Development of multifunctional flexible and structural supercapacitors based on carbon nanotube fibers

Resum

There is a great deal of interest in the study of new functional materials which can simultaneously store energy and possess augmented mechanical properties. These include electrodes for flexible and structural high power supercapacitor devices, which find application in diverse fields such as aerospace, electric/hybrid vehicles and portable electronics. Macroscopic fibers of carbon nanotubes are considered as a potential multifunctional candidate for application as flexible and tough electrodes in supercapacitors. In this thesis, various features of CNT fibers, including their structure, textural, mechanical and electrochemical properties are investigated in detail. The study particularly reveals that the complex hierarchical structure of the fibers leading to a unique combination of high mechanical properties with toughness up to 61 J g-1, highly porous CNT network with surface area of 256 m2 g-1 and broad pore size distribution from meso to macroscale. Furthermore, the low dimensional nature of CNT fibers makes their quantum capacitance observable when measuring electrochemical properties of CNT fibers in half and full cell. The properties of the material can be further tuned by well-controlled gas phase functionalization, which modify electrochemical properties through changes in wetting, electronic structure and pseudocapacitance of redox active functional groups. Besides the intrinsic physicochemical and electrochemical properties of CNT fibers, this thesis also describes the development of high-performance multifunctional flexible supercapacitor based on CNT fiber electrodes and polymer electrolytes. For this purpose, all-solid polymer electrolyte membranes based on 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) ionic liquid and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) thermoplastic were prepared and extensively characterized in terms of their ionic conductivity, mechanical and thermal properties. The assembly process of all-solid supercapacitor device is based on utilization of a pre-cast PE membrane sandwiched between two CNT fiber electrodes, with the CNT fibers acting as current collector and active material and the membrane as separator and electrolyte. The method can be applied to fabricate free-standing devices from 0.785 to 100 cm2 with good reproducibility. These devices show specific capacitance, energy and power densities of 28 F g-1, 11.4 Wh kg-1 and 46 kW kg-1, respectively, and high stability under flexural deformations. Finally, a supercapacitor prototype consisted of 4 stacked cells was manufactured and shown to comply with specifications for the target application of the sponsors of a part of this work. Another aspect of multifunctionality such as transparency was explored by making single-filament CNT fiber electrodes. The same fabrication method of devices was used to produce all-solid transparent supercapacitors with high optical transmission of 74% and outstanding electrochemical properties (power density of 1370 kW kg-1). Finally, abovementioned flexible supercapacitor devices were used to produce laminated structural supercapacitor composites. A simple fabrication method was demonstrated through embedding CNT fibers/polymer electrolyte interleaves between carbon fiber fabrics, followed by infusion and curing of a thermosetting polymer. Electrochemical behaviour of the device was evaluated during epoxy infusion, after resin curing and during flexural deformation. A high coulombic efficiency and stability were observed in all cases. Grid-shaped interleaves enable to improve interlaminar properties of the composite and offer a wide range of design parameters to obtain desired composite performance. A preliminary analysis of different configurations and architectures of interleaved composite illustrates the envelope of mechanical and energy storage properties, and the key factor to increase multifunctional efficiency and produce weight savings relative to conventional monofunctional systems.