Investigation of the Active Site for the Oxygen Reduction Reaction on the Oxide Surface Using By in-Situ XAFS Method
Authors: Saida Takahiro , Hirano Shoko , Niwa Etsuko , Sato Fumiaki , Maruyama Takahiro 
Published: February 2020

Polymer electrolyte membrane fuel cells (PEMFCs) are attracted attention as the power source in the next generation. However, its high cost and short service life are inhibiting realize the wide-spread commercialization. As one of the ultimate solution for these issues, non-platinum electrocatalyst, such as group 4 and 5 transition-metal oxides, has been reported. [1] It is thought generally that a portion of the chemical reaction on the oxide surface is progressed at the low coordination metal site. Actually, it is concluded that the oxygen reduction reaction (ORR) of group 4 and 5 transition-metal oxides has occurred at the oxygen vacancy site. [2] In addition, our results also indicated that the active site of the precious metal oxide, such as Rh2O3, IrO2, and RuO2, maybe the low-coordination metal site on the surface. It can be easily expected that the low-coordination metal-site is the active site of ORR because the low-coordination metal-site including of the oxygen vacancy site is expected to favor the adsorption of oxygen molecules. In contrast, the active site of perovskite oxide in the alkaline electrolyte was reported as the lattice O2
2-. Although the reaction site is a possible to differ between acidic and alkaline electrolyte, the adsorption behavior of oxygen molecule to oxide surface has not been observed directly in acidic conditions.

When the ORR is progressing enough on the oxide surface, the oxygen molecules would be constantly repeated absorption, reaction, and desorption. From the stand of macroscopic viewpoint, it is assumed that some of the oxygen molecules are adsorption on the oxide surface at all time. If the oxygen molecules attack the low-coordination metal site during ORR, the metal atoms of oxide should show higher oxidation state than that of non-reaction state. In this study, the change of oxidation state of target oxides, TiOx, RuO2, Rh2O3 and so on, was observed using in-situ XAFS method in order to explain the active site on oxide surface in acidic conditions.

The precious metal oxides were prepared by the modified Adams method. A titanium oxide including the oxygen vacancy site was synthesized by the chemical reductant. In-situ XAFS measurements were conducted at Aichi Synchrotron Radiation Center and SPring-8, Japan. An electrochemical cell for in-situ XAFS was based on typical three-electrode cell, which has a carbon fiber counter-electrode and a reversible hydrogen electrode as the reference. A loading of test oxide on glassy carbon plate was ~127 mg cm-2. The ORR activity was judged from difference curve of cyclic voltammograms between oxygen flow and argon flow conditions in 0.1 M HClO4.

In case of Rh2O3, the difference of oxidation state between O2 flow and N2 flow conditions was the clearest observed as the change of XANES spectrum. In the low potential region, the CV current decreased under the N2 flow conditions with decreasing the oxidation state of Rh atom. The ORR current was clearly observed under the O2 flow conditions in this potential region. The white line height, which reflects the oxidation state in XANES spectrum, was increased by switch N2 gas to O2 gas. In addition, the white line height of O2 flow conditions was almost same as that of the high potential region which does not observe the ORR current. This phenomenon means probably that the oxygen molecules are adsorbing and reacting on the Rh metal site, as a result, the oxidation state of Rh atoms is indicated higher than that of the N2 flow condition. Thus, we concluded that the active site is the low-coordination metal site in the acidic electrolyte at this time. The results of other oxides will be reported at the venue.

References

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