Oxide-Based Electrocatalysts toward Oxygen Reduction Reaction as Non Pt Cathodes for PEFC

Introduction

Development of non-noble metal cathode is required to commercialize polymer electrolyte fuel cells. We have investigated group 4 and 5 transition metal oxide-based cathodes. We reported that carbonitrides of tantalum, niobium, zirconium and titanium oxidized under low oxygen partial pressure had high catalytic activity for the oxygen reduction reaction (ORR) in acidic solution^1-4). And we revealed that active site was oxygen vacancy on the oxide⁵⁾.

In this study, we focused on tantalum whose oxide had high stability in acidic solution. We succeeded preparing the higher active catalyst by using organic metal complexes as stating materials and ball milling method⁶⁾.  In this study, we tried to find out the factors which affect ORR activity.

Experimental

Oxy-tantalum phthalocyanine (TaOPc) and multi-walled carbon nanotube (MWCNT) as a conductive material were conducted ball-mills. The powder was heat-treated for 0.5~15 h at 900^oC under N₂ containing 2% H₂ + 0.5% or 0.05% O₂ to prepare oxide-based catalyst powder (Ta-CNO).

All electrochemical measurements were performed in 0.1 mol dm^-3 H₂SO₄ at 30^oC with a 3-electrode cell. A reversible hydrogen electrode (RHE) and a glassy carbon plate were used as a reference and a counter electrode, respectively. A slow scan voltammetry was performed at a scan rate of 5 mV s^-1 from 0.2 to 1.2 V under O₂ and N₂ atmosphere. A current density was based on the amounts of catalyst powder with carbon support. The ORR activity was evaluated by using current density at 0.8 V vs RHE (｜i_ORR@0.8V｜) and potential at 5 mA g^-1 (Onset potential; E_ORR@5 mA g^-1). XRD was performed to reveal the Full Width at Half Maximum (FWHM) of Ta₂O₅ (001).

Results and discussion

A carbon made from phthalocyanine structure was deposited on tantalum oxide during heat treatment. This deposit carbon formed electron conduction path. Fig.1 shows the relationship between │i_ORR@0.8 V│ and amount of deposit carbon. Amount of deposit carbon decreased by increasing oxidation time each oxygen partial pressure. It was found that │i_ORR@0.8 V│ was higher when the amount of deposit carbon was about 20 wt%. In case of small amount of deposit carbon, electron conductivity was insufficient. On the other hand, in case of large amount of deposit carbon, the deposit carbon covered with the tantalum oxide and interfered with ORR on the tantalum oxide. Therefore, moderate amount of deposit carbon is required to obtain high ORR current density.

Fig.2 shows the relationship between E_ORR@5 mA g^-1 and FWHMs of Ta₂O₅ (001). FWHM decreased by increasing oxidation time each oxygen partial pressure. It was found that the E_ORR@5 mA g^-1 increased with increasing the FWHM. Larger FWHM meant the lower crystallinity of Ta₂O₅. Lower crystallinity of Ta₂O₅ suggested the existence of oxygen vacancies with high ORR active site.

From these results, it was found that │i_ORR@0.8 V│ was correlated with amount of deposit carbon and E_ORR@5 mA g^-1 was correlated with FWHM of Ta₂O₅ (001).

Acknowledgements

The authors thank Dainichiseika Color & Chemicals Mfg.Co., Ltd. for supply of the TaOPc, and New Energy and Industrial Technology Development Organization (NEDO) for financial support.

Reference

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