The Optimization of the Reduction Temperature and the Local Crystal Structure in Titanium-Oxide Nanosheets Model-Electrode to the Oxygen Reduction Reaction in Acidic Environmental
Authors: Saida Takahiro , Mashiyama Miyu , Maruyama Takahiro 
Published: February 2020

Non-platinum metal-oxide catalyst is expected to the next generation catalyst of polymer electrolyte fuel cell (PEFC). Because the metal oxide catalyst has high natural abundance and long service time. However, the activity of metal oxide catalyst has not achieved that of practical Pt/C catalyst, yet. This means a lot of oxide catalyst needs to the operation of PEFC if it alternatives to Pt/C, at this time. Thus, the enhancement of catalytic activity in metal oxide is urgently needed.
To enhance the catalytic activity, we should know well about the reaction mechanism on metal oxide at the first. In reality, the active site of oxygen reduction reaction (ORR) in metal oxide has not been unclear. One possibility was reported that the active site of ORR is the distortion site formed by the oxygen vacancy site in the titanium-based oxide [1]. Past research used the supported catalyst to investigate the active site. The supported catalyst has a complex substance diffusion path, which is attributed to the complicated pore structure. As a result, we should take considerations not only the catalytic reaction but also the diffusion resistivity of oxygen and generation water. However, we cannot focus attention on only the ORR on the oxide catalyst besides to remove the other phenomena in the reaction system. One of the solutions is using the model electrode which can be negligible some diffusion resistivities.
In this study, in order to understand the active site, the model electrode composed by titanium oxide nanosheet was prepared and introduced the distortion site by heat treatments. And the ORR activity and the local crystal structure as an index of the distortion site were related in the each calcination temperatures.
Titanium oxide nanosheet was prepared according to previous literature [2]. The model electrode was prepared by the layer by layer method. First, the GC plate was dipped into the PDDA solution, and then it rinsed by the ultra-pure water. Next, it dipped into the titanium nanosheet solution. After the rinsing, we obtained a monolayer titanium nanosheet electrode. The distortion site was introduced by the calcination under mixture gas of 10%H2 and 90%Ar. The crystal structure was evaluated by in-plan XRD, and the electron structure was confirmed by XPS and XAFS. The ORR activity was judged from difference curve of cyclic voltammograms between oxygen flow and argon flow conditions in 0.1 M HClO4.
The shape and height of titanium oxide nanosheet were retained after the reduction treatment at 600ºC. However, its two-dimensional size shrank a little. Additionally, its crystallinity decreased drastically by the reduction treatment. A diffraction peak of 220 phase attributed to the in-plane structure of titanium oxide nanosheet was slightly sifted by the reduction treatments. This meant that the distortion site was introduced to the titanium oxide nanosheet. The ORR activity improved by the reduction treatments at 600ºC. Thus, the distortion site probably affects to ORR and acts as the active site. On the other hand, the ORR activity of titanium oxide nanosheet decreased when the calcination temperature of reduction treatment elevated until 700ºC. From the above results, the distortion of crystal structure in titanium oxide is expected to take an optimal value.
References
[1] A. Ishihara, C. Wu, T. Nagai, K. Ohara, K. Nakada, K. Matsuzawa, T. Napporn, M. Arao, Y. Kuroda, S. Tominaka, S. Mitsushima, H. Imai, K. Ota, Electrochim. Acta, 283, 1779 (2018).
[2] T. Sasaki, M. Watanabe, Y. Michiue, Y. Komatsu, F. Izumi, S. Takenouchi, Chem. Mater., 7, 1001 (1995).