Rational Design And Electrochemical Properties Of Functional Supercapacitors Based On Transition Metal Oxides

With the continuous growth of wearable,smart and interactive electronics as well as the extension of their application scenarios,functional supercapacitors have received ever-increasing attentions in recent years,including Li-ion hybrid supercapacitors(LIHSs),flexible quasi-solid-state supercapacitors(FSSCs),transparent supercapacitors,temperature-independent supercapacitors,and so on.Usually,functional supercapacitors based on transition metal oxides(TMOs)exhibit higher capacities.Neverthless,subject to the sluggish charge transfer within TMOs,the electrochemical performance such as energy and power output is far from expectation.Especially,the charge transfer process will be sluggish and even hard when featured with various functional properties.Therefore,rational design for electrodes and devices with rapid charge transfer and revealing the underlying enhancement mechanisms are of considerable significance for developing high-performance TMOs-based functional supercapacitors,which still remains a big challenge.In this thesis,rational design has been proposed to modify the charge transfer within TMOs-based functional supercapacitors,which accelerates the charge-transfer kinetics within materials,electrodes and devices,by means of doping,nanoengineering,and integration.Moreover,the enhancement mechanisms have been revealed with various high-performance functional supercapacitors being constructed.The main contents and results are as follows:1.To accelerate the kinetics of Li-ion diffusion coefficient and electron transport within anatase Ti O₂,we proposed a one-step strategy to synthesize nanoporous carbon-modified S-Ti O₂(STO/C)hybrid nanosheets,through which both the carbon layer and sulfur doping can be simultaneously generated in situ.It is found that,benefiting from the in situ S doping,both the electronic and ionic conductivity of anatase Ti O₂ nanoparticles are enhanced.Further coated by a carbon layer,the STO/C with dominant pseudocapacitance realizes an unprecedentedly high capacity of 550 m Ah g^-1 at 0.3 C and 102 m Ah g^-1 at 50 C when used as anode materials,showcasing an excellent rate capability.The kinetics of anode is greatly improved and capable of keeping pace with that of the nonfaradic capacitive cathode,rendering the construction of high-performance Li-ion hybrid capacitors.2.To shorten the ion pathway and boost the electronic conductivity within Fe₂O₃,we fabricated tectorum-like Fe₂O₃/polypyrrole(Fe₂O₃/PPy)nanoarrays on carbon cloth with hierarchical structures.The tectorum-like morphology of Fe₂O₃ provides a high accessibility for electrolytes and shortened pathway for ions transport,therefore contributing to a high areal capacitance.Further conformally coated by PPy,the cycling stability can be prominently enhanced.In virtue of the structural synergy between Fe₂O₃and PPy,the fabricated Fe₂O₃/PPy exhibits a high areal capacitance of 382.4 m F cm^-2 at a current density of 0.5 m A cm^-2.More importantly,an asymmetric FSSC based on Fe₂O₃/PPy and MnO₂ has been assembled,delivering a high energy density of 0.22 m Wh cm^-3 at a power density of 165 m W cm^-3.3.To ensure the electron pathway within V₂O₅-based transparent electrodes,we demonstrated the fabrication of V₂O₅@poly(3,4-ethylenedioxythiophene)-Graphene(VP-G)hybrid,in which a layer of poly(3,4-ethylenedioxythiophene)(PEDOT)was coated on the surface of V₂O₅ nanowires and further assembled with graphene sheets whereby the strong conjugation interactions between PEDOT and graphene.Results show that in virtue of the nanosized elaborate structure,the electronic conductivity has been greatly boosted in the presence of graphene and PEDOT.Meanwhile,both the kinetic blocking of the PEDOT layer and the anchoring capability of graphene upon soluble vanadium ions synergistically contribute to the unusual electrochemical stability.When used as transparent electrodes,VP-G displays a high transparency of 70%and a high areal capacitance of 22.4 m F cm^-2 with an exceptional durability over 150 000 cycles.Besides,a symmetric transparent supercapacitor has been constructed and delivers a high energy density of 0.18?Wh cm^-2at 11?W cm^-2,outperforming most of reported transparent devices.4.MnO₂-based microsupercapacitors suffer from the decreased ionic conductivity of electrolytes and sluggish desolvation at low temperatures.Herein,a novel H-bonding charge-transfer complex(CAC)with a high photothermal conversion efficiency of 79.5%was fabricated via a facile wet-chemistry method,which is used to construct a self-heating supercapacitor.When combined with a MnO₂-based microsupercapacitor,the CAC layer prompts an apparent temperature increase of 22.7?under 1 sun illumination at-32.6?,effectively elevating the working temperature and charge-transfer process within the device.Moreover,the constructed self-heating supercapacitor can work normally at a low temperature of-30?in the open air,demonstrating its promising potential for applications under extreme conditions of low temperatures.