Improvement of the Electrocatalytic Activity of Zirconium Oxide-Based Catalyst for ORR
Development of non-noble metal cathodes as alternative materials of platinum is required for practical application of PEFCs. We have focused on the oxide-based compounds of group 4 and 5 metals. The oxidized carbonitrides of tantalum, niobium, zirconium and titanium have high catalytic activity for the oxygen reduction reaction (ORR) in acidic solution [1-3]. The zirconium oxide-based electrocatalysts (Zr-CNOs) made from oxy-zirconium phthalocyanines (ZrOPc) with heat treatment under low oxygen partial pressure showed higher activity than the carbonitride driven catalysts with smaller particle size of oxides . However, further enhancement of the catalytic activity with improvement dispersion of oxide on carbon support is needed.
In this study, the effect of kind of solvent in impregnation preparation for the dispersion of starting material on carbon support has been investigated to improve the ORR activity of zirconium oxide-based compounds.
Oxy-zirconium phthalocyanine (ZrOPc) is used as a staring material. The ZrOPc and multi-walled carbon nanotube (MWCNT) as a support were dispersed into an organic solvent using ultrasonic homogenizer. The organic solvent was N-methylpyrrolidone (NMP) or tetrahydrofuran (THF). The dispersion was stirred overnight, and then evaporated to deposit the ZrOPc on the MWCNT. The MWCNT supported ZrOPc was oxidized by rotary electrical furnace at 900oC in 0.05%O2 + 2%H2/N2 for several hours.
All electrochemical measurements were performed in 0.1 mol dm-3 H2SO4 at 30oC 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. In order to evaluate the catalytic activity of the ORR, slow scan voltammetry 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. The ORR current density (iORR) which based on the mass of the whole catalyst included the carbon support was the difference between the current density under oxygen and nitrogen. The current density of the ORR at the potential of 0.85 V was defined as iORR@0.85V to evaluate the catalytic activity.
Results and discussion
Figure 1 shows the comparison of the potential – ORR current density curves for the Zr-CNO prepared with THF (Zr-CNO(THF)) and with NMP (Zr-CNO(NMP)) which were oxidized for 300 min. The iORR@0.85V of Zr-CNO(THF) was two time larger than that of Zr-CNO(NMP).
Figure 2 shows the iORR@0.85V of Zr-CNO(THF) and Zr-CNO(NMP) as a function of oxidation time. The iORR@0.85V of Zr-CNO(THF) was much larger than that of Zr-CNO(NMP), and decreased with oxidation time. The amount of deposited carbon which derived from phthalocyanine decreased with oxidation time. As the amount of the deposited carbon decreases, the electronic conductivity decreases, and the surface area of ZrO2 exposed from carbon aggregation increases; therefore, there is optimum oxidation time to obtain higher iORR . The maximum iORR@0.85V of the Zr-CNO(THF) was obtained at shorter oxidation time. Higher dispersed ZrOPc might be easier for oxidation, therefore, the use of THF as solvent might higher disperse the ZrOPc than that of NMP, and give higher ORR activity.
The authors thank Dainichiseika Color & Chemicals Mfg.Co., Ltd. for supply of the ZrOPc, and the New Energy and Industrial Technology Development Organization (NEDO) for financial support.
 Y. Ohgi, A. Ishihara, K. Matsuzawa, S. Mitsushima, and K. Ota, J. Electrochem. Soc., 160, F162 (2013).
 K.-D. Nam, A. Ishihara, K. Matsuzawa, S. Mitsushima, M. Matsumoto, H. Imai, and K. Ota, Electrochim. Acta, 55, 7290 (2010).
 A. Ishihara, M. Tamura, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim. Acta, 55, 7581 (2010).