Preparation Of Hollow Porous Structured Cobalt-nitrogen/Carbon Composites And Investigation Of Oxygen Reduction Process

Concerns about the sustainability of fossil fuel use and environmental issues have led to a great deal of research into alternative energy conversion and storage systems.With the advantages of high theoretical energy density,low cost and environmental friendliness,fuel cells are widely regarded as a promising next-generation sustainable energy storage technology.However,it slow kinetics of the cathodic oxygen reduction reaction(ORR)leads to low reaction efficiency and therefore requires highly efficient electrocatalysts to accelerate the reaction.Currently,the highest efficiency electrocatalysts are precious metal Pt-based materials,but the large-scale application of precious metal catalysts has a high cost price.Therefore the development of highly active non-precious metal oxygen reduction catalysts is important to reduce the cost of fuel cells.The details of the study are as follows:(1)Study on structure regulation and ORR performance of metal-nitrogen-carbon composites with hollow hierarchical porous structure.Using polystyrene(PS)as a template,the Co-N-C ORR catalyst with hollow structure was obtained by uniformly growing ZIFs on the surface of nano-PS spheres,followed by pyrolysis.The testing and characterization of the material showed that the prepared cobalt-nitrogen co-doped hollow carbon-based catalyst(Co-NHCP-2)has a hierarchical porous structure,a large specific surface area(1817.24 m2 g-1),and pyridine-N and Graphite-N content is high.The large specific surface area and abundant hierarchical porous structure can increase the reactive sites and enhance the ORR performance.Electrochemical tests under alkaline conditions show that the onset potential(0.95 V),half-wave potential(0.84 V)and limiting current density(5.50 mA cm-2)of Co-NHCP-2 are higher than those of commercial Pt/C catalyst.Furthermore,Co-NHCP-2 exhibits a nearly four-electron ORR reaction pathway,as well as stronger methanol tolerance and higher cycling durability than commercial Pt/C.At the same time,on the basis of the previous work,the effect of m(Zn(NO3)6H2O,ZNH)/m(2-methylimidazole,2-MI)on the structure and morphology of the prepared catalysts was investigated.A series of control samples with different ratios were designed,and it was found that as m(ZNH)/m(2-MI)became smaller,the prepared samples all had hollow structures with a structure size between 160-200 nm,but agglomerated also gradually increased.In addition,when the ratio of m(ZNH,8g)/m(2-MI,8g)is close to 1,the prepared samples are all large-sized particle structures(above 1 μm),and the core-shell structure of ZIFs-coated PS is not formed.These exploration results can effectively control the size of the desired ZIFs structure during the preparation process.(3)Structural stability and catalytic performance enhancement of hollow hierarchical porous metal-nitrogen-carbon composites.Using PS spheres as sacrificial template,ZIF-8@ZIF-67 as Co/Zn/C/N source,and PVP as binder,N-doped carbon nanotubes(CNTs)-coated hollow core-shell catalysts were prepared(Co/NHCNT-160).Metal nitrides embedded in carbon nanotubes can promote the dispersion of active centers and the rapid penetration of electrolytes.Core-shell structure can improve the degree of graphitization,inhibit the dissolution and agglomeration of metal nanoparticles,enhance the stability,and improve the mass transfer efficiency.Through structural characterization,it was found that the catalyst has a large specific surface area(1359.2 m2 g-1)and abundant mesopores,which are conducive to the diffusion of electrolyte ions,thereby improving the reaction rate,catalytic activity and reaction kinetics during the catalytic process.Co/NHCNT-160 also has high content of pyridine-N and graphite-N,the more graphite-N,the better the conductivity.The electrochemical results under alkaline conditions show that the prepared electrocatalyst Co/NHCNT-160 has good catalytic activity for ORR,and its onset potential(0.96 V),half-wave potential(0.88 V)and limiting current density(6.3 mA cm-2)outperformed the Pt/C catalysts and most non-noble metal catalysts reported so far.And it has a low H2O2 yield(less than 2)and good long-term stability,with a current retention rate of 99.7%after 7 h.