Recently, CO2 electrochemical reduction (CO2R) has gained popularity, to cope with the strict environmental rules on greenhouse gas emissions, and to convert CO2 to value-added chemicals/fuels. Ionic liquids (ILs) have been considered as potential media for CO2R owing to their multi-functions in enhancing CO2R solubility and improving CO2R reaction rate and product selectivity. To date, there have been many studies related to CO2R in IL-based systems, which primarily focused on fundamental research to offer findings about CO2R performance and reaction mechanisms, and one article focused on evaluating the economic potential of the stand-alone CO2R process without considering the upstream process or integrating into other processes. In fact, the integration of CO2R with upstream or other production processes will make the evaluation more practically significant, and thus deeper knowledge about the viability of the integrated CO2R process is needed, but relevant work is still lacking. Meanwhile, how to further improve the performance of CO2R is another concern.

The goal of this work is to perform systematic studies on techno-economic assessment of the integrated CO2R process and experimental research for producing methanol (CH3OH) with ILsas electrolytes as the focus, since CH3OH is an important solvent, energy and hydrogen carrier, and feedstock.

In the first part, an intensive literature survey was conducted to summarize the research progress, identify the state-of-the-art and provide the research gap for CO2R in the IL-based systems. It shows that the multi-functions as CO2 absorbents, reaction media, and co-catalysts give ILs a distinctive boosting effect on the CO2R performance. But now the research mainly focused on lab-scale experimental studies, while the viability of this technique on a large scale is unclear.

In the second part, stand-alone CO2R producing CH3OH with IL as the absorbent and electrolyte was studied and then further integrated with biomass gasification. The economic feasibility and environmental impact were investigated and compared, under current and future conditions. Stand-alone CO2R process shows high total production cost (TPC) due to the high electrolyzer and electricity costs. The TPC could reduce from 1.44 to 1.02 €/kg-CH3OH under the current conditions after integration. Additionally, based on the analysis, electricity for CO2R is the main part of energy usage and dominates the CO2 emission of the integrated process.

In the third part, techno-economic analysis of the integrated processes that combined CO2R in IL to produce CO, syngas, and CH3OH with biomass gasification for producing CH3OH was performed and contrasted with stand-alone biomass gasification and CO2R processes. The process that integrated with CO2R to CO was identified as the optimal pathway with the lowest TPC of 0.38 €/kg-CH3OH under the current condition. Sensitivity analysis confirmed that electricity and H2 prices are two key parameters influencing the TPC of the process, which is combined with CO2R to CO followed by hydrogenation to CH3OH; while for the integrated processes with CO2R to syngas and CH3OH, simultaneously reducing stack and electricity prices as well as improving CO2R performance are significant to make these processes viable in the future.

In the fourth part, preliminary experimental research on CO2R to CH3OH with various catalysts in IL-based electrolytes was conducted to evaluate the influence of catalysts and ILs on the CO2R performance. It was found that CO2R to CH3OH by using copper-deposited nickel foam (CuNi) showed the optimal performance with current density and Faradic efficiency of CH3OH of 14 mA/cm2 and 46.31% under -1.7 V vs Ag/Ag+, respectively.

CO₂ conversion plays an important role in mitigating the issues caused by CO₂ emissions. Among different CO₂ conversion technologies, CO₂ electrochemical reduction (CO₂R) is considered as one of the most promising ways, but it still suffers from high overpotential, low reaction rate, and low selectivity. Ionic liquids (ILs) and deep eutectic solvents (DESs), as novel solvents with many unique advantages, such as wide electrochemical window, high stability, and tunable nature, have been widely used as electrolytes for CO₂R to reduce the overpotential and improve reaction performance. Currently, numerous studies on CO₂R with IL/DES-based electrolytes have primarily focused on fundamental research, providing insights into CO₂R performance improvement and reaction mechanism analysis, but research on evaluating the techno-economic and environmental feasibility of CO₂R is very limited. Meanwhile, from an experimental viewpoint, how to further improve the performance of CO₂R by designing/modifying catalysts and electrolytes is another concern.

This work aims to systematically study the techno-economic and environmental feasibility of the CO₂R process under current and future conditions, along with technology development focused on designing novel catalysts and electrolytes for CO₂R, where ILs/DESs were used as electrolytes. Since CO₂ can be converted to different chemicals/fuels, such as CO, syngas, HCOOH, CH₃OH, CH₄, and some multi-carbon compounds. The research status, difficulties, as well as the future research focuses for producing different products need to be clear. To this end, a comprehensive literature survey was conducted to highlight recent progress, to offer an overview of the state-of-the-art advancements, and to identify research gaps in CO₂R within IL-based systems. Based on the literature survey, CO, syngas, and CH₃OH have been identified as particularly attractive products, considering both CO₂R performance and the challenges associated with product separation. 

Based on the results from the literature survey, a comprehensive assessment model for the integrated (CO₂R + biomass gasification) process was established, enabling the evaluation of key performance parameters and commercialization potential for both stand-alone and combined processes. In terms of technology development, a molecule-regulated Ag catalyst was designed for CO₂R in the IL-based system, where its reaction performance and mechanisms were thoroughly analyzed. Additionally, DES-based solvents were selected as electrolytes for CO₂R with Ag as the catalyst, and the effect of electrolytes on CO₂R performance was studied. The main results are summarized below.

The stand-alone CO₂R process for producing CH₃OH in IL-based electrolytes was first established and then further integrated with biomass gasification. The economic and environmental viability of these processes under current and future conditions was thoroughly examined. The high total production cost (TPC) of the stand-alone CO₂R process is primarily driven by the significant expenses associated with the electrolyzer and electricity. After integration, TPC can be reduced from 1.44 to 1.02 €/kg-CH₃OH. In the integrated process, electricity for CO₂R constitutes the main portion of energy usage and is the predominant contributor to CO₂ emissions.

A techno-economic analysis was conducted on the integrated processes of CO₂R to CO/syngas/CH₃OH, combined with biomass gasification for CH₃OH production. Among these processes, the process combined with CO₂R to CO showed the lowest TPC of 0.38 €/kg-CH₃OH under current conditions, which is below the market price of CH₃OH (0.50 €/kg-CH₃OH). Sensitivity analysis revealed that electricity price is a crucial factor affecting TPC for all combined processes. In addition, CO₂R performance and stack price significantly impact TPC in processes that involve CO₂R to syngas and CH₃OH.

Experimental research on CO₂R to CO was conducted with a molecule-regulated catalyst in BmimPF₆-based electrolytes. Specifically, the molecule with desirable CO₂ affinity, 3-mercapto-1,2,4-triazole (m-Triz), was introduced onto the surface of Ag to obtain the Ag-m-Triz catalyst. This catalyst demonstrated desirable performance, achieving a partial CO current density of 85.0 mA/cm² and a Faradaic efficiency of CO of 99.2% at -2.3 V vs. Ag/Ag⁺. Mechanism studies revealed that the enhanced performance is due to the increased CO₂ adsorption ability and the reduced binding energy for the formation of the COOH* intermediate, resulting from the introduction of m-Triz on the surface of the Ag catalyst. 

To investigate the effect of electrolytes on CO₂R, a DES-based non-aqueous electrolyte, 0.5 M 1-butyl-3-methylimidazolium chloride/ethylene glycol (BmimCl-EG) with a mole ratio of 1:2 in acetonitrile (AcN), was used as catholyte for CO₂R to CO over Ag. The results showed that the highest Faradaic efficiency could reach 100% within the potential range of -2.0 to -2.4 V vs. Ag/Ag⁺, which is 1.43 times higher than the maximum Faradaic efficiency achieved in 0.5 M KHCO₃. Additionally, the overpotential was reduced by 150 mV, and the current densities increased significantly across various applied potentials compared to the results obtained using 0.5 M BmimCl in AcN. In situ ATR-SEIRA measurements confirmed that EG, acting as a hydrogen bond donor, participated in the CO₂R process, thereby enhancing the reaction performance.