Electrochemical devices such as fuel cell, battery and supercapacitors are potential alternatives for energy conversion and storage apart from the burning of fossil fuels. Among the different energy storage systems, the rechargeable zinc-air battery displays great promise due to many attractive features; for example, high energy density, cost-effective and environmentally friendly design, as well as low operating risks ¹. The excellent properties of rechargeable zinc-air batteries lead to potential applications in areas, such as stationary and portable power applications including electric vehicles ^2,3. In a rechargeable zinc-air battery, oxygen evolution (OER) and oxygen reduction (ORR) reactions occur on the air electrode during battery charge and discharge, respectively. The sluggish reaction kinetics and large overpotential associated with OER and ORR greatly limit the performance of rechargeable zinc-air batteries ^2,4. Among them, manganese oxide  is a particularly interesting candidate due to its rich oxidation states, chemical compositions and crystal structures. MnO2 is the most common ORR electrocatalyst in commercial zinc–air batteries. As a result, an affordable, active and stable bifunctional catalyst is in great demands to improve to the performance of rechargeable zinc-air batteries.

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，in a typical synthesis of MnO₂ nanotubes, 0.7902g KMnO₄ and 2mL concentrated HCl(37%) were added to 50mL deionized water to form a precursor solution, then the solution was transferred into a 100mL Teflon-lined stainless steel autoclave. The autoclave was sealed and hydrothermally treated at 100，140，180 and 220^oC for 12 hours. After the autoclave was cooled down to room temperature naturally, the MnO2 nanotube sample was collected and washed for 3-5 times with ethanol and deionized water, and dried in air at 70^oC 24hours. 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 by 140℃, whereas it was 1.08 V and 0.78 V for MnO₂ by 100,180 and 220℃, respectively. At 0.2 V, MnO₂ by 100, 140,180 and 220℃afforded an ORR current density of 1.2mA cm^-2 , 5.8mA cm^-2, 5.8mA cm^-2 and 5.2mA 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 by 140℃, whereas it was 1.51V,1.56V and 1.53Vfor MnO₂by100,180and 220℃,the OER current density of MnO₂ by 100, 140,180 and 220℃at 1. 7 V was 16, 9, 12 and 14mA cm^{- 2}. The battery had an open circuit voltage of 1.35 V. At a voltage of 732 mV, it showed a high current density of 450mA cm^-2. The peak power density was 293mW cm^-2. the battery discharge and discharge performance noticeably at lower current densities and through long cycle times.

In summary, MnO₂ a new air electrode material have been synthesized. 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 S.I. Smedley, X.G. Zhang, J. Power Sources 165,897 (2007).

2 V. Neburchilov, H.J. Wang, J.J. Martin, W. Qu, J. Power Sources 195,1271 (2010).

3 J. Goldstein, I. Brown, B. Koretz, J. Power Sources 80 ,171 (1999) .

4 P. Sapkota, H. Kim, J. Ind. Eng. Chem. 15, 445(2009).