Oxygen Reduction Reaction on a Titanium Suboxide Possessing Anatase or Rutile Structure

Polymer electrolyte membrane fuel cells (PEMFCs) are attracted attention as power source in the next generation. However, the wide-spread commercialization of PEMFCs is delaying by its high cost and short service life. This reason is that the platinum using as electrocatalyst is essentially high cost, and degrades under operating conditions of PEMFCs. One of the answers for these issues is development of non-platinum electrocatalyst. Recently, several non-platinum electrocatalyst, such as oxide, carbonitride, carbon alloy, metal, etc., has been reported. Ishihara and co-workers demonstrated that the partially oxidized carbonitrides showed high activity for oxygen reduction reaction (ORR). [1] In addition, the active site of ORR in the partially oxidized carbonitrides was concluded as oxygen vacancies. [2] However, carbon species and few nitrogen atoms remained in these samples after the partially oxidized processes. This means that it is hard to estimate only the influence of oxygen vacancies for ORR activity. In other hands, titanium suboxide is often researched as photocatalyst. It is well known that the color of titanium dioxide is changed to black from white in case of presence of oxygen vacancies. The famous structures of titanium suboxide are Ti₂O₃ and Ti₃O₅, Ti₄O₇. Late years, titanium suboxides possessing anatase or rutile structure were also reported. [3, 4] An issue of these titanium suboxide is that these synthesis method is too complex and/or needs special reaction equipment. But, it had been reported that the (110) plane of heat treated titanium oxide influenced to ORR activity, while it was low ORR activity in comparison with the partially oxidized carbonitrides. [5] An advantage of titanium dioxide is that it can exclude other elements. When it can innovate the oxygen vacancies to the rutile structure of titanium dioxide, the influence of oxygen vacancies is only able to estimate without other elements from the reaction field of ORR. Moreover, the impact of crystalline structure of titanium suboxide is possible to evaluate by comparison between the anatase and the rutile structure of titanium suboxide. In this study, anatase and rutile structured titanium suboxides were synthesized by simple method, and these properties and ORR activity were investigated in order to clarify effect of the oxygen vacancies and crystalline structure to ORR.

Titanium suboxides who possessed anatase or rutile structures were synthesized using a sodium tetrahydroborate as reductant. First, a titanium dioxide was mixed with sodium tetrahydroborate in nitrogen atmosphere. Little pure water was added to mixture, and then this mixture was calcined at 500ºC. Finally, obtained sample was washed with pure water. The crystalline structure of both titanium suboxides was decided by XRD and raman spectroscopy. The chemical state of both titanium suboxides was estimated by XPS. The ORR activity was judged from difference curve of cyclic voltammograms between oxygen flow and argon flow conditions in 0.5 M H₂SO₄.

The crystalline structure of synthesis samples was shown anatase or rutile structure. In addition, synthesized titanium suboxides suggested possibility that few oxygen vacancy is formed and amount of vacancy is low in comparison with Ti₄O₇ and Ti₂O₃, Ti₃O₅. ORR activity of rutile structured titanium-suboxide was higher than that of titanium dioxide. Thus, it is thought that oxygen vacancies are positive effects to the ORR activity.

References

[1] A. Ishihara, Y. Shibata, S. Mitsushima, K. Ota, J. Electrochem. Soc., 155, B400 (2008); Y. Ohgi, A. Ishihara, K. Matsuzawa, S. Mitsushima, K. Ota, J. Electrochem. Soc., 157, B885 (2010); K. Suito, A. Ishihara，M. Arao，M. Matsumoto，H. Imai, Y. Kohno，K. Matsuzawa，S. Mitsushima，K. Ota, Nenryou Denchi, 12, 130 (2013). [In Japanese]

[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] X. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science, 331, 746 (2011).

[4] Z. Zhao, H. Tan, H. Zhao, Y. Lv, L.-J. Zhou, Y. Song, Z. Sun, Chem. Commun., 50, 2755 (2014).

[5] J.-H. Kim, A. Ishihara, S. Mitsushima, N. Kamiya, K. Ota, Electrochim. Acta, 52, 2492 (2007).