| With the development of next generation flexible/bendable electronic devices, flexible energy storage devices have attracted extensive attention in past years. The supercapacitor undoubtedly would have wide application prospects in this field thanks to its high power density, short charging time and long cycle-life performance. As the core component of supercapacitor, the electrode is the key factor of overall performance quality. Therefore, exploitation of high performance flexible electrode material has great significance for the research and application of supercapacitor.Polyaniline (PANI) is considered to be one of versatile conducting polymers due to its low cost, east of synthesis, relatively high theoretical capacitance and flexibility. It could be used as electrode material in flexible supercapacitor. Nevertheless, PANI conducts poor cyclic stability, which seriously weakens its electrochemical performance.The effective method is to incorporate PANI with other kinds of electrode materials before using. On the other hand, titanium nitride (TiN) is reported to possess good electrical conductivity. Moreover, the well-aligned TiN could obtain an excellent rate performance due to its fast charges and electrons transport framework. So the TiN nanoarray has become a kind of new and promising material for electrochemical energy storage applications. This article aims to design and prepare a sort of flexible PANI composite electrode materials based on TiN nanoarray substrate. Titanium foil and carbon fiber were chose as flexible current collector to build TiN nanotube array (NTA) and nanowire array (NWA). The relationship among material manufacturing process, structure characteristics and electrochemical properties was investigated. After that, the optimal designed flexible PANI composite electrode materials were developed with an improvement in power density, energy density and cycle stability. The main contents and results are shown as follows.1. Electrochemical capacitive performance of PANI/TiN NTA deposited on titanium foil flexible electrode materialThis study focused on improving the electrochemical capacitance of PANI. The TiN NTA was prepared by anodization of titanium foil and subsequent nitridation through ammonia annealing. Then, PANI was deposited into TiN NWA using the normal pulse voltammetry method, forming the flexible electrode material of PANI/TiN NTA deposited on titanium foil.The TiN NTA could provide fast charge-transfer framework and high intrinsic surface area. The PANI covered on TiN NTA could avoid the cycling degradation caused by volume changes and can also contribute to high specific capacitance. The results show that the PANI/TiN NTA hybrid had a heterogeneous coaxial nanoarray structure. The average nanotube pore diameter and the wall thickness were about 80 and 20 nm, respectively. The nanotube length was about 5μm. The PANI filled the entire TiN nanotube and along the nanotube walls until PANI coated on the top surface of the nanotube. The PANI/TiN NTA composite showed a high capacitance of 1066 Fg-1 at a current density of 1 A g-1 in the 1 M H2SO4 aqueous solution. The corresponding specific capacitance still kept 864 F g-1 at 10 A g-1. Only 19% capacitance loss was obtained at 10 A g-1 when compared with that at 1 A g-1, indicating a remarkable rate capability. An aqueous supercapacitor, consisting of two symmetric PANI/TiN NTA composite electrodes and 1.0 M H2SO4 electrolyte solution, presenting the specific capacitance of 194.8 F g-1, energy density of 9.74 Wh kg-1 and power density of 0.3 kW kg-1, respectively. So, the PANI/TiN NTA deposited on titanium foil flexible electrode material exhibited high specific capacitance and superior rate capability. It could act as a supercapacitor electrode material for electrochemical capacitive energy storage.2. Electrochemical capacitive performance of PANI/TiN NWA deposited on carbon fiber flexible electrode materialThis study focused on keeping both high flexibility and high electrochemical capacitance of PANI/TiN NWA composite electrode material. The TiN NWA was prepared via a seed-assisted hydrothermal process and then ammonia nitridation process. PANI was then coated onto TiN NWA using a cyclic voltammetry method, forming the flexible electrode material of PANI/TiN NWA deposited on carbon fiber. The carbon fiber served as light weight current collector with excellent electrical conductivity and corrosion resistance. The TiN NWA could provide high surface area and fast charge-transfer framework. The PANI film could achieve high specific capacitance and strengthen the cyclic stability. The results show that the individual or cluster TiN NWA was formed at an angle to the carbon fiber surface. Each individual TiN nanowire had a diameter of 10-30 nm; TiN nanowire cluster had a diameter of 20-200 nm. The total length of TiN NWA was 1-1.5μm. PANI layer was uniformly coated on the surface of TiN NWA to form heterogeneous coaxial PANI/TiN NTA, which included a TiN NWA core layer and and a PANI shell layer. The PANI/TiN NWA composite showed a high capacitance of 1064 F g-1 at a current density of 1 A g-1 in the 1 M H2SO4 aqueous solution. The corresponding specific capacitance still kept 787 F g-1 at 5 Ag-1, which was about 74% of that measured at 1 Ag-1. This result indicated the good rate capability of PANI/TiN NTA. The PANI/TiN NWA also kept 95% capacity retention after 200 cycles at a current density of 5 A g-1, showing a good cyclic stability. So, the PANI/TiN NWA deposited on carbon fiber flexible electrode material not only exhibited high specific capacitance and superior rate capability, but also possessed high flexibility. It could act as a wearable supercapacitor electrode material for electrochemical capacitive energy storage.3. Electrochemical capacitive performance of PANI/MnO2/TiN NWA deposited on carbon fiber flexible electrode materialThis study focused on further improving the specific capacitance of PANI/TiN NWA composite electrode material. The TiN NWA was prepared via a seed-assisted hydrothermal process and ammonia nitridation process. The electroactive MnO2 and PANI was then layer-by-layer coated on TiN NWA through a stepwise electrodeposition process, forming the flexible electrode material of PANI/MnO2/TiN NWA deposited on carbon fiber. MnO2 layer covered on TiN NWA could yield high ionic conductivity, leading to high specific capacitance. The inclusion of PANI aimed at supplying an additional electron transport path and further improving the capacitive performance. The results show that MnO2 thin-film layer was coated on the ordered TiN NWA to form MnO2/TiN NWA. The deposited MnO2 layer had villiform surface microstructure and a thickness of 10-20nm. PANI further covered on the MnO2/TiN NWA to form PANI/MnO2/TiN NWA, the PANI layer had a thickness of 20-50 nm. The neighboring PANI layer crossed link together to form a "coral-like" structure, covering on the top surface of MnO2/TiN NWA. The individual PANI coral branch had a diameter of 100 nm. The as-prepared PANI/MnO2/TiN NWA presented a three dimensional interconnected porous structure, which would be accessible to the electrolyte ions and beneficial for full utilization of electroactive materials. The PANI/MnO2//TiN NWA shows a specific capacitance of the 614 Fg-1 at a current density of 1.0 mA cm-2 (or 2.0 A g-1) in the 0.5 M Na2SO4 aqueous solution, which was approximately a 1.50-fold and 1.20-fold enhancement when compared with the MnO2/TiN NWA (405 F g-1) and PANI/TiN NWA (512 F g-1), respectively. This PANI/MnO2/TiN NWA still kept 90% capacity retention after 1000 cycles, showing a good cyclic stability. So, the PANI/MnO2/TiN NWA deposited on carbon fiber flexible electrode material not only exhibited high flexibility and cyclic stability, but also possess the improved specific capacitance in comparison with PANI/TiN NWA or MnO2/TiN NWA. It could act as a good supercapacitor electrode material for electrochemical capacitive energy storage.4. Electrochemical capacitive performance of PANI/C/TiN NWA deposited on carbon fiber flexible electrode materialThis study focused on further improving the cyclic stability of PANI/TiN NWA composite electrode material. The TiN NWA was prepared via a seed-assisted hydrothermal process and then ammonia nitridation process. Carbon layer was then coated on TiN NWA via a glucose-assisted hydrothermal treatment. PANI layer was finally coated onto C/TiN NWA via a cyclic voltammetry method, forming the flexible electrode material of PANI/C/TiN NWA deposited on carbon fiber. The carbon layer could promote electron transport throughout the composite materials. Importantly, it could protect the TiN NWA from corrosion in acid electrolyte and also facilitate the deposition of PANI layer. The results show that the PANI/C/TiN NWA hybrid had a "shell-shell-core" structure. Herein, the carbon layer with a thickness of 5-10nm covered on the surface of TiN NWA, forming the "shell-core" structured C/TiN NWA. An external surface PANI layer with a thickness of 20-40nm covered on the carbon layer surface, forming the "shell-shell" structured PANI/C/TiN NWA. The PANI/C/TiN NWA shows a high capacitance of 1093 F g-1 at a current density of 1 A g-1 in the 1 M H2SO4 aqueous solution, and good cyclic stability with capacity retention of 98% after 2000 cycles. This capacity retention in cyclic stability test was better than that of C/PANI/TiN NWA (96%) and PANI/TiN NWA (90%). So, the PANI/C/TiN NWA deposited on carbon fiber flexible electrode material not only exhibited high specific capacitance and flexibility, but also possessed improved cyclic stability in comparison with PANI/TiN NWA. It could act as a good supercapacitor electrode material for electrochemical capacitive energy storage. |
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