Synthesis Of Transition Metal Sulfide Composites And Their Electrochemical Properties

In order to solve the energy environmental problems and energy crisis,high-efficiency electrochemical material to alternative energy storage and conversion systems are badly in need of research.Recently,transition metal（Fe,Co,Ni,and Mn）sulfides as promising electrode materials for electrochemical energy storage and conversion applications,such as supercapacitors,Li-ion rechargeable batteries,electrocatalysis and micro-electronics,have been extensively studied by various research groups.Although great achievements have been made in preparation and application of transition metal sulfides,it remains a big challenge in enhancing cycling stability.By compositing with highly conductive substrate was an effective way to solve this problem.In this work,two templates,carbonaceous and metal-organic frameworks,were used to fabricate different nanostructured transition metal sulfides.The main results are shown bellows::Firstly,porous Co₃O₄ hexagonal bipyramid have been synthesized via two simple and cost-effective approaches through calcination of cobalt-based metal-organic framework template.Two porous Co₃O₄ hexagonal bipyramid structures as the electrodes exhibit high specific capacitances of 234 and 231 F g^-1 at 1A g^-1,and the intermediate CoO_x/C hexagonal bipyramid structure electrode exhibit specific capacitance of 150 F g^-1 at 1 A g^-11 with excellent recycling stability over 2000cycles at 2 A g^-1.Well-designed morphologies and architectures,the abundant mesopores and novel structures of the porous Co₃O₄ are of great significance for the electrochemical processes,which enhance the electrochemical performance.The facile conversion approaches can be easily extended to the scaling-up synthesis of other porous metal oxide-based functional materials.Secondly,in order to further improve the electrochemical performance of Co₃O₄,simple calcination combined with in situ sulfurization of a cobalt-based metal-organic framework template was used to synthesize CoS,then phosphate ion functionalized CoS was obtained through a low temperature phosphating reaction process with CoS.The obtained phosphate ion functionalized CoS（P-CoS）exhibits excellent electrochemical activity as bifunctional materials for supercapacitor electrodes and oxygen evolution reaction（OER）catalysts.CoS and P-CoS as the electrodes exhibit high specific capacitances of 304 and 442 F g^-1,with excellent recycling stability over10000 cycles at 5 A g^-1.Meanwhile,the P-CoS reveal comparable electrocatalytic performance for OER affording a current density of 10 mA cm^-2 at overpotentials of340 mV for OER with small Tafel slope,phosphate ion functionalize is a promising method for stimulate high chemical reactivity and faster ion and electron transfer.P-CoS has excellent electrochemical and electrocatalytic activity,so it can be used as a bifunctional material to meet the needs of different energy sources.Thirdly,polymer fibers were electrospinning prepared with PAN and PVP with Ni ion-containing.Then,after preoxidation and carbonization the polymer fibers,CNFs-NiO_x was obtainted.Finally,Carbon nanofibers（CNFs）wrapped with NiS nanoparticles were obtained through calcination and in situ sulfurization using CNFs-NiO_x as template.NiS nanoparticles in composite nanofibers are covered by a layer of graphitic carbon,which not only increase the conductivity but also provide active regions for nanoparticle growth to prevent aggregation.The CNFs-NiS electrode has high specific capacity of 1275 F g^-11 at 1 A g^-1(0.41 mAh cm^-2 at 2.3 mA cm^-2)and good cycling stability,with 88.7%capacitance retention after 5000 cycles.The excellent electrochemical activity may be attributed to the accessible specific surface,unique porous structure of CNFs and high specific capacitance of NiS.In addition,the asymmetric supercapacitor has an enhanced volumetric energy density of13.32 mWh cm^-3 at a volumetric power density of 180 mW cm^-3 and high cycling stability,with 89.5%capacitance retention after 5000 cycles.It also successfully lights up a light-emitting diode.Meanwhile,the CNFs-NiS revealed comparable catalytic activity and long-term stability as ORR catalyst.