Preparation And Electrocatalytic Performance Of (Co, Co₂P)@NC Catalysts

The rapid consumption of fossil fuels and the resulting environmental pollution have increased the demand for sustainable energy conversion and storage devices.Zn-air batteries have the advantages of high energy density,good safety,low cost,environmental friendly,and are recognized as a very promising energy conversion and storage device.The performance of Zn-air batteries mainly depends on the electrocatalytic activity,stability and kinetic reaction rate of oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)of the cathode catalyst.However,the ORR and OER involve multi-electron reactions and proton transfer processes.There are problems such as unsatisfactory catalytic activity,slow kinetic reaction rate and poor stability,which lead to slow charge-discharge rate,low enery conversion efficiency and poor cycle performance of Zn-air battery.In order to solve the above problems,some bifunctional catalysts have been developed,which have broken through the early technical barriers and made gratifying progress.It is still necessary to continue efforts to apply them commercial rechargeable Zn-air batteries.Construction of composite structure is the most effective way to improve catalytic performance.On the basis of complementary performance,the components of composite can interact with each other,which endows composite with excellent catalytic performance.Therefore,in this paper,cobalt-based materials and nitrogen-doped carbon materials are used as structural units to prepare cobalt-based materials/nitrogen-doped carbon composite materials.The electrocatalytic performance of the prepared composite materials and its application in Zn-air batteries were systematically studied.The main contents and conclusions are as follows:(1)The triblock copolymer polyethylene oxide-polypropylene oxide-polyethylene oxide(P123)was used as the main carbon source,and the dicyandiamide(DCDA)was used as the nitrogen source and reducing agent to prepare cobalt nanoparticles encapsulated in nitrogen doped carbon nanotube(Co@NC)catalysts.Through the analysis of its synthesis mechanism,it is found that the material prepared after adding DCDA is Co@NC catalyst,which proves that DCDA can not only be used as a nitrogen source to prepare nitrogen-doped nanotubes,but also be used as a reducing agent to reduce cobalt ions to metal cobalt.In the process of synthesis,sheet-like nitrogen doped carbon(NC)catalyst were obtained without cobalt source,indicating that the presence of Co is not only conducive to the formation of carbon nanotubes,but also conducive to improving the graphitization degree of nitrogen doped carbon materials.It is further confirmed that the existence of Co is beneficial to the formation of carbon nanotubes by density functional theory(DFT)calculation.Through electrocatalytic performance test,it is found that the Co@NC catalyst had better ORR and OER catalytic activity,kinetic reaction rate and stability than the NC material.The electrocatalytic performance of Co@NC catalyst is comparable to commercial Pt/C(20%Pt)catalyst.More importantly,Co@NC catalysts has higher energy density,power density and better cycle stability than NC and commercial Pt/C catalysts in Zn-air batteries.(2)The increase of N content in carbon material can increase the number of active sites of catalyst to a certain extent.Therefore,on the basis of the above work,by optimizing the preparation process,Therefore,on the basis of the above work,it is expected to obtain a Co@NC catalyst with more excellent performance by optimizing the preparation process.A series of Co@NC catalysts were prepared by adjusting the amount of DCDA and fixing the amount of Co source and P123.It is found that the amount of DCDA can affect the morphology,specific surface area,pore size distribution and graphitization of carbon nanotubes to to a certain extent.More importantly,through the analysis of X-ray photoelectron spectroscopy(XPS),it is found that the Co@NC-3catalyst contains more pyridine nitrogen and graphite nitrogen,which makes it have more excellent ORR ad OER electrocatalytic activity,kinetic reaction rate and stability.As a bifunctional cathode catalyst used in alkaline Zn-air battery,Co@NC-3 catalyst exhibits the maximum power density,energy density and excellent cycle stability.(3)Cobalt phosphide(Co₂P)nanoparticles encapsulated in nitrogen doped carbon nanorods(Co₂P@NCNRs)catalyst were prepared by using dicy,phosphoric acid and P123 as nitrogen source,phosphorus source and main carbon source.Through the analysis of synthesis mechanism,it is found that in neutral medium,the bulk Co₂P nanoparticles encapsulated in nitrogen doped carbon material(Co₂P@NC)catalyst is obtained without adding graphene oxide.But by adjusting the amount of graphene,rod-shaped Co₂P@NCNRs catalysts can be obtained,and the graphitization degree of nitrogen doped carbon nanorods can be also improved,which shows better ORR and OER catalytic activity,kinetic reaction rate and stability in electrocatalytic reaction systems.The onset potential of Co₂P@NCNRs-15 catalyst is close to commercial Pt/C(20%Pt)catalyst,and its limiting current density is equal to commercial Pt/C at 0.4 V(vs RHE).Moreover,Co₂P@NCNRs-15-based Zn-air battery has the highest peak power density,specific capacity and excellent stability.