Oxide-Based Electrocatalysts as Precious Metal and Carbon Free Cathodes for PEFC

Introduction

Development of non-platinum electrocatalysts for oxygen reduction reaction (ORR) is required for real commercialization of polymer electrolyte fuel cells. We focused on high stability of group 4 and 5 metal oxide-based compounds in acidic media. Recently, we found that group 4 and 5 metal oxide-based compounds prepared by oxidation of oxy-metal phthalocyanines under low oxygen pressure with multi wall carbon nanotube (MWCNT) as support had high ORR activity^1,2). MWCNT and deposited carbon which was formed with thermal decomposition of the oxy-metal phthalocyanine works as electron conduction pass, but they are oxidized at high potential. Therefore, we need to prepare carbon free (all oxide-based) electrocatalysts to demonstrate that the oxide-based cathodes have high durability.

In this study, we prepared all oxide-based cathodes which were composed of titanium magneli phase (Ti₄O₇) as electric conductive³⁾and complex oxide with oxygen vacancies as active sites.

Experimental

TiO₂ nano-powder was mixed with transition metal oxides such as Nb₂O₅, Ta₂O₅, or ZrO₂ with the weight ratio of 8:2, respectively. The mixtures were ball-milled at 600 rpm in 180 min to form complex oxides. They were reduced at 1050^oC in 4%H₂/N₂ for several ten hours to form Ti₄O₇ and complex oxide with oxygen vacancies (Ti₄O₇+Ti_xM(Nb,Ta,Zr)_yO_x).

All electrochemical measurements were performed in 0.1 mol dm^-3 H₂SO₄ at 30^oC with a 3-electrode cell. Working electrode was glassy carbon rod with Ti₄O₇ + M(Nb,Ta,Zr)O_x of 0.25 mg. A reversible hydrogen electrode (RHE) and a glassy carbon plate were used as a reference and a counter electrode, respectively. Slow scan voltammetry (SSV) was performed with a scan rate of 5 mV s^-1 from 0.2 to 1.2 V vs. RHE under O₂ and N₂ atmosphere to obtain ORR current. ORR current density (i_ORR) was based on the mass of the catalyst powder. Triangular potential cycling between 1.0 and 1.5 V vs. RHE with a scan rate of 0.5 V s^-1 was used as degradation test. During degradation test, the SSV was performed, and the durability was evaluated by ratio of i_ORR at 0.7 V before and during degradation test, N_{-iORR@0.7 V}.

Results and discussion

Figure 1 shows the potential – i_ORR curves of the carbon free electrocatalysts in 0.1 mol dm^-3 H₂SO₄ at 30^oC. Only Ti₄O₇ had poor ORR activity. On the other hand, the ORR activity of the Ti₄O₇+Ti_xM(Nb,Ta,Zr)_yO_x had higher than that of the Ti₄O₇. Therefore, the active site might be oxygen vacancies on the Ti_xMe_yO_zs. These results revealed that complex oxide with oxygen vacancies formed on oxide conductive such as Ti₄O₇ showed ORR activity. Ti₄O₇+Ti_xNb_yO_zshowed the highest ORR activity in this study.

Figure 2 shows the degradation of |i_{ORR at 0.7 V}| during triangular potential cycling between 1.0 and 1.5 V vs. RHE of Ti₄O₇+Ti_xNb_yO_z. The degradation behavior of zirconium oxide-based compounds prepared by heat treatment of oxy-metal phthalocyanines under low oxygen pressure with MWCNT as support, Zr-CNO/MWCNT^1,2) was also plotted for comparison. The durability of Ti₄O₇+Ti_xNb_yO_zwas much higher than the durability of Zr-CNO/MWCNT in high potential region. Therefore, we successfully demonstrated that the carbon free (all oxide-based) electrocatalyst had high durability in high potential region.

Acknowledgements

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

Reference

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