| As environmental pollution and energy shortages becoming increasingly severe globally,it is urgent to find renewable energy resources and improve the utilization efficiency of energy.A lot of attention has been paid to research and develop polymer membrane fuel cells(PMFCs)and Zn-air batteries(ZABs).PMFCs have the advantages of high-power density,high efficiency and environment-friendliness.ZABs possess the merits of low cost and high energy density.Those energy conversion devices offer a wide range of applications from electric vehicles to portable devices;however,more efforts should be paid for their extensive commercialization.The most urgent problems to be solved are sluggish kinetics oxygen electrode reactions such as oxygen reduction reaction(ORR)and oxygen evolution reaction(OER).The keys to those problems are the development of new electrocatalysts.Precious metal-based materials can effectively catalyze the electrocatalytic reactions.However,their high cost and scarce reserves are not conducive to large-scale use.Therefore,the development of ORR catalysts and bifunction OER/ORR no-precious catalysts with high activity and stability is of great significance for the commercial application of PMFCs and ZABs.Transitional metal-nitrogen-carbon(M-N-C,M=Fe,Co,Ni,Cu,etc.)materials exhibit fine electrocatalytic activity for the oxygen electrode reactions,and are the most promising materials to replace precious metal-based catalysts.However,M-N-C electrocatalysts still have the disadvantages of low active site density,low site availability and poor intrinsic activity ang stability.Therefore,in order to solve the above problems,in this paper,we used a series of strategies such as heteroatom doping,metal component coupling,morphology regulation and construction of single atom to adjust the electronic structure and synergies of the active sites,which could further optimize the electrochemical properties of catalysts,and improve the catalytic activity and stability of the catalysts.The relationship between structure and catalytic performance and the catalytic reaction mechanism were studied and some meaningful results were obtained.The main research contents and results of this paper are as follows:(1)Nanoporous M-N-C catalysts were prepared by using functional carbon black assembled with MIL-101(Fe)as a precursor,followed by pyrolysis,and their catalytic performances for ORR were studied.Firstly,by changing the amount of melamine,different nitrogen-doped carbon materials were derived by metal organic frameworks(MOF).The structure and morphology characterization results of derived materials showed that Fe and Fe3 C nanoparticles were encapsulated in nitrogen-doping mesoporous carbon and connected to functional carbon black(Fe/Fe3C@NC).The hierarchical structure improved the electronic conductivity of the catalyst,exposed more active sites and enhanced the stability of metal species.XPS results showed that different contents of Nx-C active sites were formed in the catalysts.The best catalyst exhibited excellent ORR activity with onset potentials(Eonste)and half-wave potentials(E1/2)of 0.85 and 0.70 V,respectively.Furthermore,it showed much higher stability and better methanol tolerance than those of the state-of-the-art Pt/C.Through further analysis of the effect of nitrogen doping on active site density and catalytic activity,we found that the improvement of catalytic activity of Fe/Fe3C@NC is mainly attributed to the role of nitrogen doping in adjusting the electronic structure,and highefficiency nitrogen dopants form more active centers.At the same time,the enhancement of the synergistic effect between Fe/Fe3 C nanoparticles and Nx-C further promotes the ORR process.(2)On the basis of(1),we explored the influence of preparation conditions on the performance of iron-based catalysts,and obtained the optimized conditions and the catalysts with Fe/Fe3O4@NC structure.Iron-based catalysts containing different metal species were obtained at different pyrolysis temperatures(600 ℃,650 ℃ and 700 ℃)by choosing MIL-101(Fe)/C as precursor.The experimental results showed that the Fe/Fe3O4@NC obtained by pyrolysis at 650 ℃ had a nitrogen-doped carbon structure which regulated by the Fe/Fe3O4,showing higher electrocatalytic activity and stability as compared with the other iron-based catalysts.Fe/Fe3O4@NC conducted a close to four-electron pathway with Eonste of 0.90 V and E1/2 of 0.77 V,and its ORR activity was better than commercial Pt/C.Studies have shown that the pyrolysis temperature has an important influence on the metal compositions and unique structures.Fe/Fe3O4 has an excellent modulation effect on the electronic structure of the carbon layer,which can reduce the local work function of the carbon layer,enhance the intrinsic activity of the surface active sites and improve the ORR catalytic activity.On the whole,the effect of metal components coupling on the electronic structure of active sites in M-N-C catalysts was studied in this work.It is important to provide study value for the design and synthesis of high-performance non-noble metal catalysts.(3)Iron-nitrogen co-doped hierarchically porous carbon(Fe-N-HPC-0.05)electrocatalytic catalyst was synthesized through a dual-template assisted method and its catalytic performances for ORR under acid and alkaline conditions were studied.By controlling the compositions and contents of MOFs,morphologically controlled M-N-C catalyst was obtained through applying silica as a hard template together with MOFs as a self-sacrificing template.The characterization results show that Fe/Fe3 C nanocrystals,Fe-Nx and nitrogen-doped carbon all exist as active sites in the catalyst;the hierarchical porous structure facilitates the exposure of active sites and increases the accessibility of reactants.Fe-N-HPC-0.05 showed enhanced ORR performance in both alkaline and acidic conditions.In alkaline conditions,Fe-N-HPC-0.05 exhibited onset and half-wave potentials of 1.02 and 0.85 V,which were similar to those of commercial Pt/C.In addition,the stability of Fe-N-HPC-0.05 is better than Pt/C.In acidic conditions,Fe-N-HPC-0.05 possessed half-wave potential of 0.78 V,only 60 m V lower than that of Pt/C(0.84 V).The analysis shows that the hierarchically porous structure and synergistic effect of active sites could optimize the ORR reaction pathway.Fe-N-HPC-0.05 has potential application prospects as a fuel cell cathode catalyst.At the same time,this work also provides the design principle for increasing the ORR catalytic activity of hierarchically porous structure with exposed active sites.(4)Atomically dispersed Fe and Co doping 3D nitrogen-doped carbon nanosheets(A-Fe Co@NCNs)was prepared by using modified MOFs(Si O2@Fe-ZIF-8/67)as precursor,and its bi-functional ORR/OER catalytic activity and performance of zincair batteries were studied.Firstly,A-Fe Co@NCNs catalyst was obtained by using Si O2@Fe-ZIF-8/67 as the precursor.The morphology and phase structures analysis results showed that Fe or Co single atoms were identified to be coordinated with N atoms and formed Fe N4,Co N4 and N3Fe-Co N3 anchoring on 3D defect carbon.AFe Co@NCNs exhibited excellent electrochemical performance with ORR/OER potential gap of 0.80 V.The performance of A-Fe Co@NCNs-based ZAB was better than that of a zinc-air battery assembled with a commercial Pt/C+Ir O2 catalyst.AFe Co@NCNs-based ZAB had a specific capacity of 736.2 m Ah g-1Zn,a power density of 132 m Wcm-2 and outstanding cycle stability.The results shown that bi-metal SACs were prepared to further increase the density of active sites,and the synergistic coupling effect between metal atoms can effectively improve the bifunctional activity of the catalyst.Density functional theory(DFT)study showed that the electron density of Fe atoms in A-Fe Co@NCNs apparently increases after the introduction of Co atoms,which is conducive to the adsorption and activation of O2 molecules.Fe N4-Co N4 shows a positive thermodynamic limiting potential of 0.85 V.It is further clarified that the synergetic coupling between Fe and Co can optimize the ORR reaction path and enhance the ORR performance.Results from this study may provide a facile method for precious control of dual metal doped carbon with highly activity and durability for bifunctional electrocatalysis. |
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