Preparation And Electrochemical Properties Of Novel Carbon-based Composite Electrodes

With the rapid development of industrial civilization and economic society,the increased fossil energy consumption and energy demand have stimulated in-depth research on high-performance energy storage systems.Carbon-based materials are widely used as electrode materials for supercapacitors and electrocatalysis due to their high conductivity,good stability and low cost.In this dissertation,a series of carbon-based composite electrode materials are prepared,which exhibit improved electrochemical properties due to the synergistic effect between each component.Meanwhile,the structure,morphology and composition of the electrode materials are adjusted to improve the energy storage performance of supercapacitors and catalytic activity of electrocatalysts,and the performance enhancement mechanisms are investigated.New ideas and methods are proposed for the preparation of carbon-based electrode materials for electrochemical energy storage and conversion applications.The main research contents are as follows:(1)Three-dimensional(3D)nitrogen-doped carbon materials with hierarchically porous structure are prepared by the introduction of nitrogen-doped carbon spheres(NCS)into the inter-sheets space of graphitic carbon nitride nanosheets(g-CN).The as-prepared graphitic carbon nitride/nitrogen-doped carbon spheres(g-CN/NCS)composites present a high nitrogen doping level,a unique hierarchically porous structure,and a high specific surface area of 448 m² g^-1.Such features makes the g-CN/NCS composite an ideal material for supercapacitor electrodes,which could deliver a high specific capacitance of 403.6 F g^-1 at 0.1 A g^-1,an excellent rate capability of220 F g^-1at 10 A g^-1,and a high cycling stability with almost 100%capacitance retention after 5000 cycles at 20 A g^-1.Furthermore,the g-CN/NCS electrode based symmetric supercapacitor exhibits a decent energy density of 6.75 Wh kg^-1at the power density of 1000 W kg^-1.The enhanced performances are mainly attributed to the high nitrogen doping level and the hierarchically porous structure of the 3D structured g-CN/NCS composites,which provide efficient pathway for the transportation of ions and electrons,and endow more active sites for electrochemical redox processes.(2)The binder-free CC@g-CN-900 electrode by growing graphite carbon nitride(g-CN)nanosheets onto the flexible carbon cloth(CC)is synthesized through a facile strategy.The CC@g-CN-900 electrode delivers a remarkable specific capacitance(499F g^-1 at 1 A g^-1),an excellent rate capability(292 F g^-1 at 20 A g^-1)and outstanding cyclic performance(capacitance loss of only 4.4%after 10000 cycles).The symmetric supercapacitor device based on CC@g-CN electrodes exhibits decent energy density at ultrahigh power density(10.1 Wh kg^-1 at 10000 W kg^-1),and superb cyclic performance(no capacitance reduction after 10000 cycles).The superior electrochemical performance of the CC@g-CN electrode can be ascribed to its self-supported binder-free character,high nitrogen doping level and strong coupling between CC and g-CN nanosheets,which results in a small charge transfer resistance,highly stability,and remarkable pseudocapacitance.Therefore,the CC@g-CN,as an outstanding carbon-based binder-free electrode,promises potential applications in high-performance supercapacitors.(3)Three-dimensional(3D)graphene aerogel and its composite with interconnected porous structure have aroused continuous interests as electrode for energy storage devices owning to its large surface area and hierarchical pore structure.Herein,we report the preparation of 3D nitrogen-doped graphene/polyaniline(N-GE/PANI)composite foam as electrode material with greatly improved electrochemical capacitive performance.The 3D porous structure can allow the quick diffusion of the electrolyte ions.The doped nitrogen atoms and redox-active polyaniline can enhance the psedocapacitive behavior of the electrode material,and can also ensure the high electrical conductivity of the electrode.Moreover,the synergistic effect between N-GE and PANI materials also play an important role on improving the electrochemical performance of electrode.Therefore,the as-prepared composite foam could deliver a high specific capacitance of 528 F g^-1 at 0.1 A g^-1and a high cyclic stability with 95.9%capacitance retention after 5000 charge-discharge cycles.This study provides a new idea on improving the energy storage capacity of supercapacitors by using 3D graphene-based psedocapacitive electrode materials.(4)ZIF-67 derived Co P-decorated nitrogen-doped porous carbon(NPC)polyhedrons anchored on reduced graphene oxide(RGO)sheets has been successfully prepared through an efficient pyrolysis-phosphidation-assembly strategy.The resulted Co P-NPC/RGO composite as electrode material for supercapacitors shows an enhanced electrochemical performance with high capacitances of 466.6 F g^-1 at 1 A g^-1 and 252F g^-1 at 20 A g^-1,as well as 94.7%of capacitance retention after 10000 cycles in 1 M H₂SO₄ solution.Moreover,the symmetrical two-electrode device assembled from Co P-NPC/RGO electrodes delivers a high energy density of 12 Wh kg^-1 at the power density of 500 W kg^-1,and excellent long-term cycling stability(93%of the initial capacitance after 10,000 cycles at 10 A g^-1).The superior electrochemical performance of Co P-NPC/RGO can be ascribed to its 3D interconnected porous structure,and the synergistic effect between Co P and nitrogen-doped carbon matrix.The unique architecture of the composites can effectively enhance electrochemical performance by shortening the diffusion distance of electrolyte ions,improving the electrical conductivity and the contact area between active materials and the electrolyte.The excellent electrochemical performances make Co P-NPC/RGO a promising electrode material for high-performance supercapacitors.(5)Developing earth-abundant,low-cost and high-efficient non-noble metal catalysts plays key role in realizing the practical application of overall water splitting.Herein,nitrogen doped porous carbon embedded Co nanoparticles(Co-NPC)were in-situ grown on carbon cloth(CC)to form CC@Co-NPC composite.Benefiting from the exposure of abundant Co active sites and the synergistic effect between Co particles and nitrogen doped carbon species,the CC@Co@NPC composite exhibits high bifunctional catalytic activity toward oxygen evolution reactions(OER)and hydrogen evolution reactions(HER)in alkaline solution,The optimized CC@Co-NPC electrode delivers small overpotentials of 286 and 340 m V at current density of 100 m A cm^-2 with Tafel values of 59 and 97 m V dec^-1 for OER and HER,respectively.The electrolytic cell based on the CC@Co-NPC||CC@Co-NPC couple requires only a small voltage of 1.49 V to achieve the current density of 10 m A cm^-2,indicating that the CC@Co-NPC is a highly efficient bifunctional electrocatalyst for overall water splitting.This work provides a feasible method for the preparation of transition metal/nano-carbon composites as efficient electrochemical catalysts.