Preparation Of Nanoreactors Based On M-N-C(M=Fe,Ni) Structure For Oxygen Reduction Reaction

The increasing consumption of fossil energy and the resulting climate change and environmental pollution urgently require the development of clean,sustainable new energy sources.However,the practical application of renewable energy conversion and storage devices such as metal-air cells and fuel cells are limited by the cathodic oxygen reduction reaction（ORR）,which has slow kinetics and a complex electron transfer process,leading to a decrease in catalytic efficiency.Currently,platinum-based materials remain the most efficient ORR catalysts for commercial applications,but their scarcity and high cost severely limit their large-scale adoption.Therefore,it is necessary to prepare non-precious metal catalysts with low cost and ORR activity instead of precious metal catalysts.In recent years,due to the unique electronic structure of the active center of single-atom catalysts,their preparation and performance studies have become a hot topic in the field of electrochemistry.One of the bottlenecks limiting the further improvement of the performance of single-atom catalysts is the problem of insufficient exposure of active sites.Based on this,in this thesis,nanoreactors with hollow nanostructures are prepared and transition metal single-atom catalytic sites are placed on the inner wall of the reactor.The catalyst can both fully expose the active site through its own three-dimensional structure and increase the reaction rate by increasing the collision frequency between reactants using the domain-limiting effect.In this work,transition metal nanoparticles（M=Ni and Fe）were used as precursors,which were coated with dopamine hydrochloride polymerization,and polydopamine（PDA）was used as a carbon and nitrogen carrier to construct the walls of the nanoreactor,and then the central metal template was removed by acid etching to prepare hollow carbon nanostructures for oxygen reduction reaction.The nitrogen atoms contained in PDA not only act as an anchor site to stabilize the metal atom,but also play an important role in regulating the electronic structure of the active site.By using metal nanoparticles as a precursor template,the metal morphology is effectively utilized while leaving the metal active center.The microstructure and chemical composition of the nanoreactors were studied by changing the experimental parameters to reasonably regulate the microscopic morphology of the nanoreactors and to investigate the influence law of its microstructure and chemical composition on the catalytic activity of ORR.The details of the study are as follows:（1）Uniformly dispersed nickel nanoparticles（Ni-NPs）were prepared by the reduction of nickel chloride hexahydrate using hydrazine hydrate.Ni,N co-doped carbon nanoreactors（Ni-NC）were prepared by the self-templating method using Ni-NPs as precursors and using dopamine to coat them by oxidative polymerization on the surface,and the nitrogen-doped carbon-coated nickel metal core-shell material was obtained by heat treatment,and the Ni template was removed by acid etching to leave a hollow structure.The effect on the microscopic morphology and oxygen reduction activity of the catalyst was investigated by regulating the heat treatment conditions.It was found that the Ni-NC-500 catalyst at the heat treatment condition of 500℃was a uniform and complete hollow carbon nanostructure with the best microscopic morphology and oxygen reduction activity compared with the control sample,which proved the feasibility of the self-templating method to synthesize hollow nanoreactors.（2）Fe,N co-doped carbon nanoreactors（Fe-NHC）were prepared using a similar template synthesis strategy by coating a layer of PDA on the surface of iron nanoparticles（Fe-NPs）,followed by carbonization and acid leaching to remove the central iron template.The microstructure and electrochemical properties of the samples were compared by adjusting the mass ratio of Fe-NPs to PDA to form hollow carbon nanostructures with different shell layer thicknesses.The results showed that Fe-NHC-3 had an optimal carbon shell layer thickness（～12 nm）,larger specific surface area and microporous volume,which facilitated the transport of molecules during the reaction.The presence of Fe-NHC-3 as single atoms on the inner wall of the nanoreactor verified by spherical differential electron microscopy.In the electrochemical tests,the onset and half-wave potentials of Fe-NHC-3 were 0.919 V and 0.812 V,respectively,with a Tafel slope of 70.83 m V dec^–1 and an electron transfer number close to 4.In addition,the samples showed good ORR durability and methanol resistance.（3）Based on the above work,Fe,N and S co-doped carbon nanoreactors（Fe-N/S-C）were prepared for catalytic ORR by further introducing the heteroatom S.On the basis of ensuring the hollow nanostructure of the catalyst,the effect on the microstructure and electrochemical activity of the catalyst was investigated by comparing two different doping conditions.The structural characterization showed that the addition of S source along with the cladding of PDA had minimal effect on the microscopic morphology of the samples,and the electrochemical test results demonstrated that the additional S doping was beneficial to enhance the oxygen reduction performance.