Synthesis and Electocatalytic Activity of MnO2/RuO2/Carbon Nanocomposites to Enhance Lithium-Oxygen Battery Performance

Abstract

Lithium-oxygen battery has received much attention as a potential energy storage system because of its ultra-high theoretical energy density (5,200 Wh kg−1), which remains to be the most promising future technology. However, there are several scientific/technical hurdles that must be overcome in order for a rechargeable Li–air battery to be commercialized in the market. In particular, the incomplete decomposition of solid discharge products such as Li2O2 etc. formed during discharge process passivates the cathode surface by clogging pores of the porous carbon cathode, resulting in diminishing the overall performance of battery. In order to solve these issues, great effort has been devoted to designing and developing novel electrocatalysts. It has been reported that electrocatalysts play an important role in ORR and OER, and hence improve the kinetics and efficiency of the Li–air battery.In this work, a suitable bifunctional catalyst was synthesized with MnO2 and RuO2, which are the well-known catalysts for oxygen reduction reaction(ORR) and oxygen evolution reaction(OER), respectively, by supporting on two different carbon supports such as graphite nanofiber(GNF) and graphene oxide(GO) for the application to oxygen cathode bifunctional catalysts of lithium-air batteries. MnO2/GNF was first prepared by a simple hydrothermal synthesis and was then decorated with various decorating ratios of RuO2 using a simple redox reaction technique. A good electrocatalytic performance was observed from a weight ratio of RuO2:MnO2/GNF=5:5. The Li-oxygen battery fabricated with the synthesized catalyst composition delivered a reduced overpotential with good cyclability and high specific capacity. In order to understand the effect of carbon, urchin-shaped α-MnO2/RuO2 nanostructures was first synthesized by a hydrothermal synthesis and was then loaded on GO with a weight ratio of 5:5 using a redox reaction technique. The synthesized α-MnO2/RuO2@GO (5:5) composite delivered high specific capacity with excellent electrocatalytic activity. Between the two different carbon supported composites, RuO2@MnO2/GNF showed excellent catalytic activity than α-MnO2/RuO2@GO. The increase in the surface area of carbon supports greatly influenced the performance of Li-air battery. Catalysts with carbon supports showed higher activity than without carbon. RuO2@MnO2/GNF was found to be the best catalyst which has two or tree times larger surface area than α-MnO2/RuO2@GO. Thus, RuO2@MnO2/GNF with 5:5 weight percentage was found to be the best electrocatalyst than all other synthesized catalysts.