Preparation Of Bifunctional Oxygen Electrode Catalyst Based On G-C₃N₄ Nanostructure And Its Application Research In Zn-air Battery

The increasing energy demand and the depletion of traditional fossil fuels have caused increasingly serious energy crises and environmental pollution.Therefore,the development of clean and renewable energy has become an urgent task.As an efficient and clean power generation equipment,Zn air battery has a very high theoretical energy density,but also has the advantages of low cost,high safety,good charge-discharge cycle stability and so on,is the main research direction of the new generation energy conversion.Zn air batteries generate electricity through the oxidation reduction reaction between the cathode oxygen and the zinc anode.However,the oxygen reaction kinetics involved in these two processes is extremely slow,which seriously hinders the energy conversion efficiency of the battery.Therefore,high-efficiency bifunctional electrocatalysts must be used to reduce the activation energy of the oxygen reaction process and effectively improve the energy conversion efficiency of the battery.Currently,the effective catalysts used in Zn air batteries are precious metal-based catalysts such as Pt,Ru and Ir.But the high cost,scarcity and poor durability of precious metals limit their practical applications.In this paper,three graphitic carbon nitride（g-C₃N₄）-based transition metal,nickel cobalt sulfur and copper single atom composite nanocatalysts have been prepared,and their oxygen reduction and oxygen evolution properties and their application capabilities in zinc air batteries have been studied.The main contents are as follows:（1）By acid-assisted peeling off bulk g-C₃N₄ and solid-phase pyrolysis with transition metal salts（Fe,Co and Ni）,a series of high-efficiency bifunctional electrocatalysts with transition metal nanocrystals embedded in g-C₃N₄ nanosheets（M-CNNs）were synthesized,which can be used as the cathode of rechargeable Zn air batteries.The ratios between metallic acetylacetonates and the g-C₃N₄ precursor can be controlled where Fe-CNNs_-x,Ni-CNNs_-x and Co-CNNs_-x composites have been optimized to exhibit superior ORR/OER bifunctional electrocatalytic activities.Specifically,Co-CNNs_-0.7 exhibited not only a comparable half-wave potential（0.803 V vs.RHE）to that of the commercial Pt/C catalyst with a larger current density for the ORR but also a lower overpotential（η=440 m V）toward the OER compared with the commercial IrO₂ catalyst（η=460 m V）,revealing impressive application in rechargeable Zn air batteries.As a result,rechargeable Zn air batteries using Co-CNNs_-0.7 as the cathode exhibited an excellent peak power density of 85.3 m W cm^-2 with a specific capacity of 675.7m Ah g^-1 and remarkable cycling stability of 1000 cycles,outperforming the commercially available Pt/C+IrO₂ catalysts.（2）The composite materials（NiCo₂S₄/CNNs）with nickel-cobalt-sulfur compounds embedded in g-C₃N₄ nanosheets were prepared by in-situ growth,pyrolysis and carbonization methods.The in-situ synthesized Ni Co₂S₄ nanocrystals are confined in the g-C₃N₄ nanocage,which can produce a clear electron transfer heterogeneous interface and more exposed catalytic sites,resulting in higher ORR/OER bifunctional electrocatalytic activity.The rechargeable Zn air battery equipped with a Ni Co₂S₄/CNNs_-1 catalyst cathode shows a high peak power density(92 m W cm^-2)and an excellent energy density close to the theoretical value(1025 Wh kg^-1).（3）Atomically dispersed Cu-N/HPC-second catalyst was constructed using ion exchange and two-step adsorption strategies.The active sites of Cu-N₄ are captured on the defect sites generated through the volatilization of the zinc node in ZIF-8,thereby effectively inhibiting the migration and aggregation of metal atoms.In the first adsorption process,Cu ions are combined with Zn ions to prepare Cu-doped ZIF-8 precursor.After high temperature Zn evaporation and acid leaching to remove excess Cu nanoparticles,the derived porous 3D carbon framework is used as the main body.By introducing tubular g-C₃N₄ and Cu ions to disperse the 3D carbon framework and perform a second adsorption,then thermal activation and simultaneous decomposition of uncoordinated tubular g-C₃N₄.The copper single atoms catalyst obtained by two-step adsorption has a unique 3D layered pore structure,a specific surface area of up to 872m² g^-1,and more exposed catalytic active sites.Moreover,compared with Pt/C catalyst,this Cu-N/HPC-second catalyst shows excellent ORR activity and stability in alkaline medium.The primary Zn air battery based on Cu-N/HPC-second has an extremely high power density of215.8 mW cm^-2.