Controllable Synthesis Of Nitrogen-doped Carbon Nanotubes Supported Transition Metal Catalysts For Oxygen Reduction Reaction

Oxygen reduction reaction(ORR)as a key half-reaction that affects the energy conversion efficiency of fuel cells and metal-air batteries,requires a large amount of noble metal-based catalysts,such as Pt/C,to reduce the overpotential required for the reaction.However,the Pt/C is expensive and has poor tolerance to CH₃OH and CO.Therefore,it is of great significance to develop low-cost,high-activity and stable non-precious metal catalysts.Recently,nitrogen-doped carbon-supported transition metal catalysts are considered to have great potential to replace Pt/C.The density and utilization of active sites in catalysts are the key factors affecting its catalytic activity.Carbon nanotubes(CNTs)can effectively improve the utilization of active sites due to the good electrical conductivity and unique hollow structure which can be used as a fast channel for mass transfer.In this thesis,nitrogen-doped CNTs supported transition metal catalysts were studied.The ORR properties of the catalysts were optimized from the perspective of electronic and geometric structure regulation of the catalysts.On one side,the density of active sites is improved by adjusting the electronic structure of the catalysts through heteroelement S doping and Co-Zn alloying.On another side,the utilization of active sites is improved by regulating the geometric structure of the catalysts with controlled sizes of CNTs and metal particles,special pore structures,and optimized specific surface areas.The main research contents and conclusions are as follows:(1)Synthesis of S,and N co-doped CNTs supported iron nanoparticle catalyst and its ORR performance.Fe SNC catalysts were synthesized in situ by one-step pyrolysis at 700?,800?,900?and 1000?using melamine and ferrous sulfate as catalysts precursors.It was found that S was successfully doped into the carbon matrix with structures with potential ORR activity such as S-O_x and C-S-C.The diameter of the prepared CNTs showed an increasing trend as the pyrolysis temperature increased.In the selected pyrolysis temperature,the catalyst prepared at900°C(Fe SNC-900)shows the best ORR activity with the half-wave potential(E_1/2)of 0816 V(vs.RHE)in 0.1 M KOH,The ORR activity was further improved through acid treatment and the E_1/2 increases to 0.839 V.The pore density and specific surface area of the catalysts are effectively improved after acid treatment due to removal of unstable metals,thereby optimizing the activity of the catalysts.(2)Control of the diameter of CNTs in Fe SNC catalysts by two-step pyrolysis to improve ORR performance.We designed a simple two-step pyrolysis method to synthesize S,and N co-doped small-diameter CNTs supported iron nanoparticle.The synthesis involves a first-step pyrolysis at 700?to form small-diameter CNTs,a subsequent acid-etching to remove the majority of the metal nanoparticles,and a second-step pyrolysis at 900?to improve the crystallinity.The two-step pyrolysis is proved to be effective in controlling the diameter of the synthesized CNTs and significantly increasing their specific surface area and sulfur content.Owing to the network of small-diameter CNTs,the high specific surface area,and the synergistic effect between the S functionalities and Fe-N_x sites,the catalyst exhibits 4-electron selective ORR activity with a E_1/2 of 0.854 V in 0.1 M KOH,which is better than commercial Pt/C.(3)Synthesis and ORR performance of bubble-like nitrogen-doped CNTs supported Co Zn alloy nanoparticle catalyst.The Co_2-xZn_x-N/C(x=2,1.5,1,0.5,0)catalysts with different molar ratios of Co/Zn were synthesized by pyrolyzing a mixture of cobalt nitrate,zinc nitrate and melamine at 700?,and their ORR activity were explored.It was found that the Zn will form inactive or low activity cyanide and oxide with C,N and O on the surface of catalysts.The unique CNTs structure formed by a series of bubble-like carbon spheres can be synthesized when the ratio of Co to Zn is 1:1,which has an important relationship with the formation of Co_xZn_y alloy nanoparticles.The obtained Co₁Zn₁-N/C has a high nitrogen content(12.32%)and mainly pyridine nitrogen.The E_1/2 of the acid-treated Co₁Zn₁-N/C in 0.1 M KOH is0.801 V.The ring-disk electrode test shows that the catalyst has excellent ORR activity based on the 4-electron catalytic pathway.