(Invited) Advanced Zinc-Air Betters Based on Extremely Durable Bifunctional Air Electrodes
Authors: Qiao Jinli 
Published: February 2020

Environmental pollution and shortage of fossil fuels have made it a current trend to develop electrical cars and use renewable energy. Recently, rechargeable metal−air batteries such as Zn-air batteries could be ideal energy storage devices have attracted much attention as a high energy and power density, high safety and economic viability. 1,2Active and durable electrocatalysts on the cathode side are required to catalyse oxygen reduction reaction during discharge and oxygen evolution reaction during charge for rechargeable batteries.3

A rotating disc electrode (RDE) half-cell setup was used to investigate the ORR and OER catalytic activity of the samples. The working electrode was fabricated by casting Nafion-impregnated catalyst ink onto a glassy carbon disk electrode (5.6 mm in diameter). 10 mg of the catalyst was ultrasonically dispersed into 1mL ethanol 8 and uL 5 wt% Nafion solution to form a catalyst ink. 5uL of the catalyst ink was deposited on the disk and dried at room temperature. The working electrode was allowed to achieve a catalyst loading of 0.1 mg cm-2. Electrochemical activity of the samples was studied using linear sweep voltammetry. The working electrode was immersed in a glass cell containing 0.1 M KOH aqueous electrolyte. A platinum foil and an Hg/HgO electrode were used as the counter and reference electrodes, respectively. Catalyst activity toward the ORR and OER was evaluated in oxygen-saturated electrolyte solution from 1.67 to 0.1 V vs. RHE. The potential of the reference electrode was normalized with respect to the potential of the reversible hydrogen electrode (RHE). The rotation rate is 1600 rpm and the scan rate is 5 mV*s-1. A commercial Pt/C catalyst (30 wt% platinum on carbon) was tested using the same procedure.First, MnO2 nanotubes were prepared by a facile hydrothermal method. Then Co3O4 nanoparticles were modified on the MnO2 nanotubes by a Co-precipitation method combined with post-heat treatment. In a typical synthesis of MnO2/Co3O4 hybrid nanomaterials,0.25 g Co(NO3)2 4H2O was dissolved in 40 mL of 0.13 mol L-1 ammonia solution. Afterwards, 0.125 g as-prepared MnO2 nanotubes and 0.125g CNTs were dispersed in the above solution by stirring for 60 min. The resulting products. were separated by centrifugation, washed with deionized water, dried at 60 ℃ for 5 h, and then calcined in air at 400℃ for 1 h.
As shown in Fig.1, the onset potential for was detected at 1.04 V for MnO2/Co3O4-CNTs,whereas it was 1.08 V and 0.78 V for Pt/C and CNTs, respectively. At 0.2 V, MnO2/Co3O4-CNTs, Pt/C and CNTs afforded an ORR current density of 3.8mA cm-2, 3.6mA cm-2 and 2.2mA cm-2 .Apart from the ORR activity, excellent OER activity is particularly critical for bi-functional catalysts. As shown , the onset potential for was detected at 1.45V for (MnO2/Co3O4) /CNTs, whereas it was 1.59V and 1.60V for CNTs and 20%Pt/C,the OER current density of MnO2/Co3O4-CNTs,20%Pt/C and CNT at 1.7 V was 8 and 0.9 and 0.4mA cm- 2.
The battery had an open circuit voltage of 1.35 V. At a voltage of 600 mV, it showed a high current density of 450mA cm-2. The peak power density was 247mW cm-2. the battery discharge and discharge performance noticeably at lower current densities and through long cycle times.
In summary, Co3O4/ MnO2-CNTs a new air electrode material have been synthesized via a two-step hydrothermal method. These hybrid nanomaterials display good bifunctional ORR/OER activity and cyclic stability in the discharge and charge process. Further studies are ongoing to improve the ZABs performance by manipulating the hybrid structure.
References
1 D. U. Lee, J.-Y. Choi, K. Feng, H. W. Park, Z. Chen, Advanced Energy Materials, 4,1301389(2014).
2 P.C. Pei, Z. Ma, K.L. Wang, X.Z. Wang, M.C.Song, H.C.Xu, Journal Of Power Sources, 249. 13(2014)
3 Y.G. Li, M. Gong,Y. Liang, J. Feng, J.E. Kim, H.. Wang, G.S.Hong,B.Zhang ,H.J.Dai, Nature Communications, 249. 1805( 2013)

Figure 1