Polymer electrolyte membrane fuel cell (PEMFC) are attracted attention as power source in the next generation. However, PEMFC should be achieved the low cost and the long service time in order to realize the wide-spread commercialization. As one of the ultimate solution for these issues, non-platinum electrocatalyst has been reported. In particular, group 4 or 5 transition-metal based-oxide such as Ta, Zr or Ti-based oxide showed high activity of oxygen reduction reaction (ORR) under the acidic conditions. [1] In these oxide catalyst, it is concluded that ORR is occurred at the oxygen vacancy site. [2]

Ti based oxide is attracted attention especially as the candidate of non-platinum cathode catalyst, because its chemical stability and natural abundance are higher than other material. Titanium dioxide is known as semiconductor, therefore, it needs to create the electron conductivity when used as electrocatalyst. Fortunately, the electron conductivity of titanium dioxide is improved by the oxygen vacancy site; the most famous example is Ti₂O₃, Ti₃O₅, and Ti₄O₇ as known as titanium suboxides. Thus, the concept of many researchers is efficiently creation of a lot of oxygen vacancy site.

On the other hand, titanium dioxide is as well known as photocatalyst material. In water oxidation of photocatalytic reaction, it is reported that a four coordinated titanium showed higher activity that a six coordinated titanium. [3] Moreover, the reaction site in case of the theoretical calculation and using the single crystal is thought low coordination site on surface. [4] Although the low coordination site play important role in water oxidation which is reverse reaction of ORR, it is left out of consideration in ORR.

In this study, the effect of coordination number of titanium atom for ORR activity in acidic conditions was investigated. In addition, the relationship between its bandgap and ORR activity was also examined, because bandgap of titanium oxide is narrowed by introduction of oxygen vacancy site to crystal structure.

The oxygen vacancy site into titanium oxide was introduced by using the chemical reductant. First, a titanium oxide was mixed with sodium tetrahydroborate as the chemical reductant. Little pure water was added to mixture, and then this mixture was calcined at 300ºC, 400 ºC, 500ºC, 600ºC, and 700ºC. Finally, obtained sample was washed with pure water. Those crystalline structure were decided by XRD and raman spectroscopy. The coordination number and the atomic valence were estimated by XAFS analysis. The bandgap was evaluated from diffuse reflection spectrum by UV-vis spectroscopy. The ORR activity was judged from difference curve of cyclic voltammograms between oxygen flow and argon flow conditions in 0.1 M HClO₄.

The clearly relationship between the coordination number of titanium and ORR activity was not founded. At this time, it is thought that this difference between ORR and photocatalytic water oxidation may attribute presence or absence of photoexcitation process. In contrast, the bandgap of preparation material and ORR activity was associated. This suggested presumably that the change of band structure by formation of the donner state introduced oxygen vacancy plays important role for ORR.

References

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[2] A. Ishihara, M. Tamura,Y. Ohgi, M. Matsumoto, K. Matsuzawa, S. Mitsushima, H. Imai, K. Ota, J. Phys. Chem. C, 117, 18837 (2013).

[3] M. Anpo, M. Takeuchi, J. Catalysis, 216, 505 (2003).

[4] e.g. W. N. Zhao, Z. P. Liu, Chem. Sci., 5, 2256 (2014); A. Imanishi, T. Okamura, N. Ohashi, R. Nakamura, Y. Nakato, J. Am. Chem. Soc., 129, 11569 (2007).