Facile Fabrication of functional nanocomposites for supercapacitor applications

Abstract

To address the energy crisis and environmental pollution issues, urgently need to develop an inexpensive, environmentally-friendly, and safe electrochemical energy storage system (EES). Supercapacitors (SCs) are considered promising energy storage device candidates due to their fast delivery rate, high power density, and long cycling life. However, finding an eco-friendly, low-cost, and high-energy density SC to meet commercial applications is still a challenge. To target the above-mentioned challenges, four aspects were investigated in this research: 1) energy storage mechanism through these functional nanocomposites, especially characterization and understanding of the electron and ionic transport behavior in complex interface, which could give us better design of the nanocomposite to improve the performances of the devices; 2) low-cost, sustainable raw materials (graphene, metal oxide, conducting polymer, cellulose) were chosen to synthesize electrodes for lower production cost; 3) functional nanocomposite electrodes were designed and fabricated to overcome the disadvantages of single component electrode material to enhance the performance of SC; 4) establish of facile, ultra-fast and high energy efficient approach for manufacturing of functional nanocomposites, with scale-up possibilities. In the first project (Chapter 2), a high energy efficient and ultra-fast microwave heating technique was used to fabricate NiOx@graphene electrode. Electrochemical studies indicated that the combination of NiOx and graphene leads to a high specific capacitance of 623 F/g and excellent cycling stability due to the synergistic effect between NiOx and graphene. Furthermore, three different types metal precursors ((Ni(OH)2, Ni(Ac)2·4H2O and Ni(NO3)2·6H2O) were used to prepare the NiOx@graphene nanocomposite, and explore the electrode formation mechanism. It demonstrated that the microstructure and morphology of electrode materials were metal precursor-dependent, and directly related to the electrochemical performance. This work provides experimental support for the design of nanocomposite electrode architectures with high electrochemical performance for next generation energy storage devices. Vanadium pentoxide (V2O5) possesses layered structure, a high theoretical specific capacity (2120 F/g), and a wide working potential window (up to 1.2 V in H2O). According to the definition of energy density (E=1/2 CV^2), which is proportional to the electrochemical potential window, V2O5 is believed to be one of the most promising electrode materials for the preparation of supercapacitors. In project 2 (Chapter 3), V2O5@polypyrrole (V2O5/PPy) core-shell nanofiber was synthesized by combining an economical, easy-to-process, and eco-friendly sol-gel with an in situ polymerization method to improve the energy density of SC. The PPy coating, with high conductivity, facilitated charge transfer and protected the dissolution of V2O5 in the aqueous solution. The symmetric device of V2O5/PPy device exhibited a maximum energy density of 37 Wh/Kg when the power density was 161 W/kg. The synergistic effect between the V2O5 and PPy and the individual role of each component in the electrochemical process were studied to further understand the growth mechanism and provide the rational design electrode material fundamental in the future. With the development of rechargeable consumer electronics, portable and wearable electronic devices are rapidly appearing in our life, such as roll-up displays, smart textiles, etc. Flexible supercapacitor has become an emerging frontier research area. In project 3 (Chapter 4), nature abundant, renewable, non-toxic, biocompatible and biodegradable nanocellulose was used as building blocks to fabricate the freestanding, binder-free flexible polypyrrole/poly(styrene sulfonate)/cellulose nanopaper (PPy:PSS/CNP) electrode by a facile and fast vacuum filtration method. The optimized PPy:PSS/CNP exhibited high areal specific capacitance of 3.8 F/cm2, an energy density of 122 μWh/cm2 and good cycling stability (80.9% capacitance retention rate, 5,000 cycles), which was superior to other cellulose-based composite materials. It is worth noting that PPy:PSS/CNP functions well as a flexible supercapacitor electrode material because of the following reasons: 1) poly(styrene sulfonate) (PSS) serves as a dopant and forms a water-soluble polymer network (PPy:PSS) with PPy, effectively improving the dispersity and processibility of PPy; 2) PPy with high conductivity enhances the charge transport rates and electrochemical properties (high specific capacitance, long cycling life, high power density); 3) Cellulose (CNF) is chosen as the flexible substrate, which provides good flexibility and mechanical strength for the electrodes due to large number of hydrogen bonds among cellulose molecules. Compared to PPy, polyaniline (PANI) has a higher doping level of 0.5 (i.e. two monomer units per dopant) and a higher theoretical specific capacitance (750 F/g). In project 4 (Chapter 5), we used PANI, instead of PPy used in project 3, to synthesize another promising flexible PANI:PSS/CNP electrode. The optimized PANI:PSS/CNP electrode exhibited a lower specific capacitance (2.56 F/cm2), better cycling stability (81.5% capacitance retention rate, 8000 cycles), and higher mechanical strength (29.1 MPa) than PPy:PSS/CNP. The lower specific capacitance of PANI:PSS/CNP can be attributed to the lower conductivity of PANI (0.1-5 S/cm) than PPy (10-50 S/cm). The better cycling stability and higher mechanical strength properties of PANI:PSS/CNP might be explained by a stronger interaction between PANI and CNF. In comparison to the PANI:PSS/CNP, the cross-sectional morphology of PPy:PSS/CNP displayed obviously expanded interior lamellar structures, indicating a possible interruption of more hydrogen bonds among CNFs. The number of intermolecular and intramolecular hydrogen bonds reducing in nanocellulose will affect the mechanical properties of the nanopaper. In summary, we have successfully designed and prepared four high-performance nanocomposite electrodes through simple, fast, high energy efficient, and low-cost approaches (microwave heating, in situ polymerization, and vacuum filtration). The charge transport and energy storage mechanism in the nanocomposites were investigated during the charge-discharge process. Our work is expected to provide experimental support to design functional nanocomposite electrodes with high electrochemical performance for next-generation energy storage devices.