Enhancement of Oxygen Reduction Activity of Titanium Nano-Oxide-Based Electrocatalysts for PEFC Cathode

Introduction

Development of non-noble metal cathodes as alternative materials of platinum is required for practical application of PEFCs. We focused on the oxide-based compounds of group 4 and 5 metals because of their high chemical stability. We reported that carbonitrides of tantalum, niobium, zirconium and titanium oxidized under low oxygen partial pressure had high catalytic activity for oxygen reduction reaction (ORR) in acidic solution [1-3].

Recently, we found that titanium oxide-based electrocatalysts (Ti-CNOs) made from organic titanium complexes with heat treatment under low oxygen partial pressure showed high oxygen reduction reaction (ORR) activity [4].

In this study, we prepared two kinds of Ti-CNOs, that is, Ti-CNO(TPPz_NMP) prepared by the impregnation method using N-methylpyrrolidone (NMP) as solvent and Ti-CNO(TPPz_dry) prepared by ball-milling in dry process, and compared them to obtain principle of catalyst design.

Experimental

 Oxy-titanium tetrapyrazino porphyrazine (TiOTPPz) was used as a starting material.

TiOTPPz was supported on multi-walled carbon nanotube (MWCNT) as a conductive support by impregnation method using NMP as solvent. The powder was heat-treated at 900^oC under N₂ containing 2% H₂ + 0.05% O₂ to prepare oxide-based catalyst powder (Ti-CNO(TPPz_NMP)). TiOTPPz was mixed with MWCNT and the mixture was conducted ball-mill in dry process. The powder with dry ball-mills was also heat-treated under the same condition to obtain Ti-CNO(TPPz_dry).

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. As pre-treatment, the potential was scanned 200 cycles from 0.05 to 1.2 V with a scan rate of 150 mV s^-1 in O₂ atmosphere.  Then, the rest potential was measured in O₂ atmosphere. Next, the potential was scanned 3 cycles from 0.2 to 1.2 V at a scan rate of 5 mV s^-1 in both O₂ and N₂ atmosphere, and the cathodic current density of 3rd cycle was used for the evaluation of the ORR activity.

Results and discussion

Figure 1 shows the potential – ORR current density curves and the rest potential of the Ti-CNO(TPPz_dry) and Ti-CNO(TPPz_NMP) which were oxidized 10 h. The ORR current density, i_ORR, of the Ti-CNO(TPPz_dry) was larger than that of the Ti-CNO(TPPz_NMP). The rest potential of the Ti-CNO(TPPz_dry) was also higher than that of Ti-CNO(TPPz_NMP).

Figure 2 shows the relationship between the rest potential and the |i_ORR@0.85 V| of the Ti-CNOs(TPPz_NMP) and the Ti-CNOs(TPPz_dry). Positive correlation was observed. The Ti-CNOs (TPPz_dry) had higher rest potentials and larger ORR current than the Ti-CNOs(TPPz_NMP). The secondary particle size of the Ti-CNOs(TPPz_dry) was smaller than that of the Ti-CNO(TPPz_NMP). Therefore, the precursor prepared by ball-milling oxidized more uniformly due to small secondary particles, resulting that the Ti-CNOs(TPPz_dry) had high ORR activity. The highest rest potential was over 1.0 V vs. RHE.

Acknowledgements

The authors wish to thank ORIENT CHEMICAL INDUSTRIES CO., LTD for supply of oxy-titanium tetrapyrazino porphyrazine (TiOTPPz), and the 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).

[3] K.-D. Nam, et al.,Electrochim. Acta, 55, 7290 (2010).

[4] K. Suito, et al., J. Fuel Cell Technol., 12, 130 (2013). [in Japanese]