Climate change necessitates significant reductions in CO2 emissions. Carbon capture and utilization (CCU) technologies have emerged as a promising solution, offering the potential for net-zero emissions by directly removing and utilizing CO2. The global roadmap projects CCU could reduce emissions by over 7 gigatons by 2030, with a market potential of $800 billion USD. However, current CO2 conversion technologies face challenges, including low technological maturity, high production costs, and significant energy consumption. One promising approach is the sequential conversion of CO2 via syngas production, which allows for the creation of various chemicals by adjusting the H2-to-CO ratio. Conventional syngas production methods are highly endothermic, limiting their economic viability and raising concerns about their overall CO2 reduction capabilities. Electrochemical CO2 reduction (eCO2R) using renewable energy offers a pathway to net-zero emissions during syngas production. However, eCO2R processes typically involve expensive CO2 capture and product conditioning, hindering economic attractiveness. Recent research focuses on eliminating pre- and post-conditioning processes by using low-concentration CO2 sources (e.g., flue gas) and directly converting CO2 captured in amine solutions. Direct eCO2R in amine solutions is particularly promising as it can eliminate energy-intensive thermal amine regeneration and minimize pressurization energy. While attempts have been made using various alkanolamines, these solvents often capture CO2 as carbamates, which are difficult to convert electrochemically due to strong C-N bonds. This study introduces a new approach, Reaction Swing Absorption (RSA), aiming to overcome these limitations and provide an economically feasible and environmentally benign method for CO2 reduction and syngas production.

Existing literature highlights the potential of CCU to mitigate climate change but also underscores the limitations of current CO2 conversion technologies. The high energy demand and costs associated with conventional methods, such as those based on the reverse water gas shift reaction (RWGS), are significant barriers to commercialization. Electrochemical CO2 reduction (eCO2R) offers an alternative, but its economic viability is limited by the need for CO2 capture and product purification. Studies have explored direct eCO2R in amine solutions, aiming to eliminate the energy-intensive thermal regeneration step. However, challenges remain due to the use of traditional alkanolamines that form carbamates, hindering efficient CO2 conversion. Previous work using monoethanolamine and diethanolamine showed limited success, with the active carbon source being free CO2 rather than the carbamate. While studies have explored the use of bicarbonate as a potential carbon source, issues such as salt formation and corrosion in highly alkaline conditions need to be addressed. Importantly, crucial analyses such as techno-economic analysis (TEA) and life cycle assessment (LCA) are often lacking in these studies, despite the potential for net-zero CO2 conversion and high-pressure syngas production. This gap in comprehensive analysis is a key driver for this current research, aiming to assess the economic and environmental feasibility of a novel approach to CCU.

This study introduces Reaction Swing Absorption (RSA), a novel approach to syngas production from CO2. The methodology consists of three main stages: chemisorption, pressurization, and electrochemical conversion. The selection of an appropriate amine solvent is crucial for the effectiveness of RSA. After screening several amines, triethylamine (TREA) was identified as the most suitable solvent due to its ability to capture CO2 as bicarbonate, which is more easily converted electrochemically compared to carbamates formed by other alkanolamines. The CO2 absorption capacity and rate of TREA were experimentally determined using a bench-scale absorption column with varying liquid-to-gas ratios and CO2 concentrations. A membrane electrode assembly (MEA) electrolyzer was employed for the electrochemical conversion of CO2 to syngas. The electrolyzer's configuration was optimized by systematically investigating various parameters, including the catalyst material, electrolyte solution, and membrane type. Silver nanoparticles (Ag NPs) supported on carbon (Ag/C) were initially used as the cathode catalyst, and the performance was further enhanced by creating a coral-like structure through electrochemical oxidation and reduction, resulting in improved CO Faradaic efficiency. The choice of a bipolar membrane was critical to prevent bicarbonate crossover and maintain electrolyte stability. A 3M TREA solution served as the catholyte, and 1M KOH solution was used as the anolyte. Electrochemical CO2 reduction experiments were conducted using chronopotentiometry to assess the Faradaic efficiency of CO and H2 production at various current densities. Long-term stability tests were performed to evaluate the system's performance over extended operation periods. Techno-economic analysis (TEA) and life cycle assessment (LCA) were performed to compare the RSA process with conventional syngas production methods, such as RWGS and gas-phase eCO2R. A global sensitivity analysis (GSA) was conducted to identify the key factors influencing the economic and environmental performance of the RSA process, enabling targeted improvements and policy recommendations.

The key findings of this study demonstrate the significant potential of RSA for economically viable and environmentally friendly syngas production. First, triethylamine (TREA) exhibited exceptionally high CO2 absorption rates, exceeding 84% from low-concentration flue gas. NMR analysis confirmed that TREA captures CO2 primarily as bicarbonate, facilitating electrochemical reduction. The MEA electrolyzer using TREA achieved a CO Faradaic efficiency of approximately 30% at a high current density of -200 mA cm2, significantly higher than conventional alkanolamine-based systems. Optimization of the cathode catalyst (coral-Ag/C) and the use of a bipolar membrane further improved the performance, achieving a CO FE of up to 70% at -20 mA cm2. Long-term stability tests showed consistent performance over 70 hours of operation. Techno-economic analysis (TEA) revealed that the RSA process outperforms RWGS and gas-phase eCO2R in terms of operating expenditure (OPEX) and break-even syngas price under optimistic scenarios, particularly when coupled with renewable energy sources (wind and solar). In an optimistic scenario, the break-even price of syngas could drop to $0.65/kg (wind) or $0.56/kg (solar), becoming competitive with fossil fuel-based syngas production. Life cycle assessment (LCA) indicated lower global warming potential (GWP) for RSA compared to the other methods when using renewable energy sources, with a potential GWP100 of 0.27 kg CO2 eq./kg syngas for the wind energy case. Global sensitivity analysis (GSA) identified key parameters that influence the economic and environmental performance, such as electricity generation cost, CO2 capture rate, and improvements in electrolyzer technology. These findings strongly support the potential of RSA as a game-changing technology for carbon capture and utilization.

The results demonstrate that Reaction Swing Absorption (RSA) provides a substantial advancement in carbon capture and utilization (CCU) technology. By directly converting CO2 captured in a novel amine solution (TREA), RSA circumvents the energy-intensive thermal regeneration steps and complex separation processes associated with conventional methods. The high CO2 absorption rates and significantly improved Faradaic efficiency compared to existing electrochemical CO2 reduction systems highlight the superior performance of the RSA approach. The economic analysis further underscores the potential for RSA to become a cost-competitive and sustainable alternative to fossil fuel-based syngas production. The integration of renewable energy sources further enhances its environmental benefits, leading to near-zero CO2 emissions. The sensitivity analysis provides valuable insights for optimizing the process and identifying areas for future research and development. The findings suggest that focused improvements in bicarbonate electrolyzer efficiency and CO2 capture rates will further enhance the economic and environmental performance of the system.

This study successfully demonstrates the feasibility and advantages of Reaction Swing Absorption (RSA) for sustainable syngas production. The use of TREA as a CO2 absorbent and the optimized electrochemical system configuration lead to high CO2 absorption rates and significantly improved CO Faradaic efficiency, compared to conventional methods. Techno-economic and life cycle analyses indicate that RSA offers a promising pathway to cost-competitive and low-emission syngas production, especially when coupled with renewable energy sources. Future research should focus on further enhancing the performance of the bicarbonate electrolyzer, improving CO2 capture rates, and exploring the scalability and robustness of the RSA technology for industrial applications.

While the RSA process shows considerable promise, several limitations should be considered. The current study focuses on a laboratory-scale system, and scaling up the process to an industrial level might introduce challenges related to mass transfer, heat management, and material durability. Furthermore, the long-term stability and durability of the bipolar membrane under continuous operation need further investigation. The sensitivity analysis highlights the impact of electricity cost on the economic viability of RSA, emphasizing the importance of integrating with cost-effective renewable energy sources. The influence of trace impurities on the bicarbonate electrolysis system also requires further study before large-scale implementation.