Oxygen Reduction Activity of TiO2 Single Crystals in Acidic Media
Authors: Ota Kenichiro , Hirata Junji , Nagai Takaaki , Arao Masazumi , Matsumoto Masashi , Kuroda Yoshiyuki , Matsuzawa Koichi , Imai Hideto , Mitsushima Shigenori , Ishihara Akimitsu 
Published: February 2020

Low temperature fuel cells have theoretically higher efficiency compared to higher temperature fuel cells such as SOFCs. Polymer electrolyte fuel cells are expected for the residential and transportable applications, due to their high power density and low operating temperature. Many ENEFARMs (micro CHP) are operating and fuel cell vehicles are also commercially available in Japan. However, the estimated amount of Pt reserve is limited and its cost is high. The instability of Pt cathode and carbon might be the big problems to improve the stability of the present PEFC system. A stable non-precious metal oxide cathode with stable metal oxide support might be the final goal for the cathode of PEFC for fuel cell vehicles. In the future energy system fuel cells should be operated at higher efficiency such as 60 %(HHV) since their theoretical efficiency is very high. To get this high efficiency, fuel cells should be operated at 0.9 V or higher. To get this high operation voltage, their operation temperature might be higher than 120 oC for future PEFCs. At these high potential and temperature Pt and carbon are no more stable. We need new materials, such as metal oxides that are stable in acid and oxygen atmosphere.

We have reported that partially oxidized group 4 and 5 metal carbonitrides and organometallic complexes are stable in an acid solution and have definite catalytic activity for the oxygen reduction reaction (ORR) (1-8). In this paper we will report our recent advancement of the group 4 and 5 metal oxide catalyst with metal oxide support without carbon.

All electrochemical measurements were performed in 0.1 mol dm-3 H2SO4 at 30 oC with a 3-electrode cell. Chronoamperometry (CA) was performed from 0.2 to 1.2 V vs. RHE under O2 atmosphere to obtain ORR current. The ORR current density was normalized by the electric charge of the double layer capacitance under N2 atmosphere

The ORR activity of the TixNbyOz + Ti4O7 is higher than that of the Ti4O7, indicating that the TixNbyOz might have active sites for the ORR. The highest onset potential of the TixNbyOz +Ti4O7 was over 1.1 V vs. RHE. No degradation of the ORR performance of TixNbyOz + Ti4O7 was observed during both start-stop and load cycle tests. Therefore, the group 4 and 5 oxide base cathodes had superior ORR activity and durability compared to Pt cathode under the cathode conditions of PEFCs.

There are some difference in ORR activities of TiOx which treated in reductive atmosphere and oxidative atmosphere at high temperature. Ti3+ on the surface may be the active site for the ORR reaction.

TiOx which was treated in reducing atmosphere at high temperature showed mostly the 4 electron reaction for ORR. The reaction rate of 4 electrons was 10 to 30 times faster than that of 2 electrons and 5 times faster than that of Pt.

The ORR activity of ZrOx catalyst that was made by the arc-plasma deposition depended on the thickness of the catalysis layer. The maximum ORR current was obtained at 2 nm. The tunneling current might help the electron conduction for ORR.

Considering these factors we are going to improve the ORR activity of group 4 and 5 metal oxide cathodes with oxide support.

The authors wish to thank to the New Energy and Industrial Technology Development Organization (NEDO) for their financial support.

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