Stability of Group 4 and 5 Metal Oxide Cathode with Titanium Oxide Support for PEFCs
Authors: Ota Ken-ichro , Matsuzawa Koichi , Nagai Takaaki , Ishihara Akimitsu , Mitsushima Shigenori 
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 commercially available in Japan.

However, the estimated amount of Pt reserve is too small to supply for the huge number of fuel cell systems. In order to commercialize PEFCs systems more widely, the development of a non-precious metal cathode is strongly required. Additionally, the instability of carbon support is also a big problem especially 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 the stability of the group 4 and 5 metal oxide catalyst with metal oxide support without carbon.

Powders of group 4 and 5 metal (Metal: Ta, Zr, Nb, Ti) compounds were mixed with Ti oxide, and were heat-treated at 600-1200oC under hydrogen gas mixtures that containing small amount of oxygen.  After heat treatment, the compounds changed to oxides that contained small amount of carbon and nitrogen.  Heat treated powder was mixed with alcohol and Nafion.  The mixture was dipped on a glassy carbon rod (5 mm diameter) and the working electrode was made. All electrochemical measurements were examined in 0.1 M H2SO4 at 30oC and 80oC under atmospheric pressure using a conventional 3-electrode cell. The RHE was used for the reference in the same solution.  Slow scan voltammetry (scan rate: 5 mVs-1) was performed under O2 and N2 atmosphere respectively to obtain the current for the oxygen reduction reaction (ORR).  Potential cycling tests were conducted by the rectangular wave between 1.0 V and 1.5 V and by the triangular wave between 0.6 V and 1.0 V in order to check the durability of our materials. 

TixNbyOz and Ti4O7 mixture was formed by the heat treatment of a mixture of Ti and Nb compounds.  The Ti4O7 is an electronic conductor.  We obtained clear results of ORR current with using these cathodes.  The onset potentials of partially oxidized Ti and Nb compound with the electron conductive oxide support have reached over 1.15 V vs RHE that is more than that of commercial Pt-C.  The active point of our materials might have a better activity compared to Pt. 

Potential cycling test was conducted between 1.0 V and 1.5 V using triangler wave, and between 0.6 V and 1.0 V using rectangular wave, respectively.  We have not detected any decay of activity in both cases even at 80oC, although the current was small.  Modified group 4 and 5 metal oxides have a great potential for the better activity for ORR than Pt as well as the better stability than Pt. 

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

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