The design and fabrication of bifunctional catalysts with low cost and high efficiency is a great challenge for the practical application of Li-O 2 batteries. This work presents a bifunctional electrocatalyst consisting of RuFe nanoparticles embedded in high-surface-area nitrogen-doped mesoporous carbon (RuFe@NC). The RuFe@NC-900 catalyst exhibits a specific surface area (677 m 2 g −1 ), pore diameter (9.52 nm), and high pore volume (0.3 cm 3 g −1 ). The catalyst displays high oxygen reduction and evolution reaction activity and exhibited excellent bifunctional activity (ΔE) of 0.73 V vs RHE compared to the benchmark catalyst, 40 wt % Pt/C + RuO 2 substantiates the excellent catalytic activity as an oxygen electrode. The excellent bifunctional activity is attributed to the synergistic effect arising from RuFe@NC type sites, and the high electrical conductivity of the support material was key to tuning the catalytic activity. The potential practical application is further demonstrated by using it as an air cathode for rechargeable metal−air batteries. The Li-O 2 battery constructed with the optimized RuFe@NC-900(5h) cathode exhibited robust reversibility with negligible discharge voltage loss. As a result, the discharge-specific capacity of 11,129 mAh g −1 at a current density of 100 mA g −1 shows a practical approach to explore the high-rate capability by constructing optimal cathode electrodes. In addition, the rechargeable zinc−air battery with RuFe@NC-900(5h) as a bifunctional catalyst exhibits high activity and stability during battery discharge, charge, and cycling processes. Therefore, RuFe@NC can be a potential air cathode for non−aqueous and aqueous rechargeable metal−air batteries.

Li-O 2 batteries are attracting considerable attention as a promising power source for electric vehicles as they have the highest theoretical energy density among reported rechargeable batteries. However, the low energy density and efficiency of Li-O 2 batteries still act as limiting factors in real cell implementations. This study proposes the cathode structure engineering strategy by tuning the thickness of a catalyst layer to enhance the Li-O 2 battery performance. The construction of the Li-O 2 battery with a thinner porous cathode leads less parasitic reactions at the solid electrolyte interface, maximization of the catalyst utilization, and facile transport of oxygen gas into the cathode. A remarkably high specific capacity of 33,009 mAh g −1 and the extended electrochemical stability for 75 cycles at a 1000 mAh g −1 limited capacity and 100 mA g −1 were achieved when using the porous Co/ CeO 1.88 -nitrogen-doped carbon nanorod cathode. Further, a high discharge capacity of 20,279 mAh g −1 was also achieved at a relatively higher current density of 300 mA g −1 . This work suggests the ideal cathode structure and the feasibility of the Co/CeO 1.88nitrogen-doped carbon nanorod as the cathode material, which can minimize the areal cathode catalyst loading and maximize the gravimetric energy density.

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