2024 Theses Doctoral

Development of Nanomaterials Sorbents for CO₂ Capture and Conversion

Wu, Xiaolong

Excessive carbon dioxide (CO₂) emission into the atmosphere is the dominant factor in the global warming effect. The quick development in industries, anthropogenic activities, expansion of electric cars, and AI-Generated Content (AIGC) market significantly increase the energy demand. The emission of CO₂ gas into the atmosphere bounced back to a new high level after the economic recovery across the world following COVID-19. The zero-carbon policies were carried out by more countries to keep the temperature rise within 1.5 ℃ according to the Paris Agreement. However, fossil fuels still occupy the first place in emitting CO₂ into the air, though a lot of renewable appliances have started to run in recent years. Apart from controlling and diminishing the emission of CO2, capture and utilization technologies are the most significant strategy to achieve carbon neutrality before 2050. The utilization of CO₂ has become various, including direct CO2 electrolysis, two-step tandem CO₂ electrolysis, and hybrid process. Some technologies require concentrated CO₂, and the desorption of CO₂ is now becoming an unavoidable and energy-consuming process. In response to the excessive CO₂ emissions into the atmosphere, 2019 is the ninth year in a row that the global mean sea level has risen compared to the year before, setting a new record with a peak of 87.6 mm in the middle of the year. To meet the 1.5 ℃ objective, we should develop novel technologies to capture and convert CO₂ with new heating technology for tandem utilization or skip the specific desorption process to directly produce the value-added chemicals.

Carbon materials are widely used in different industrial fields. Since graphene, graphene oxide, and carbon nanotubes have such remarkable properties, these three carbon-based materials have been highly interesting research subjects in recent years. Graphene stands out for its toughness, flexibility, lightness, high resistance, electrical conductivity, and heat conduction. The applications of graphene are very broad: they include electronics to improve the chip performance, flexible screens to enhance the user experience, construction materials to improve safety and save energy, and medical treatment for drug delivery. Thus, the process of producing premium graphene is the remaining problem blocking the development of graphene applications. Graphene is primarily made through two methods: chemical vapor deposition (CVD) and oxidation-reduction (Redox). The substance needed to make graphene using CVD is methane (CH4), and the temperature is around 1000℃, which makes this an energy-intensive method. The oxidation-reduction (Redox) method will need much stronger acids and oxidants to oxidize the graphite and then reduce the graphene oxide to get reduced graphene with defects, which also has a huge demand for energy. The proposed strategy is preparing the graphene by re-carbonization of asphaltene molecules extracted from crude oil or asphalt paving materials. This strategy saves a lot of energy and improves the use rate of abundant asphalt materials. During the synthesis process of graphene, we choose the natural montmorillonite as the substrate material to provide the designed space for re-carbonization reactions. Using this new method and these materials, we can get low-layer (< 3) graphene sheets with remarkable 2D scale. Using a scalable process to generate graphene of superior quality with neglectable defects, the applications of graphene in various fields would be largely enhanced and facilitated. Besides the above applications, graphene has become familiar as a composition of absorbent in the Carbon Capture Utilization and Storage (CCUS) field.

To reach the goal of carbon-zero before 2050, the studies of CCUS have prevailed in the past years. The emission of CO2 comes from industry, agriculture, transportation, and other human activities. To overcome the challenge of removing the greenhouse effect, the CO₂ capture technologies are the most important part of the realm of CCUS. CO₂ capture technologies are primarily designed based on the main existing form, such as 3 to 8 % emitted by natural gas power plants, 10 to 18 % coal-fired plants, and 400 ppm (0.04%) in the atmosphere. Hence, a lot of sorbents were designed and produced for various CO₂ sources. Compared with the capture process, the demand for CO₂ desorption is becoming the most energy-intensive process.

For example, the amine sorbents can only desorb the CO₂ at a high temperature around 100 to 120 ℃. The alkaline sorbents need protons to generate CO₂. With regard to the heating method, conventional heating, including vapor heating and induction heating, also consumes a lot of energy. To overcome the energy demand challenge, using microwave irradiation to desorb CO₂ becomes a potential solution to reduce the energy demand. Therefore, modified graphene-doped solvent-impregnated polymers (SIPs) were created to capture CO₂ and desorb CO₂ with a faster speed and lower energy consumption.

Graphene is extraordinarily responsive to microwave irradiation. Carbon can be quickly heated to >1200℃ using a power of 1000 W at 2.45 GHz within 1 minute. Among the graphene (GN), graphene oxide (GO), and carbon nanotubes (CNTs), the low-layer graphene demonstrates a remarkable ability to absorb the microwave to generate heat to desorb CO₂ and a highly efficient ability to dissipate heat. The synthesized SIPs/GN absorbent can capture 400 ppm, 7%, 15%, and 100% CO₂ with great capacities and desorb the CO₂ with a power of 100 W microwave irradiation within 5 minutes at 50 ℃ for long-term use (50 cycles), accompanying no degradation of amine groups. With a fast and energy-saving heating method, the SIPs/GN sorbents would be one of the longest-lasting materials for industrial applications. In the tandem utilization of CO₂, the high concentration of CO₂ is a foundational reactant for following reactions to value-added products. So, the graphene-doped SIPs give a valuable strategy to lessen the energy demand.

Except for the necessity of pure or high concentration of CO₂ for subsequent CO₂ utilizations, the direct conversion of CO₂ is also the other solution to convert the captured CO₂. The dual functional solid materials are one of the current feasible approaches to directly reduce CO₂ by introducing H₂ and catalyst at high temperatures to get CO or CH₄. With the rising trend of electrochemical electrolysis of CO₂, a dual functional liquid absorbent-based electrolyte can be the candidate to directly reduce CO₂. With a relatively slow CO₂ desorption rate compared to 2M MEA and 8 wt.% PEI solutions, the 10 wt.% NOHMs is made to serve as the electrolyte. This is an excellent electrolyte possessing a high conductivity to directly reduce the CO₂ to CO with 0.5 M NaCl and 83.3% less concentration than 2 M MEA + 3 M KCl. Thus, it realizes the same ∼ 20 % Faradaic Efficiency of CO and production rate of CO at 25 ℃. The study of direct reduction of 10 wt.% NOHMs gives inspiration for future large-scale continuous reduction in a flow cell. The high CO2 partial pressure in the reservoir and suitable pH value range would be adopted to convert the CO₂. In addition, the 10 wt.% NOHMs also saves at least 38.63% and 77.71% energy compared to 8 wt.% PEI and 2 M MEA solutions. When the adjusted cathode contact area and the stable current density of CO can give the same product rate of CO with the CO₂ desorption rate, the balance between the desorption rate and conversion rate would be achieved.

In sum, the results of this thesis show the combined strategy to capture and convert various CO₂ sources efficiently. Due to the limitation of liquid NOHMs solution to capture and convert the 400 ppm CO₂, the 10 wt.% NOHMs solution is a good approach for 10 to 18 % CO₂ source. The solid SIPs/GN absorbent has a high capacity when capturing 400 ppm CO₂, which is one of the most effective ways for direct air capture (DAC). With the comprehensive capture technologies and excellent conversion rate to CO, the future CCUS technologies will be focused on the combinations of capture, conversion, and storage, with considerations of energy efficiency, process cost and production efficiency.