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Hierarchical Pt-based Metals (HPMs): Hierarchies Controlling, Stability Enhancing and ORR-Performance Promoting

Subject Area Inorganic Molecular Chemistry - Synthesis and Characterisation
Term from 2019 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 410864487
 
Final Report Year 2024

Final Report Abstract

Porous and non-porous nanoparticles (NPs) of primarily platinum, but also of nickel, palladium, rhodium, ruthenium, tin and of PtPd alloys and iridium oxide were synthesized in ionic liquid media and fully characterized inter alia for their particle size and size distribution. For handling and stabilization, the NPs were in addition supported on carbonaceous materials such as covalent triazine frameworks (CTFs), reduced graphite oxide (rGO), Ketjen Black (KB), metal-organic frameworks (MOFs, here ZIF-8). In addition, mono- and mixed-metal (Ni/Co, Ni/Fe) oxide-hydroxides were prepared from MOFs as sacrificial agents. The NPs and composites were tested for their performance in the oxygen reduction reaction (ORR) but also in the oxygen and hydrogen evolution reaction (OER, HER) which are all relevant for fuel cells and electrocatalytic water splitting. Here we summarize only the results concerning ORR: The hierarchical PtPd bimetallic nanoparticles with different nanosizes (40, 60 and 80 nm) exhibited about 10-fold higher activity over commercial Pt/C and 82% or maintained catalytic activity after 5,000 cycles, also much higher than that of commercial Pt/C. Thereby, the morphology and crystal structure of these hierarchical PtPd-NPs did not change significantly after the long-cycle stability test. Hierarchical PtPd-NPs with a particle size of ~5 nm exhibited an about 5-fold higher activity compared to commercial Pt/C catalyst towards ORR. After 8,000 cycles of stability tests, the mass activity and specific activity of the hierarchical PtPd quantum dots became ca. 8-times higher than those of commercial Pt/C. Again, after the stability tests, there were no distinct changes to the morphology and particle size of hierarchical PtPd-NPs, while severe aggregation was found for the initially highly dispersed commercial Pt/C. For the efficient ORR performance of hierarchical Pt-based materials, a d-π catalytic mechanism of the PtPd-NPs was suggested from the detailed characterization of the surface interface structure and theoretical calculations. By achieving the hierarchical Pt-based nanoalloy to form a strong d-π effect, a strong distortion of the electronic structure was realized to enhance the electron transfer from the Pt-based alloy and optimization of the Pt 5d orbital, which thus significantly improve the ORR catalytic activity and stability of Pt-based materials. Lattice fusion was found to explain the high stability of hierarchical PtPd-NPs in ORR through high-resolution TEM characterization. In Pt-NP@rGO the Pt-NPs are homogeneously distributed on the rGO, without any aggregation while in the absence of rGO dense aggregates of Pt-NPs are formed. Pt-NP@rGO samples exhibited a 2-fold activity for ORR, compared with standard Pt/C 60%. Iridium oxide (IrOx-NP) and Pd-NPs supported on a 2,6-dicyanopyridine-based CTF (DCP-CTF) performed with high half-wave potentials between 810 and 872 mV which can compete with the high performance of Pt/C. Exemplary cyclic voltammetry durability tests with 1000 cycles and subsequent TEM analyses showed good long-term stability of the materials. Ni-NPs supported on CTF-1 showed activity for ORR. with a half-wave potential of 775 mV which is only slightly lower than commercial Pt/C (890 mV). Additionally, accelerated durability tests of 2000 cycles indicated only a slight decrease in activity, demonstrating superior stability. In total 28 publications originated from this project.

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