On Dec. 2020, new MIRAI of fuel cell vehicle (FCV) from Toyota started to sell in Japan. However, the price of new model is almost same as previous model. For wide commercialization of FCV, it should be required for more development. One of the barriers of innovative system for the FCV is conventional platinum-based electrocatalyst. The cost of Pt is very expensive due to its very poor resources. Moreover, the conventional Pt-based electrocatalyst cannot reach enough performance for next generation FCV that is proposed in new roadmap released from NEDO, Japan. From this point of view, the non-precious metal electrocatalyst should be required for the breakthrough against above issue. We have focused and studied group 4 and 5 metal oxide-based electrocatalyst as non-platinum catalysts for the oxygen reduction reaction (ORR) because of low-cost, abundant reserves, and high stability in acidic electrolytes [1-2]. We found that titanium oxide prepared from TiOTPyzPz supported on multi-walled carbon nanotubes had superior ORR activity [3]. On the other hand, it was published as an international patent that the addition of other elements such as Fe and Ni is affected to enhance the ORR activity of Ti oxide-based electrocatalyst [4]. In this study, we have investigated to apply for the TiOTPyzPz as a starting material with and without Fe, Ni, and Zn addition to enhance the ORR activity of Ti oxide-based electrocatalyst.

2,3-Dicyanopyrazine, urea, and Ti isopropoxide were dissolved in quinoline and refluxed to synthesize TiOTPyzPz. Iron acetate, Nickel acetate, and Zinc acetate were also added to dissolve in quinoline to obtain the Fe, Ni, and Zn-added TiOTPyzPz as a starting material. The molar ratio of Ti:Fe:Ni:Zn was set to constant. These starting materials were mixed with carbon nanotube by ball-milling to prepare the precursors. These precursors were heat-treated under low oxygen partial pressure for 3 h to obtain titanium oxide-based catalysts. The catalyst powder was dispersed into 1-propanol with Nafion solution to prepare a catalyst ink. The ink was dropped on a glassy carbon rod, and dried for an hour to use as a working electrode in electrochemical measurement.

Electrochemical measurements were performed in 0.5 mol dm^-3 H₂SO₄ at 30 ^oC with a conventional 3-electrode cell. A reversible hydrogen electrode (RHE) and a glassy carbon plate were used as used as a reference and counter electrode, respectively. Slow scan voltammetry (SSV) was performed at a scan rate of 5 mV s^-1 from 0.2 V to 1.2 V vs. RHE under O₂ and N₂. The ORR current (i_ORR) was determined by calculating the difference between the current under O₂ and N₂.

Figure 1 shows the effect of Fe, Ni, and Zn addition to TiOTPyzPz as a starting material on the ORR polarization curves of the titanium oxide-based catalysts. The vertical axis is based on catalyst weight. Ti oxid-based electrocatalyst prepared from Fe, Ni, and Zn addition to TiOTPyzPz (TiO_x-Fe, Ni, Zn) resulted in the increase in the onset potential for the ORR and ORR current compared to that without addition (TiO_x), revealing that the addition of Fe, Ni, and Zn was found to be effective in improving the ORR activity. XRD pattern of TiO_x shows several peaks identified TiO₂-Rutile, TiO₂-Anatanse and TiO₂-Brookite while the XRD pattern of TiO_x-Fe, Ni, Zn also shows several peaks identified similar to TiO_x. In the case of XRD pattern of TiO_x-Fe, Ni, Zn, the peak identified compounds relating to additional elements such as Fe, Ni and Zn does not detect strongly. It is suggested that the addition of Fe, Ni and Zn to TiOTPyzPz conduce to the distortion of crystal phase of TiO₂ [5] and it affected to suppress the crystal growth of Ti oxide-based electrocatalyst. These facts are possibly contributed to enhance the ORR activity of TiO_x-Fe, Ni, Zn compared to that of TiO_x.

Acknowledgement: The authors thank New Energy and Industrial Technology Development Organization (NEDO) and ENEOS Tonen General Research / Development Encouragement & Scholarship Foundation for financial support.

Reference

[1] A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim. Acta, 55, 8005 (2010).

[2] A. Ishihara, S. Tominaka, S. Mitsushima, H. Imai, H. Imai, O. Sugino, and K. Ota, Curr. Opin. Electrochem., 21, 234 (2020).

[3] S. Tominaka, A. Ishihara, T. Nagai, and K. Ota, ACS Omega, 2, 5209 (2017).

[4] K. Takahashi, T. Imai, R. Monden, Y. Wakisaka, and S. Sato, Oxygen Reduction Catalyst, Process for Producing Same, and Polymer Electrolyte Membrane Fuel Cell, WO/2013/008501.

[5] Y. Yamamoto, S. Kasamatsu, and O. Sugino, J. Phys. Chem. C, 123, 19486 (2019).