(Digital Presentation) Improvement of Oxygen Reduction Activity on Ti Oxide-Based Electrocatalyst

For widespread utilization of fuel cell system in the future, we have focused on non-precious metal electrocatalyst against high cost of system. Especially, we have studied and reported 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 have found excellent ORR activity of Zr oxide-based electrocatalysts prepared from pyrazinecarboxylic acid as a precursor [3-4]. In the case of Ti oxide-based electrocatalysts, the sample prepared under low partial pressure of oxygen with the heat treatment shows high ORR activity when it uses nitrogen-containing porphyrin cyclic organic Ti complexes (TiOTPPz) with multi-walled carbon nanotubes (MWCNTs) as a precursor. However, the heat treatment conditions have not widely investigated yet [5]. In this study, we have investigated to improve ORR activity of Ti oxide-based electrocatalyst prepared from TiOTPPz by varying the oxygen concentration during heat treatment. Oxytitanium tetrapyrazinoporphyrazine (TiOTPPz, [TiOC 24 H 8 N16]), a porphyrin cyclic nitrogen-containing organic Ti complex, was used as a starting material. 0.39 g of TiOTPPz and 0.26 g of MWCNT were mixed and treated in a dry ball mill for 1 h to obtain the precursor. The precursor was heated up to 900 o C in an Ar atmosphere and then annealed at 900 o C for 3 h in a low-oxygen atmosphere to obtain a catalyst with precipitated carbon and Ti oxide dispersed on the MWCNTs. The conditions of the hypoxic atmosphere were 2 %H 2 + n %O 2 / inert gas, with n = 0.05, 0.1, 0.5, 1.0, and 2.0. Catalysts are denoted by TiCNO_(O 2 concentration during heat treatment). 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, and dried for an hour to use as a working electrode in electrochemical measurement. Electrochemical measurements were performed in 0.5 mol dm -3 H 2 SO 4 at 30 o C 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 O 2 and N 2 . The ORR current ( i ORR ) was determined by calculating the difference between the current under O 2 and N 2 . Figure 1 shows oxygen reduction activity of Ti oxide-based electrocatalyst. The vertical axis shows the | i ORR @0.8 V|. The | i ORR @0.8 V| was less than 100 mA g cat -1 at O 2 concentrations of 0.05 and 0.1 %, and was greatest at 0.5 % at 413 mA g cat -1 . The concentration of O 2 further decreased with increasing O 2 concentration, and was the smallest at 2.0%. It is suggested that the O 2 concentration during heat treatment may have an optimum value around 0.5%. We have also analyzed the catalysts by the XRD before electrochemical measurements. According to XRD spectra, the presence of TiC 0.3 O 0.7 was confirmed in all catalysts. In Ti-CNO_0.5 %, the most active catalyst, the diffraction peaks identified TiO 2 rutile and TiO 2 anatase were observed. In the case of Ti-CNO_1 and 2 %, the peak intensities of TiO 2 anatase increased and the ORR activity decreased. This indicates that the degree of Ti oxidation can be controlled by the atmosphere during heat treatment. Acknowledgement: The authors thank for providing TiOTPPz from Dainichiseika Color & Chemicals Mfg. Co., Ltd. Reference [1] A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim. Acta , 55 , 8005 (2010). [2] A. Ishihara, S. Tominaka, S. Mitsushima, H. Imai, H. Imai, O. Sugino, and K. Ota, Curr. Opin. Electrochem. , 21 , 234 (2020). [3] Y. Takeuchi, K. Matsuzawa, T. Nagai, K. Ikegami, Y. Kuroda, R. Monden and A. Ishihara, Bull. Chem. Soc. Jpn. , 96 , 175 (2023). [4] Y. Takeuchi, K. Matsuzawa, Y. Matsuoka, K. Watanabe, T. Nagai, R. Monden and A. Ishihara, Electrochemistry , (2023) in press [5] T. Hayashi, A. Ishihara, T. Nagai, M. Arao, H. Imai, Y. Kohno, K. Matsuzawa, S. Mitsushima and K. Ota, Electrochim. Acta , 209 , 1 (2016). Figure 1