Design,and Study On Properties And Reaction Mechanisms Of Non-noble Matal Porous Carbon For Oxygen Reduction Reaction

In recent years,great efforts have been made to explore non-noble metal catalysts in order to reduce the catalyst cost,which hindering the large-scale commercialization of fuel cell.With the rapid development of electric catalytic technology,carbon as a kind of main material in the application has been attracting significant attention in the academic world and industries.Regarding non-precious metal catalysts,metal,especially cobalt or iron atoms,and N doped carbon materials（M-Nx-C）with large specific surface area,reasonable pore size distribution,high oxygen reduction of catalytic activity,low cost,being environment friendly,safe and stable,which is considered to be one of the most promising alternative to platinum non-noble metal catalysts.However,all of these reported carbon-based catalysts are still inferior to the state-of-the-art Pt/C catalyst in terms of ORR activity and stability in both alkaline and acid medium.At present,the commercialization of fuel cells are still based on the acid medium,which using acidic proton exchange membrane as the main body of fuel cell.Therefore,preparation of doped carbon material showing good catalytic activity and stability in both alkaline condition and acid condition is significant.In this paper,a new type of N/S/Fe multi-doped carbon materials is synthesized using silica colloid as template,FeSO₄·7H₂O as Fe and S sources,and polyquaternary ammonium salt as N and C source,respectively.By using both rotating disk electrode（RDE）and rotating ring-disk electrode（RRDE）techniques,combined with SEM,XRD,low temperature N2 absorption,XPS and ICP methods,we focuses on determining the parameters of catalyst preparation and studying the effect of the introduction of the template agent and S elements for catalysts’ activity and structure,and the influence of different metal salts on the catalysts’ structure and active site.Moreover,by changing the amount of metal ferrous salt sulfate adding during the process of preparing,we explored that the structure and chemical composition of catalysts are how to influence the catalysts’ activity.Using electrochemical tests and physical characterization methods to study the physical properties and oxygen reduction performance of the catalysts in both the alkaline acidic medium,the main points are summarized as follows:Firstly,a hard-templating method combining a simple and green two-step graphitization of the impregnated carbon,using polyquaternium^-2（PQ-2）as nitrogen and carbon precursor,Si O2 nanosphereas as template agent,FeCl₂·4H₂O and FeSO4 7 H2 O as the precursor of metal and sulfur,respectively,was used to synthesize the N & S-doped hierarchically porous carbon catalysts（N–S-HPC）for oxygen reduction.Through the introduction of template agent,metal and sulfur element,the structure and the active site of the catalyst can optimized greatly.N–S-HPC contains microporous,mesoporous and large pores,and has a BET surface area as high as 1201 m2g-1.The obtained N–S-HPC shows a superior catalytic ORR performance to the commercial Pt/C catalyst in alkaline media,including high catalytic activity,remarkable long-term stability and strong methanol tolerance.Even in acidic media where most non-precious metal catalysts suffer from high overpotential and low durability,our N–S-HPC exhibits an amazing ORR activity with a half-wave potential of 0.73 V,and 40% enhanced limited diffusion-current density when compared to the Pt/C catalyst.Particularly,when used for constructing a zinc–air battery cathode,such an N–S-HPC catalyst can give a discharge peak power density as high as 536 m W cm^-2.At 1.0 V of cell voltage,a current density of 317 m A cm^-2 is achieved.This performance is superior to all reported non-precious metal catalysts in the literature for zinc–air batteries and significantly outperforms the state-of-the-art platinum based catalyst.The outstanding electrochemical performance of such catalysts thereby makes it very promising for this non-noble catalyst to replace the commercial Pt/C in both fuel cell systems and metal–air batteries.Secondly,A series of novel and high-performing oxygen reduction reaction（ORR）catalysts based on N/S-Me-doped（Me = Fe,Co,Ni）hierarchical porous carbons（N/S-Fe-HPC,N/S-Co-HPC and N/S-Ni-HPC）have been synthesized by a green and cost-effective method combining a simple silicate templated two-step graphitization of the impregnated carbon,using polyquaternium^-2（PQ-2）as nitrogen and carbon precursor,Si O2 nanosphereas as template agent,FeSO4 7 H2 O,Co SO4 7 H2 O and Ni SO4 ·H2O as different precursors of metal and sulfur.we can conclude that different metal salts can make a huge difference on the structure and chemical composition and the active site of the obtained catalysts.Metal salts play a vital role on the optimization of structure and surface functional groups.Compared with Co SO4 7 H2 O and Ni SO₄·7H₂O,FeSO₄·7H₂O can greatly improve the pore structure of catalyst,increase the specific area and the proportion of active sites,which can expose more active sites.N/S-Co-HPC and N/S-Ni-HPC has a BET surface area 607 and 432 m² g^-1,respectively,but N/S-Fe-HPC has a BET surface area as high as 1201 m² g^-1.N/S-Fe-HPC has more abundant microporous and mesoporous structure by observing N2 adsorption–desorption plots.The obtained three catalyses have the same level of total nitrogen content,while the proportion of pyridine nitrogen and graphitization nitrogen being considered to be the active sites is up to 91% for N/S-Fe-HPC,and N/S-Co-HPC and N/S-Ni-HPC have only 79% and 79%,respectively.At the same time,we found that the metal content and sulfur content had a positive relationship.It is worth noting that Co content of N/S-Co-HPC reached 15.6%,far higher than N/S-Fe-HPC and N/S-Ni-HPC.Through half cell performance test,N/S-Fe-HPC shows more superior catalytic properties in alkaline medium with oneset potential and half wave potential of 0.98 V and 0.86 V,respectively.Thirdly,we use polyquaternium-7（PAD）as nitrogen and carbon precursor,and Si O2 nanosphereas as template agent.By adding different amounts of FeSO4 7H2O（with ratios of FeSO4 7H2O/PAD: 0.05,0.25,0.5,1.0,and 1.5,respectively）,a series of precursors samples could be obtained.These obtained catalysts are designated as Fe（x）/N/S-PAD（X=0.05,0.25,0.5,1.0,1.5）.To elucidate the joint effect of template agent of colloidal silica and FeSO₄·7H₂O,Fe（1.5）/N/S-PAD was also prepared without using the silica template,named Fe（1.5）/N/S-PAD-（w/o-silica）.We concluded that the joint effect of template agent of colloidal silica and the activating agent of FeSO₄·7H₂O can contribute to the formation of the complicated mesoporous system and optimization of chemical composition,and thus promotes the improvement of ORR performance.Along with the increase of the metal salt,BET,porosity and graphitization degree can be improved greatly,while nitrogen content showed a trend of decline in the process.At the same time,the specific surface area and the catalytic performance present positive correlation,while catalytic performance has no direct relationship with the metal content and nitrogen content.We proved that the structure plays a crucial role in improving the performance of catalyst,and optimizing of the pore structure can provide more effective active sites.