Nano Platinum Decorated Graphene Wrapped a-MnO2 Nanocomposite Lithium Air Breathing Cathode for Li-O2 Batteries

Currently, battery research and development have focused on energy storage and conversion with high energy, high power density, and reliable safety. Among batteries, the lithium air battery has recently captured worldwide attention due to its ultrahigh specific energy density of 11.140 Whkg^-1 rivaling that of gasoline (1, 2). However, Li⁺ reduces molecular O₂ to form insoluble lithium peroxide (Li₂O₂) or lithium oxide (Li₂O) at the cathode during discharge, which gradually blocks the electrolyte and oxygen pathways and eventually also limits the rate capability, capacity, and cyclic life of Li-O₂batteries. Therefore, the two properties of the cathode materials are the important features. The first one is the porosity of the air electrode and the second one is the oxygen reduction (ORR) and oxygen evolution reaction (OER) behaviors of the cathode (2, 3).

In this study, nano platinum decorated Graphene wrapped a-MnO₂ nanocomposite cathode was produced to increase cycle life and the capacity of the Li-O₂ batteries. Before production of the composite cathodes, Graphene Oxide (GO) was synthesized by Hummers Method and a-MnO₂ nanowires was synthesized using microwave hydrothermal synthesis method. To production of the composite cathodes, GO and a-MnO₂ dispersed in the distilled water and homogenously mixed using ultrasonication for 1 h. After then H₂PtCl₆ was added in the suspension under stirring. After then, NaBH₄ solution was slowly added in the suspension to reduce H₂PtCl₆ and GO into platinum and Graphene, respectively. After centrifugation and drying of the products, Graphene/a-MnO₂/Pt nanocomposite was obtained. Using this composite, Graphene/a-MnO₂/Pt nanocomposite cathode was prepared. To prepare Graphene/a-MnO₂/Pt nanocomposite cathode, 15wt.% PVDF dissolved in NMP solution and 85 wt.%  Graphene/a-MnO₂/Pt nanocomposite were mixed and obtained slurry cast on the nickel foam. After drying of the samples in vacuum oven, Graphene/a-MnO₂/Pt nanocomposite cathode was obtained. The structure of the composite was characterized using X-ray diffraction pattern, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. Electrochemical characterization of the Graphene/a-MnO₂/Pt nanocomposite cathode was performed between 2.15 V and 4.25 V at a constant current density of 0.1 mAcm^-2 in ECC-Air test cell (purchased from EL-Cell Company in Germany). In order to investigate electrochemical reaction of the cathode with electrolyte, cyclic voltammetry test was carried out at a scan rate of 0.1 mV s^-1. For further electrochemical characterization, electrochemical impedance spectroscopy of the cathode was investigated after selected cycles in the frequency range of 1000 kHz to 0.1 Hz with AC amplitude of 10 mV.

References

1. T. Cetinkaya, S. Ozcan, M. Uysal, M.O. Guler, H. Akbulut, J. Power Sources, 267 (2014)   140-147.

2. Z.L. Wang, D. Xu, J.J. Xu, L.L. Zhang, X.B. Zhang, Adv. Funct. Mater., 22 (2012) 3699–3705.

3. Y. Li, J. Wang, X. Li, D. Geng, R. Li, X. Sun, Chem. Commun., 47 (2011), 9438–9440.