Factors Affecting ORR Activity of Nb-Added TiOx Catalyst Using Carbon Support for PEFC
Authors: Adachi Yoshinori , Nagai Takaaki , Kuroda Yoshiyuki , Ohgi Yoshiro , Matsuzawa Koichi , Mitsushima Shigenori , Ota Ken-ichiro , Ishihara Akimitsu 
Published: December 2020

1. Introduction

Carbon-supported platinum catalysts used for the cathode of present polymer electrolyte fuel cells (PEFCs) have some problems such as a large amount of platinum use and low durability under operation condition. Therefore, we have attempted to develop substitute of platinum based on group 4 and 5 transition metal oxides. We found that stable oxygen vacancies on the surface and lattice distortion of these metal oxides could act as active sites of oxygen reduction reaction (ORR)1). On the other hand, cup-stacked carbon nanotubes (CSCNTs) coated with TiO2 have been investigated as durable supports for a platinum-based catalysts2). We already revealed that Nb-added titanium oxide with lattice distortion showed some ORR activity1). Then, we thought that CSCNTs coated with Nb-added titanium oxide might show definite ORR activity. In this study, we prepared CSCNTs coated with Nb-added TiOx and evaluated their behavior to investigate the factors which affect the ORR activity.

2. Experimental

The CSCNTs (GSI Creos) without amorphous carbon on the surface was added to 2-propanol and dispersed ultrasonically, then a desired amount of Ti isopropoxide and Nb ethoxide were added with stirring. A sufficient amount of water mixed with 2-propanol were added to proceed hydrolysis and stirred overnight. Suction filtration was performed to obtain a precursor. In this experiment, Nb was added into TiO2 with an atomic% of 10 against Ti, and a loading of oxide was adjusted to 30 wt% of total catalyst. The precursor was heat-treated under an Ar gas containing 4% hydrogen at desired temperature and time to obtain Nb-added TiOx catalysts, designated as Nb-added TiOx/CSCNTs.

The Nb-added TiOx/CSCNT was supported on a glassy carbon rod to make a working electrode. Electrochemical measurements were carried out using a three-electrode cell at 30 ± 1 °C in 0.1 M H2SO4. Cyclic voltammetry was performed with 5 mV sec-1 in the potential range from 0.2 to 1.2 V in an oxygen and nitrogen atmosphere to obtain oxygen reduction current. ORR current based on oxides was obtained by subtracting the reduction current of carbon from the reduction current of the entire catalyst. The oxygen reduction current density, iORR, was normalized by the mass of the oxides.

3. Results and discussion

Figure 1 shows the dependence of iORR at 0.6 V of Nb-added TiOX/CSCNTs on heat treatment temperature. The results of Nb-added TiOx powders prepared without carbon were also plotted. Because the electro-conductivities of the Nb-added TiOx powders were very low, Ketjenblack was mixed as an electro-conductive material to evaluate the ORR activity. The current value of Nb-added TiOx/CSCNT is significantly improved compared to oxide powder. This is because the electron supply increased due to the deposition of oxides on the CSCNT. The highest ORR current density was observed at 800 oC.

Figure 2 shows the SEM images of Nb-added TiOX/CSCNTs. Amorphous TiO2 covers with the surface of CSCNTs in the unheated catalyst. At 700 ° C and 800 ° C, the oxide particles of around 10 nanometers were supported with high dispersion. However, at 900 ° C, the particles aggregated to become larger.

Figure 3 shows the Ti 2p XPS spectra of Nb-added TiOx/CSCNTs heat-treated at 600-1000 °C. Ti2p peak shifted to the lower binding energy from 600 ° C to 800 ° C and to the higher energy from 800 ° C to 1000 ° C. The proportion of low-valence state Ti3+ increased as the temperature increased from 600 to 800 ° C. The reason why the shift to the higher binding energy occurred above 800 ° C was that the oxide surface prepared at higher heat-treatment temperature was oxidized by exposure of the air.

Figure 4 shows the relationship between ratio of Ti3+ / (Ti3+ + Ti4+) and ORR current density at 0.6 V. The ratio of Ti3+ was calculated from the Ti 2p XPS spectra. The iORR at 0.6 V increases as ratio of Ti3+ increases. The ratio of Ti3+ indicates the presence of oxygen vacancies on the surface of the oxide. Therefore, it is considered that the increase of oxygen vacancies on the oxide surface strongly affected the ORR activity.

Acknowledgement

The authors wish to thank the supply of the CSCNT from GSI Creos and the support of the New Energy and Industrial Technology Development Organization (NEDO), JSPS grants-in-aid for scientific research, Suzuki Foundation, and Tonen General Sekiyu Research / Development Encouragement & Scholarship Foundation

References

1) A. Ishihara, et. al., Electrochim. Acta, 283,1779 (2018).

2) F. Ando, et. al., Electrochim. Acta, 404, 232 (2010)

Figure 1