Study of bismuth based material for supercapacitor applications

Abstract

The rapid development of industrial production has made the energy supply increasingly tight and accelerated the exploitation of fossil energy, which not only caused a sharp decline in the reserves of fossil energy, but also brought environmental pollution, which strongly promoted the development of new energy sources such as wind, water, and solar energy. However, the storage and transportation of new energy are affected by time and space, which limits its application. In order to ensure the effective storage and stable output of new energy, the development of high-performance energy storage devices has become a current research focus. As a new type of energy storage device, supercapacitors have great application prospects in the fields of renewable energy, transportation, and wearable portable devices because of their high power density, fast charge and discharge, and stable cycle performance. As one of the key factors affecting the performance of supercapacitors, electrode materials are the focus of research in the scientific community. The electrode materials currently studied include carbon materials, conductive polymer materials, and metal compound materials. Compared with carbon materials and conductive polymers, metal compounds have become an important research object for supercapacitor electrode materials because of their higher specific capacitance. Bi-based compounds are favored by researchers because of their relatively rich content in the crust, wide sources, and high theoretical specific capacitance. This thesis focuses on Bi-based materials and studies the application of Bi2S3, Bi2O3, BiSI and Bil13S18I2 as supercapacitor materials.

The first chapter introduces the background of supercapacitors and the production process of supercapacitors, and the comparison with other main energy storage materials, ion batteries and fuel cells. The classification of supercapacitors, the current main supercapacitor structure, electrode, and electrolyte materials are explained.

Chapter 2 introduces the characterization methods used in the experiments, including X-ray diffraction (XRD), elemental analysis (EDS), scanning electron microscope (SEM), transmission electron microscope (TEM) and other physical characterization methods were also introduced to study the phase structure and morphology of the material. Moreover, the electrode assembly process and various electrochemical tests are described in detail, including cyclic voltammetry, constant current charge and discharge, and electrochemical impedance spectroscopy.

In chapter 3, there are three main parts. First, a novel synthetic route for two 3-D hexagonal bismuth chalcogenide materials Bi13S18I2 and BiSI is demonstrated, and their potential as the active electrode material for supercapacitors is investigated. Both pure BiSI and Bi13S18I2 powder were obtained for the first time at a relatively low temperature (120 °C) in a solution system. The effects of different reaction conditions on the surface morphology of the product were studied, showing that at a relatively low temperature of 120 °C under atmospheric pressure, the materials can yield smaller crystallite size and higher specific surface area, further increasing the capacitance compared to the synthesis under hydrothermal conditions. The galvanostatic charge-discharge measurement results show that Bi13S18I2 electrode has a maximum capacitance of 50 C g-1 at the current density of 1.0 A g-1 and excellent capacitance retention of 98.4% after 5000 cycles at the current density of 10.0 A g-1 in 3.0 M KOH electrolyte as a two-electrode electrical double-layer capacitor system (EDLC). This facile route to the synthesis of both Bi13S18I2 and BiSI with superior stability has promising potential for low-cost and effective electrochemical supercapacitor applications. In chapter 4, a novel method was used to employ the -OH group on the graphene oxide (GO) surface and ethylene glycol (EG) as a linker to anchor Bi ions on the GO surface, and further in situ generate BiSI crystals to obtain a BiSI composite structure with a uniform surface coating of reduced graphene oxide (rGO). Through the electrochemical testing of BiSI-rGO, it has both the pseudocapacitance of BiSI and the EDLC performance of rGO, which greatly improves the capacitance from 88 C g-1 to 195 C g-1, greater than the sum of the simple BiSI and rGO capacitances, which shows that BISI-rGO produces a synergistic energy storage effect. By assembling an asymmetric capacitor with Ni(OH)2 as a counter electrode, it exhibits a high energy density of 12.8 W h kg−1 at a power density of 8 KW kg−1. The synthesis mechanism is further discussed, showing that Bi ions are anchored on the GO surface first, then GO is bent to finally form BiSI-rGO uniformly coated with rGO, instead of first generating BiSI crystals and then coating with GO during the formation of BiSI crystals. This is the first time that chemical energy has been used to coat Bi-based substances with rGO instead of a simple physical attachment. This route uniformly coats the Bi-based active substances with GO, which improves new ideas and synthetic routes for Bismuth-based material surface modification.

In chapter 5, there are three parts. In the first part, three 3-D structured Bismuth-MOFs on carbon paper BA, PA and MBA were prepared through a facile solvothermal method. The as-prepared MOF/CP electrodes were directly used as an electrode material with superior capacitive behavior in 2 M KOH. All of the three bismuth-MOF materials show high specific capacitance in three electrodes system as pseudo-capacitor, BA 153 C g-1, PA 246 C g-1 and MBA 267 C g-1 at 1 A g-1 discharge density; BA 56 F g-1, PA 58 F g-1 and MBA 62 F g-1 at 1 A g-1 discharge density in two-electrode symmetric devices. It is the first time that these bismuth MOF materials have been proven to have the potential for supercapacitor applications. In the second part, three unique Bi2S3 negative electrode materials for SCs derived from different Bi-MOF precursors are reported through a facile self-sacrificing template strategy and in-situ synthesized them on Bi-MOF coated carbon paper without any bonding material. The prepared Bi2S3 nanorods electrode exhibits a large specific surface area (54.3 m2 g−1) and an ultrahigh specific capacity (664 C g−1 at 1 A g−1) that is the highest among all Bi2S3 reported. Moreover, a hybrid supercapacitor (HSC) device using a layered double hydroxide Ni(OH)2 as a positive electrode delivers an excellent energy density of 41.5 Wh kg-1 at a power density of 8 KW kg−1. This is the first study to control the structure of synthetic Bi2S3 using the structure of Bi-MOF materials as the precursor and in-situ grow inorganics Bi2S3 on the surface of hydrophobic carbon cloth without adding adhesives. In the third part, Bismuth-MOF MBA was used as a precursor to synthesize Bi2O3-C composite material by a simple heating process. The prepared Bi2O3-C composite material has a capacity of 1233 C g-1 at 1 A g-1 discharge density. In addition, a hybrid supercapacitor (HSC) device using a layered double hydroxide Ni(OH)2 as a positive electrode delivers an excellent capacity of 263 C g-1 at 1 A g-1 discharge density. The above advantages suggest that the current strategy for the derivation of Bi-MOFs will provide a valuable route for the preparation of bismuth-based inorganic nanomaterials for use in high-performance energy storage technologies and other areas.