Advancement of Group 4 and 5 Metal Oxide Cathode with Oxide Support for PEFCs
Authors: Ota Kenichiro , Tamura Yuko , Nagai Takaaki , Matsuzawa Koichi , 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. The ENEFARMs (micro-co-generation system using 700 W PEFC system) are operating more than 150,000 units in Japan. Fuel cell vehicles are also commercially available in Japan.
However, the estimated amount of Pt reserve is too small to supply for the huge number of fuel cell devices. Pt is not stable in acid solution although Pt is relatively stable among the platinum group metals. The dissolution of Pt cathode might be the final problem to be solved related to the stability in the present PEFC system. In order to commercialize PEFCs systems more widely, the development of non-precious metal cathode is strongly required. 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 without carbon 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). The stability of metal oxide were evaluated by the solubility of a metal oxide in the acid solution. In oxygen containing atmosphere most of metals except Au form metal oxide thermochemically. 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 (TixNbyOz + Ti4O7) (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 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%, 800oC 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%, 800oC 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%_800oC 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 authors wish to thank to the New Energy and Industrial Technology Development Organization (NEDO) for their financial support.
REFERENCES
1) A. Ishihara, Y. Shibata, S. Mitsushima, K. Ota, Journal of Electrochemical Society, 155, 2008, B400-B406 (2008).
2) A. Ishihara, M. Tamura, Y. Ohgi, M. Matsumoto, K. Matsuzawa, S. Mitsushima, H. Imai, K. Ota,

Journal of Physical Chemistry, ser. C, 117, 18837-18844 (2013).
3) A. Ishihara, M. Chisaka, Y. Ohgi, K. Matsuzawa, S. Mitsushima, K. Ota, Physical Chemistry Chemical Physics, 17, 7643-7647 (2015).
4) N Uehara, A. Ishihara, M Matsumoto, H. Imai, Y. Kohno, K. Matsuzawa, S. Mitsushima, K. Ota, Electrochimica Acta, doi:10.1016/j.electacta.2015.03.125.
5) A. Ishihara, M. Hamazaki, M. Arao, M. Matsumoto, H. Imai, Y. Kohno, K. Matsuzawa, S. Mitsushima, and K. Ota, Journal of The Electrochemical Society, 163 (7) F603-F609 (2016)