Oxygen Electrocatalysis of Perovskite Oxyfluorides in Alkaline Media
Authors: Masutani Kanata , Matsumoto Kazuhiko , Sakamoto Ryo , Lee Changhee , Miyahara Yuto , Miyazaki Kohei , Abe Takeshi 
Published: December 2024

Introduction

Fossil fuels are indispensable as the main energy source in modern society, but there are concerns about the environmental problems and depletion caused by increased use of them. To realize a sustainable society, high-performance energy conversion devices are required, and various secondary batteries are being developed. Among these, metal-air secondary batteries, which use atmospheric oxygen as the active material, are attracting a lot of attention. Their theoretical energy density are approximately 3 to 30 times higher than that of lithium-ion batteries.[1] At the air electrode of this battery, oxygen evolution reaction (OER) occurs during charging and oxygen reduction reaction (ORR) occurs during discharging, but the overpotentials of these reactions are high, which is an issue for practical application that needs high current density. Therefore, cost effective bifunctional catalysts that can promote both reactions are being extensively researched.

Perovskites have high catalytic activity in alkaline solutions and are inexpensive compared to noble metal catalysts such as Pt and IrO2.[2] It has been reported that Ruddlesden-Popper-type layered perovskite oxychlorides showed high OER and ORR activities,[3] and this study focused on fluorine, which has high electronegativity. The purpose is to clarify the effects of partial fluorination of SrFeO3
-δ and SrCo0.8Fe0.2O3
-δ containing Fe, which is one of the 3d transition metal elements reported to have OER and ORR activity.[4]

Experimental

SrFeO3
-δ and SrCo0.8Fe0.2O3
-δ were synthesized by the sol-gel method.[5] Fluorination was performed by filling a fluororesin tube with the synthesized catalyst powder and fluorine gas at various pressures, and allowing it to stand overnight at room temperature or 100 °C. As for the characterization of the obtained catalyst, X-ray diffraction measurements were used to analyze the crystal structure, and a fluorine ion meter was used for the solution in which the catalyst powder was dissolved to quantify the amount of F- introduced.

Catalytic activity was evaluated by using cyclic voltammetry (CV) measurements. The catalyst ink was prepared by dispersing the catalyst, acetylene black, and a cation-conducting ionomer (Nafion) in tetrahydrofuran. This catalyst ink coated on a glassy carbon rotating disk electrode was used as a working electrode. A platinum wire was used as the counter electrode, a reversible hydrogen electrode (RHE) was used as the reference electrode, and an oxygen-saturated 1 mol dm-
3 potassium hydroxide aqueous solution was used as the electrolyte. For OER activity measurement, the rotational speed of the working electrode was 1600 rpm, the scanning speed was 10 mV s-
1, and the scanning range was set from 1.1 V to 1.6 V vs. RHE, and measurements were performed for 50 cycles. For ORR activity measurement, the rotational speed of the working electrode was 1600, 1200, 800, and 400 rpm, the scanning speed was 10 mV s-
1, and the scanning range was set from 0.4 V to 1.0 V vs. RHE, and measurements were performed for 1 cycle at each rotational speed.

Results

As a result of X-ray diffraction measurements, it was confirmed that SrFeO3
-d and SrCo0.8Fe0.2O3
-d were synthesized before the reaction with F2. After the reaction, the peak positions were shifted, which confirmed that F- was doped, and it became clear that SrF2 was produced as an impurity.

Figure 1 shows the results of CV measurements normalized by the electrochemically active surface area (ECSA) obtained in the potential range of 1.1 – 1.2 V vs. RHE (non-Faradaic reaction region) or electrode surface area. Regarding OER, the activity and cycling stability of SrFeO3
-δ decreased due to fluorination. Although the surface area of SrCo0.8Fe0.2O3
-δ increased due to fluorination, the activity normalized by ECSA decreased. Regarding ORR, it was confirmed that fluorination increased the electron transfer number and improved the ORR onset potential for all catalysts. Although the details are not clear so far, it is assumed that fluorination changes the valence of transition metal, local structure, and amount of oxygen vacancies, which may change the catalytic activity.

Conclusion

As a result of evaluating the catalytic activity of SrFeO3
-δ and SrCo0.8Fe0.2O3
-δ in which F- was doped, it was revealed that the OER activity decreased, but the ORR activity was improved.

References

[1] Y. Li et al., ACS.
Energy. Lett., 2 (2017) 1370-1377.

[2] D. Liu et al., Small., 17 (2021) 2101605.

[3] Y. Miyahara et al., Chem.
Commun., 53 (2017) 2713-2716.

[4] C. Feng et al., ACS Catal., 10 (2020) 4019-4047.

[5] K. Tamai et al., Phys. Chem. Chem. Phys., 22 (2020) 24181-24190.

Figure 1