3D-structured materials for efficient supercapacitors

Abstract

In this thesis, I observe the effect of metal oxide morphology on the supercapacitor properties, and introduce many facile approaches to improve the capacitor properties by fabricating the three dimensional structure of active materials and current collector with forming the 3D conducting network system. First, we prepared various shape of NiO such as nanoparticle-, nanoslice-, and nanoflower- by a sol-gel method and their morphology-dependent supercapacitor properties were exploited. The nanoflower- shaped NiO have highest portion of pore volume and three-dimensional (3D) network, which causes the best supercapacitor properties with the advantage in contact and transport of the electrolyte. The XPS and EIS data of the NiO nanostructure confirm that flower-shaped NiO, which has the lowest surface area among the three morphologies, was effectively optimized as a superior electrode and yielded the greatest pseudocapacitance. This study indicates that forming a three-dimensional nano-network is a straightforward means of improving the electrochemical properties of a supercapacitor. Second, I improve the capacitor properties of nickel hydroxide (Ni(OH)2) by simply coating gold nanoparticles (Au NPs) on the surface of Ni(OH)2. Au NP-deposited Ni(OH)2 (Au/Ni(OH)2) has been prepared by application of a conventional colloidal coating of Au NPs on the surface of 3D-Ni(OH)2 synthesized via a hydrothermal method. Compared with pristine Ni(OH)2, Au/Ni(OH)2 shows a 41% enhanced capacitance value, excellent rate capacitance behavior at high current density conditions, and greatly improved cycling stability for supercapacitor applications. The outstanding performance of Au/Ni(OH)2 as a supercapacitor is attributed to the presence of metal Au NPs on the surface of semiconductor Ni(OH)2; this permits the creation of virtual 3D conducting networks via metal/semiconductor contact, which induces fast electron and ion transport by acting as a bridge between Ni(OH)2 nanostructures, thus eventually leading to significantly improved electrochemical capacitive behaviors, as confirmed by the EIS and I-V characteristic data. Third, I devise a straightforward strategy to simultaneously improve the capacitance, rate capability, and cycle life of a supercapacitor by simply electrodepositing Ni-nanoparticles (Ni-NPs) on an as-prepared electrode. 3D-structured current collectors such as metal foams, metal meshes, and carbon meshes have been widely used in supercapacitors, secondary batteries, glucose sensors, etc. In particular, the 3D-metal foam readily improves device properties due to its unique 3D-nature and high surface area. However, there are practical constraints when applying 3D-current collectors to the industrial world, including high cost. Here, by simply electrodepositing Ni-NPs in a cost-efficient manner, a similar effect to that derived with the use of 3D-metal foam was realized. The deposited Ni-NPs are preferentially located near the contact area between the active materials and a plate-type current collector, which allows for tight binding between the active materials and the current collector as well as facile charge transfer and high capacitance. Given the simplicity and cost-efficiency of this method, it can be readily applied to other energy storage devices with practical applications in the industrial world. Last, I report an outstanding positive electrode for supercapacitor having both high energy density and high power density as well as good stability, by fabricating a low resistance Ohmic contact between Ni(OH)2 active materials and a 3-D current collector, nanoporous gold (NPG). The Ni(OH)2/NPG electrode was optimized by finely adjusting the portion of two different parts of the deposited Ni(OH)2, one part in direct contact with the NPG and the other part on top of the NPG. The optimized Ni(OH)2/NPG electrode exhibited 2,223 F/cm3 of volumetric capacitance (considering both the active material and the current collector) at a current density of 5 A/g, values which are beyond the theoretical capacitance value, and the device retained 90% capacitance of the initial value at 500 A/g and after 30,000 cycles, respectively. This electrode showed an excellent energy density of 98 Wh/kg and power density of 50 kW/kg in a Ni(OH)2/NPG//MnO2/NPG two electrode supercapacitor system. The excellent performance of the Ni(OH)2/NPG electrode is attributed not only to the increased surface area of the Ni(OH)2 active materials, but also to the favorable path for charge transport created between the Ni(OH)2 and the gold electrode due to the presence of Ohmic contact, which eventually leads to the good kinetic properties and good stability.