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Synthesis and Electrocatalysis of ternary shape-controlled octahedral Pt alloy Nano catalysts for the Oxygen Reduction Reaction

Subject Area Synthesis and Properties of Functional Materials
Term from 2021 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 461853506
 
Final Report Year 2024

Final Report Abstract

Fuel cell technology advancements are critical for future energy conversion and storage applications. Fuel cells with proton exchange membranes (PEMs) are becoming increasingly popular as clean energy sources for mobile and stationary applications. The slow oxygen reduction reaction (ORR) at the cathodes of fuel cells is a major obstacle that currently limits their performance. Due to the extremely high ORR activity of the Pt3Ni (111) surface, the (111)-facetted octahedral Pt-Ni nanoparticles have attracted considerable attention. A major drawback of using octahedral Pt-Ni nanoparticles as ORR catalysts is their insufficient long-term stability. Furthermore, the transfer of their high activities to an MEA setup was insufficient, which was accompanied by a significant decrease in activity and stability. The aim of this DFG project was to overcome these drawbacks by developing new synthesis strategies for the preparation of larger octahedral shaped electrocatalysts. Additionally, doping of the octahedral Pt-Ni particles with added elements such as Mo and/or Rh as well as thermal post-synthesis treatments were used to improve the activity and stability of the as-prepared shaped catalysts. Another goal was to gain a better knowledge of their use in MEAs under real PEMFC conditions, such as high current densities, as well as to evaluate their intrinsic activity under relevant potentials for PEMFC cathode catalysts. To ensure the high quality of the fabricated material, we combined state-of-the-art structural and compositional characterization at the atomic level by aberration-corrected analytical electron microscopy with advanced material synthesis. In addition, electrochemical activity and stability tests were performed at both the RDE and GDE levels, which were closely linked to the microstructural analysis of nanoparticle formation and degradation during electrocatalysis. Iterative synthesis, electrochemical measurements, and state-of-the-art ex-situ, identical location (IL), in-situ, and operando analytical high-resolution scanning transmission electron microscopy (STEM) in combination with X-ray spectroscopy (EDX), X-ray absorption spectroscopy (XAS), and magnetic measurements were used to study the catalysts. Based on a robust yet customizable one-pot synthesis technique, a new synthetic control strategy was developed to obtain NPs with longer edge lengths (up to 17 nm) and high Ni content. Mo- and Rhdoped monodisperse large octahedral Pt-Ni nanoparticles and bimetallic and trimetallic octahedral Pt- Ni nanoparticles were developed. We extended the size window and controlled the catalyst surface design by performing different post-synthesis treatments, i.e., leaching and annealing in different atmospheres in tandem or as a single treatment. The relationship between the preparation of the ternary and quaternary octahedral Pt-Ni electrocatalyst, particle size, elemental content, and catalytic ORR reactivity at low and high current densities was investigated. This catalyst family outperformed the bimetallic PtNi counterpart and the reference Pt/C catalysts in terms of ORR performance and electrochemical stability at both RDE and GDE levels. The octahedral PtNi, PtNiMo, and PtNiMoRh nanoparticles were further investigated by in situ electron microscopy in vacuum and H2 environments to follow their morphological and compositional changes during thermal annealing. It was convincingly demonstrated that surface doping with Mo or Mo/Rh improved the morphological shape stability after thermal treatment. The main objectives of the project, the preparation of shaped ORR cathode catalysts for PEMFCs and the testing of these catalysts under PEMFC-relevant conditions, were achieved. In addition, the methodological and conceptual developments in the analysis of surface-doped octahedral PtNi nanoparticles, with respect to their synthesis, post-synthesis treatment, and electrochemical tests, combined with ex- and in-situ high-resolution electron microscopic characterization were accomplished. These results could pave the way for the application of a new class of large, welldefined, active, and stable surface-doped and post-synthesis treated optimized catalysts for use in PEM fuel cell cathodes. The results of this project have been published in nine journals and presented at various national and international scientific conferences.

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