Energy storage and conversion devices attracted growing interests to reduce emissions from vehicles by replacing conventional combustion engines to electric motors. Recent analyses on the system cost and performance for both current/post lithium ion batteries and polymer electrolyte fuel cells (PEFCs) indicate that PEFCs are suitable to drive vehicles which operate over 200 miles or need high loads such as buses and tracks.^1,2 Several minutes of refuelling time also makes this energy conversion technology promising; however, current usage of platinum in 114 kW-PEFC stack for a generic sedan is estimated in the range of 22–38 g,² which is unacceptably high considering the world reserve of platinum-group-metals (PGMs), 67,000 tons.³ Further, corrosion of carbon, which is currently used to support platinum-based nanoparticle catalysts, is known to more severe to degrade the performance of PEFC than platinum dissolution.⁴

Most non-PGM catalysts developed in the last decade is a so-called Fe/N/C catalysts, which utilize graphitic carbon materials originated from carbonized polymers,⁵ carbonized metal organic frameworks⁶ or both.^7,8 All these carbons are conductive as they contained graphitic layers with edges terminated by pyriginic nitrogen atoms. Another non-PGM catalyst type is oxide/oxynitride containing group IV or V metals. As these catalysts are not conductive, they have mostly been supported on carbon materials.^9–11 Therefore, both these non-PGM catalyst types should be protected from corrosion of carbon supports accelerated during the startup/shutdown of the cell. Develompment of carbon-support-free non-PGM catalysts is a challenging, attractive option to replace conventional carbon-supported platinum-based catalysts. We therefore recently reported titanium oxynitride (TiO_xN_y) catalyst free from carbon-supports.¹² The activity in acidic media was the highest among the ever reported carbon-support-free oxide based catalysts, whereas the minimum catalyst loading to uniformly coat the grassy carbon disk electrode was also the highest, 2 mg cm^–2 due to the high density.

In this study, various attempts have been made to lower the density and thus decrease the catalyst loading. The minimum catalyst loading was successfully reduced to 0.6 mg cm^–2, same as the standard of Fe/N/C catalysts. Effect of the loading on activity and visual reaction mechanism will be discussed at the meeting.

 

Acknowledgments

The authors gratefully acknowledge Mr. Yusei Tsushima for his help with acquisition of transmission/scanning electron microscopy images. 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, KJ-2539, from the Kato Foundation for Promotion of Science in Japan, a research grant from Nippon Life Insurance Foundation in Japan and a research grant from Yashima Environment Technology Foundation. The X-ray photoelectron spectra were acquired with the support by Nanotechnology Platform of the MEXT of Japan.

 

References

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