(Invited) CoFe2O4@CNTs As High-Performance Air-Cathode Bifunctional Catalysts for Rechargeable Zinc-Air Batteries
Authors: Xu Nengneng , Peng Luwei , Qiao Jinli , Zhou Xiao-Dong 
Published: February 2020

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).