Abstract
This study focuses on the preparation and performance of the supercapacitor for energy storage devices due to their special characteristics. The research topics of this dissertation are related to the preparation and properties of the electrodes of supercapacitor. There are three parts in this study: 1.Asymmetric supercapacitors based on functional electrospun carbon nanofiber/manganese oxide electrodes with high power density and energy density Carbon nanofibers modified with carboxyl groups (CNF-COOH) possessing good wettability and high porosity which are homogeneously deposited with amorphous manganese dioxide (amorphous MnO2) by potentiodynamic deposition for asymmetric super-capacitors (ASCs). The potential-cycling in 1 M H2SO4 successfully enhances the hydrophilicity of carbonized polymer nanofibers and facilitates the access of electrolytes within the CNF-COOH matrix. This modification favors the deposition of amorphous MnO2 and improves its electrochemical utilization. In this composite, MnO2 was homogeneously dispersed onto CNF-COOH which provides desirable pseudocapacitance and the CNF-COOH network works as the electron conductor. The composite of CNF-COOH@MnO2-20 shows a high specific capacitance of 415 F g-1 at 5 mV s-1. The capacitance retention of this composite is 94% in a 10,000-cycle test. An ASC cell consisting of this composite and activated carbon as positive and negative electrodes can be reversibly charged/discharged to a cell voltage of 2.0 V in 1 M Na2SO4 and 4 mM NaHCO3 with specific energy and power of 36.7 Wh kg-1 and 354.9 W kg-1, respectively. This ASC also shows excellent cell capacitance retention (8% decay) in the 2V, 10,000-cycle stability test, revealing superior performance. 2.Asymmetric supercapacitors based on electrospun carbon nanofiber/sodium-pre-intercalated manganese oxide electrodes with high power density and energy density This study first presents that the sodium-pre-intercalated δ-MnO2 is in-situ grown on carbon nanofiber via a simple, one-step method for the application of asymmetric supercapacitors. The pre-intercalation of Na ions into layered structure of δ-MnO2 reduces the crystallinity, beneficial for Na+ diffusion into/out the interlayer structure and pseudocapacitive utilization of MnO2. This NaxMnO2@CNF nano-composite with desirable pseudo-capacitance from δ-NaxMnO2 and high electric conductivity from CNF network shows a high specific capacitance of 321 F g-1 at 1 A g-1 with ca. 75.2 % capacitance retention from 1 A g-1 to 32 A g-1. An ASC cell consisting of this nanocomposite and activated carbon as the positive and negative electrodes can be reversibly charged and discharged to a cell voltage of 2.0 V in 1 M Na2SO4 and 4 mM NaHCO3 with specific energy and power of 21 Wh kg-1 and 1 kW kg-1, respectively. This ASC also shows excellent cell capacitance retention (7 % decay) in the 2V, 10,000-cycle stability test, revealing superior performance. 3.Novel, flexible supercapacitors based on activated carbon nanofiber and carbon nanofiber/potassium-pre-intercalated manganese oxide In this study, potassium-pre-intercalated MnO2 is grown on carbon nanofibers (KxMnO2@CNF) for the positive electrode of asymmetric supercapacitors (ASCs) and an electrospun CNF is chemically activated with KOH at 800°C (ACNF) for the negative electrode. The crystallinity of MnO2 is significantly reduced by the pre-intercalation of K ions into its layered structure. This textural characteristic is beneficial to the K+ diffusion into/out the interlayer structure, leading to effective utilization of the electroactive material of KxMnO2. This unique composite electrode provides both ideal pseudo-capacitive behavior from KxMnO2 and excellent electric conductivity from the CNF network, exhibiting a fairly high specific capacitance value of 279 F g-1 at 1 A g-1 with ca. 82.3 % capacitance retention from 1 A g-1 to 32 A g-1. A flexible ASC consisting of the positive KxMnO2@CNF electrode, a paper separator, and the negative ACNF electrode is successfully assembled. This cell shows superior ASC performance between 0 and 2 V for 10,000 cycles (10 % decay) at 2 A g-1 with specific energy and power of 23.5 Wh kg-1 and 211.4 W kg-1, respectively. The charge storage behavior of such a cell without bending and with a bending angle of 90o shows no apparent difference, demonstrating its potential in the next-generation flexible energy storage devices.