Preparation,characterization Of Carbon Nanotubes-based Composites And Their Supercapacitive Properties

Supercapacitor is a good energy-storage device with high power property and long cycling life, however,its energy density is relatively poor.Thus,it turns out to be a research spot how to further obtain the larger energy density but not sacrifice its immanent higher power density.A hybrid of carbon nanotubes (CNTs) with good electronic double layer capacitance and electroactive materials with good Faradaic pseudocapacitance has been proposed as ideal electrode materials for supercapacitors , because such hybrid can both utilize the fast and reversible Faradaic pseudocapacitance and the indefinitely reversible double-layer capacitance at the electrolyte/CNTs interface so as to obtain large energy density at high rate.Therefore,the thesis is focused upon the preparation and characterization of the CNTs-based composites and their application in electrochemical capacitors (ECs) in order for the simultaneous achievement of large specific energy density and good power property.1.The wrapping modification of CNTs by poly (sodium 4-styrene sulfonate) (PSS) and their use in electrode materials for ECs.PSS was originally applied to solubilize CNTs well into the aqueous solution and noncovalently functionalize CNTs through a polymer-wrapping mechnism.As a consequence,the noncovalent sidewall functionalization of CNTs with negatively charged PSS,as an"anchor",could create much more electroactive sites facilitating the subsequent nuclearation,growth and good dispersion of electroactive materials onto their surfaces.In addition,the PSS-functionalized CNTs (PSS-CNTs) would be solubilized well in the aqueous solution. The PSS-CNTs were used as a support for the electroactive materials with redox Faradaic psuedocapacitance,which is important in the design and efficient construction of the CNTs-based composites for ECs application.2 . Preparation and investigation of ruthenium oxide-based nanocomposites with high electrochemical utilization for ECs.The RuO₂·nH₂O nanodots,in the case of high loadings,were originally dispersed onto the surface of PSS-CNTs well under the mild hydrothermal treatment.Electrochemical results demonstrated that the synthesized RuO₂·nH₂O nanodots/PSS-CNTs nanocomposite (10 wt.% loading) could deliver a specific capacitance (SC) of 1474 F g^-1 for the Ru species,resulting in an electrochemical utilization of ca. 71%.The composites with even more loadings still maintained the good dispersion of RuO₂·nH₂O nanodots,such as,25 wt.% and 45 wt.%, whose SCs were 774 and 703 F g^-1,respectively,for the Ru species.It indicated that the applied method made the RuO₂·nH₂O nanodots well dispersed and large electrochemical utilization,even in the case of high loadings.Such RuO₂·nH₂O nanodots/PSS-CNTs nanostructures facilitated electrolyte ions and electrons contact much more RuO₂·nH₂O nanodots with high electroactive activity for more efficient Faradaic reactions to realize their high electrochemical energy storage.Binary Ru-based oxides doped with homovalent and/or heterovalent substitution had become the research hot,because they could not only reduce the amount of Ru species but also enhance the utilization of ruthenium oxide . The Ru_1-xIn_xO_y·nH₂O/PSS-CNTs nanocomposite was first synthesized under the mild hydrothermal treatment. Electrochemical data showed a good electrochemical performance for the nanocomposite.3 . Construction and electrochemical performance of the"sandwich-like"polyaniline (PANI)/MnO₂/PSS-CNTs hybrids and core-shell PANI/PSS-CNTs composites for ECs in strong acidic electrolytes.MnO₂ was dispersed uniformly onto the surface of CNTs under the assistance of PSS.The MnO₂/PSS-CNTs composites could operate stably in the strong acidic medium due to the protective modification of PANI coating layer onto their surface.The electrochemical performance of the PANI/MnO₂/PSS-CNTs was greatly dependent upon the concentration of protons in the acidic electrolytes.PANI not only served as a physical barrier to restrain the underlying MnO₂ phase from reductive-dissolution process so as to make the novel ternary hybrid material work in strongly acidic medium to enhance the utilization of MnO₂ as much as possible,but also was another electroactive material for energy storage in the acidic mixed electrolytes.It was due to the existence of PNAI layer that an even larger SC of 384 F g^-1 and a much better SC degradation of ca. 18% over 1000 continuous charge/discharge cycles were delivered by the hybrid in the optimum 0.5 M Na₂SO_4-0.5 M H2SO4 mixed electrolyte.Particularly,a SC contributed by the MnO₂ reached about 880 F g^-1 . Furthermore , the core-shell PANI/PSS-CNTs nanocomposite was synthesized by the reacting-template method based on the MnO₂/PSS-CNTs as a template and an oxidant.A SC of 296 F g^-1 could be obtained at 2 A g^-1 and even 220 F g^-1 at 5 A g^-1 for the nanocomposite,revealing that it owned large energy density and good power property.4. Synthesis and electrochemical performance of porous nickel oxide/PSS-CNTs composites for ECs application.First,a facile and efficiency route was described to synthesize NiO microspheres with hierarchical (meso- and macro-) porosity by following thermal decomposition of the precursor obtained via simply refluxing process.Such superstructure could not only make full use of the favorable kinetics and high capacities of the nanosized building blocks but also guarantee its good stability,easiness to fabrication and tap density.The formation mechanism of such superstructure was proposed attentively that the hierarchical structured NiO microspheres were obtained by the self-assembly of two-dimensional and mesoporous NiO petal building blocks based on the coalescence and Ostwald-ripening mechanisms.Such macroporous structure,due to its great role of"ion-buffering reservoirs",could maintain the sustentation of OH- ions and make sure that the enough Faradaic reactions could take place at high current densities for larger energy storage.To further enhance its conductivity and obtain even better electrochemical behavior,PSS-CNTs were added during the synthetic process.The ordered mesoporous NiO/PSS-CNTs composites were unexpectedly formed.PSS-CNTs,as a good three-dimensional conducting network,not only enhanced the conductivity of the composite,but facilitated electrolyte soaking into particles,maintained the sustentation of it,and created much more porous channel for electrolyte ions to transport and electrochemically access even more electroactive sites of the ordered mesoporous NiO for energy storage at larger current densities.Electrochemical data demonstrated that the unique composite (ca. 48 wt.% NiO) could deliver large energy density and high power property,and a SC of 439 F g^-1 could be delivered at 6 A g^-1.5.Interface hydrothermal synthesis of the CoSx/PSS-CNTs nanocomposites for ECs and minute investigation of their real energy-storage mechanisms in alkaline KOH solution . Novel CoSx/PSS-CNTs composites were first synthesized in the unique H2O/CS2 interface under mild hydrothermal treatment.Electrochemical data demonstrated that the CoSx itself did not own energy-storage ability in the alkaline electrolyte,but the new phase Co(OH)2 formed during the continuous CV scanning in the KOH solution should be responsible for its good energy-storage property in the KOH solution.To obtain much better electrochemical performance, a good dispersed CoSx/PSS-CNTs composite was further obtained. And the CoSx/PSS-CNTs composite delivered even higher energy density and better power properties after the addition of PSS-CNTs.