Paper Title
ENGINEERING DUAL SINGLE-ATOMSITES FE-MNON ULTRATHIN CARBON NANOSHEETS FOR ULTRA-STABLE ZINC-AIR BATTERIES

Abstract
The utilization of renewable energy sources and deployment of advanced electricity storage/conversion technologies are of crucial importance to achieve the sustainable development of human society. In recent years,rechargeable zinc-air batteries (ZABs) have emerged as a promising energy storage technology owing to their high energy density and environmental friendliness together with the abundance of Znresources. Besides, the aqueous ZABs combine the advantages of safety in aqueous flow batteries and favorable stability in metal-air batteries, making them the desirable power source in various fields, including distributed energy storage, portable devices, and transportation.[1] However, the sluggishkinetics and high activation energy of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) involved in the device result in high overpotential, greatly hindering the practical performances of ZABs. At the air cathode, the effective bifunctional oxygen electrocatalysts play a vital role in achieving ZABs with high round-trip efficiency and high power-density.The platinum-group metal (PGM)-based catalysts such as Pt, Ru and Ir are currently the most used to facilitate the ORR/OER rates, but their scarcity, high cost and undesirable stability causemajor roadblocks inthe large-scalecommercialization of ZABs. In this context, developing robust and cost-efficient bifunctional oxygen electrocatalysts is of great concernfor attaining high-performance ZABs and achieving their commercial deployment.[2] Transition metal single atoms (especially Fe, Co, Ni, and Mn) anchored on electroactive N-doped carbon matrices, represented as M-N-C species, have been regarded as promising ORR/OER electrocatalysts for ZABs. These single atom catalysts (SACs) offer unique advantages including strongmetal-substrate interactions, tunable electronic structure ofthe active metal site as well as high electric conductivity, oftenresulting in excellent redox-catalytic performance. Also, the nearly molecular design of these materials enables optimum metal-atom utilization and -rational active site design at theatomic level. For instance, by introduction of a second metal center to fabricateadjacent hetero-metal pairs (as called dual atom catalysts, DACs), is a common strategy to modify the asymmetric charge distribution at the active sites and enhance overall catalytic performance forboth ORR and OER.[3] Herein, we demonstrate the fabrication of a cost-effective OER/ORR bifunctional catalyst by anchoring atomic Fe–Mn dual metal pairs into nitrogen-doped carbon matrices.Multiple characterizations confirm that electronic configuration of the active sites Fe-Nx and Mn-Nx were tuned mutually, facilitating high intrinsic activity towards oxygen electrocatalysis. Its hierarchical structure and stable atomic dispersion also enable highly efficient and durable catalytic performance in ZABs application. As results, the bifunctional oxygen catalyst FeMn-NC shows a voltage difference (ΔE) of 0.68 V with half-wave potential of ORR: 0.89 V and the potential of OER at 10 mA cm-2: 1.57 V in 0.1 M KOH. For the ZABs study, the battery with the FeMn-NC achieves a high open circuit voltage of 1.45 V, has a good specific capacity of 795mAh/g at current density of 15 mA cm-2 and the excellent power density of 225 mW cm-2 at current density of 170 mA cm-2. Moreover, this rechargeable ZAB exhibits remarkable long-term durability over the charge-discharge cyclic test (700 cycles, 350 h) with negligible voltage gap increasing. Keywords - Energy storage and conversion, Zn-air battery, Bifunctional electrocatalyst, Single-atom catalyst.