Rechargeable zinc-air batteries have received strong interest for use in electric vehicles thanks to their cost-efficient, high theoretical energy density and open cell configuration that uses O₂ as a fuel¹. However, to overcome the high activation energy during the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is one of the important kinetic challenges in rechargeable zinc-air batteries. Thus, many researches have been dedicated to the design for high-efficiency bifunctional catalysts². Spinel type metal oxides (AB₂O₄) have attracted much attention in the literature due to their redox stability, variable valence states and simple synthetic routes³. However, it's known that the electrochemistry activity of AB₂O₄ is unpromising due to their low electrical conductivity. It is a useful measure to improve the electronic conductivity of AB₂O₄ by growing or supporting itself on carbon based materials. Here, CoFe₂O₄@CNTs was designed to be as an efficient bifunctional catalyst for both ORR and OER, which was synthesized by a simple one-step hydrothermal method.

First of all, 0.20 g Co(NO₃)₂ 4H₂O, 0.20 g Fe(NO₃)₃ 9H₂O and CNTs (the weight ratio of CNT to Fe and Co varied from 0.04) were dissolved in 30 ml NH₃ H₂O (28.0-30.0 vol%) and stirred for 1 h. Then, it was transferred to autoclave and heated at 160 ^oC for 6 h in a drying oven. After the heated procedure, the autoclave was cool to room temperature naturally. These solid products were collected, washed with water several times and then dried in a drying oven. Finally, these dried precursors were calcined in air at 350 ^oC for 1 h to obtain catalyst (denoted as CoFe₂O₄@CNTs).

The electrochemical activity of samples was measured in a three-electrode cell reactor. The catalyst ink was then drop-cast onto a clean GC electrode surface to a catalyst loading of 100 mg cm^-2 and as work electrode. For IrO₂ 20 wt% Pt/C (Johnson Matthey), (Johnson Matthey) were also prepared at same state. A Hg/HgO (SCE) reference electrode and a Pt rod counter electrode were used together with the GC working electrode. The measurements were carried out under an O₂-saturated environment. All potentials in this paper was referenced to the reversible hydrogen electrode (RHE) at pH 13 (= E [vs. SCE] + 0.9934 V). ORR were recorded from 0.2 to 1.1 V vs RHE and OER curves were evaluated from 1 to 2 V vs RHE at a rotation speed of 1500 rpm of 5 mV s^-1.

The CoFe₂O₄@CNTs hybrid shows the excellent ORR performance among Pt/C and IrO₂. Particularly, the CoFe₂O₄@CNTs shows the largest limiting current density of ORR than Pt/C and IrO₂. Meanwhile, the CoFe₂O₄@CNTs catalyst also shows the much higher OER activity than Pt/C, and is even superior to IrO₂ (the state-of-the art commercial OER catalyst) in high potential.

In summary, the developed high-performance reversible oxygen electrode (CoFe₂O₄@CNTs) might open new avenues for advanced renewable energy systems.

References

1 N.N. Xu, X.M. Li, H.R. Li, Y.N. Wei, J. L. Qiao, Sci. Bull., 62, 1216 (2017).

2 N.N. Xu, J.L. Qiao, X. Zhang, C.Y. Ma, S.S. Jian, Y.Y. Liu, P.C. Pei, Appl. Energy, 175, 495 (2016).

3 N.N. Xu, Y.Y. Liu, X. Zhang, X.M. Li, A.J. Li, J.L. Qiao, J.J. Zhang, Sci. Rep., 6, 33590 (2016).