Development of Group 4 and 5 Metal Oxide Cathode with Electron Conductive Oxide

A polymer electrolyte fuel cell (PEFC) system was first invented in 1950s in USA for space use.  PEFCs are now commercially available in Japan.  Total number of installed ENE-FARMs, the home cogeneration system using PEFC, is over 12,000 units and the thermal efficiency is over 90 %.  The commercialization of fuel cell vehicles has been started at the end of last 2014 as the ultimate clean car due to their high power density and low operating temperature.  However, a cost reduction is needed and the cost of Pt is a key for the reduction.  And the estimated amount of Pt reserve is too small to supply for the huge number of fuel cell vehicles.  As a cathode catalyst, Pt has large oxygen over potential compared to the anode.  Pt dissolves in acid and oxygen containing atmosphere in nature and this causes some instability.  In order to commercialize the fuel cell systems widely, these problems should overcome and the development of a non-precious metal cathode is strongly required.

We believe that high stability in cathodic condition is essentially required for the cathode catalyst. We started this study by searching stable materials in acid and in oxygen containing atmosphere by measuring the solubility in sulfuric acid.  The solubilities of group 4 and 5 metal oxides are smaller than that of Pt in acidic and oxidative atmosphere. These oxides might be more stable than Pt at PEFC cathode condition.  However, these oxides are generally insulator. In order to get some electrical conductivity, these oxides should be modified by the formation of the oxygen vacancy and/or the substitution of foreign atoms. We have reported that partially oxidized group 4 and 5 metal carbonitrides and metal complexes which contain nitrogen were stable in an acid solution and had a definite catalytic activity for the oxygen reduction reaction (ORR) (1-4).  In this paper we will report our recent advancements of group 4 and 5 transition metal oxide cathode for PEFCs using electron conductive oxides as the catalyst support.

Powders of metal (Metal: Ta, Zr, Nb, Ti) compounds (carbonitride or metal complexes that contain nitrogen) were heat-treated at 800-1200^oC under different flowing rate of the H₂/N₂ gas mixtures that containing small amount of oxygen to obtain specimens with different oxidation state.  After heat treatment, the compounds changed to oxides that contained small amount of carbon and nitrogen.  Heat treated powder was mixed with alcohol, carbon 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 30^oC 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 to obtain the current for the oxygen reduction reaction (ORR).

An appropriate oxidation is essential to have a definite catalytic activity for the ORR.  The onset potentials of partially oxidized Zr and Ti compounds with the electron conductive oxide support have reached over 1.1 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 the decay of acitivity in both cases even at 80^oC, although the current was small.  These materials have great potential for PEFC cathode, especially for the cost reduction that is most important among the present PEFC technologies.

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

References

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