Much attention has been devoted recently to rechargeable lithium-oxygen batteries because of their very high energy density compared to that of other rechargeable systems. Theoretically, it can reach up to 2-3 kWh kg~(-1) with respect to the mass of the discharged electrode, although practical values are expected to be in the range of 1000 Whkg~(-1). However, high overpotential on charge that results in poor round-trip efficiency, along with poor cyclability makes the use of Li-O2 batteries impractical until these problems are solved. Various catalysts have been examined to lower the activation barriers, such as nanowire α-MnO2, Co3O4, Mn3O4, and PtAu, with many studies having been conducted in alkyl carbonate electrolytes. Spinel-based M3O4 (M = Mn,Co) catalysts supported on graphene were reported to be promising bifunctional catalysts in aqueous media. However, these electrolyte systems do not rely on Li2O2 formation and its subsequent OER as would be the case in an aprotic Li-O2 cell. Moreover, some recent articles question the efficacy or necessity of catalysis in the aprotic system. Our studies were aimed at addressing this. Here, we show that nanocrystalline Co3O4, grown on reduced graphene oxide (Co3O4/RGO) and employed as part of a carbon-based oxygen electrode membrane, results in significant reduction of overpotentials for OER (up to 350 mV), and improved cycling performance. The material acts as a promoter, rather than as a classic electron-transfer catalyst for the reactions involved in the Li-O2 cell.
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