Tatsuya Takeguchi (*,a), Ryo Kobayashi (a), and Koichi Ui (a)

(*) Corresponding author

(a) Department of Chemistry and Biological Sciences, Faculty of Science and Engineering, Iwate University, Iwate 020-8551, Japan.

Introduction: Rechargeable Zn-air batteries have been expected as the next-generation energy storage devices because of its high theoretical energy density of 1300 Wh/kg¹⁾. However, it is desired to reduce overpotentials for ORR and OER on the positive electrode during discharging and charging reactions. Carbon supports on the positive electrode are easily oxidized during the charging reaction. Therefore, high-performance and carbon-free positive-electrode catalysts are indispensable. It is reported that a perovskite-type LaSr₃Fe₃O₁₀ (LSFO) has a high electrical conductivity and high activities for ORR and OER²⁾. In this study, the carbon-free LSFO pellets were used as positive electrode for Secondary Zn-air batteries, charge and discharge test was conducted.

Experimental: In this study, the LSFO pellets (Santoku Co.) were used as the catalysts of the positive electrode. The LSFO powders were pressed at 15 MPa for 10 min and pelletized into disks of 10 mm in diameter for LSFO pellet sample. For porous LSFO pellet, mixtures of LSFO powders and C powder were used. These pellets were calcined in air at 900 ^oC for 10 h. The OER and ORR activities of the as-prepared pellets were determined by charge (12 h) and discharge (12 h) cycle at 0.5 mA cm^-2 between 0.5 and 2.5 V at 25 ^oC in 4.0 mol dm^-3 - KOH 0.3 mol dm^-3 ZnO electrolyte using a H-type cell. The LSFO pellet, the porous LSFO pellet, or Pt/C plate was used as the working electrode, while a Zn plate was used as counter electrode, respectively.

Results and Discussion: Charge and discharge profiles at 0.5 mA cm^-2 for secondary Zn-air battery with carbon-free LSFO positive electrode are shown in Fig. 1. For the LSFO pellet and porous LSFO pellet, overpotentials of OER and ORR are much lower than reference Pt/C because of the easily removable oxygen present in LSFO particles. Although these LSFO pellets are carbon-free, their activities are much higher than those of Pt/C. Activates of LSFO for both ORR and OER are stable, although Pt/C is deactivated owing to the oxidation of carbon during OER. Porous structures

Energy efficiency based on discharge voltage/charge voltage is as followed:

Porous LSFO pellet > LSFO pellet >> Pt/C (87.1% > 83.8% > 51.7%)

Carbon-free LSFO clearly reduces the overpotential of Zn-air battery and enhances the energy efficiency. Porous structure improved the efficiency of both ORR and OER. Perovskite-type oxide catalysts with a high electrical conductivity and high activities should be potential candidates as positive electrode for the next-generation metal-air battery.

References

1) Z. Chen, et al., Nano Lett., 12, 1946 (2012).

2) T. Takeguchi, et al., J. Am. Chem. Soc., 135, 11127 (2013).

Acknowledgements

This research was partially supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan.