. Kumaraguru, J. Electrochem. Soc. 2021, 168, 024520. Figure 1