Influence of Strong Metal-Support Interaction to the Noble Metal Catalyst for the Activity and Durability Oxygen Reduction Reaction
Authors: Kano Chika , Nimi Arisa , Maruyama Takahiro , Saida Takahiro 
Published: December 2024

The stability and the activity of electrocatalysts in fuel cells and other applications can be improved by the strong interaction between noble metal nanoparticles and transition metal oxide, a phenomenon known as “strong metal-support interaction (SMSI).” Understanding and harnessing this interaction could lead to the development of more efficient and durable electrocatalysts. When SMSI occurs, the d-band center of noble metal as the catalyst changes to the favored state for the catalytic reaction or stability. Naturally, the variation of the d-band center depends on the combination of noble metal elements and transition metal oxide species, and the optimum state of the d-band is determined by the reaction. We focused on the oxygen reduction reaction (ORR), which is the cathode reaction of polymer electrolyte fuel cells (PEFCs). Because PEFCs require highly active and durable electrocatalysts.

Generally, in catalysts where SMSI occurs, the electron-poor metal oxide covers the electron-rich noble metal particles in the reduction environment. On the other hand, the covered noble metal particles recover by conducting the oxidation treatment. An essential point for this phenomenon is that noble metal particles are smaller enough than metal oxide as the support or substrate. Therefore, the metal oxide layer can cover the noble metal particles. Theoretically, if the oxide layer is a few nanometers thick, it is difficult to cover the noble metal particles because the oxide layer is insufficient around them. In other words, it is expected that the electron structure of catalyst particles may be dramatically changed.

The aim of this study was to find the optimum combination between noble metal and oxide for ORR in cathode catalysts in PEFCs. As the first step, Ir nanoparticles were deposited on the TiO2 layer, which was a few nanometers thick, and its ORR activity was evaluated. Electrooxidation by potential scanning was employed as the oxidation method for a model electrode. Moreover, the prepared Ir/TiO2 model electrode was conducted in the thermal reduction treatments, and the XPS analysis was performed to investigate the change in its electron structure. Other combinations, for example, Pt/TiO2, Ir/CeO2, etc., will be also done.

Ir/TiO2 model electrode was composed of a glassy carbon substrate, a 1 nm TiO2 middle layer made from titanium oxide nanosheet, and Ir nanoparticles on the top deposited using the arc plasma deposition method. The electrochemical measurements were conducted using a three-electrode cell with a working electrode, a carbon rod counter electrode, an RHE reference electrode, and 0.1 M HClO4 at room temp. The reduction method was calcination at 500ºC under the reduction environment.

The XPS results indicated that the TiO2 middle layer suppressed the valence variation of Ir nanoparticles. In this study, it was confirmed that the effects on ORR by introducing the TiO2 middle layer were stabilization of ORR activity in the Ir/TiO2 model electrode and inhabitation of the dissolution of Ir nanoparticles. In the day's presentations, we will compare with other combination model electrodes and discuss the relationship between valence variation and ORR activities of several model electrodes.