Transition Metal Oxide Catalyst for Low Temperature Electrochemical Systems in Acid Media
Polymer electrolyte fuel cells are expected for the residential and transportable applications, due to their high power density and low operating temperature. Polymer electrolyte membrane electrolysis is developing for hydrogen production for the storage and transport of renewable energies because it has high efficiency and power density. The electrolyte of both systems is a proton conductive membrane and it is acid.
Platinum group metals or oxides are used for the low temperature electrochemical systems in acid media because of their catalytic activity and stability. However their cost is very high and their resources are limited. In order to commercialize these systems widely, the developments of non-precious metal based catalysts are strongly required.
We think that new non-precious metal catalysts should have both high stability and high catalytic activity.. In particular, we believe that high stability is essentially required. Some transition metal oxides could be used for the catalyst. However, most of the transition metal oxides are not stable in the acidic and oxidative atmosphere.
We started this study by searching stable materials in acid and in oxygen. Group 4 and 5 metal oxides, which are well known as valve metals, are stable even in acidic and oxidative atmosphere. However, these oxides are generally insulator. In order to get some electrical conductivity, these oxides should be modified by the formation of oxygen vacancies and/or the substitution of foreign atoms.
We have reported that partially oxidized group 4 and 5 metal carbonitrides were stable in an acid solution and had a definite catalytic activity for the oxygen reduction reaction (ORR) . We have tried to apply group 4 and 5 metal oxide-based compounds to the catalyst in the low temperature electrochemical systems in acid media. In this paper we will report our recent results of oxygen electrodes for fuel cells and water electrolyzer.
Powders of Ta, Zr, Nb and Ti compounds (carbonitride or metal complexes that contain nitrogen) were heat-treated at 800-1200oC under different flowing rate of the H2/N2gas 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 used for working electrode for ORR and OER. All electrochemical measurements were examined in 0.1 M H2SO4 at 30oC under atmospheric pressure using a conventional 3-electrode cell. The RHE was used for the reference in the same solution.
Results and Discussion
The catalytic activity of our material strongly depended on the degree of oxidation (DOO) for these compounds. An appropriate oxidation is essential to have a definite catalytic activity.The onset potentials of ORR for partially oxidized Zr carbonitride (Zr-CNO) were 1.05 V vs RHE that is the onset potential of commercial Pt-C. In order to improve the current density especially at high potentials we have tried to use metal complexes that have nitrogen as starting materials. More than 100 times improvements have been obtained using these materials for ORR. Although the stability of these materials are very high even at a high potentials, the activities of OER are not enough for water electrolysis. We have to improve the OER activity.
The authors wish to thank to the New Energy and Industrial Technology Development Organization (NEDO) for their financial support.