Catalytic Properties of Ir-Decorated Pt Nanoparticles As Bifunctional Catalyst for Oxygen Reduction and Evolution Reactions.
Authors: Blanco-Redondo Lucinda , Matolín Vladimír , Lobko Yevheniia 
Published: May 2020

The challenge of slowing down the climate change and ensuring the worldwide energy supply requires the progress in using renewable energy sources. This imminent global power shift has focused the attention on the hydrogen technologies such as the Unitized Regenerative Fuel Cell (URFC). It is an electrochemical device that can operate alternately as a fuel cell (FC, to produce electricity) or water electrolyser (WE, to produce hydrogen) [1]. However, the way of developing an efficient and durable unit is still opened. Oxygen electrode in URFC is a limiting component due to the complicated and slow kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) that take place there. Moreover, there is not a common material that catalyses these reactions: the most suitable materials for the ORR and OER are the costly noble metals platinum and iridium [2], respectively. As a consequence, it is necessary to reach a compromise in the composition of a bifunctional catalyst [3] based on platinum and iridium able to improve, or at least sustain, the activity and stability with the lowest metal loading possible. Many researchers have studied the efficiency of bimetallic Pt-IrO2 [4] and Pt-Ir [5] catalysts. Nevertheless, most of them were obtained by mechanical mixing of commercial powders. Consequently, the bad dispersion of the components led to a low bifunctional catalyst performance. To overcome this problem, in this study Ir-decorated Pt nanoparticles (NPs) have been produced by polyol method [6]. Briefly, the Pt precursor has been reduced at elevated temperature in ethylene glycol (EG) to form Pt NPs. After that, the Ir precursor dissolved in EG has been introduced into the synthesis flask and reduced on the surface of the existing Pt NPs. In addition, certain amount of polyvinylpyrrolidone (PVP) has been added to the precursor solutions to stabilize the nanoparticles and reduce its energy surface. Thus, the agglomeration has been prevented and the average particle size decreased [7]. In order to compare the activity of the bimetallic nanoparticles, pure Pt and Ir have been synthesized and characterized via Rotating Disk Electrode (RDE) technique, X-Ray Photoelectron Spectroscopy (XPS), and Transmission Electron Microscopy (TEM). Ir-decorated Pt NPs with various compositions (Pt15Ir85, Pt50Ir50 and Pt85Ir15) have been prepared, and the results show that the nanoparticles with a Pt:Ir ratio 50:50 display the best results. The activity towards the ORR is comparable with pure Pt nanoparticles and the OER activity is lower than pure Ir nanoparticles. Nonetheless, the Ir-decorated Pt NPs has demonstrated better bifunctionality than commercial Ptblack-Irblack mixture.

References:

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