The Controlled Synthesis Of Porous Carbon-based Electrocatalysts And Their Enhanced Oxygen Reduction Reaction Performance

Rechargeable zinc–air batteries have been acknowledged as the next-generation energy conversion and storage devices owing to a high theoretical energy density and an environmentally friendly nature.One critical reaction occurs at the air cathode of rechargeable zinc–air batteries is the oxygen reduction reaction（ORR）,includes multistep electron transfers processes of inherently sluggish kinetics,so the design of electrocatalysts with high kinetic activity and durability is the key to improve the efficiency of energy conversion and storage device.Noble metal-based electrocatalysts（e.g.,Pt）have been vigorously developed for decades owing to their high catalytic activity towards ORR,but the high cost and low abundance on earth have largely restricted their large-scale applications.Single-atom electrocatalysts（SACs）are considered as a highly efficient electrocatalyst that can replace PGM.In this paper,SACs electrocatalysts of metal-nitrogen-carbon（M-N_x-C_y）configuration was used to construct SACs with porous structure to regulate the coordination environment of the central metal atoms,so as to obtain high activity and high stability of SACs.Finally,we explore the application of synthetic SACs in rechargeable Zn-air batteries.The key research results are show as follows:（1）:We introduced a facile sol-gel pore-sealing strategy.Firstly,using KOH as etching agent,carbon spheres（CSs）were activated by potassium hydroxide through one-step calcination to produce a large number of micropores.The pore sealing was then performed using gels containing Ru³⁺and Zn.Single-atom electrocatalysts（Ru-SAs@N-ACSs）with a heteroatom N-doped carbon substrate simultaneously anchored to a single-atom Ru were obtained by pyrolysis.The Zn²⁺in the gel can effectively prevent the agglomeration of the Ru atoms during pyrolysis.Our constructed carbon-based porous structure can not only provide abundant Ru single-atomic anchor sites,but also improve the exposure probability of the active site of Ru-N₄located at the edge.Therefore,our synthesized SACs have excellent electrocatalytic activity and stability.The calculation of density functional theory（DFT）shows that regulating the second coordination sphere of Ru atoms can effectively optimize the binding energy of ORR intermediates,thus improving the ORR performance.（2）:We first synthesized hydroxy-rich porous carbon spheres with a large specific surface area.Then,high-performance ORR single-atomic electrocatalysts:Cu SAs@DCSs were synthesized under high temperature pyrolysis,using NH₄Cl as nitrogen source and porator,Cu Cl₂as metal source,and porous carbon sphere as template.Porous carbon spheres rich in hydroxyl groups can significantly increase metal ion loading.NH₄Cl as heteroatom N source and etch agent significantly increased the specific surface area and porosity of carbon substrate.The number of Cu-N₄monatomic active sites anchored at pore edges was increased.At a lower catalyst load,Cu-SAs@DCSs had a larger initial potential（1.02 V）,half-wave potential（0.9 V）and limiting current(6.1 m A cm^-2).The Zinc-Air battery assembled with Cu-SAs@DCSs as cathode has excellent power density and charge-discharge cycle stability.This study showed that abundant Cu-N₄active sites anchored at pore edge and good material transport capacity were the main reasons for the improvement of ORR performance.