Fe and Zn Addition Effects of Ti Oxide-Based Electrocatalyst on Catalytic Activity for Oxygen Reduction Reaction
Authors: Matsuzawa Koichi , Obata Momo , Takeuchi Yuu , Ohgi Yoshiro , Nagai Takaaki , Monden Ryuji , Ishihara Akimitsu 
Published: September 2023

The conventional catalyst of polymer electrolyte fuel cell (PEFC) is a platinum which is categorized as precious metal with very high-cost material, and its performance is limited in principal. From this point of view, the non-precious metal electrocatalyst should be required. We have focused and studied group 4 and 5 metal oxide-based electrocatalyst as non-platinum catalysts for the oxygen reduction reaction (ORR) because of low-cost, abundant reserves, and high stability in acidic electrolytes [1-2]. We found that titanium oxide prepared from TiOTPyzPz supported on multi-walled carbon nanotubes had superior ORR activity [3]. On the other hand, it was published as an international patent that the addition of other elements such as Fe and Ni is affected to enhance the ORR activity of Ti oxide-based electrocatalyst [4]. We have also applied for the TiOTPyzPz as a starting material with Fe, Ni, and Zn addition to enhance the ORR activity of Ti oxide-based electrocatalyst [5-6]. In this study, we have investigated the addition effects of Zn and Fe for Ti oxide-based electrocatalyst on the catalytic activity for the ORR.

2,3-Dicyanopyrazine, urea, and Ti isopropoxide were dissolved in quinoline and refluxed to synthesize TiOTPyzPz. Iron acetate and zinc acetate were also added to dissolve in quinoline to obtain the Fe and Zn-added TiOTPyzPz as a starting material. These starting materials were mixed with carbon nanotube by ball-milling to prepare the precursors. These precursors were heat-treated under low oxygen partial pressure at 900 oC for 3 h to obtain the oxide-based electrocatalyst powder. The catalyst powder was dispersed into 1-propanol with Nafion solution to prepare a catalyst ink. The ink was dropped on a glassy carbon rod to use as a working electrode in electrochemical measurement.

Electrochemical measurements were performed in 0.5 mol dm-3 H2SO4 at 30 oC with a conventional 3-electrode cell. A reversible hydrogen electrode (RHE) and a glassy carbon plate were used as used as a reference and counter electrode, respectively. Slow scan voltammetry (SSV) was performed at a scan rate of 5 mV s-1 from 0.2 V to 1.2 V vs. RHE under O2 and N2. The ORR current (i
ORR) was determined by calculating the difference between the current under O2 and N2.

Figure 1 shows the ORR polarization curves Fe and/or Zn addition to the Ti oxide-based electrocatalysts. The Fe and Zn added Ti oxide-based electrocatalyst (Fe,Zn-TiOx) was obviously the highest activity for the ORR, and it was higher ORR activity than Fe added Ti oxide-based electrocatalyst (Fe-TiOx), Zn added Ti oxide-based electrocatalyst (Zn-TiOx), and Ti oxide-based electrocatalyst (TiOx) without addition. It reveals that the addition of Fe and Zn was found to be effective for enhancing the ORR activity, and Fe addition was more effective than Zn addition for enhancing the ORR activity. XRD pattern of TiOx shows several peaks identified TiO2-Rutile and TiC0.3N0.7 while the XRD pattern of Fe-TiOx, Zn-TiOx, and Fe,Zn-TiOx also shows several peaks identified similar to TiOx. According to results from ICP analysis, the contain amount of Ti and Fe in Fe,Zn-TiOx was similar to preparation amount in precursor, but that of Zn showed below detection limit. From SEM images, Zn-TiOx and Fe,Zn-TiOx have more space in its powder than Fe-TiOx and TiOx. Based on above results, it is suggested that Zn addition affected to increased electrochemical surface area of TiOx, and Fe addition affected to form active site [7] in TiOx as similar case as Zr oxide-based electrocatalyst [8].

Acknowledgement: The authors thank New Energy and Industrial Technology Development Organization (NEDO) and ENEOS Tonen General Research / Development Encouragement & Scholarship Foundation for financial support.

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Figure 1