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)