We report and study the translation of exceptionally high catalytic oxygen electroreduction activities of molybdenum-doped octahedrally shaped PtNi­(Mo) nanoparticles from conventional thin-film rotating disk electrode screenings (3.43 ± 0.35 A mgPt –1 at 0.9 VRHE) to membrane electrode assembly (MEA)-based single fuel cell tests with sustained Pt mass activities of 0.45 A mgPt –1 at 0.9 Vcell, one of the highest ever reported performances for advanced shaped Pt alloys in real devices. Scanning transmission electron microscopy with energy dispersive X-ray analysis (STEM-EDX) reveals that Mo preferentially occupies the Pt-rich edges and vertices of the element-anisotropic octahedral PtNi particles. Furthermore, by combining in situ wide-angle X-ray spectroscopy, X-ray fluorescence, and STEM-EDX elemental mapping with electrochemical measurements, we finally succeeded to realize high Ni retention in activated PtNiMo nanoparticles even after prolonged potential-cycling stability tests. Stability losses at the anodic potential limits were mainly attributed to the loss of the octahedral particle shape. Extending the anodic potential limits of the tests to the Pt oxidation region induced detectable Ni losses and structural changes. Our study shows on an atomic level how Mo adatoms on the surface impact the Ni surface composition, which, in turn, gives rise to the exceptionally high experimental catalytic ORR reactivity and calls for strategies on how to preserve this particular surface composition to arrive at performance stabilities comparable with state-of-the-art spherical dealloyed Pt core–shell catalysts.

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Cost reduction and fast scale-up of electrolyzer technologies are essential for decarbonizing several crucial branches of industry. For polymer electrolyte water electrolysis, this requires a dramatic reduction of the expensive and scarce iridium-based catalyst, making its efficient utilization a key factor. The interfacial properties between the porous transport layer (PTL) and the catalyst layer (CL) are crucial for optimal catalyst utilization. Therefore, it is essential to understand the relationship between this interface and electrochemical performance. In this study, we fabricated a matrix of two-dimensional interface layers with a well-known model structure, integrating them as an additional layer between the PTL and the CL. By characterizing the performance and conducting an in-depth analysis of the overpotentials, we were able to estimate the catalyst utilization at different current densities, correlating them to the geometric properties of the model PTLs. We found that large areas of the CL become inactive at increasing current density either due to dry-out, oxygen saturation (under the PTL), or the high resistance of the CL away from the pore edges. We experimentally estimated the water penetration in the CL under the PTL to be ≈20 μm. Experimental results were corroborated using a 3D-multiphysics model to calculate the current distribution in the CL and estimate the impact of membrane dry-out. Finally, we observed a strong pressure dependency on performance and high-frequency resistance, which indicates that with the employed model PTLs, a significant gas phase accumulates in the CL under the lands, hindering the distribution of liquid water. The findings of this work can be extrapolated to improve and engineer PTLs with advanced interface properties, helping to reach the required target goals in cost and iridium loadings.

The sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode of proton exchange membrane fuel cells (PEMFCs) and the harsh environment pose challenges on the development of cheap, active and stable catalysts. As a consequence, Pt catalysts are the most utilized catalysts in commercial PEMFC stacks for automotive applications. While the progress in the recent years allowed a considerable decrease of Pt loadings (from 0.4-0.8 to ~0.1 mgPt cm-2), other challenges emerged at such and lower Pt loadings at high current densities.1 For example, octahedral PtNi nanoparticles have been reported to achieve extremely high mass activity in rotating disk electrode (RDE) experiments2 and the introduction of a third metal as surface dopant has been shown to have beneficial effects on the RDE performance.3 Despite these promising steps toward shape-stable PtNiX octahedral nanoparticles, the morphological stability and the performance in membrane electrode assembly (MEA)-based fuel cell measurements still need to be improved to match and surpass the state of the art Pt and dealloyed Pt-alloy catalysts.4 In this contribution we will show our recent efforts in improving the performance of PtNi based octahedral nanoparticle catalysts towards integration in low Pt loading cathodes for PEMFC. In particular, two strategies will be presented. First, Rh surface doping is introduced to improve the morphological stability. Second, new carbon modified supports by nitrogen plasma treatment have been investigated. References A. Kongkanand and M. F. Mathias, J Phys Chem Lett, 2016, 7, 1127-1137. P. Strasser, Science, 2015, 349, 379-380. X. Q. Huang, Z. P. Zhao, L. Cao, Y. Chen, E. B. Zhu, Z. Y. Lin, M. F. Li, A. M. Yan, A. Zettl, Y. M. Wang, X. F. Duan, T. Mueller and Y. Huang, Science, 2015, 348, 1230-1234. F. Dionigi, C. C. Weber, M. Primbs, M. Gocyla, A. M. Bonastre, C. Spöri, H. Schmies, E. Hornberger, S. Kühl, J. Drnec, M. Heggen, J. Sharman, R. E. Dunin-Borkowski and P. Strasser, Nano Lett, 2019, 19, 6876-6885. Acknowledgements The GAIA project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 826097. This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe Research. Figure 1. Mass activity from RDE measurements as a function of the electrochemically active surface area obtained by hydrogen under potential deposition, before (black) and after (red) stability tests. The blue arrow and the percentage indicate the loss after stability tests. Pt/Cv and Oh-PtNiMo/Cv data from ref.4. Figure 1

Polymer electrolyte water electrolysis (PEWE) is a key technology in the realization of a future hydrogen economy. The thorough understanding and optimization of each of its components in a synergistic matter is essential in the advancement of this technology. In this study, we aim at understanding the role of the porous transport layer (PTL) interface in contact with the catalyst layer (CL) [1] and its impact on PEWE performance losses. We addressed the PTL/CL interface by creating a matrix of well-defined two-dimensional PTL materials [2]. These 2D-PTLs are fabricated by precise laser drillings of varying sizes on thin Ti-sheets, allowing us to accurately control geometrical properties such as interfacial contact area, pore size, triple-phase boundaries (TPB), and surface roughness. This systematic approach, combined with in-depth electrochemical analysis, provides insights into how the interface parameters affect the catalyst layer utilization at different current densities leading to severe differences in mass transport losses. Furthermore, by combining operando X-ray tomographic microscopy [3] and electrochemical characterization under selective conditions, we were able to get insights into the water transport mechanism inside the CL as a function of the different interfacial regions at the PTL/CL interface. The findings related to this work will potentially contribute to a future more rational design of PTL and CL structures. [1] T. Schuler, T.J. Schmidt, F.N. Büchi, J. Electrochem. Soc. 166 (2019) F555–F565. [2] Z. Kang, J. Mo, G. Yang, S.T. Retterer, D.A. Cullen, T.J. Toops, J.B. Green Jr, M.M. Mench, F.-Y. Zhang, Energy Environ. Sci. 10 (2017) 166–175. [3] S. De Angelis, T. Schuler, M.A. Charalambous, F. Marone, T.J. Schmidt, F.N. Büchi, J. Mater. Chem. A 9 (2021) 22102–22113. Figure 1