| Electrochemical capacitors (ECs), also known as supercapacitors, have superiorpower density, fast charge/discharge rates and long cycle lifetime and have attractedmuch attention for various applications, including energy sources for hybrid electricalvehicles, power back-up in portable electronics, load-leveling and other successivepower supplies. However, commercial supercapacitors suffer from low energy density.To meet the increasing energy demands for next-generation ECs, the energy densityshould be substantially increased without sacrificing the power density and cycle life.Energy-storage mechanisms for electrochemical capacitors are fundamentally differentfrom those of batteries. Electrochemical double layer capacitors (EDLCs) are based onion adsorption at the interface between the electrode, typically carbon, and the liquidelectrolyte. A second capacitive storage mechanism is that of pseudocapacitance whichinvolves reversible faradaic reactions. These reactions can occur from ions adsorbingonto the surface of the material or, if the material has a layered or tunnel structure,being inserted within specific ion-conducting planes or channels. The specificcapacitance values can be significantly larger for pseudocapacitive materials (>1,000Fg1) compared to EDLCs because of the greater level of charge storage associated withredox reactions. Thus, there is considerable interest in investigating pseudocapacitivematerials inculding conductive polymers and transtion metal oxides. Polyaniline(PANI) is one of the promising electrode materials for ECs due to the highpseudocapacitance, easy synthesis, high conductivity, and low cost. Unfortunately, thecycling stability of PANI, especially nanostructured PANI, is poor because of swellingand shrinkage of the polymer backbone during the charge/discharge processes therebyseverely hindering application of PANI to ECs. Transtion metal oxides are anotherclass pseudocapacitive materials, however, the intrinsic poor conductivity limits therate performance. In order to overcome the shortages of high pseudocapacitivematerials, hybrid nano-materials arrays electrodes that combine highly conductiveEDLC electrode with pseudocapacitive materials are widely investigated.In this thesis, we fabricated nanoporous TiN nanotube arrays (NTAs) byanodization of Ti foil and then nitridation under NH3. Compared with carbon nanotube arrays and carbon nanofiber arrays, the TiN NTAs have mespoous wall structure withstable physical and chemical properties as well as high conductivity. Thus they shouldbe a better substrate for loading high pseudocapacitive materials such as PANI andMoOxto achieve excellent supercapacitive properties. Importantly, the PANI or MoOxcould be deposited on the outer and inner surfaces as well as nanopores in the wall ofthe TiN NTAs to form an interpenetrating three-dimensional (3D) network. Thisspecial structure cannot be obtained with the CNTs or CNFs scaffold structure.Additionally, the TiN NTAs produced on the Ti foil which serves as a current collectorprovide both high electrical conductivity for rapid electron movement and strength toavoid mechanical breakdown of the pseudocapacitive materials during thecharging/discharging process thus yielding good cycling stability and rate capability.The detailed studies are as followings:(1) By electrochemical polymerization method, PANI has been successfullypolymerized on the outer and inner surfaces as well as wall nanoholes of TiN nanotubeto form an interpenetrating3D PANI network, forming PANI/TiN hybridnanostructures. The influence of electrochemial polymerization condition on themorphology of hybrid nano-material and their corresponding electrochemicalproperties has been systematically investigated. The coaxial and interpenetratingPANI/TiN/PANI NTAs provide abundant space between the nanotubes and inside eachtube so that electrolytes can readily access the capacitive PANI. The nanocompositeelectrode exhibits a high specific capacitance of242mF cm2at a current density of0.2mA cm2and good rate capability with the specific capacitance remaining at69%when the current density is increased50fold from0.2to10mA cm2. Aftercharging/discharging for3,000cycles,83%of the initial capacitance is retained. Theexcellent results suggest promising applications of the electrode materials in energystorage.(2) By electrochemical depositing method, MoOxhas been deposited on the outerand inner surfaces as well as wall nanoholes of TiN nanotube to form an3D MoOxnetwork interpenetrating into TiN, forming MoOx/TiN hybrid nanodtructurs. Themulti-layered MoOx/TiN NTAs exhibit excellent electrochemical performance due tothe3D penetrating structure of MoOxand TiN. The MoOx/TiN hybrid electrodeperforms high specific capacitance of97mF cm-2and an excellent rate capability with the specific capacitance remains65%when the current density increased20timesfrom1to20mA cm-2. The device assembled with MoOx/TiN exhibits high specificcapacitance of1.2F cm-3without obvious fading after charging/discharging for10,000cycles. The outstanding results indicate it would be an excellent candidate for nextgeneration ECs. |
