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.

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) (1-4). In this paper we will report our recent advancement of the group 4 and 5 metal oxide catalyst with metal oxide support without carbon.

Recently, we published the results of precious-metal-free and carbon-free cathodes based on oxides and demonstrated the superior durability of oxide-based cathodes by preparing titanium-niobium oxides mixed with Ti₄O₇ (Ti_xNb_yO_z + Ti₄O₇) (5). The ORR activity of the Ti_xNb_yO_z + Ti₄O₇ is higher than that of the Ti₄O₇, indicating that the Ti_xNb_yO_z might have active sites for the ORR. The highest onset potential of the Ti_xNb_yO_z +Ti₄O₇ was over 1.1 V with respect to reversible hydrogen electrode. No degradation of the ORR performance of Ti_xNb_yO_z + 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 a polymer electrolyte fuel cell.

In order to qualify the role of Nb oxide for ORR, we used TiO₂-Nb (Nb; 0.5 or 5atm%) rods (TOSHIMA Manufacturing Co., Ltd) as working electrodes. In addition, TiO₂-Nb (0.5atm%) rod was heat-treated at 800 ^oC in 4%H₂/Ar to examine the effect of heat-treatment under reductive atmosphere (0.5atm%, 800^oC reduction). 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.

From the results of the potential-ORR current curves from 0.2 to 1.0 V of TiO₂-Nb(0.5 or 5atm%) and (0.5atm%, 800^oC reduction) electrodes, the ORR activity of the TiO₂-Nb(0.5atm%) electrode was higher than that of the TiO₂-Nb(5atm%) electrode. Although Nb doping is necessary to have some electric conductivity, small amount of Nb doping might be enough to get high ORR activity. In addition, the ORR activity of the TiO₂-Nb(0.5atm%_800^oC reduction) was higher than that of the TiO₂-Nb(0.5atm%). Therefore, we found that the heat-treatment under reductive atmosphere enhanced the ORR activity.

The catalytic activity might be affected by many factors. Among them we are studying the effects of crystal size, surface area, and crystal lattice. The results will be reported at the symposium. Our final target of our project is to obtain the catalyst that has better ORR activity and better durability compared to the present Pt with carbon support. We are moving towards the target.

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

REFERENCES

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