Synthesis of porous carbon materials via CO2 conversion for electrochemical energy storage

Abstract

Since the Paris Agreement launched in 2015, the regulations on greenhouse gas emission have been negotiated to overcome global warming. Accordingly, many studies have been developed to reduce carbon dioxide emissions or to treat the emitted carbon dioxide, which is one of the major greenhouse gases. In the former case, the recyclable electrochemical storage systems gain attraction to replace fossil fuel, one of the main causes of carbon dioxide emissions. However, most of the electrochemical storage systems represented by lithium-ion batteries contain the electrodes composed of the metal-based and graphite-based materials with high prices. Thus, these materials restrict the complete replacement of the low-cost fossil fuel, and more economical material is necessary than the conventional electrode materials. In the latter case, the technology that converts the emitted carbon dioxide into valuable materials has great potentiality. However, since the carbon dioxide has very stable structure, it is difficult to convert within a mild range of temperature and pressure, which implies that extreme condition such as a supercritical point is required. In this regard, even if the material produced from carbon dioxide is valuable, commercialization could be problematical if the cost incurred during the conversion process exceeds the value of the product. Therefore, when developing these technologies, the cost of the conversion process and the value of the converted material should be compared to determine the overall efficiency, economic feasibility, and commercial viability. This dissertation concentrates on the control of the morphology and surface atom of the produced carbon materials from carbon dioxide while alleviating the extreme temperature and pressure conditions, which are the main obstacles to the commercialization. The carbon dioxide is converted under atmospheric pressure with a reducing agent, and the porosity of resultant carbon is controlled successfully through calcium carbonate as a pore template. The electrochemical potentiality as an electrode material of supercapacitor and lithium-sulfur battery was confirmed. In addition, by applying the carbon dioxide conversion technology to an electrospinning process, the carbon fibers were synthesized in form of paper with significantly higher porosity and surface nitrogen content than conventional electrospun fibers. The modified carbon paper is applied as an interlayer, to improve the overall stability and output performance of the lithium-sulfur battery. Further, after impregnating sulfur into the fiber, the modified carbon paper was applied as a cathode of a lithium-sulfur battery without a slurrying process requiring binders, conductive materials, and current collectors. This paper-type cathode not only simplifies the manufacturing process, but also reduces the weight and the cost of the assembled cell while having high sulfur content and capacity per area compared to previous studies. Finally, in this dissertation, the amounts of carbon dioxide produced and consumed in the synthesis process of each material are calculated, and their values are contemplated environmentally. Consequently, by converting emitted carbon dioxide into a porous carbon material and applying to the electrochemical storage system, which can replace the fossil fuel generating carbon dioxide, this investigation can contribute to the reduction of carbon dioxide emissions in two aspects simultaneously.