Oxidative environments of acidic polymer electrolyte fuel cell (PEFC) cathodes have been a barrier to the development of active and stable platinum group metal (PGM)-free catalysts. In an alkaline environment, many non-PGM catalysts have exhibited higher oxygen reduction reaction (ORR) activity in comparison with commercial carbon-supported platinum (Pt/C) catalysts. In most cases, the higher activity of non-PGM catalysts has been reported at higher non-PGM catalyst loading than Pt/C. But in some recent reports, non-PGM catalysts displayed higher activity at catalyst loading identical to that of Pt/C.^1,2 Due to the difference in reaction mechanism, such high ORR activity has not been reported in acidic PEFC cathodes. Thus scarce and high cost carbon supported platinum cobalt catalyst has been used in automotive PEFCs and the reduction of the usage is still needed for the widespread use.³

Stability is another critical factor to introduce PEFCs into global automotive markets as corrosion of carbon supports during startup/shutdown of vehicles has been more serious than platinum dissolution.⁴ We therefore recently developed a non-PGM catalyst free from carbon supports using titanium. The bulk of the catalyst is TiN whereas the surface was oxidized naturally to form so-called titanium oxynitride (TiO_xN_y).^5,6 The optimization of the synthesis conditions enhanced the ORR activity to the level of the best carbon-supported oxide-based catalyst, zirconium oxynitride on multiwalled carbon nanotube⁷ in 0.1 mol dm^–3 H₂SO₄ solution.⁶ The activity did not change after 20 000 potential cycles between 0.6 and 1.0 V versus reversible hydrogen electrode, and the aforementioned stability was significantly improved from the previously reported carbon-supported TiO_xN_y.⁸ The disadvantage of the TiO_xN_y catalyst is small surface area to require high loading of 2.0 mg cm^–2 for evaluating the ORR activity.^5,6 For the reduction of catalyst loading via increasing the surface area, phosphor atoms were doped into the TiO_xN_y catalyst in this study. Phosphor doping has been utilized to reduce the size of PtRu nanoparticles,⁹ TiO₂ photocatalysts,¹⁰ etc. and thus increase their surface area. The X-ray diffraction and Raman spectroscopy analyses revealed that crystal structure of the bulk was TiN and the surface was rutile TiO₂ with oxygen vacancy. The effect of phosphor to titanium molar ratio on the ORR activity, stability and the reaction mechanism will be presented at the meeting.

Acknowledgments

The author acknowledges Dr Kohei Okitsu, Prof Hojun Im and Mr Yusei Tsushima for providing assistance in obtaining the X-ray photoelectron (XP) spectra, Raman spectra and microscopy images, respectively. This work was partially supported by a Grant-in-Aid for Scientific Research (C) (17K06180) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan; a research grant from Nippon Sheet Glass Foundation for Materials Science and Engineering in Japan; a research grant from Yashima Environment Technology Foundation and a research grant from Suzuki Foundation in Japan. The XP spectra were acquired with the support by Nanotechnology Platform of the MEXT of Japan.

References

1. A. Ferrero, K. Preuss, A. Marinovic, A. B. Jorge, N. Mansor, D. J. L. Brett, A. B. Fuertes, M. Sevilla and Maria-M. Titirici, ACS Nano, 10, 5922 (2016).
2. Li, Q. Gao, W. Qian, W. Tian, H. Zhang, Q. Zhang and Z. Liu, Sci. Rep., 8, 2863 (2018).
3. Yoshida and K. Kojima, ECS Interface 24, 45 (2015).
4. Parrondo, T. Han, E. Niangar, C. Wang, N. Dale, K. Adjemian and V. Ramani, Proc. Natl Acad. Sci., 111, 45 (2014).
5. Chisaka, Y. Ando, Y. Yamamoto and N. Itagaki, Electrochim. Acta, 214, 165 (2016).
6. Chisaka, Y. Yamamoto, N. Itagaki and Y. Hattori, ACS Appl. Energy Mater., 1, 211 (2018).
7. Chisaka, A. Ishihara, H. Morioka, T. Nagai, S. Yin, Y. Ohgi, K. Matsuzawa, S. Mitsushima and K. Ota, ACS Omega, 2, 678 (2017).
8. Chisaka, Y. Ando and N. Itagaki, J. Mater Chem. A, 4, 2501 (2016).
9. Daimon and Y. Kurobe, Catal. Today, 111, 182 (2006).
10. C. Yu, L. Zhang, Z. Zheng and J. Zhao, Chem. Mater., 15, 2280 (2003).