Advanced electrocatalysts for oxygen reduction reaction

Abstract

Proton exchange membrane (PEM) fuel cells have gained great attention due to environmentally benign products and unlimited sources of reactants. The application of costly platinum as the catalysts in PEM fuel cells, however, results in high cell cost and consequently hinders their commercialization. To reduce the cost and improve the activity of Pt-based electrocatalysts, morphology controlled nanocrystals for oxygen reduction reaction (ORR), including Pt-Ni octahedral nanoparticles, Pd@Pt-Ni core-shell octahedral nanoparticles, and Ru@Pt-Ni nanoparticles were explored in this work. The physicochemical properties and ORR activities were characterized by transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), cyclic voltammetric (CV) and rotating disk electrode (RDE). Firstly, the formation mechanism of Pt-Ni octahedra was investigated by scanning TEM-EELS, XPS and ICP-MS. The morphology evolution and composition change as a function of temperature during synthesis were recorded. Pure Pt sphere with a particle size of ~3 nm was formed at a low temperature of 140°C. The Pt-Ni octahedral with a particle size of ~5 nm started to form at 170°C. Most of Pt and Ni precursors were reduced at temperatures higher than 180°C. At 190°C, the particle size grew to ~8 nm, and finally reached ~10 nm at 230°C. As a result, the Pt-Ni octahedra obtained at 230°C presented a Pt-rich center and a uniform Pt-Ni shell. The octahedral Pt-Ni catalysts showed a 10-fold higher activity than commercial Pt/C. Then, to further reduce the Pt loading in the octahedral Pt-Ni particles, the synthesis of shaped controlled core-shell nanostructures with various cores were explored. Octahedral and spherical Pd nanoparticles were synthesized and used as seeds for Pt-Ni deposition, resulting in a Pd@Pt-Ni core-shell structure. This novel structure also showed excellent ORR activities. Then I tried to synthesize 2-4 nm Ru nanoparticles as cores, which was much cheaper than Pd. The synthesis of Ru@Pt-Ni was much more difficult than that of Pd@Pt-Ni. The deposition of Pt-Ni was found non-uniform on the Ru core resulting in a much lower activity. Great effort is needed to adjust the synthesis conditions to obtain more perfect core-shell structures with higher ORR activities.

Date of Award	2016
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology