Controlled Synthesis And Electrocatalytic Performance Of Co@N-doped Carbon Hollow Spheres

The energy crisis and environmental pollution caused by high speed economic development promote the unprecedented development of renewable and clean energy technology.Oxygen Reduction Reaction（ORR）and Oxygen Evolution Reaction（OER）are the key reactions of renewable and clean energy conversion devices represented by metal-air batteries and fuel cells.At present,Ru,Ir and Pt based materials have been commercialized as high-performance ORR and OER electrocatalysts.However,the most intractable problem is that the high cost and scarce resources of these precious metals electrocatalysts make it difficult to realize large-scale industrial production and application.Cobalt-based non-noble metal catalysts have attracted wide attention because of their remarkable efficiency,high stability and low cost,and have excellent performance in the highly efficient electrolysis of water to produce hydrogen and oxygen.However,it’s ORR and OER properties are still not ideal,which is limited by the slow particle diffusion rate and low active point utilization rate.To this,this paper from the nitrogen doping carbon load and the two aspects of pore structure regulation,the double metal organic framework based on zinc cobalt（Zn Co-MOF）polyhedron and the competitive growth of dopamine,the preparation of nitrogen doped carbon（N-C）package Co metal particle porous hollow ball,in order to increase together ORR activity sites,reactant/electronic transmission,upgrade product diffusion rate,thus improve the oxygen of the efficiency of catalytic reaction.The main research contents and conclusions of this paper are summarized as follows:（1）Growth and structure regulation of PDA-Zn Co-MOF hollow spheres.By adjusting the relative molar ratio of Zn/Co in Zn Co-MOF,a series of Zn Co-MOF-n with different relative molar ratios of Zn/Co（n=1,2,3,4,5）were obtained.The results show that the Zn Co-MOF-3 samples corresponding to the molar ratio of zinc and cobalt（n=3）have relatively regular morphology,smooth surface and uniform particle size.With the increase of relative moles,Zn Co-MOF-n agglomerates to varying degrees,and the particle size ranges from 500nm to 1μm.It can be seen that the Zn/Co ratio has a significant effect on the morphology and structure of Zn Co-MOF.This is because the addition of Zn can distance Co atoms in space and realize the uniform distribution of metal cobalt elemental matter.Therefore,the sample can maintain a uniform cube morphology when the proportion of zinc and Co is balanced.Then,the Zn Co-MOF-3precursor was combined with polydopamine（PDA）.By adjusting the ratio of dopamine and MOF,the hollow spheres with rhomboid dodecahedron structure were successfully prepared.The particle diameter was about 200nm,the shell thickness was about 40nm,and the main component of the shell was PDA-Zn Co-MOF.When the addition amount of DA was 100mg/200mg Zn Co-MOF,the polyhedron structure collapsed,indicating that when the concentration of DA was larger,PDA competed with cobalt ions as the dominant reaction with a higher rate,leading to rapid dissolution of Zn Co-MOF and then collapse.（2）Electrocatalytic performance analysis of PDA-Zn Co-MOF derived Co@NC hollow spheres.PDA-Zn Co-MOF The carbonization product was named Co CHS-N（n=1,2,3,4,5）.Co CHS-3 has a regular hollow spherical structure,which is a typical hierarchical porous structure with large pore cavity.The cavity is large pore,and the spherical wall is rich in micropores and mesopores.It has a large specific surface area(480.84m²g^-1)and an ideal nitrogen doping form.The initial ORR potential and half-wave potential of Co CHS-3 at 0.1M KOH were 0.89V and 0.80V,respectively,which were better than those of Pt/C catalyst（E0=0.95V and E1/2=0.84V）,and the limiting current density of Co CHS-3 was 7.7 m A cm^-2.The OER overpotential of Co CHS-3sample at the current density of 10 m A cm^-2 is 1.61V,which is similar to that of commercial Ru O2 catalyst（Ej10=1.56V）,indicating that Co CHS-3 has excellent bifunctional electrocatalytic properties.