Preparation And Capacitive Performance Of Carbon-based Composite Materials

Supercapacitor,as a kind of new type of energy storage device,has many advantages including long lifespan,high power density and safety operation.However,the biggest bottleneck that limits its widespread application is its relatively low energy density.The nanostructure and composition of electrode materials play an important role in the capacitive performance of supercapacitor.Large specific surface or abundant surface active sites are essential to achieve high energy densities.In this respect,how to ameliorate the energy density of supercapacitor while maintain its high power density is still the research objective in the future.In our study,we explored the strategies of how to improve the power density of supercapacitor by designing several different nanostructures.On this basis,Faridic energy storage mechanisim was introduced to further improve the energy density.As a result,the electrodes with both high energy density and power density were obtained.Herein,this study firstly reports a hierarchical three-dimensional graphene/porous carbon composite with superhigh surface area.K⁺dispersed atomically in the precursor through an ion exchange process,which can not only avoid additional activation process,but also enhance the micropores generation efficiency.The as-prepared composite presented a super high specific surface area of 2034 m²·g^-1.The graphene skeleton endowed the composite abundant mesopores and macropores,constructing rapid electrolyte ions transport channels.Thus,the composite possessed a high specific capacitance(354 F·g^-1)and excellent rate performance(157 F·g^-1 at 50 A·g^-1).Furthermore,we report a micropore-enriched graphene/porous carbon composite with a reticular nanostructure.Carbon nanofibers with uniform diameter interweaved with each other on the graphene surface.Abundant micropores with short pore channels can be generated across the nanofirbers by a chemical activation process.The short pore channel is beneficial to the repaid ions transport,leading to an enhanced rate performance of the composite.The specific capacitance still maintained 87.8%even at a super high current density of 50 A·g^-1.Then,we prepared graphene/polypyrrole/NiAl-layered double hydroxides?GPPY/NiAl-LDH?composites to further improve the energy density by introducing faradic energy stroage mechanism.NiAl-LDH nanowires were intertwined with each other on the surface of GPPY which can prevent LDH from aggregating together.The nanowire structure of NiAl-LDH helps active Ni²⁺expose to electrolyte as much as possible,leading to efficient and rapid redox reactions of Ni²⁺.A much improved specific capacitance of 845 F·g^-1 was achieved,which could still maintain 67%at 30A·g^-1.The synergistic effect of different metal ions on the electrochemical behaivor was also investigated.It is found that the substitution for Al³⁺in the host layers by other electrochemical active metals?such as Ni,Mn and Co?has a big influence on the electrochemical behavior.LDH with dual transition metals in host layers could be more easily oxidized or reduced during the redox reactions due to the reduced band-gap energies and improved conductivities.From the spin-polarized density functional theory calculations,the hybridization between 3d-orbitals of different transition atoms endows the corresponding LDH half-metallic characteristics,leading to enhanced electronic properties.Especially,NiMn-LDH presented the best capacitive behavior owe to its good layer structure and retalively low band gap energy.Finally,a three-dimensional flexible carbon foam@NiMn-LDH@graphene?SC@NiMn-LDH@G?composite is reported.The integrative carbon foam can not only serve as substrate for the deposition of NiMn-LDH nanosheets,but also provide as connected electron transfer channels.The NiMn-LDH nanopetals with a thickness of6-8 nm were perpendicularly grown on the carbon foam ligaments to construct fast ion transport channels.The graphene coating can further facilitate the electron transfer along the LDH nanosheets and connect adjacent LDH plates to enhance the mechanical stability of the composite.The as-prepared flexible electrode,combining a highly open scaffold-like structure with well-designed electron transfer channels,exhibited significantly enhanced energy storage performance(1314 F·g^-1),excellent cyclic stability?99.6%of the initial activity maintained after 5000 cycles?and high energy density(41.0 Wh·kg^-1)in aqueous electrolyte.The assembled SC@NiMn-LDH@G//nitrogen-doped porous carbon asymmetric supercapacitor can lighten a LED indicator for more than 15 minutes,signifying its practical application in the future.