Preparation And Electrochemical Performance Of Cobalt-based Oxygen Reduction Catalyst

Nowadays,direct methanol fuel cells（DMFCs）play a pivotal role in the development of modern society.As an oxygen reduction catalyst in fuel cells,Pt/C is not only expensive,but also shows poorly resistance to methanol,which makes it impossible for application on a large scale.Therefore,it is critical to design and prepare efficient,low cost,electrochemically and chemically stable catalysts for oxygen reduction reaction（ORR）.Metal complexes have the characteristics of large surface area and diversity,and are interesting precursors for preparing metal or metal composite materials with carbon.In this paper,starting from cobalt-based complexes（Co-NTA）,we proved an efficient and simple method to prepare cobalt-based high-efficiency catalystss.The Co-NTA wire was synthesized by solvothermal method and pyrolyzed in situ in an inert atmosphere with sulfur powder to prepare porous CoS_1.097-C composite nanowire.The composite nanowire is composed by carbon-coated CoS_1.097nanoparticles.The carbon layer and the particles are closely connected.The composite nanowire shows large specific surface area and unique 1-D porous structure,which provides many active sites and improves the electron transport property between CoS_1.097 and carbon layer.Electrochemical tests show that onset potential and half-wave potential of the optimized CoS_1.097-C composite nanowire catalyst are 0.90 V and 0.79 V,respectively.The oxygen reduction reaction is carried out in a nearly four-electron approach.In addition,after（3 hours）stability test with chronoamperometric method,the CoS_1.097-C composite nanowire catalyst has a current retention of 93%and exhibits methanol tolerance superior to commercially available Pt/C.Porous Co@Co_0.85Se-C composite nanowire was prepared by in-situ pyrolysis of Co-NTA in an inert atmosphere with selenium powder.The TEM result shows that the Co@Co_0.85Se-C composite nanowire is composed by carbon-coated Co@Co_0.85Se nanoparticles with a particle size of approximately 20-30 nm.The synthesis conditions of Co@Co_0.85Se-C composite nanowire is optimized by changing the selenization temperature and selenium powder content.The electrochemical test results show that onset potential and half-wave potential of the optimized Co@Co_0.85Se-C composite nanowire catalyst are 0.93 V and 0.80 V,respectively.And the oxygen reduction reaction is carried out by a four-electron route.The catalyst has higher catalytic performance than Co-C nanowire and exhibits excellent stability（current retention of95%）and methanol tolerance property,which maybe derived from the synergetic effect of Co@Co_0.85Se-C composite nanowire.Since the particle size has a large influence on the performance of the catalyst,porous Co-CN composite nanowire with tiny cobalt metal particles was successfully prepared by in-situ pyrolysis of Co-NTA precursor together with urea in an inert atmosphere.This composite nanowire is composed by carbon-coated Co nanoparticles with average particle size of 5.8 nm,which is significantly smaller than that of the catalyst obtained from the pyrolysis of Co-NTA precursor without urea,indicating that the addition of urea can alleviate the agglomeration problem of the nanoparticles during sintering and reduce the particle size.At the same time,it is found in the XPS test results that the addition of urea can increase the content of pyridin and pyrrole nitrogen in the material.The electrochemical test results show that onset potential and half-wave potential of the optimized Co-CN composite nanowire catalyst are 0.93 V and 0.80 V,respectively.The oxygen reduction reaction is carried out by a four-electron approach with a dynamic limiting current density of 5.21 mA cm^-2.