Introduction Oxygen evolution reaction (OER) is the fundamental processes for water splitting and rechargeable metal-air batteries in alkaline media. Ir-based catalysts are regarded as one of the promising active OER catalysts, however the use of such noble metal-based materials leads to high cost. Perovskite metal oxide has been attracted much attention as the alternative low cost materials. Among them, the layered perovskite, LaSr₃Fe₃O₁₀ (LSFO) shows enhanced ORR activity thus is expected as a bifunctional catalyst for ORR and OER¹⁾. Moreover, ABxB’1−xO3 structure shows improved OER activity compared to the simple perovskites ²⁾. In this study, we prepared the layered perovskite LaSr₃Fe_3-xCo_xO_10-δ (x = 0, 0.15, 0.3, 0.6, 1.5, 3.0). Their OER activity and performance as a cathode catalyst for Zn-air batteries were studied.

Experimental Synthesis of LaSr₃Fe_3-xCo_xO_10-δ was carried out by a nitrate-citrate method. Stoichiometric metal nitrates and citric acid were dissolved in water (metal : citric acid = 1 : 1 mol) and the solution was concentrated at 120 ^oC for several hours, followed by drying at 450°C for 1 hour. After drying, the obtained powders were pelletized into a disk, then calcined in air at 1400 ^oC for 20 hours.

LaSr₃Fe_3-xCo_xO_10-δ electrode was fabricated by pressing the calcined pellet at 15 MPa for 10 minutes and sintering at 900 ^oC for 10 hours. H type-cell consisted of the prepared LaSr₃Fe_3-xCo_xO_10-δ as a cathode, platinum plate as an anode, 4.0 mol dm^-3 KOH as an electrolyte, and Celgard #3401 as the separator was used for OER measurement under air atomosphere. For zinc-air battery tests under O₂ atmosphere, zinc foil and ZnO saturated 4.0 mol dm^-3 KOH were instead used as anode electrode and electrolyte, respectively.

Results and Discussion For choronoamperometry tests, the LaSr₃Fe_3-xCo_xO_10-δ electrode showed higher OER activity than the Pt/C and Ir/C electrodes. As shown in Figure 1(a), the OER activity of LaSr₃Fe_3-xCo_xO_10-δ electrodes is sensitive to the amount of Co substitution. This result suggests that Co substitution into Fe-occupied site is effective in improvement of the OER activity of LSFO. The LaSr₃Fe_3-xCo_xO_10-δ electrode also shows superior charge/discharge energy efficiency than the Pt/C electrode in zinc-air battery cycling. Moreover, it is found that Co substitution to LSFO unfortunately led to decrease in ORR activity while somewhat effective on OER activity improvement under O₂ atmosphere. The catalytic behavior of LaSr₃Fe_3-xCo_xO_10-δ is highly complicated, however the valence of B-site elements probably has a significant effect on determining OER and ORR activity.

References

1) T. Takeguchi, et al., J. Am. Chem. Soc., 135, 11125 (2013).

2) A. Vignesh, et al., ACS Appl. Mater. Interfaces, 8, 6019 (2016).

Acknowledgement This work is partly supported by NEDO.

Fig.1. (a) Current density of LaSr₃Fe_3-xCo_xO_10-δ electrodes as a function of the amount of Co substitution (x). Current densities were recorded at 10 hours after applying potential at 1.55 V vs. RHE. (b) Charge/discharge curves of Pt/C and LaSr₃Fe_3-xCo_xO_10-δ (x = 1.5) electrodes in zinc-air battery. Current density of 0.1 mA cm^-2 for 12 hours each.