The whole world has been in transition from a fossil fuel based economy to a clean energy economy. Especially in today's society, the demand for energy is increasing, the fossil fuel is exhausted and the environmental safety is becoming more and more important. ^[1-3]This inevitable process is being accelerated by recent active research on sustainable energy scavenging, conversion and storage. Batteries have long been accepted for their capacity to efficiently convert and store electrical energy^[4] . To the best of our knowledge, there has been no report on the combination among La₂O₃, MnO₂, CaO and carbon-based materials to form hybrid catalysts for improving the electro-catalytic performance of the materials in rechargeable metal-air batteries. It is expected that by combining the transition metal oxides with carbon-based material via a facile hydrothermal process would lead to a strong hybrid effect to enhance the catalytic performance. Based on this strategy, we have designed and synthesized the hybrid material via a facial method where the precursors of the metal-oxide (La, Mn and Co) and CNTs are justly mixed into a single reaction to produce the final catalysts.

First, MnO2 nanotubes were prepared by a facile hydrothermal method. Then CaO nanoparticles and La₂O₃ nanoparticles were modified on the MnO₂ nanotubes by a hydrothermal method combined with post-heat treatment. In a typical synthesis of La₂O₃/CaO/MnO₂-CNTs hybrid nanomaterials, 0.25 g Ca(NO₃)₂▪4H₂O and 0.25g  La(NO₃)₂▪6H₂O were dissolved in 15 mL of 1.3 mol L-1 ammonia solution. Afterwards, 0.125 g as-prepared MnO2 nanotubes and 0.125g CNTs were dispersed in the above solution by hydrothermal 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 0.94 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, La₂O₃/CaO/MnO₂-CNTs, Pt/C and CNTs afforded an ORR current density of 5.2mA cm^-2 ,3.6mA cm-2and 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 1.46V for La₂O₃/CaO/MnO₂-CNTs, whereas it was 1.59V and 1.60V for CNTs and 20%Pt/C,the OER current density of La₂O₃/CaO/MnO₂-CNTs,20%Pt/C and CNT at 1.67 V was 8 and 0.7 and 0.3mA cm^{- 2}.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.

The battery had an open circuit voltage of 1.35 V. At a voltage of 600 mV, it showed a high current density of 375mA cm^-2. The peak power density was 258mW cm^-2. the battery discharge and discharge performance noticeably at lower current densities and through long cycle times.

In summary, La₂O₃/CaO/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 M.Armand, J. M.Tarascon, Nature, 451, 652(2008).

2 A. S.Arico, P.Bruce, B.Scrosati, J. M.Tarascon, W. Van Schalkwijk, Nat. Mater., 4, 366(2005).

3 G.Girishkumar, B.McCloskey, A. C.Luntz, S.Swanson, W.Wilcke, J. Phys. Chem. Lett., 1, 2193(2010).

4 R.Cao, J.Lee, M. Liu, J.Cho, Adv. Energy Mater., 2, 816(2012)