Preparation Of Cobalt Based Complexes And Electrochemical Properties Of Their Derivatives

In recent years,the researches on complexes are no longer limited to the increases in the number of compounds with novel structures.Researchers have paid more attention to the excellent properties obtained by the special structures of the complexes,especially which have been applied in catalysis,adsorption and separation,sensors,electrochemical energy storage materials and so on.And with the wide uses of portable devices,the researches and developments of energy storage equipments have received more and more attentions from researchers.In this paper,twenty three novel complexes were synthesized by selecting different ligands and metal salts,and the suitable complexes were selected as the precursors for preparing the electrode materials of supercapacitor.The contents were listed as follows:1.Using pyridine 2,5-dicarboxylic acid（2,5-Pda）as the first ligand and phenanthroline（Phen）as the second ligand,four novel complexes（trans-O,O-1,trans-O,O-2,cis-O,O-1,cis-O,O-2）were prepared with cobalt chloride hexahydrate（Co Cl₂·6H₂O）and cobalt nitrate hexahydrate（Co（NO₃）₂·6H₂O）as cobalt sources.The effects of different hydrothermal temperature,hydrothermal time and raw material ratio on the structures or morphologies of the obtained complexes were studied.The structures of the products were confirmed by X-ray single crystal diffractometer（SXRD）.The powder samples were characterized by X-ray photoelectron spectroscopy（XPS）,X-ray powder diffractometer（PXRD）,Fourier transform infrared spectroscopy（FT-IR）and ultraviolet visible spectrophotometer（Uv-vis）.Due to the poor conductivity of the complex reducing its electrochemical performance,trans-O,O-1 was selected as the precursor for preparing electrode material of supercapacitor through analyzing thermogravimetric（TG）,scanning electron microscope（SEM）and PXRD of calcinations products.The mesoporous composite（C（N,Cl）/Co O/Co）was prepared by calcining the precursor at 500℃for 4 h in the atmosphere of nitrogen gas flow.The product was used as the electrode material of supercapacitor with 6 M KOH solution as the electrolyte,which showed a specific capacity of 685.6 F·g^-1 when the current density was 2 A·g^-1.After 2000cycle-experiments at a current density of 20 A·g^-1,the capacitance retention was well.2.By changing the second ligand when replacing Phen with 2,2’-bipyridine（2,2’-Bpy）,three complexes（Co-1,Co-2,Co-3）whose structures were similar but heterogeneous as trans-O,O-1,trans-O,O-2,cis-O,O-1 were prepared.The effects of different hydrothermal temperature,hydrothermal time and raw material ratio on the structures or morphologies of the prepared complexes were studied.The structures of the products were confirmed by SXRD.The powder samples of the complexes were characterized by XPS,PXRD,FT-IR and Uv-vis.Through the analyses of TG,SEM and PXRD,Co-3 was selected as the precursor for preparing the electrode material of supercapacitor.The composite（C（N,O）/Co_xO_y）in which Co_xO_yuniformly dispersed on carbon substrate was prepared by calcining the precursor at 420℃for 2h in the atmosphere of nitrogen gas flow.The product was used as the electrode material of supercapacitor with 6 M KOH solution as the electrolyte,which showed a specific capacity of544 F·g^-1 when the current density was 2 A·g^-1.After 2000 cycle-experiments at a current density of 20 A·g^-1,the capacitance retention was well.3.Three series of Co₂Lns₂（2,5-Pda）₆ complexes with different structures were prepared by adding different rare earth salts（Lns）and Co（NO₃）₂·6H₂O.The first series was four kinds of Co₂Ln1₂（2,5-Pda）₆ acicular complexes（Ln1=Ce,Pr,Nd,Sm）.The second type was eight kinds of Co₂Ln2₂（2,5-Pda）₆ acicular complexes（Ln2=Y,Eu,Gd,Tb,Dy,Ho,Er,Tm）.Another was four kinds of Co-Ln（Ln=Y,Er,Tm,Yb）block complexes which were always mixed in the corresponding products of Co₂Ln2₂（2,5-Pda）₆.The effects of different hydrothermal temperature,hydrothermal time and raw material ratio on the structures or morphologies of the synthesized products were studied.The structures of the products were confirmed by SXRD.The representative complexes such as Co₂Pr₂（2,5-Pda）₆ and Co₂Tb₂（2,5-Pda）₆ which were selected from Co₂Ln1₂（2,5-Pda）₆ and Co₂Ln2₂（2,5-Pda）₆ respectively were further characterized by XPS,PXRD,FT-IR and Uv-vis.Through the analyses of TG,SEM and PXRD,Co₂Pr₂（2,5-Pda）₆ was selected as the precursor for preparing the electrode material of supercapacitor.Under the atmosphere of nitrogen gas flow,the composite with blade structure（C/Pr₂O₂CO₃/Co O/Co）was formed when the precursor were calcined at 800℃for 2 h.The product was used as the electrode material of supercapacitor with 6 M KOH solution as the electrolyte,which showed a specific capacity of 1440 F·g^-1 when the current density was 2A·g^-1.After 2000 cycle-experiments at a current density of 20 A·g^-1,the capacitance was 88.5%of the initial capacitance.4.The complexes were theoretically studied by quantitative calculation method.Trans-O,O-2 and Co-2 were selected as the research objects.Based on gaussrian 9 software package,6-31G（d）basis set was selected for ligands and lanl2dz pseudopotential basis set for center atom.The results showed that HOMO of trans-O,O-2 and Co-2 was mainly concentrated on part of 2,5-Pda,while LUMO was mainly concentrated on parts of Phen and 2,2’-Bpy.It showed that the electron transition from HOMO to LUMO mainly occured in the charge transform process from 2,5-Pda to Phen or 2,2’-Bpy.Furthermore,compared with Co-2,the stability of trans-O,O-2 was higher than that of Co-2,and the charge transform of trans-O,O-2was more obvious.The analyses of the above research results can be illustrated as follows.Firstly,porous materials can provide large specific surfaces,which are conducive to the diffusion of electrolyte and benifited to the transmission of ions and electrons,so as to improve the energy storage performance of materials in supercapacitors.Secondly,the product calcined in an inert atmosphere can retain the carbon from the precursor,which can improve the conductivity and utilization of the electrode material.Finally,bimetallic complex can be obtained by introducing rare earth element into Co based complexes.Bimetallic complexes as precursors can give full play to the joint action of transition element Co and rare earth elements,which are conducive to improving the specific capacity of supercapacitor.