Undoubtedly, shape-controlled Pt nanoparticles are of great interest in today´s fuel cell research, because they build a unique bridge between UHV-prepared single crystals and industry-related catalysts. Since most of the fundamental studies for the oxygen reduction reaction have been performed by using single crystals, the knowledge transfer to real nanoparticle catalyst system is heavily limited.[1-3] However, surfactants and capping agents are required to control the growth, orientation and size of the seeds to form nano-scaled single crystals with high mono-dispersity. Therefore, simple strategies to clean the superficial surfaces of the single nanocrystals without any structural and electro-catalytic losses are needed.

In this work, we investigated shape-controlled Pt nanoparticles like nanocubes for the ORR by using various synthetic routes. We revealed that the electronic structure of these shape-controlled Pt nanoparticles is modified in dependence on the kind of surfactant or capping agent used in the preparation. To understand the interaction between surfactant and metallic surface, which strongly affect the number of catalytically active centers and thus the catalytic ORR activity, we used a combination of different techniques like high resolution (scanning) transmission electron microscopy (HR-(S)TEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR). In addition, we provide cleaning procedures to successfully remove the surfactant without any particle shape loss. For instance, a combination of solvent washing and electrochemical cleaning increase the electrochemically surface area of the Pt nanocubes by a factor of 10 – 15. This immeasurably boosts the catalytic properties of the Pt nanocubes for the ORR. Nevertheless, after successful cleaning process the structural stability and electrochemical durability of the “naked” Pt single-nanocrystals were investigated by applying accelerated degradation protocols. A structural transformation of the single nano-crystals to near-spherical nanoparticles was identified to be dependent on the electrochemical potential and the number of scans. Our goal is to better understand the interplay between surfactant-metal interaction and electrochemical reaction process for the development of highly active nano-crystal surfaces and to fill the knowledge gap between single crystal surfaces and nano-scaled polycrystalline materials for the ORR.

Reference

[1] M. Oezaslan, F. Hasché, P. Strasser, The Journal of Physical Chemistry Letters, 4 (2013) 3273-3291.

[2] D. Li, C. Wang, D.S. Strmcnik, D.V. Tripkovic, X. Sun, Y. Kang, M. Chi, J.D. Snyder, D. van der Vliet, Y. Tsai, V.R. Stamenkovic, S. Sun, N.M. Markovic, Energy & Environmental Science, 7 (2014) 4061-4069.

[3] I. A. Safo, M. Oezaslan, Electrochemical acta, 241 (2017) 544 - 552