Preparation And Electrocatalytic Activity Of Metal-free/Cobalt Nitrogen-doped Porous Carbon Materials Based On Metal-Organic Framework

Nowadays,with the growth of population,the elevation of resource demand,and the development and progress of society,increasingly serious energy and environmental problems have endangered the entire world.In order to achieve the balanced development of industry and environment,sustainable clean energy has attracted wide attentions.In particular,electrochemical energy storage and conversion devices such as supercapacitors,lithium-ion batteries,metal-air batteries,fuel cells and electrolytic water devices are extensively concerned and studied due to their high transfer efficiency,abundant capacity and environmental friendliness.In these advanced technological devices,metal-air batteries and fuel cells can directly transform chemical energy into electricity,which mainly relies on the cathode oxygen reduction reaction(ORR)in the electrochemical process.During the reaction,oxygen can be converted into water directly through four-electron transfer pathway,water is clean and zero pollution,thus sustainable development can be realized.However,oxygen reduction reaction(1/202+2H+→H2O)in the cathode is complicated,and the kinetic mechanism is relatively sluggish,these factors have limited the potential commercial application of metal-air batteries and fuel cells.Therefore,it is essential to explore a favourable catalyst to improve the reaction rate of ORR.Traditionally commercial Pt/C,as a precious metal material,which is deemed as a superior ORR catalyst,whereas it suffers from some issues,such as high price,CO poisoning,poor stability and methanol durability,these drawbacks hinder the use and development of metal-air batteries and fuel cells.Consequently,it is demanded to explore an electrocatalyst with low cost,high activity and stability to substitute commercial Pt/C.Metal-organic frameworks(MOFs)are a crystal material formed by the coordination bond between metal centers and organic ligands.Given MOFs have some unique advantages,such as adjustable pore size,plentiful porosity,large specific surface area and diversity,as a result,MOFs can be a promising precursor to derive porous carbon materials,which play an important role in different fields.Recently,MOFs-derived oxygen reduction catalysts have attached concerns from researchers and are extensively applied in energy conversion devices.In this work,bio-MOF-1[Zn8(Ad)4(Bpdc)6O·2Me2NH2]was selected as a precursor to prepare novel metal-free/cobalt-nitrogen doped porous carbon materials.The application of as-prepared materials as ORR electrocatalysts in the field of electrocatalysis was earnestly studied by adjusting calcination temperature,holding time and doping content of cobalt.The main research contents in this work are as follows:(1)Adenine(Ad),biphenyl dicarboxylic acid(H2Bpdc)and zinc acetate dihydrate(Zn(OAC)2·2H2O)were used as raw materials,and N,N’-dimethylformamide(DMF)was utilized as solvent,bio-MOF-1 microcrystals were successfully fabricated by solvothermal method.The composition,morphology and thermal stability of bio-MOF-1 were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM)and thermogravimetry(TG).The results showed that bio-MOF-1 has good crystallinity,rod-like morphology,uniform size and highly thermal stability.(2)Given abundant carbon and nitrogen sources existing in the bio-MOF-1,bio-MOF-1 precursor was carbonized at high temperature under inert atmosphere(argon)to successfully prepare metal-free highly graphitized nitrogen-doped porous carbon materials(NPCs).By adjusting the calcination temperature,it is found that the optimal carbonation temperature is 1000℃.Subsequently,bio-MOF-1 precursor was calcined at 1000℃ for different carbonization time(2,3 and 4 h)to obtain products(NPC-1000-ts).The ORR electrocatalytic performance of NPC-1000-ts was investigated,and testing results demonstrated NPC-1000-ts have good catalytic activity.Especially,the NPC-1000-4 has not only the highest catalytic performance,but also excellent methanol durability and stability.After the cycling stability testing for 18000 s,NPC-1000-4 still maintains a higher current density than that of commercial Pt/C catalyst.NPC-1000-4 has a higher degree of graphitization and a high proportion of graphitized nitrogen,which can augment the electronic conductivity of catalyst and provide favorable active sites of ORR catalysis.Moreover,NPC-1000-4 has large surface area and porous structures,which is conducive to reactant transport and electron transfer.These merits collectively contribute to the superior electrocatalytic activity of NPC-1000-4,even exceeded that of Pt/C catalyst.(3)CoxZn1-x-bio-MOF-1(x reflects molar ratio)precursor was successfully prepared by different amounts of cobalt salts substituting Zn salts in bio-MOF-1 under solvothermal condition.We explored the optimal Co-substituted amount is 5%.By means of XRD,SEM and TG characterization tests,confirming cobalt salts were successfully incorporated into bio-MOF-1,and Co5Zn95-bio-MOF-1 has rod-like morphology and highly thermal stability.(4)Metal cobalt-nitrogen doped carbon core-shell nanostructural materials(Co5-N-C-Ts)were successfully prepared by high-temperature pyrolysis of Co5Zn95-bio-MOF-1 precursor at different temperatures(800,900,1000℃).The ORR electrochemical tests demonstrated that Co5-N-C-Ts have good electrocatalytic behaviors.Among the Co5-N-C-Ts,Co5-N-C-900 has the highest catalytic activity,which was superior to those of NPC-1000-4(prepared by bio-MOF-1)and commercial Pt/C electrocatalysts.Meanwhile,Co5-N-C-900 also has high cyclic stability and excellent methanol tolerance.The introduction of metal Co plays a key role in enhancing the catalytic activity of Co5-N-C-Ts,and Co-Nx active sites were formed by Co binding with N to further catalyze ORR.