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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