Biowaste-Derived Highly Porous N-Doped Carbon as a Low-Cost Bifunctional Electrocatalyst for Hybrid Sodium-Air Batteries

Noble-metal-free bifunctional catalysts are vital to improve high-performance, cost-effective metal-air batteries. This work presents highly porous carbon (HPC), derived from waste tree leaves, as a low-cost carbon-based bifunctional electrocatalyst. To further improve the catalytic activity, nitrogen and sulfur doping in HPC is achieved by treating it with urea (CO(NH2)(2)) and thiourea (CS(NH2)(2)), respectively. The electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity of the HPC, N-doped HPC, and S-doped HPC have been investigated. Among them, the N-doped HPC was found to show excellent bifunctional electrocatalytic (ORR/OER) activity. The N-doped HPC exhibited a superior bifunctional catalytic activity with an onset potential of 0.95 V (vs RHE) at a current density of 6.31 mA cm(-2), whereas in the case of the OER, the observed onset potential was 1.4 V (vs RHE), which is comparable to that of the benchmark RuO2 (1.45 V vs RHE) catalyst. The assembled hybrid Na-air battery exhibited reversible electrochemical performance with a round-trip efficiency of similar to 83% over 30 cycles. These economical bifunctional HPC-based catalysts can be effectively employed as air cathodes in hybrid sodium-air battery applications.