Engineering Nanomaterials For High-performance Supercapacitor Electrodes

Supercapacitors,also known as electrochemical capacitors,consisting of electrical double-layer capacitors?EDLCs?and pesudocapacitors?PCs?,have received extensive attention as a promising energy storage device due to their high power density,long cycle life and rapid charging/discharging capability.However,the limited energy density?⁵Wh/kg?of supercapacitor restricts its applications to those demanded by ubiquitous portable electronic devices and electrical equipment.Based on the energy density formula.it is feasible to realize the supercapacitor with high energy density by fabricating new capacitor configuration and engineering new electrode materials for increasing both the specific capacitance C and the work voltage window V.Therefore,in this thesis,a series of electrode materials with different size and morphology were fabricated and investigated electrochemically as EDLCs or PCs,exploring their mechanism of energy storage and conversion.Furthermore,combinating the lithium ion battery and supercapacitor,a novel PCs – lithium ion hybrid supercapacitor was designed and investigated,exhibiting a potential prospect in the energy storage and conversion.The detailed contents are as followings:1.Four kinds of carboneous materials including graphene oxide?GO?,reduced graphene oxide?r GO?,carbon BP-2000 and carbon CMK-3 were investigated systematically and comparatively as EDLCs electrode materials.The results show that the electrochemical performance of such traditional supercapacitor is dependent on the existential state,morphology and size dimension.Thereinto,carbon BP-2000 exhibits higher specific capacitance,better cycling stability and higher coulombic efficiency compared to other carboneous materials due to its large specific surface area and excellent electronic conductivity.2.Hollow Co₉S₈ nanospheres were fabricated by a facile solvothermal method and investigated electrochemically as supercapacitors in KOH aqueous solution.The results indicate that hollow Co₉S₈ nanospheres deliver a initial specific capacitance of as high as 234.7F/g at a current density of 0.5A/g and exhibit a high energy density of 17.96Wh/kg at a power density of 32.82W/kg,which is much higher than traditional EDLCs.This may be due to the dual function of hollow Co₉S₈ nanospheres including both capacitive property of EDLCs and Faraday electrochemical energy storage,showing the good property of pseudocapacitor.3.Hollow LiFePO₄ nanospheres were synthesized using the monodispersed hollow Li3PO4 microspheres as a template and the electrochemical performance of lithium ion hybrid supercapacitor with the as-prepared hollow LiFePO₄ nanospheres or its mixture with activated carbon as positive electrode and activated carbon as negative electrode was investigated.The results show the power energy of such supercapacitor with mass ratio 1:2 of positive to negative material reaches as much as 19.4Wh/kg as the power energy is 90W/kg,exhibiting the advantageous combination of both high energy density of lithium ion battery and high power density of supercapacitro.4.Mesoporous TiO₂ nanoparticles were fabricated by a template-free hydrothermal method and an asymmetrical hybrid supercapacitor was constructed in organic electrolyte with the as-prepared TiO₂ as negative electrode and activated carbon as positive electrode.The effect of different mass ratio of positive material to negative material on the electrochemical performance of such supercapacitor was explored.The results show that the supercapacitor with mass ratio 1:2 deliver a specific capacitance of 48.6F/g and the energy density is 60.75Wh/kg as the power density is 150W/kg,indicating the potential application of mesoporous TiO₂ nanoparticles to the pseudocapacitor.