Electroactive surface-controlled functional nanomaterials for electrochemical energy applications

Abstract

For the sustainable development, reducing the use of fossil fuels and increasing the proportion of renewable energy sources such as solar and wind resources are key tasks. However, since the supply of such eco-friendly energy resources is intermittent and volatile, it is essential to build a system that can store and reuse energy in other forms of energy. An electrochemical energy device is a device that uses an electrochemical reaction to store energy in the form of electricity or converts it into chemical fuel and then reuses it as electrical energy. Electrochemical energy devices can be classified into energy storage devices such as supercapacitors or batteries, and energy conversion devices such as electrocatalysts and fuel cells. Each device has a different principle of operation, but the common and most critical element is the working electrode. In the electrochemical storage and conversion reaction, the reaction in which the charge is stored or transferred occurs at the electrode surface in contact with the ions in the electrolyte. The electrochemical properties of the device are determined by how ions are transferred and adsorbed to the electrode surface and how well electrons are transferred from the substrate. This study relates to the development of functional nanomaterials with controlled active surfaces for improving the performance of electrochemical energy devices, and is composed of three chapters according to the type of applied device. The first topic about controlling electroactive surface area is a study on the synthesis of porous metal oxide nanoclusters having a large surface area by reacting metal oxide nanoparticles with lithium ions and an aqueous asymmetric supercapacitor using two types of the metal oxide as a cathode and an anode. Realizing a stable energy storage device with high energy and power density is a major challenge. In this study, a strategy to fabricate a high-performance aqueous energy storage device was developed by synthesizing porous manganese oxide cathode and iron oxide anode induced through the conversion reaction of metal oxide nanoparticles with lithium ions on a graphene sheet. Porous nanoclusters composed of 2-3 nanometers of crystals in random orientation have a larger electroactive surface area than nanoparticles, and have enlarged reaction sites and enhanced ion transfer characteristics due to large amounts of gaps among the nanoclusters created during the lithiation process. The asymmetric supercapacitor combining porous manganese oxide and iron oxide nanoclusters exhibits an energy density of about 2.5 times that of the same device composed of manganese and iron oxide nanoparticles without lithium ion reaction, and shows excellent stability even after charging and discharging more than 30,000 times. In addition, it can be charged within a few seconds by a high-speed USB charger, showing a high power density that outperforms a typical aqueous battery. The second study is on the strategy to increase the reactivity of a water oxidation photocatalyst through nitrogen plasma treatment on layered double hydroxide (LDH). Layered double hydroxide is a promising water oxidation photocatalyst essential for solar-powered fuel production. However, there are problems such as limited accessibility to the reaction site, slow charge transfer, and easy activity degradation. In this study, as a method to overcome these drawbacks, ultrathin nanosheets exfoliated into mono- or dual-layers and doped with nitrogen were prepared through nitrogen plasma treatment on a layered double hydroxide. Since the nanosheet has abundant oxygen vacancy, the number of bonds between nickel and oxygen decreases and the bond length between nickel and metal is shortened, and the doped nitrogen induces metal-nitrogen or nitrogen-oxygen bonds. Oxygen vacancies allow the intermediates of water oxidation reactions to be easily adsorbed to nitrogen of low coordination, and the doped nitrogen plays a role of attracting nitrogen in the reaction process. The ultrathin nanosheets were coated on the hematite nanorods to act as a cocatalyst for the water oxidation photocatalytic reaction, and the reactivity was improved about twice as compared to the layered double hydroxide before plasma treatment. The last study is on the development of a porous carbon material that maximizes ion storage by controlling the properties of the metal-organic framework and carbonization. The hybrid ion capacitor uses a high power capacitor-type cathode and a high energy battery-type anode, which is a next-generation energy storage device that can overcome the low power and short lifespan of batteries, and low energy density of supercapacitors. The performance of the hybrid ion capacitor is determined by the ion storage capacity of capacitor-type cathode and the rate capabilities of the battery-type anode. In this study, a metal-organic framework (MOF) consisting of mixed metal ion cluster and 2-methylimidazole ligand was pyrolyzed to derive a porous carbon structure with nitrogen doping, hierarchical porosity, and high degree of graphitization. In the pyrolysis process, the vaporization of zinc forms micropores, and cobalt grows into nanoparticles, and the site of the cobalt particles removed through acid treatment acts as a mesopore, which is an ion transmission channel. Furthermore, the catalytic reaction of cobalt grows graphitic carbon and changes the doping type of nitrogen. By adjusting the ratio of zinc and cobalt, the cathode material optimized for ion storage was developed by simultaneously changing the ratio of micropores and mesopores, crystallinity of carbon, and nitrogen doping. In addition, tin oxide nanoparticles with a size of 3 to 5 nanometers were grown on the MOF-derived carbons for an anode of hybrid ion capacitors. The electrical conductivity is improved by nitrogen doping and crystallization, and the carbon support with the optimized pore structure rapidly transfers ions and electrons to the tin oxide particles, and it is physically firmly supported to store lithium and sodium ions quickly and stably. The strategies to synthesize functional nanomaterials with controlled electroactive surfaces presented in this study are simple to process, have versatility that can be applied to various materials, and can result in high energy storage performance or conversion efficiency. It is expected to be used in a variety of electrochemical energy fields such as Li-S batteries, lithium air cells, and fuel cells.