Possibility of Oxide-Based Electrocatalyst toward Oxygen Reduction Reaction as Non Pt Cathode for PEFC

Introduction

Development of non-noble metal cathode is required to commercialize polymer electrolyte fuel cells. We focused on group 4 and 5 transition metal oxide-based cathodes, because of their high chemical stability in acidic media (1,2). We have investigated the possibility of the oxide-based cathodes from two viewpoints such as activity and durability.

Regarding the enhancement of oxygen reduction reaction (ORR) activity, we used carbon materials such as deposited carbon derived from starting materials and multi-walled carbon nanotube (MWCNT) to obtain sufficient electrical conductivity. We prepared the oxide-based cathodes with high ORR activity by heat treatment of organic complexes such as oxy-metal phthalocyanine or oxy-metal tetrapyrazino porphyrazine under low oxygen partial pressure. Nano-sized oxide-based compounds were successfully obtained to have high ORR activity.

On the other hand, regarding the durability, we prepared the oxide-based compounds without carbon to examine the durability at high potential region.

In this paper, we introduce the status of development of the oxide-based cathodes from these two viewpoints.

Experimental

For the ORR activity, oxy-titanium tetrapyrazino porphyrazine (TiOTPPz) and oxy-titanium phthalocyanine (TiOPc) were used as different starting materials. Multi-walled carbon nanotube (MWCNT) was used as catalyst support. TiOTPPz or TiOPc and MWCNT were conducted ball-mills in dry process. The powder was heat-treated for 0.5~10 h at 900^oC under N₂ containing 2% H₂ + 0.05% O₂ to prepare oxide-based catalyst powder. Ti-CNO(TPPz) and Ti-CNO(Pc) were designated oxide-based compounds made from TiOTPPz and TiOPc, respectively.

     For the durability, TiO2 and Nb₂O₅ were mixed to conduct dry ball-milling to prepare all oxide-based cathodes. The obtained powder was heat-treated at 1050^oC for 40 h under 4% H₂ to obtain reduced complex oxide (Ti_xNb_yO_z) on Ti₄O₇.

All electrochemical measurements were performed in 0.1 mol dm^-3 H₂SO₄ at 30^oC with a 3-electrode cell. 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 amount of catalyst powder. Triangular potential cycling between 1.0 and 1.5 V vs. RHE with a scan rate of 0.5 Vs^-1 was used as degradation test. During degradation test, the SSV was performed, and the durability was evaluated by degradation rate of ORR current at 0.7 V.

Results and discussion

Fig.1 shows the dependence of the ORR current at 0.85 V and the rest potential of the Ti-CNO(TPPz) and the Ti-CNO(Pc). Both Ti-CNO(TPPz) and Ti-CNO(Pc) had rutile TiO₂ structure. The ORR current at 0.85 V and the rest potential had maximum at oxidation time of 2-3 h. The rest potentials were same behavior between the Ti-CNO(TPPz) and the Ti-CNO(Pc), indicating that the quality of the active sites was almost the same. On the other hand, the ORR current at 0.85 V of the Ti-CNO(TPPz) was higher than that of the Ti-CNO(Pc). This is because the oxide particles of the Ti-CNO(TPPz) dispersed higher than the Ti-CNO(Pc). The starting materials affected the dispersion of the oxide particles.

Figure 2 shows the degradation rate of the Ti₄O₇+Ti_xNb_yO_z and the Zr-CNO(Pc)/MWCNT (zirconium oxide-based compounds prepared by heat treatment of oxy-zirconium phthalocyanines under low oxygen pressure with MWCNT as support) during triangular potential cycling between 1.0 and 1.5 V vs. RHE. The Zr-CNO(Pc)/MWCNT violently deteriorated. On the other hand, Ti₄O₇+Ti_xNb_yO_z maintained ORR activity during degradation test. The durability of Ti₄O₇+Ti_xNb_yO_z was much higher than that of Zr-CNO/MWCNT in high potential region.

Acknowledgements

The authors wish to thank ORIENT CHEMICAL INDUSTRIES CO., LTD for supply of TiOTPPz and TiOPc, and New Energy and Industrial Technology Development Organization (NEDO) for financial support.

Reference

1) A. Ishihara et al., J Phys Chem C, 117, 18837 (2013).

2) Y. Ohgi et al., J. Electrochem. Soc., 160, F162 (2013) (2013).