Stevens Institute of Technology,
Keywords: flexible supercapacitors, all-solid-state, polypyrrole, vertially aligned carbon nanotubes
Summary:Flexible electronics have a wide range of applications in wearable and multifunctional electronics , including flexible displays, curved smart phones, electronic skins, and implantable medical devices. Consequently technologies for flexible energy storage have to be developed for such flexible electronic devices . Carbon nanotubes are a promising material for electrodes of flexible supercapacitors owing to their excellent electrical, optical and mechanical properties . Here, we develop a supercapacitor with polypyrrole-vertically aligned carbon nanotube (VACNT) hybrid partially embedded into PDMS utilizing a facile fabrication technique. Our unique technique ensures a strong hold of the partially embedded PPy-VACNTs hybrid into Polydimethylsiloxane (PDMS), which facilitates a stable charge/discharge under varied strains. To fabricate the stretchable supercapacitor with embedded VACNTs, we synthesized VACNTs using atmospheric-pressure chemical vapor deposition (APCVD) into carpet-like structures and transferred them onto partially cured PDMS. We optimized the curing condition of PDMS, where the partially cured PDMS was tacky but not fully wet. The grown VACNTs were then placed onto partially cured PDMS and the tips of CNTs were partially immersed into PDMS. During the curing process, the embedded CNTs were eventually wetted by PDMS and the Si/SiO₂ substrate was successfully peeled off from VACNT-PDMS structure after PDMS is fully cured. The entire fabrication process allows a rapid and facile fabrication and integration of VACNT-PDMS substrate. The VACNT structures are vertically aligned in general, but interwoven at an individual level. Therefore the VACNT-PDMS structure can still have a good electrical conductivity under stretching/bending deformations. The polypyrrole film was fabricated on the surface of VACNTs by electropolymerization. The all-solid-state flexible supercapacitor was fabricated by stacking two PPy-VACNT-PDMS structures face-to-face with PVA-KOH gel electrolyte in the middle. The electrochemical property of the flexible supercapacitor was measured at different scan rates from 50 mV/s to 1000 mV/s using cyclic voltammetry. The measured capacitance of our structure was approximately 158 µF/cm^2 at a high scan rate of 1 V/s. In addition, the partially embedded VACNT-PDMS structure was sustained under various strains (including stretching, bending and twisting) during the testing. The structure was seamlessly stretched up to 160%, bent and twist to 180˚, and the capacitance of this structure at 160% stretching was attenuated by 30%, and was found consistent under the bending/twisting angles varying from 0 to 180˚. As next steps, the performance of this flexible supercapacitor will be fully characterized at various applied strain values (stretching, bending and twisting at different frequencies under different temperatures and humidity values). The cyclic behavior of supercapacitors will be investigated under such applied strains. The droplet behavior of the surface of PPy-VACNT-PDMS structure under redox and under different stretching strains at varying PPy thickness and surface morphology will be characterized. References  Shao, Yuanlong, et al. Chemical Society Reviews 44.11 (2015): 3639-3665.  Peng, Xu, et al. Chemical Society Reviews 43.10 (2014): 3303-3323.  Chen, Tao, et al. Scientific reports 4 (2014): 3612.