The primary cause of numerous environmental issues, such as increasing CO2 emissions, can potentially be addressed through electrochemical CO2 reduction (CO2R) to produce valuable carbon-based chemicals. Ionic liquids (ILs) have garnered significant attention as electrolytes and co-catalysts in promoting CO2R due to their unique advantages.
Among the potential products of CO2R, the so-called C1 products, which consist of only one carbon atom, such as CO, CH3OH, CH4, and syngas, are comparatively easier to achieve. In recent years, there has been considerable progress in the development of CO2R-to-C1 products, with numerous experimental studies and reviews contributing to this advancement.
Nevertheless, to the best of our knowledge, there has been no comprehensive work conducted to discuss and systematically compare the economic benefits of different C1 products (CO, CH3OH, CH4, H2/CO(1:1), and H2/CO(2:1)) in the context of IL-based electrolyte systems. Additionally, an analysis of the environmental impacts is lacking, which could provide guidance for the future commercialization of CO2R technology.
Therefore, a team of scientists has undertaken a comprehensive evaluation of the economic benefits and environmental influences of CO2R-to-C1 products based on IL-based electrolytes. They have summarized the research progress in this field, examined the state-of-the-art technology, and projected the potential for future advancements. Their findings have been published in the journal Industrial Chemistry & Materials.
Xiaoyan Ji, a professor at Luleå University of Technology, explained, “As CO2R with IL-based electrolytes has witnessed rapid development in the laboratory, it is crucial to gain a clear understanding of its commercial value when scaling it up to an industrial level.”
“In our review, we have summarized the experimental achievements of CO2R-to-C1 products using IL-based electrolytes, evaluated their performance from economic and environmental perspectives, considering both the current state-of-the-art technology and the anticipated improvements in the near future. Furthermore, we have identified the commercialization potential of these products and proposed strategies to enhance their performance and profitability in the future.”
CO2R stands out as one of the most promising methods for converting CO2 into value-added chemicals due to its mild conditions and controllability. Moreover, it can be powered by renewable energy sources such as solar, wind, and hydropower.
The performance of CO2R can be assessed based on three main parameters: current density, Faradaic efficiency (FE), and cell voltage. These parameters can be improved by designing and optimizing electrocatalysts and electrolytes.
ILs, with their adjustable structures and properties, wide electrochemical windows, and high electrical conductivities, offer the advantage of low overpotential and high current density, thereby enhancing the selectivity of CO2R products. Notably, ILs effectively inhibit the hydrogen evolution reaction (HER), which competes with CO2R.
“Among the studied C1 products, only CO demonstrates profitability, whereas the total production costs (TPC) of other products, particularly CH4 and H2/CO(2:1), are prohibitively high,” Ji highlighted.
The performance of CO2R for each product aligns with the observed phenomenon. CO exhibits remarkably high current density and Faradaic efficiency (FE) values of 182.2 mA cm-2 and 99.7%, respectively. Conversely, CH4 and H2/CO(2:1) display significantly lower current densities of 25.6 and 11.4 mA cm-2, respectively. However, with anticipated advancements in CO2R performance, CH3OH and H2/CO(1:1) hold the potential for profitability in the near future. On the other hand, it remains challenging for CH4 and H2/CO(2:1) to achieve profitability, even in the most ideal scenarios, partially due to their relatively low market prices. Notably, the production of CH4 necessitates the transfer of the highest number of electrons (8e-) among the studied C1 products.
Xiangping Zhang, a professor at the Institute of Process Engineering, Chinese Academy of Sciences, emphasized, “Compared to other pathways, CO2R-to-CH4 is the most environmentally friendly. However, when considering both economic and environmental aspects, CO emerges as the most attractive product. To realize the commercial value of other C1 products, further enhancements in CO2R performance or the development of more advanced electrolyzers are required.”
Furthermore, there are several avenues for future exploration of ILs in CO2R. First, the adjustable nature of ILs in terms of structure and properties offers unique advantages and opportunities for designing more efficient and suitable electrolytes for CO2R. Second, the capability of ILs to dissolve various solvents and electrolytes allows for integration with other components, thereby further improving the performance of CO2R. Third, the design and synthesis of cleaner ILs can be employed to mitigate the environmental impact associated with CO2R. Finally, besides serving as electrolytes, ILs can also act as co-catalysts or modifiers for catalysts, exhibiting superior performance.
“In this review, our primary objective is to provide readers with a comprehensive understanding of the commercial potential of CO2R-to-C1 products utilizing ILs as electrolytes. We present insights from both economic and environmental perspectives, considering the state-of-the-art technology as well as future scenarios,” said Ji.