Synthesis And Electrochemical Performance Of Bifunctional Electrocatalvsts Derived From ZIF-67

Due to the sluggish kinetics of oxygen evolution reaction(OER)at the anodes of water electrolysis devices and oxygen reduction reaction(ORR)at the cathodes of the fuel cells,efficient catalysts are needed to promte the reaction.Transition metal carbon composites have many advantages,such as low cost,high catalytic activity and good stability.They are considered to be one of the most potential electrocatalyst materials.Research shows hat the carbon based materials of porous structure are beneficial to the exposure of more active sites and promote mass transfer,while nitrogen doping can increase the conductivity and activity of carbon based materials.In this paper,ZIF-67 is used as a precursor for the preparation of porous dodecahedron carbon based composites Co@NC and Co1-xS@C,and carbon nanotube Co@BNC.All the materials were synthesized by simple and convenient methods such as high temperature pyrolysis and chemical water bath.And then we evaluated the electrocatalytic properties towards OER/ORR of these materials,including catalytic activity,stability,methanol resistance and so on.The main contents are as follows:(1)Nitrogen doped carbon based composite material Co@NC was prepared by high temperature carbonization of ZIF-67.The morphology,composition and crystallization of Co@NC were analyzed,and the electrocatalytic properties towards OER/ORR were investigated under alkaline conditions.The results show that Co@NC has a porous dodecahedron structure,and the composition of elements changes at different carbonization temperatures.The atomic content of Co elements is about 6%～9%,and the atomic content of N elements decreases while the content of C elements increases with the increase of carbonization temperature.The electrochemical test results showed that the Co@NC prepared at 700℃ exhibited efficient OER activity with low overpotential of 266 mV to obtain 10 mA cm-2,and had a good stability over 40 h.The catalytic activity towards ORR reaction was similar to that of commercial Pt/C,with the stability and methanol tolerance superior to commercial Pt/C.(2)Co1-xS@C was successfully fabricated through the a two-step transformation method by using ZIF-67 as precursor.A lot of characterization methods were used to characterize the structure and morphology of the materials,the OER/ORR electrocatalytic tests were carried out under alkaline conditions.The Co1-xS@C shows a porous dodecahedron hexagonal structure.Co2+ and Co3+ ions coexist in Co1-xS while S exists as S2-.It only need an overpotential of 260 mV to achieve 10 mA cm-2 for OER,and the Faraday current efficiency is close to 100%,and it has withstood the stability test of 80 h.In addition,Co1-xS@C exhibit comparable ORR catalytic activity to commercial Pt/C and better stability and methanol tolerance.(3)Cobalt nanoparticles uniformly dispersed boron nitrogen co-doped carbon nanotubes Co@BNC were prepared through high temperature pyrolysis by using ZIF-67 as the metal source.The effects of the content of ZIF-67 in the precursor on the formation of nanotube in Co@BNC and the distribution of Co nanoparticles were investigated.Co@BNC-100 has the best morphology and structure,with Co nanoparticles evenly distributed in the nanotubes.The catalytic activity and stability of OER/ORR were studied under alkaline conditions.The overpotential for Co@BNC-100 to obtaine 10 mA cm-2 towards OER reaction is only 288 mV,and the stability is up to 80 h.It also shows high catalytic activity for ORR reaction,the current value is still 100%after the stability test of 4 h,and the current in the methanol tolerance performance test can also be kept 95%.