Synthesis Of Transition Metal(Fe,Cu,Ni)/Nitrogen Co-Doped Carbon Nanotubes And Their ORR/OER Properties

The application of fuel cell is an effective way to deal with the problems of the increasing fossil energy crisis and greenhouse effect.Pt-based noble metal are widely used as electrocatalysts to solve the slow kinetics of the cathodic oxygen reduction reaction in fuel cells.However,the shortcomings of scarce storage,high cost of the noble-catalysts limit their commercial appications.Therefore,it is urgent to develop alternative non-noble-metal-based electrocatalysts.Carbon-based catalysts have attracted wide attention due to their low cost,abundant storage and stable electrochemical performance.It is of great theoretical and practical significance to synthesize low-cost and stable carbon-based catalysts with high catalytic performance to substitute noble-metal catalysts.Focusing on this purpose,high conductive carbon nanotubes have been used as substrates to support various transition metals（Fe,Cu,Ni）and N atoms as the active sites,to develop new non-noble-metal electrocatalysts.This work investigated the transition metal/nitrogen co-doped carbon nanotubes forelectrocatalysts used in oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）with high catalytic activity and stability.The main contents and conclusions are as follows:（1）Preparation and electrocatalytic ORR performance of Fe/nitrogen-codoped carbon nanotubes nanocomposites.Fe-C-N species modified carbon nanotubes（F/N-CNT）nanocomposites were prepared by pyrolyzing the mixture of CNT,melamine and Fe species.The effect of Fe content and heat conditions on the synthesis of F/N-CNT was investigated,and a highly active ORR/OER sample was prepared under the optimal condtions（0.24 wt%Fe,900℃,Ar）.The compositon characterization results show that F/N-CNT has a large number of different active sites,like pyridine-N,Fe-N₄ and Fe-C species.The synergistic effect of active sites and high conductivity of the carbon nanotubes improve the ORR catalytic performance of the Fe/N-CNT catalysts.Under the optimal synthetic conditons（0.24 wt%Fe,900℃,Ar）and the catalyst loading amount of 0.28 mg cm^-2 on the electrode,the as-obtained F/N-CNT catalyst performs an ORR activity with an onset potential of 1.06 V and a half-wave potential of 0.91 V（vs.RHE）in an alkaline condition,more positive than those（1.01 and 0.88 V）of the commercial Pt/C catalyst.The catalyst was also used as the ORR catalyst in a Zn-air battery（ZAB）that exhibits a power density of 76.5 m W cm^-2,much higher than that of the Pt/C catalyst,making F/N-CNT a promising candidate as the efficient air electrode in ZABs.（2）Preparation and electrocatalytic ORR performance of Cu/nitrogen-doped carbon nanotube nanocomposites.A serial of copper-nitrogen-modified carbon nanotube electrocatalysts（Cu/N-CNTs）were prepared by pyrolyzing the freeze-dried mixture of CNTs,melamine and Cu species at 700～900℃in Ar.The effect of heating temperatures on the phase,structure and electrocatalytic ORR performance of Cu/N-CNTs was investigated.The results show that when the heating temperature is 850℃in Ar,the as-obtained Cu/N-CNT-850 catalyst presents a high ORR performance:In an alkaline solution with a loading of 0.14 mg cm^-2,the onset and half-wave potentials of the Cu/N-CNT-850 electrode are 0.95 and 0.83 V（vs.RHE）,respectivley,which are close to the ORR data（0.95 and 0.84 V）of the commercial Pt/C.The limiting current density of Cu/N-CNT-850 is 7.25 m A cm^-2,much higher than that(4.95 m A cm^-2)of Pt/C.The sample of Cu/N-CNT-850 also has a smaller electron transfer resistance（5.54Ω）than that（5.67Ω）of the comercial Pt/C catalyst.（3）Preparation and ORR/OER performance of Ni/nitrogen-doped carbon nanotube nanocomposites.Ni/nitrogen co-doped carbon nanotube catalysts were prepared by pyrolyzing a complex of Ni,Zn-ZIF-8 and carbon nanotubes.The Ni-Nx/CNT obtained at 750℃（Ni-Nx/CNT-750）in Ar shows high ORR and OER properties.For ORR with a loading of 0.14 mg cm^-2in 0.1 M KOH solution,the Ni-Nx/CNT-750 electrode shows an initial potential of 0.92 V（vs.RHE）and a half-wave potential of 0.81 V（vs.RHE）,close to the Pt/C electrode（0.95 and 0.84 V）.For OER with a current density of 10 m A cm^-2 in 0.1 M KOH,the potential(E@j=10 m A cm^-2)of Ni-Nx/CNT-750 is 1.81 V,which is comparable to the OER performance of Ir/C（1.79 V vs.RHE）.