Smart Supercapacitors

Renowned as high specific capacitance, high power density, long life stability and eco-friendly, the supercapacitor gains an increasing popularity in a wide range of areas including portable and wearable electronics. Many efforts are devoted to func-tionalizing and broadening the application of supercapacitors. For example, develop-ing a smart supercapacitor, that can visually indicate its energy storage state in real time, is innovative and never reported yet. Presented in this dissertation, by incorpo-rating sensitive polymer materials, a series of smart supercapacitors in planar or fiber shape were fabricated.In this dissertation, two typical sensitive polymer materials, polydiacetylene and polyaniline, were investigated to improve their sensitivity through introducing inor-ganic functional nanomaterials. Ferroferric oxide, a magnetic responsible nanoparticle, was incorporated with polydiacetylene to produce a novel composite that can change color in alternating current magnetic field. Briefly, diacetylenic monomers were linked, through hydrogen bonds, to the surface of ferroferric oxide nanoparticles which were synthesized through chemical co-precipitation. Then the diacetylenic moieties were topochemically polymerized under UV light, yielding the polydiacety-lene/ferroferric oxide composite. Because of the Neel relaxation of ferroferric oxide particles in alternating current magnetic field, the ferroferric oxide particles produced heat and thus heated the polydiacetylene, which consequentially induced a confor-mation change of the polydiacetylene backbone and leads to a color change. Polyani-line, a polymer exhibiting different colors at different oxidation states, was adopted along with carbon nanotubes to prepare a reversible electrochromatic composite. Aligned carbon nanotube sheet with excellent conductivity was first dry-spun from spinnable carbon nanotube array synthesized by chemical vapor deposition and then polyaniline was electrochemically deposited onto the aligned carbon nanotube sheet. The produced polyaniline/carbon nanotube composite exhibited a rapid and reversible color change as a result of electrochemical redox reaction. The visible color changes and high sensibility endowed polydiacetylene and polyaniline with great potentials in electronics and sensing. Considering the excellent electrochemical properties, polyan-iline was mainly investigated to realize the intellectualization of supercapacitor.Supercapacitor. Aligned carbon nanotube fibers and sheets were first adopted to fabricate stretchable planar and wire-shaped supercapacitors with outstanding flexi- bility and excellent electrochemical performances, which laid the foundation for the smart supercapacitors. Considering planar-shaped supercapacitors have been widely studied, wire-shaped supercapacitor was mainly investigated. Carbon nanotube fiber and carbon nanotube sheet with excellent mechanical and electrical properties were dry-spun from a spinnable carbon nanotube array. Afterwards, the carbon nanotube fiber was coated with a layer of phosphoric acid/poly(vinyl alcohol) gel electrolyte followed by scrolling carbon nanotube sheet on the resulting fiber. The as-prepared supercapacitor displayed a coaxial structure with carbon nanotube fiber and carbon nanotube sheet as two electrodes. The coaxial wire-shaped supercapacitor performed as a typical electric double-layer capacitor with a specific capacitance of 59 F/g. Moreover, attributing to the mechanical property of carbon nanotube as well as its unique coaxial configuration, the wire-shaped supercapacitor deserved an excellent flexibility and considerable stretchability.Planar-shaped smart supercapacitor. The smart supercapacitor with good sen-sitivity to its energy storage state, started with preparing a composite electrode. First, layers of aligned carbon nanotube sheets were paved onto a transparent polydime-thylsiloxane film, which was then electrochemically deposited with polyaniline. Two as-prepared composite electrodes were assembled into a supercapacitor using phos-phoric acid/poly(vinyl alcohol) gel as the electrolyte. A specific capacitance of 308.4 F/g was achieved due to the pseudocapacitance of the polyaniline. The high specific capacitance remained unchanged after repeatedly stretched to 100% for 200 cycles, and it remained 95.8% after bending for 1000 cycles. The incorporation of polyaniline made the supercapacitor exhibit different colors relative to different voltages during charge-discharge processes. For instance, the positive electrode displayed blue, green, light green and light yellow at 1 V,0.5 V,0 V and -1 V, respectively. Therefore, the supercapacitor can visually indicate its energy storage state in situ through different colors exhibited.Wire-shaped smart supercapacitor. The concept of smart supercapacitor was further expanded to the wire-shaped supercapacitor, a potential alternative for portable and miniature devices. The electrodes of the wire-shaped smart supercacitor were prepared by winding continuous aligned carbon nanotube sheet onto a stretched elas-tic rubber fiber followed by electrochemical depositing polyaniline onto the carbon nanotube sheet. phosphoric acid/poly(vinyl alcohol) gel electrolyte was adopted for the supercapacitor. The resulting wire-shaped supercapacitor exhibited a specific ca- pacitance of 255.5 F/g, which can be well maintained under bending and stretching. Likewise, the wire-shaped smart supercapacitor showed a color change in response to the variation of voltages and can indicate the energy storage state of itself. Moreover, the fiber-like structure endowed the wire-shaped smart supercapacitor with unique advantages. For instance, these wire-shaped supercapacitors can be easily woven into textiles for applications in wearable electronic devices. Each fiber in the textile is an independent unit sensible to voltage, the textiles, therefore, can display various signals or symbols.In summary, this dissertation has developed planar and wire-shaped smart su-percapacitors by incorporating sensing polymer materials. The smart supercapacitor displayed different colors at different voltages during charge-discharge process. Therefore, it can visually indicate its energy storage state in real time. The dissertation provided theoretical and experimental support for the development and application of the supercapacitor.