Key Factors to Display Oxygen Reduction Reaction Activity on Support-Free Group IV Metal Oxynitride Catalysts in Acidic Media
Authors: Chisaka Mitsuharu , Yamamoto Yusuke , Itagaki Noriaki 
Published: February 2020

Conductive carbon black powders have been used to support platinum-group-metal (PGM) catalysts in polymer electrolyte fuel cells (PEFCs). The PEFC-powered vehicles have been commercialized recently, yet their prices remain high due to the scarce and expensive PGMs in the catalysts, as well as the need to protect carbon-black supports from corrosion.1,2 In PEFC, the PGM loading required for oxygen reduction reaction (ORR) at the cathode is an order of magnitude higher than that for hydrogen oxidation reaction at the anode.3 At potentials above 0.207 V versus the standard hydrogen electrode, the carbon support corrodes. The corrosion rate is especially high during the startup/shutdown of the cell, due to the so-called reverse-current decay mechanism which increases the cathode potential up to ~1.5 V.4 Therefore, non-PGM cathode catalysts and/or PGM catalysts on carbon-free supports have been extensively developed to reduce the cost of PEFC. Most of the non-PGM catalysts developed to date utilize graphitic carbon materials such as carbon black,5 carbon nanotube,6,7 graphene,7 carbonized polymers,5 carbonized heterocyclic compounds8 or carbonized Metal Organic Frameworks.9 Development of non-PGM cathode catalysts free from carbon supports remains challenging and only a few papers have been published, in which the reported geometrical current densities were moderate, the order of micro ampere per square centimeters in a practical potential range.10

In this work, support-free titanium oxynitride and zirconium oxynitride catalysts were synthesized using a recently developed solution phase combustion route11 with the modification of removing carbon-support. In 0.1 mol dm–3 H2SO4 solution, titanium oxynitride showed three orders of magnitude larger current densities than those studies.10 Compared with the previously synthesized carbon-supported titanium oxynitride,11 the activity was enhanced by 0.05 V of half-wave potential but the selectivity was similar, indicating that ORR proceeded on titanium oxynitride and carbon-support was not necessary. The support-free zirconium oxynitride showed no activity owing to the insulating nature, indicating that activity of the titanium oxynitride is not from the carbon traces from precursors. The higher conductivity of titanium oxynitrides as compared with zirconium oxynitrides is due to amenable nature to the incorporation of oxygen defects. The effect of the source of titanium, dispersant of precursors on the activity will be shown at the meeting.

Acknowledgments

The authors gratefully acknowledge Mr. Yusei Tsushima for his help with acquisition of transmission electron microscopy images. This work was partially supported by a Grant-in-Aid for Scientific Research (C) (26420132) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan; a research grant from the Naoji Iwatani Foundation of Japan; a grant for chemical research from the Foundation for Japanese Chemical Research and a research grant from Nippon Sheet Glass Foundation for Materials Science and Engineering in Japan. The X-ray photoelectron spectra were acquired with the support by Nanotechnology Platform, 12024046 of the MEXT of Japan.

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