Hierarchical porous energy storage material controlling the active surface of nanometal materials and 3D graphene structure composite

Abstract

For reducing environmental pollution and sustainable development, the establishment of a system that can store and reuse the energy produced is a core task. Currently, the two energy storage mechanisms being researched most due to its high development potential and utility are electrochemical energy storage systems and hydrogen energy storage. An electrochemical energy device stores energy in an electric form using an electrochemical reaction or converts it into chemical fuel and reuses it as electrical energy, which can be classified into energy storage devices such as supercapacitors and batteries and energy conversion devices such as fuel cells. Each device has a different principle of operation, but the common and most important element is the working electrode in contact with ions in the electrolyte, in which charge is stored or transferred in electrochemical storage and conversion reactions. The performance of a device depends on how ions or molecules are transferred and adsorbed to the electrode surface and how well electrons are transported from the substrate. This study relates to the development of active surface of nanometal materials and 3D graphene structure composite to improve the performance of energy devices and is composed of three chapters according to the type of device applied. The first topic is to overcome the low energy density of conventional supercapacitor by configuration a thorn-covered core-shell structure composed of nano-metal sulfide and nano-sized sulfide loaded on a three-dimensional graphene aerogel structure as a cathode and an anode of an aqueous asymmetric supercapacitor, respectively. Implementing stable energy storage devices with high energy and power density is a major challenge. In this study, a strategy was achieved in which metal sulfides were composed of nano-sized materials and thus had a high active surface area to cause as many redox reactions as possible in the corresponding working voltage range. In addition, the structure of the anode and cathode is a hollow core-shell structure and a porous three-dimensional graphene structure, which is rich in meso-macropores that function as diffusion channels for electrolyte ions, so that the device can be operated even at a fast charge-discharge rate. In particular, since the metal sulfides of both electrodes are composed of multivalent states, the redox reaction occurs more abundantly in the working voltage range, which greatly affects the improvement of the capacity performance. The asymmetric supercapacitor combining the porous high surface area and multivalent metal sulfide nanocluster exhibits energy density about three times as compared to other metal oxides and carbon electrode capacitors operating in water, and exhibits excellent stability even after charging and discharging 100,000 times. In addition, it can be charged with a high-speed USB charger in seconds, showing a high-power density that surpasses a typical water-based battery. The second study is to prepare many active surfaces and diffusion channels inside and outside the palladium metal by intercalating lithium ions into the palladium metal particles loaded on the three-dimensional graphene structure. Due to the high surface area of the porous palladium particles prepared in this way, more hydrogen molecules can be adsorbed, and the number of palladium particles involved in the hydrogen molecules is large, so that the activation energy can be lowered to make dissociation more easily. The dissociated hydrogen atom is stored by a spillover mechanism that is migrated to a nitrogen-doped graphene sheet. The hydrogen storage material manufactured through this study exhibits three times the hydrogen storage performance of palladium particles without lithium insertion process and much greater hydrogen storage performance than single atom-sized palladium particles. Similarly, by comparing the performance with single atom-sized palladium particles with a large surface area, the importance of the surface structure to help dissociation of hydrogen molecules as well as a large surface area could be confirmed through this study. Both the adsorption and desorption of hydrogen at a temperature of 90℃ are stable, so there is the potential of attracting hydrogen storage materials that can be actually used when the hydrogen industry arrives in the future. In the last study, we synthesize high-capacity/high-rate anode and cathode materials based on the same precursor structure enabling to realize high-energy density and ultrafast rechargeable hybrid PICs. First, a three-dimensional rGOA structure rich in macropores and mesopores is synthesized through the hydrothermal process of graphene oxide (GO). Using this synthesized rGOA as a framework, micropore-rich ZIF-8 is formed on the rGO sheet to synthesize the ZIF-8/rGOA precursor structure. The ratio of mesopores and micropores in the hierarchical porous precursor structure is controlled by changing the amount of ZIF-8 loaded on the rGOA structure. This ZIF-8/rGOA precursor was carbonized in an N2 atmosphere at 700°C to form a Zn embedded ZIF-8 derived hierarchical porous 3D carbon structure (ZZHPC). In the ZZHPC structure, abundant sub-nanometer zinc metal is embedded in the carbon structure and the hierarchical pore structure of the precursor is maintained even after the carbonization process. In addition, a similar carbonization process was performed at 900°C to obtain a ZIF-8 derived hierarchical porous 3D carbon structure (ZHPC) in which all zinc metals were vaporized. Through these abundant mesopores, it has a fast ion transport channel and forms a high-capacity electrode material at many reaction sites derived from a high surface area. In addition, through a carbonization process, nitrogen-containing ligand (2-methylimidazole) provides nitrogen doping effect to carbon structure to provide not only excellent electrolyte wettability but also abundant pseudo-capacity sites for high capacity. With these features, a PIC full-cell was assembled using the synthesized ZZHPC and ZZPC as anode and cathode, respectively to attain high energy density. In addition, in order to prove the high-speed charging capability of a full-cell hybrid capacitor, it further demonstrates the high-speed charging capability by using a high-speed universal serial bus (USB) charger. 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. It is expected to be used in a variety of energy storage fields such as batteries, supercapacitors and fuel cells.