Factors Which Affect Oxygen Reduction Activity of Niobium Added Titanium Oxides as Non-Platinum Cathodes for Polymer Electrolyte Fuel Cells
Authors: Ota Kenichiro , Nagai Takaaki , Tamura Yuko , Arao Masazumi , Matsumoto Masashi , Matsuzawa Koichi , Imai Hideto , Napporn Teko W. , Mitsushima Shigenori , Ishihara Akimitsu 
Published: February 2020

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 dissolution of Pt cathode might be the final problem to be solved related to the stability in the present PEFC system. Additionally, the instability of carbon support is also a big problem especially for fuel cell vehicles. Carbon including graphite is thermochemically unstable at room temperature in air or oxygen containing atmosphere. 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 PEFCs. At these high potential and temperature Pt and carbon are no more stable. We need new materials, such as metal oxides that is 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) (1-7). In this paper we will report our recent advancement of the group 4 and 5 metal oxide catalyst with metal oxide support without carbon.
The precious-metal-free and carbon-free cathodes based on oxides (titanium-niobium oxides mixed with Ti4O7 (TixNbyOz + Ti4O7) showed the superior durability (5). 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 with respect to reversible hydrogen electrode. No degradation of the ORR performance of TixNbyOz + Ti4O7 was observed during both start-stop and load cycle tests. Therefore, we successfully demonstrated that the precious-metal and carbon-free oxide-based cathodes had superior durability under the cathode conditions of PEFCs.
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
In order to qualify the role of Nb oxide for ORR, we used TiO2-Nb (Nb; 0.5, 1 and 5atm%) rods as working electrodes. In reducing atmosphere treatment, TiO2-Nb (0.5atm%) rod had best ORR activity that was heat-treated at 800 oC. However TiO2-Nb(5%) showed the best result in air treatment at 800oC. The heat-treatment temperature and the atmosphere are important to get high ORR activity. In order to study the effect of crystal planes of TiO2, we used the single crystal of TiO2-Nb (0.5 atm.% Nb). The ORR activity depended on the crystal plane significantly.
The activity might be affected by many factors. We are looking for the important factor for ORR.
The authors wish to thank to the New Energy and Industrial Technology Development Organization (NEDO) for their financial support.
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