Synthesis of Heteroatom doped Carbon Nanomaterials using Sono-Cavitation & Nebulization Method for Energy Conversion and Storage

Abstract

The controlled carbon-heteroatom bonding of carbon nanomaterials has received much attention over the past decade owing to their unique chemical, electrical, magnetic, and optical properties. Among the carbon nanomaterials, graphene is 200 times stronger than steel, 1,000 times more electrical conductive than copper, 97.7% transparent at visible light, and the thinnest material on earth. However, widespread chemical and semiconducting industrial applications are not expected for a long time because of the lack of band gap, stacking to each other by Van der Waals forces, low polarizability and low ionic accessibility. In this context, heteroatom doping strategy is of great significance in terms of tuning the inherent properties (physical, chemical, electrical, and optical) of carbon nanomaterials. However, the honeycomb sp2-hybridized network in the carbon nanomaterials is thermodynamically very stable; therefore, expensive processes and/or harsh conditions are required to achieve heteroatom doping, such as atomic layered deposition, chemical vapor deposition, and plasma treatment. Taken together, the formation of carbon-heteroatom bonds under mild reaction conditions remains a major challenge. In this dissertation, new synthesis process for direct formation of carbon-heteroatom bond have been developed and named sono-cavitation and nebulization synthesis (SNS). Using the SNS method, new heteroatom doped carbon nanomaterials have been designed and prepared for supercapacitor and electrochemical CO2 reduction reaction (CO2R) where the supercapacitor is an important for high-powered energy storage system, while CO2R is the key electrocatalytic reaction for CO2 resourceization and net-zero carbon growth. Chapter 2 presents facile and scalable SNS method that has synergetic effect of extreme single hot zone of sono-cavitation and continuous multiple hot zones of ultrasonic spray pyrolysis. The SNS method has few restrictions on the choice of dopant, because liquid dopant can be used in cavitation process, and gas (N2, O2, etc.) dopant also can be fixed by nebulization process. Furthermore, we can precisely control the heteroatom doping ratio through a change of liquid dopant concentration and gas dopant pressure, from 1.0 atom% to 6.0 atom%. Based on the SNS method, we built up the library included in single-, dual-, and metal-nitrogen-doped carbon nanomaterials. The strategy of multi-heteroatom doping can induce the polarization and distortion in sp2-hybridized framework of carbon nanomaterials, resulting in enhanced redox activity and electrical sensitivity for supercapacitor and CO2R activity. Ocean acidification due to the absorption of 40% of the world’s anthropogenic CO2 emissions severely affects the faltering marine ecosystem and the economy. However, there are few reports on reducing CO2 dissolved in seawater. In Chapter 3, we introduce a new CO2R battery in seawater (CBS) system comprising a graphitic frustrated Lewis pair (GFLP) cathode, a Na metal anode, and natural seawater electrolyte, which can drastically recover the pH of seawater during discharging. In general, the properties of frustrated Lewis pairs (FLPs) are ideal for capturing CO2 because the sterically constrained Lewis acid (LA) and Lewis base (LB) pair strongly binds to CO2. Therefore, we modified the FLP system for seawater applications using a carbon nanomaterial with an immobilized LA and LB pair on a graphitic framework, which we refer to as ‘GFLP’. The sp2-hybridized graphitic framework of GFLP can donate π-electrons to FLP sites, thereby enhancing the catalytic activity and electrochemical stability in seawater. The GFLP synthesized by SNS method converts CO2 dissolved in seawater to multi-carbon products during discharging of CBS, thereby increasing the pH of intentionally acidified seawater from 6.4 to 8.0 with more than 87% Faradaic efficiency. BN-GFLP afford dual CO2 binding mode that enables exothermic C–C coupling to deliver 95% selectivity for valuable multi-carbon products from CO2R. Based on this results, we suggest a molecular design strategy for next-generation CO2 reduction catalysts for both green oceans and the atmosphere. The direct formation of C-N and C-O bonds from inert gases is essential for chemical/biological processes and energy storage systems. However, its application to carbon nanomaterials for improved energy storage remains technologically challenging. Chapter 4 describes a simple and fast method to form C-N and C-O bonds in reduced graphene oxide (RGO) and carbon nanotubes (CNTs) by the SNS process. Electrodes of nitrogen- or oxygen-doped RGO (N-RGO or O-RGO, respectively) are fabricated via the fixation between N2 or O2 carrier gas molecules and ultrasonically activated RGO and CNT. The materials exhibit much higher capacitance after doping (133, 284, and 74 F g−1 for O-RGO, N-RGO, and RGO, respectively). Furthermore, the doped 2D RGO and 1D CNT materials are prepared by layer-by-layer deposition using double injection line of SNS to form 3D porous electrodes. These electrodes demonstrate very high specific capacitances (62.8 mF cm−2 and 621 F g−1 at 10 mV s−1 for N-RGO/N-CNT at 1:1, v/v), high cycling stability, and structural flexibility.