Oxygen Reduction Durability and Activity of Tantalum Oxide-Based Electrocatalysts for Cathode of Polymer Electrolyte Fuel Cells

Introduction

Because of high cost, limited availability, and insufficient stability, the use of Pt as a cathode electrocatalyst for polymer electrolyte fuel cell (PEFC) is the one of major problems. In order to successfully commercialize PEFCs, non-platinum cathode catalysts with high stability and low cost must be developed.

We have previously described the high stability in acidic media and high catalytic activity of 4, 5^th group metal oxide-based materials including tantalum oxide-based materials for oxygen reduction reaction (ORR) [1]. On the other hand, the surface areas of the catalysts prepared by heat treatment under low oxygen partial pressures remained as low as several m²×g^-1 [2]. We also indicated that the carbon and nitrogen components of the carbonitride precursors are considered to be effective for creating the oxygen-vacancy active sites [3]. However, a further investigation for the increase of the ORR current by increasing the surface areas of the catalysts should be needed.

In this study, we prepared tantalum oxide nanoparticles using the organometallic complex oxy-tantalum phthalocyanine (TaOPc) as a precursor. In addition, we investigated the relationship between the physico-chemical properties and the ORR activity to clarify the factors which influence the ORR activity.

Experimental

Oxy-tantalum phthalocyanine (TaOPc [TaOC₃₂H₁₆N₈],) was used as the starting material. The TaOPc and multi-wall carbon nanotubes (MWCNTs) were dry ball milled to obtain the catalyst precursor (TaOPc/MWCNT), which was heat-treated at 900^oC. After reaching 900°C under N₂ atmosphere, a reactive gas mixture consisting of 2% H₂ with either 0.5 or 0.05%O₂ was introduced for 0.5 – 15 h. The obtained catalysts are designated in the form of “TaCxNyOz/MWCNT_oxygen partial pressure-oxidation time”. Transmission electron microscopy (TEM; JEOL LEM-2100F) was used to observe the morphology of the catalysts.

The catalyst was supported at a loading of ca. 0.37 – 0.51 mg×cm^-2 on top of the glassy carbon (GC). The electrode measurements were performed with a 3-electrode cell contained 0.1 mol dm³ H₂SO₄ at 30°C. A reversible hydrogen electrode (RHE) and a glassy carbon plate were used as the reference and counter electrodes, respectively. Slow scan voltammetry (SSV) was performed at a scan rate of 5 mV×s^-1 from 0.2 to 1.2 V under O₂ and N₂ atmospheres. The current density was based on the weight of the oxides: mA g^-1. For the evaluation of catalyst activity, we used the current density of oxygen reduction (j_ORR) at 0.8 V vs. RHE (|j_ORR@0.8 V|) .

 Results and discussion

Figure 1 shows the tantalum oxide-based particles supported on the MWCNTs. From this figure, we successfully prepared nano-sized tantalum oxide-based catalysts using TaOPc as the precursor. In addition, the surface of the oxide-based particles was partially covered with an amorphous material, which was identified as the deposited amorphous carbon derived from the phthalocyanine structures. The deposited carbon seems to restrict the increase in the size of the oxide-based particles. In addition, the deposited carbon seems to form local electron conduction paths near the active sites. Therefore, to obtain a large ORR current, carbon deposition must be controlled to optimize the balance between the exposure of the oxide surface and the local electron conduction paths.

Figure 2 shows the relationship between the amount of deposited carbon and |j_ORR@0.8 V|. Compared with our past results [1], the ORR current was dramatically increased compared to that of TaC_xN_yO_z(CN) made from the carbonitride. The |j_ORR@0.8 V| reaches a maximum value at around 20 wt% deposited carbon. For a small amount of deposited carbon, the local electron conductivity seems to be insufficient, although the exposure area of the oxide surface increased. On the other hand, for a large amount of deposited carbon, the deposits cover the tantalum oxide and interfere with the ORR. Both TaC_xN_yO_z/MWCNT_0.5%O₂ and 0.05%O₂ have maximum |j_ORR@0.8 V| values near 20 wt% deposited carbon. This result suggests that the exposure of the oxide surface and the formation of the electron conduction paths are similar for the same amount of supplied oxygen. The difference in the |j_ORR@0.8 V| values essentially reflects the oxygen vacancies on the oxide surface.

 Acknowledgement

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

[1]    A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim. Acta, 55, 8005 (2010).

[2]    A. Ishihara, M. Tamura, K. Matsuzawa, S. Mitsushima, K. Ota, J. Fuel Cell Sci. Technol., 8, 031005 (2011).

[3]    A. Ishihara, M. Tamura, Y. Ohgi, M. Matsumoto, K. Matsuzawa, S. Mitsushima, H. Imai, K. Ota, J. Phys. Chem. C, 117, 18837 (2013).