| Protection and aesthetics are the two basic functions of textiles. With the ever-increasing demands of the consumers, “intelligence” has become the new needed attribute. Smart textiles are able to convert the solar energy, heat energy and mechanical energy during wear’s movements into electric energy and store, then deliver the electricity to other imbedded electronics, accomplishing monitoring the physical condition of the wearer, the sensing, information-processing, andcalculating. The emerged wearable electronic systems are expected to satisfy people’s needs in the innovative applications ranging from the military, public, healthcare, space exploration, sports, to consumer fitness fields.As one type of crucial energy storage devices, supercapacitors possess high power density, fast discharge rate, excellent cyclic stability, environment-friendly ability and and long life. Recently, the stretchable, wearable miniatured supercapacitors have aroused the interests of the academia. If the supercapacitors were used in textiles, the deformations, such as the stretching and twisting, could be inevitable. However, the majority of the traditional supercapacitors are made of rigid electrode materials. The deformations are supposed to cause structure damage, the deterioration of the electrochemical performance and even the function lose of charging and discharging. Therefore, realizing stretchability is the prerequisite to enable the wearabilty of the electronics. In order to realize the stretchability, it is necessary for the supercapacitors to possess both stable electrochemical performance and robust mechanical properties. The comprehensive and in-depth research on nanocarbon materials has greatly benefited the progress of the supercapacitor technology. Under this background, it is believed that the stretchable supercapacitors fabricated taking advantage of emerged highly flexible nanocarbon materials(CNT fiber and graphene film) hold the potential to be used in the field of smart textiles. Hence, this research project is very meaningful.The macroscopic one-dimensional assembly of CNTs---CNT fiber and the two-dimensional ultrathin graphene film were utilized as the research objects. Based on them, one-dimensional wire-shaped and two-dimensional planar supercapacitors were fabricated and the feasibility of endowing them with stretchability was discussed. The stretchable supercapacitors at different stretching states were evaluated in the terms of specific capacitance, energy density, power density and long cyle stability. The contents include the following aspects:(1) Stretchable supercapacitor consisted of two twisted CNT fibers, H2SO4-PVA gel electrolyte and spandex fibers was successfully fabricated using the prestraining-then-buckling method and revealed a sinusoidal wavy shape. The electrochemical performance of the stretchable supercapacitor at tensile strains of 0%, 40%, 70% and 100% as well as after 20 mechanical stretching-releasing cycles were investigated by electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic charge-discharge tests.It is worth noting that the electrochemical performance of the wire-shaped supercapacitor at different tensile strains of up to 100% or after 20 mechanical stretching-releasing cycles did not degrade, but slightly increased. The specific capacitance was enhanced by 8.2%, and the power density was increased by 25.8 – 55% with the energy density increased from 0.40×10-7-1.13×10-7 Wh cm-2 to 0.47×10-7- 1.44×10-7 Wh cm-2. Especially, after 20 mechanical stretchingreleasing cycles with tensile strains of 100% and 10000 charge-discharge cycles, the capacitance retention of the stretchable wire-shaped supercapacitor still maintained 108%, implying the outstanding electrochemical stability. The excellent intrinsic mechanical and physical properties of the CNT fibers, such as excellent conductivity, porosity, flexibility and chemical inertness as well as the stretchability of gel electrolyte were of great significance in maintaining the electrochemical performance of the stretchable WSS.(2) Taking advantage of the CNT sizing and graphene oxide sheet aqueous suspension, the randomly-oriented CNTs network or graphene sheets with 3D structure were coated on the CNT fibers after surface modification. The effects of each above-mentioned modification methods on the electrochemical performance of the resulting wire-shaped supercapacitors were evaluated. Then, the supercapacitors were endowed with stretchability through the prestrainning-thenbuckling strategy.The experimental results indicated that the dip-coating process of graphene oxide sheet aqueous suspension can hardly improve the performance of the wire-shaped supercapacitors, while the electrodeposition of graphene oxide sheets has been proved to be an effective way to enhance the electrochemical performance of the supercapacitors.In addition, CNT sizing(SIZICYLTM XCR2G) provided by Nanocyl was utilized to modify the CNT fiber surface by means of dip-coating. At the scan rates ranging from 5 m V s-1 to 1000 m V s-1, the specific capacitance value of wire-shaped supercapacitor based on CNT sizing/CNT fiber increased to 4.78 – 11.40 m F cm-2, which was higher than that of the wire-shaped supercapacitor based on pristine CNT fibers(2.02 – 3.45 m F cm-2). Tensile strains of up to 100% only caused slight variation of the electrochemical performance to wire-shaped supercapacitor based on CNT sizing/CNT fiber.(3) An asymmetric supercapacitor with Mn O2/CNT hybrid fiber as the positive electrode and pristine aerogel CNT fiber as the negative electrode, and KOH-poly(vinyl alcohol)(KOH-PVA) gel as the electrolyte has been fabricated. The gel electrolyte was coated over the pristine CNT fiber and Mn O2/CNT hybrid fiber along the length of 5.5 cm and 2 cm, respectively. The supercapacitor was assembled by twisting the gel-coated parts together and then was endowed with stretchability of up to 100% by applying the simple prestrainning-then-buckling strategy.As the potential window extended from 1.5 V, the energy density and the power density of the asymmetric supercapacitor were tripled and doubled respectively, compared to its asymmetric counterpart with potential window of 0.8 V. It possessed a high specific capacitance of around 157.53 μF cm-1 at 50 m V s-1 and a high energy density varying from 17.26 to 46.59 n Wh cm-1 with the corresponding power density changing from 7.63 to 61.55 μW cm-1. Remarkably, a cyclic tensile strain of up to 100% exerted negligible effects on the electrochemical performance of the stretchable asymmetric wire-shaped supercapacitor. Moreover, after 10000 galvanostatic charge-discharge cycles, the specific capacitance retained over 99%, demonstrating a long cyclic stability.The asymmetric configuration has been demonstrated to be the preferable supercapacitor structure to achieve high operating voltage, and high energy density without sacrificing the power delivery and cycle stability.(4) The 4-layer CVD graphene film was processed by synthesizing individual graphene films, followed by a film transporting and laying-up process. 4-layer graphene films on an elastomeric PDMS substrate were endowed with stretchablility using prestraining-then-buckling approach. Upon releasing the prestrain of PDMS substrate, the 4-layer graphene film formed a nanoscale ripple-like structure on the PDSM substrate. The proposed stretchable supercapacitor in the form of simple parallel-plate geometry comprised two pieces of transparent buckled graphene/PDMS films, which were electrically separated by H2SO4-PVA gel electrolyte. Optical transmittance of up to 72.9% at a wavelength of 550 nm and stretchability of 40% were achieved. As the tensile strain increased up to 40%, the specific capacitance showed no degradation and even increased slightly.Especially, the excellent rate capability was further analyzed by the Bode plots of imaginary specific capacitance using the series resistor-capacitor model. The frequency response slowed down from f0 = 34.3 Hz(at tensile strain of 0%) to f0 = 7.92 Hz(at tensile strain of 40%), while the time constant increased correspondingly from 29 ms to 126 ms, respectively, which was still shorter than many supercapacitors based on carbon materials. In addition, the high rate capability of supercapacitors was demonstrated by better capacitance retention at high scan rates in the CV measurements. The supercapacitor possessed an excellent electrochemical stability with the capacitance retention still reaching 98% after 10000 cycles.(5) Buckled 4-layer graphene on PDMS film with the sheet resistance of 3.43 k? sq-1 and the optical transmittance of 88.1% was the research object. Observed by SEM, the buckled structure showed uniform ripple-like geometry. According to the AFM images, as the applied tensile strain varied from 0% to 40%, the maximum amplitude of the ripples in the wavy graphene films decreased from 146.8 nm at 0% to 124.5 nm at 20% and 120 nm at 40%. As the applied tensile stain increased from 0% to 40%, the strain variation of the buckled 4-layer graphene film was investigated by means of micro-Raman maps. A higher applied tensile strain caused the overall redshift of the 2D band in the targeted area of 30μm by 12μm, implying that the compressive strain induced in the buckling process was relieved gradually when the applied strain increased. Calculated from all the spectra in the targeted area, the 2D band positions were 2687.51 ± 1.39 cm-1 at unbuckled state, 2686.33 ± 1.84 cm-1 at 0%, 2685.71 ± 1.26 cm-1 at 20% and 2684.48 ± 1.03 cm-1 at 40% strain, which also indicated the marginal Raman shift induced by the applied tensile strains. The 2D band in the range from 2680 cm-1 to 2692 cm-1 led to a uniaxial strain variation of less than 0.2% in the graphene film, which should not damage the graphene film. This study has confirmed that the graphene film in the buckled state is suitable for its application as a stretchable electrode. |
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