Design,Construction And Electrocatalytic Performance Of Transition Metal-based Heteroatom-doped Carbon Nanocomposites

Polymer electrolyte fuel cell(PMFC)is one of the most promising green power generation devices in the future.Its cathodic oxygen reduction reaction(ORR)catalysts usually require much platinum,which increases the cost of PMFC and therefore hindering its development to some extent.Therefore,the development of non-precious metal-based ORR catalysts with low cost and excellent catalytic performance has become the focus of research.Among these catalysts,heteroatom-doped transition metal-based carbon nanocomposites,especially iron/cobalt-nitrogen-carbon(Fe/Co-N-C)materials,are of great potential.However,the ORR catalytic activity and stability of Fe/Co-N-C catalysts still need to be improved.Based on this issue,different iron/cobalt-nitrogen-carbon catalysts are designed and constructed,and the effects of the composition,structure and interface modification of the catalyst on the ORR catalytic performance are discussed.The main research contents are as follows:1.Iron-nitrogen-carbon composites generally have good ORR catalytic activity.In the preparation of such catalysts,various components such as iron nitride,Fe-N₄moieties and nitrogen-doped carbon are often produced.The Fe-N₄ and nitrogen-doped carbon have been recognized as active components of ORR,while whether iron nitrides(eg.Fe₂N)contributes to ORR catalysis remains controversial.To study the role of iron nitrides in Fe-N-C ORR catalyst,the mesoporous N-doped carbon spheres mainly embedded with Fe₂N nanoparticles are designed and constructed by combining hydrothermal synthesis and calcination method.By characterization techniques such as M?ssbauer spectroscopy,it is proved that only inorganic iron phase(mainly Fe₂N,including?-Fe₂N and?-Fe₂N)and nitrogen-doped carbon are present in the material,while no Fe-N₄ moieties can be found.The ORR activity of the material is comparable to Pt/C in an alkaline medium.It is found that Fe₂N and nitrogen-doped carbon are ORR active components and the synergistic effect between these two components significantly improves the overall ORR activity of the composite.In addition,it is found by density functional theory calculations that?-Fe₂N has a more moderate oxygen adsorption energy than?-Fe₂N,which is more favorable to the occurrence of ORR.2.The carbonized products of the zeolitic imidazole metal organic framework(ZIF)have the large specific surface areas and rich Me-N₄(Me=transition metal)ORR active sites,and therefore showing good ORR catalytic activity.However,the ORR activity of such catalysts in acidic media is difficult to be further improved.In addition,the active sites of such catalysts are easily corroded by acidic electrolytes,resulting in poor catalyst stability.To solves these problems,an aprotic ionic liquid[BMIM][NTf₂]is selected to creatively modify the surface of Zn,Co,N-doped microporous carbon(Zn Co NC)derived from a ZIF by an immersion-vacuum drying method.It is found that,[BMIM][NTf₂]can be stably adsorbed on Zn Co NC substrate and the ionic liquid-surface-modification leads to increased electron-transfer number,stability in acid electrolyte and increased peak power density in the proton exchange membrane fuel cell test.This is because the C-F chain of[NTf₂]^-anion makes the ionic liquid oxophilic and hydrophobic.The oxophilic nature of[BMIM][NTf₂]helps to increase the oxygen concentration near the active site,while the hydrophobicity helps to expel the water generated near the active site and protects the active site from rapid corrosion by the electrolyte.In addition,[BMIM][NTf₂]shows a complicated effect on the ORR activity of the catalyst under alkaline conditions.The peak power density of the anion exchange membrane fuel cell is increased,but the onset potential is slightly decreased.3.The protic ionic liquids have excellent proton conductivity,electrochemical stability,and thermal stability.Based on the work in last section,the protic ionic liquid([MTBD][NTf₂],PIL)and aprotic ionic liquid([BMIM][NTf₂],AIL)with the same anion([NTf₂]^-)are selected to modify the surface of N-doped carbon nanofibers encapsulating Co/Co O nanoparticles(Co CNFs).It is found that the effect of AIL-modification to ORR activity of Co CNFs catalyst is similar to that observed in last section.The catalyst modified by PIL shows improved electron-transfer number and half-wave potential in both acidic and alkaline electrolyte,and the PIL is better than AIL in terms of improving ORR activity.This is because,compared to AIL,PIL not only has oxophilicity and hydrophobicity which can help to increase the diffusion current,but also has excellent proton conductivity which can promote the ORR process and therefore increasing the half-wave potential.In addition,the stability of the catalyst modified by PIL in the acidic electrolyte is remarkably improved.4.The application of Fe/Co-N-C materials in ORR catalysis and other electrocatalysis fields is further explored.In order to enable the Fe/Co-N-C catalyst with high multi-functional electrocatalytic performance,we design and construct the N,P co-doped carbon nanofibers encapsulating Fe₃O₄ and Fe P nanoparticles(Fe CNFs-NP)by electrospinning technology combining with carbonization and phosphating heat treatment process.In addition to excellent ORR catalytic activity and stability,Fe CNFs-NP also exhibits good hydrogen evolution and oxygen evolution catalytic activity.The electrospinning method can help to controllably obtain the encapsulation structure which is good for improving the multifunctional catalytic activity of the catalyst.The Fe P produced by the phosphating process can significantly improve the hydrogen evolution catalytic activity of the catalyst in both alkaline and acidic electrolytes.In addition,the zinc-air battery using Fe CNFs-NP as catalyst also shows good performance.Similarly,the N,P co-doped microporous carbon nanofibers encapsulating Co/Co P nanoparticle(Co CNFs-NP)catalyst prepared by the same strategy also has good multifunctional catalytic activity.