Electrochemical CO₂ reduction with ionic liquids: review and evaluation

Abstract

The increasing CO₂ emission, as the chief culprit causing numerous environmental problems, could be addressed by the electrochemical CO₂ reduction (CO₂R) to the added-value carbon-based chemicals. Ionic liquids (ILs) as electrolytes and co-catalysts have been widely studied to promote CO₂R owing to their unique advantages. Among the potential products of CO₂R, those only containing one carbon atom, named C1 products, including CO, CH₃OH, CH₄, and syngas, are easier to achieve than others. In this study, we first summarized the research status on CO₂R to these C1 products, and then, the state-of-the-art experimental results were used to evaluate the economic potential and environmental impact. Considering the rapid development in CO₂R, future scenarios with better CO₂R performances were reasonably assumed to predict the future business for each product. Among the studied C1 products, the research focuses on CO, where satisfactory results have been achieved. The evaluation shows that producing CO via CO₂R is the only profitable route at present. CH₃OH and syngas of H₂/CO (1 : 1) as the targeted products can become profitable in the foreseen future. In addition, the life cycle assessment (LCA) was used to evaluate the environmental impact, showing that CO₂R to CH₄ is the most environmentally friendly pathway, followed by the syngas of H₂/CO (2 : 1) and CO, and the further improvement of the CO₂R performance can make all the studied C1 products more environmentally friendly. Overall, CO is the most promising product from both economic and environmental impact aspects.

Keywords: Electrochemical-CO₂-reduction; Ionic-liquids; C1-product; Economic-evaluation; Environmental-impact.

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Yangshuo Li

Yangshuo Li received her BE degree in Chemistry from Henan Normal University in 2018, and ME degree in Physical Chemistry from Henan Normal University in 2021. She is currently a Ph.D. candidate in Energy Science under the supervision of Prof. Xiaoyan Ji at Luleå University of Technology. Her doctoral research focuses on the electrochemical CO2 reduction to CO with ionic liquids as electrolytes.

Fangfang Li

Fangfang Li received her BE degree in Chemistry from Zhengzhou University in 2016, and ME degree in Chemical Engineering from University of Chinese Academy of Sciences in 2019. She is currently a Ph.D. candidate in energy science under the supervision of Prof. Xiaoyan Ji at Luleå University of Technology. Her doctoral research focuses on the electrochemical CO2 reduction to CH3OH with ionic liquids as electrolytes.

Xiaoyan Ji

Xiaoyan Ji got her Ph.D. degree in Chemical Engineering in 2000 with more than 25 years of experience in research and development in the area of Chemical Engineering and Energy Engineering. She has taken part in projects in research groups in China, Sweden, Germany, and the US. Her research work is from theoretical modeling to technology development, as well as process simulation and evaluation. Xiaoyan Ji started working at the division of Energy Science, Lulea University of Technology, in 2008. She created a research group on “Advanced fluid materials for energy applications”, where CO2 capture/separation, CO2 electrochemical conversion, advanced electrolytes for batteries, and lignin conversion coupling H2 production via electrochemical process are the focus.

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1. Introduction

On the one hand, the increasing anthropogenic CO₂ emissions due to the long and extensive burning of fossil fuels have caused severe environmental problems, such as the greenhouse effect leading to extreme weather events due to the rise of Earth's surface temperature.¹ On the other hand, CO₂ itself is an abundant carbon resource that can be converted to chemicals and fuels. Therefore, carbon capture and utilization via conversion (CCU) have become an important strategy to mitigate CO₂ emissions and produce carbon-based chemicals and fuels.^2–4 Several methods have been proposed to convert CO₂, including thermal,⁵ chemical,⁶ and bio-conversion⁷ as well as electrocatalytic,^8–10 photocatalytic,¹¹ and photoelectric¹² conversions and co-conversions. Among them, electrochemical CO₂ reduction (CO₂R) is among the most promising owing to the mild reaction conditions, as well as easy and flexible controllability. Also, its driving force, electricity, can be potentially integrated with renewable energy sources, for example, solar and wind power.^13–15

The performance of CO₂R is mainly characterized by the faradaic efficiency (FE), current density, and cell voltage, reflecting product selectivity, reaction rate, and energy usage.¹⁶ Therefore, higher FE and current density as well as lower cell voltage are being pursued in the performance of CO₂R, where the suitable electrocatalyst, electrolyte, electrolyzer, and applied potential, as well as reaction temperature and pressure, can all contribute to the performance. Currently, the research on electrocatalysts has focused on noble metals (Au, Ag, Pt),^17–22 transition metal disulfide compounds (MoS₂, MoSe₂, WS₂),^23–26 metal organic frames,²⁷ graphene-based synthetic materials,^28–30 and molecular and single-atom catalysts^31,32 for their desirable catalytic performances.³³ While in CO₂R, the electrolyte not only enriches the dissolved CO₂ (as the carbon source) but also provides protons for the reduction, so that their concentration, pH, and buffer capacity can affect the local reaction conditions, thereby a variety of products.^10,34–36 Thus, it is desirable to have an electrolyte having the ability to dissolve CO₂ and then stabilize it as the reaction intermediate. Previously, the aqueous solutions of sodium and potassium salts have been widely used as electrolytes, but their solubility of CO₂ is limited.^33,37–40 Recently, it has been discovered that ionic liquids (ILs) can greatly promote the performance of CO₂R because IL is a powerful CO₂ absorbent, and additionally, ILs can also activate CO₂ to facilitate the further conversion (co-catalyst).^41–45 ILs themselves are electrolytes, which can also be readily mixed with other electrolytes. Besides, ILs, with their tunable structures and properties, wide electrochemical windows, and high electrical conductivities, can provide a lower overpotential, a higher current density, and improved product selectivity for CO₂R.^46–49 Significantly, ILs can effectively inhibit the hydrogen evolution reaction (HER), which is a competitive reaction with CO₂R.

Developing IL-based CO₂R has been studied. It was found that, when ILs were immobilized into the cathode catalyst, the cell operating current was increased by a factor of two or more, and FE was enhanced by 20–30%, indicating that ILs could effectively promote CO₂R.⁴² Rosen et al.⁵⁰ were the first to report that the use of 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]) increased the selectivity for CO and lowered the overpotential. Since then, developing ILs as electrolytes have been intensively studied, and their efficiency has grown tremendously. Kumar et al.⁵¹ performed CO₂R to CO in the [BMIM][BF₄] electrolyte, showing an extremely low overpotential of 0.17 V over the metal-free carbon nanofiber electrode. Choi et al.⁵² demonstrated that the addition of [EMIM][BF₄] into an aprotic electrolyte could reduce the overpotential and enhance the kinetics of electron transfer for CO₂R, and the turnover frequency (TOF) in the IL-based systems was 4 times higher than without ILs. Min et al.⁵³ reported that the current density of syngas could reach up to 644.7 mA cm⁻² in the gas diffusion electrode (GDE) electrolyzer with the imidazolium-based ILs as the electrolytes. Furthermore, the product of CO₂R was changed into CO from syngas by replacing the C2–H of the imidazolium cation with methyl, and the current density could reach up to 528.3 mA cm⁻², which has already achieved the industrial standard, indicating that the IL-based electrolytes are efficient in GDE. All these studies indicate that ILs can play an important role in CO₂R.

Among the potential products that can be obtained from CO₂R with ILs, the ones containing just one carbon atom, named C1 products, including CO, CH₃OH, CH₄, or syngas, are easier to achieve. Much research work has already been conducted and even summarized in review articles.^46,54–57 However, we feel that the recent studies now lead to a rapidly improving performance call for a fresh update. Also, such constant improvements in technological development attract great attention and interest toward novel implementations. To understand the economic feasibility of CO₂R with ILs as electrolytes, Chang et al.⁵⁸ evaluated CO₂R to CO, pointing out that it could be competitive with fossil fuels when the current density reached 200 mA cm⁻² at the condition of 99% FE. Rumayor et al.⁵⁹ analyzed the competitiveness of CHOOH generation from CO₂R in comparison to traditional methods, concluding that the CO₂R pathway was more economically feasible. On the other hand, Jouny et al.⁶⁰ evaluated the commercial value for CO₂R without limiting the types of systems, including non-IL-based electrolytes, and products, including syngas, CO, CHOOH, CH₃OH, CH₄, C₂H₄, C₂H₅OH, and n-C₃H₈O by calculating the end-of-life net present value (NPV) using the state-of-the-art indicators in 2018. However, IL, as an efficient and green solvent that can replace toxic organic solvents and conventional aqueous carbonate solutions with limited CO₂ solubility, has always been sufficiently interested and concerned as novel electrolytes for CO₂R, and the research on IL-based electrolytes is extensively booming in recent years due to the unique advantages of ILs in CO₂R. To the best of our knowledge, no work has been conducted to discuss and systematically compare the economic benefits of different C1 products focusing on IL-based electrolyte systems, as well as analyzing environmental impacts.

Therefore, in this work, to evaluate and compare the economic potential and environmental impact of CO₂R to C1 with ILs as electrolytes, a thorough literature survey was conducted to update the research progress and provide state-of-the-art research achievements for different target C1 products, including CO, syngas, CH₄, and CH₃OH, which was performed in H-cell. Subsequently, the economic performance was evaluated, and the environmental impact was analyzed based on the state-of-the-art research achievement. Considering the rapid development in CO₂R, several future scenarios with improved FE and current density, as well as decreased cell voltage for each target C1 product, were created, and their performances from both economic and environmental aspects were predicted. Additionally, a sensitivity analysis was conducted to provide a comprehensive understanding of how the main parameters would affect TPC for each product. Finally, the traditional and IL-based electrolytes were compared from economic and environmental aspects.

2. IL-based electrochemical CO₂ reduction to C1 products

In general, aqueous solutions containing carbonate sodium and potassium salts are used as the electrolyte for CO₂R, but the product selectivity and current density are often unsatisfactory. To address these problems, IL-based electrolytes can be used to improve the absorption capacity of CO₂ and stabilize the intermediate, and reconstruct the electrode interface.^61–63 Herein, the achievements of CO₂R to CO, CH₃OH, CH₄, and syngas in IL-based electrolytes are summarized in the following subsections, and the best performance for each target product was identified for further evaluation.

2.1. CO as the target product

The electrochemical conversion of CO₂ to CO is the most promising one among the various CO₂R pathways due to the lower electron demand (2e⁻) for the formation of CO and the pivotal role of CO in the industry as the primary ingredient for the synthesis of hydrocarbons and alcohols.^64–66 Recently, relevant research is growing fast, and previous important research work has been summarized in review articles published in the past few years.⁶⁷ Therefore, we focused only on the latest research, reported mainly since 2020, as summarized in Fig. 1a.

Fig. 1 Performance, including FE and current density, of CO₂R to CO (a); CH₃OH (b); CH₄ (c); and syngas (d) in the IL-base electrolytes.

For the IL-based CO₂R to CO, Liang et al.⁶⁸ reported that the FE of CO and current density could be up to 93.1% and 122.0 mA cm⁻², respectively, in the 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆])/acetonitrile (MeCN) electrolyte. In the work of Oguma and Azumi,⁶⁹ 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM][EtSO₄]) was added into the 0.1 mol dm⁻³ K₂CO₃ aqueous solution for CO₂R on the Ag electrode, and observable improvements in the FE of CO and current density were obtained, owing to the high complexation ability of EMIM with CO₂ that was intensively adsorbed on the electrode surface. Guo et al.⁷⁰ studied CO₂R in 0.5 mol L⁻¹ (M) [BMIM][PF₆]/MeCN on the Cu–Co bimetallic electrode, and the FE of CO and the current density could reach 97.4% and 62.1 mA cm⁻², respectively. Zhang et al.⁷¹ designed a catalyst with ZIF-8 films on Zn foils (ZIF-8/Zn) to promote CO₂R, and FE_CO of 91.8% along with the current density of 12.6 mA cm⁻² was obtained at −1.9 V in the electrolyte with (30 wt% [BMIM][PF₆] + 65 wt% MeCN + 5 wt% water). Kunene and co-workers⁷² explored the CO₂R to CO in the MeCN/100 mmol L⁻¹ (mM) tetrabutylammonium hexafluorophosphate ([TBA][PF₆]) electrolyte with 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM][OTf]) as the additive on the soldering alloy Bi₅₀Sn₂₂Pb₂₈ as the anode, where the influences of [BMIM][Otf] concentrations (50, 100, and 200 mM) and the applied potentials (−1.85, −1.95, and −2.05 V) on CO₂R were investigated. The result showed that the FE of 95% and current density of 10.2 mA cm⁻² were obtained when the concentration of [BMIM][Otf] was 100 mM at −2.05 V. To make the CO₂R to CO more economical with ILs as the electrolytes, Ganesh⁷³ synthesized the low-cost highly pure 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]) using the inexpensively accessible raw materials (1-bromobutane (BB) and 1-methylimidazole (MI)). The 20 mM synthesized [BMIM][BF₄] was mixed with 0.1 M [TBA][PF₆] and MeCN and then used as the electrolyte to perform CO₂R on the Sn and MoSi₂ electrodes, obtaining FE of 91.2% and the current density of 27 mA cm⁻² at 622 mV potential.

The linear CO₂ being very thermodynamically stable and kinetically inert is difficult to be reduced, and the formation of CO˙⁻ radical in the first-step one-electron reduction of CO₂ is the major obstacle, which requires a high potential.⁷⁴ It was reported that the formation of the CO₂-imidazolium intermediate compound could help to reduce the overpotential of CO₂R. Ju and co-workers⁷⁵ discovered that the imidazolium ILs could be adsorbed on the electrode and form a film layer during CO₂R, which can help CO₂ to contact the catalyst and stabilize the generated CO₂˙⁻ after the CO molecule obtains an electron according to the comparison of six imidazolium-ILs, consisting of the same anion [BF₄]⁻ and different cations ([EMIM]⁺, [BMIM]⁺, 1-hexyl-3-methyl-imidazolium ([HMIM]⁺), 1-methyl-3-octyl-imidazolium ([OMIM]⁺) and 1,3-dimethyl-imidazolium ([DMIM]⁺)), with [TBA][BF₄] in propylene-carbonate (PC) as the electrolytes. Among them, [BMIM][BF₄] was identified as the most efficient electrolyte, which may be attributed to its suitable chain length at the N1-position of the imidazolium cation. Hu et al.⁷⁶ found that, in the system of N-octyltrimethyl ammonium 1,2,4-triazole ([N₁₁₁₈][TRIZ])/MeCN electrolyte, the linear CO₂ molecule could change into a bend because the extra electron was filled in the lowest unoccupied molecular orbital (LUMO) from [TRIZ]⁻. The formation of the [TRIZ–CO₂] complex was emphasized and demonstrated via Fourier transform infrared (FTIR), which could result in the high solubility of CO₂ and low energy barrier for CO₂ activation.

To confirm the role of imidazolium-2-carboxylate species, Ratschmeier and Braunschweig⁷⁷ detected the produced CO molecule adsorbed on the Pt electrode in the 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][NTf₂]) with 0.5 M H₂O electrolyte via the in situ vibrational sum-frequency generation (SFG). Also, the in operando Fourier transform infrared reflection absorption spectroscopy (FT-IRAS) was used to determine the formation of imidazolium-2-carboxylic acid species. The investigation revealed that the mechanism strongly depended on the types of ILs. For [BMIM][NTf₂], providing an active C2 position of the imidazolium ring, the [BMIM]-2-carboxylic acid species could be generated through the carbene intermediate. While for 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([BMMIM][NTf₂]) with protected C2 position by the methyl group and 1-butyl-1-methyl pyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMPyrr][NTf₂]) without the active C2 position (their structures were shown in Fig. 2), when 0.5 M H₂O was used as the electrolyte, the stabilization of CO₂˙⁻ was attributed to the formation of bicarbonate and the Coulomb interactions between CO₂˙⁻ and the IL-cations. Meanwhile, the anion can also effectively facilitate CO₂R to CO. For example, Hu and co-workers⁷⁶ developed a novel IL [N₁₁₁₈][TRIZ] and used it as the electrolyte to explore the effect on CO₂R, where the electrochemical methods, FTIR spectroscopy, and the density functional theory (DFT) were combined to analyze the mechanism. It showed that the 1,2,4-triazole ([TRIZ]) anion was equally effective for facilitating CO₂R to CO on the Ag electrode.

Fig. 2 The chemical structures of [BMIM]⁺, [BMMM]⁺, [BMPyrr]⁺, and [NTf₂]⁻.⁷⁷ Copyright 2021 American Chemical Society.

To overcome the high viscosity of ILs, adding H₂O or organic solvent is an efficient way, and their content plays an important role. In the work of Wang et al.,⁷⁸ CO₂R to CO was explored with various contents of [BMIM][PF₆] and H₂O in the MeCN electrolyte, where a series of porous zinc oxide nanosheets grafted with the hydroxyl groups on the carbon paper substrates were used as the catalyst. They found that the current density first raised and then dropped with increasing H₂O concentrations. This observation was caused by the appropriate electrostatic attraction between the anions and cations, availing the ion movement and charge transport and thereby promoting CO₂R. In this work, the best performance was obtained at a low overpotential of 340 mV when 5 wt% H₂O was added into 30 wt% [BMIM][PF₆]-based electrolytes, where the addition of H₂O might increase the electrolyte conductivity and the CO₂ solubility by changing the microstructure of the electrolyte. Similarly, Hu et al.⁷⁶ explored the influence of the H₂O contents on the current density, drawing analogical conclusions. With increasing water content from 0 to 7.5 wt%, the onset potential of CO₂R was changed from −1.93 to −1.75 V vs. Ag/Ag⁺, indicating that the addition of water reduced the initial reduction energy barrier.

Apart from the co-catalysis concept of ILs as the electrolyte, the coupling of electrocatalyst and IL-based electrolyte can be another important accelerant for CO₂R. To further provide a comprehensive insight into the synergistic effect of catalysts and IL-based electrolytes, Rudnev and co-workers⁷⁹ explored the performance of thirteen catalyst materials, including Pt, Ag, Bi, Sn, Mo, Zn, Pd, Cu, Au, Pb, Ni, Fe, and glassy carbon (GC), in three IL-based electrolytes ([BMIM][BF₄], [BMIM][NTf₂] and [BMPyrr][NTf₂]). It was found that the co-catalyst effect required balanced interactions between the IL and the electrode surface, depending on the chemical properties of both the IL and catalyst material. Only the proper combination of ILs and electrode materials could enhance the performance of CO₂R. Besides, IL-anions significantly influenced the CO₂R peak current density in the order of [BMIM][BF₄] ≤ [BMIM][NTf₂] ≪ [BMPyrr][NTf₂], being consistent with the decreasing trend of their viscosities. This observation indicates that low viscosity leads to a high diffusion rate and then a high current density. Among their studied cases, the best performance (99.7% FE of CO) was obtained at −1.84 V when Ag was used as the electrode in the [BMIM][BF₄] electrolyte. Zeng et al.⁸⁰ also found that the combination of a nanoporous Au film electrode with [EMIM][BF₄]/H₂O as the electrolyte could efficiently perform electrocatalytic CO₂ to CO with high selectivity of 92.5% at a low overpotential of 440 mV. In this work, the CO₂ reduction rate was strongly affected by the transport rate of CO₂ within the nanopores of the Au film. The adsorption of [EMIM][BF₄] on the nanopore, confirmed by X-ray photoelectron spectroscopy (XPS), led to the high solubility of CO₂, improving the driving force in the mass transfer of CO₂ and thereby promoting CO₂ conversion. They also found that the performance of CO₂R in [EMIM][BF₄]/H₂O was superior compared to that in 1 M NaHCO₃ aqueous electrolyte, indicating the important role of IL.

Additionally, the catalyst itself is of importance in CO₂R. The research group of Han⁸¹ conducted several studies on the catalysts. They found that the catalyst of the atomic anchored on the N-doped carbon (In_A/NC) could significantly promote CO₂R to CO in 0.5 M [BMIM][PF₆]/MeCN. As a result, the FE_CO of 97.2%, a total current density of 39.4 mA cm⁻², and a TOF of 40 000 h⁻¹ were obtained. Comparatively, FE_CO and current density decreased to 53.3% and 0.9 mA cm⁻², respectively, when the electrolyte was replaced by 0.5 M KHCO₃ aqueous solution, also indicating the indispensable role of [BMIM][PF₆]. Later, Han's group⁸² further improved the performance greatly by synthesizing a novel Cd single-atom catalyst (SACs) CdN₄S₁/CN, where 0.5 M [BMIM][PF₆]/MeCN was used as the electrolyte. The FE of CO could reach up to 99.7% along with the high current density of 182.2 mA cm⁻² and the TOF of 73 000 h⁻¹ at a low overpotential of 0.6 V. This is the most outstanding performance of CO₂R to CO in the H-type electrolysis cell so far. During this process, both the introduction of an axial coordination structure to the Cd SACs and the formation of [BMIM–CO₂]⁺ in [BMIM][PF₆]/MeCN electrolyte could reduce the reaction barrier of CO₂R and suppress the competitive HER in the aprotic solvent.

2.2. CH₃OH as the target product

CH₃OH as the important platform molecule with high energy density can be converted to different chemicals and fuels.^35,83 The formation of one CH₃OH via CO₂R needs a transfer of six electrons (6e⁻), making it more difficult to perform the reduction compared to CO₂R to CO.^56,57 Only a few related articles from Han's group reported improved performance by designing and synthesizing efficient electrocatalysts in the IL-based electrolytes. These achievements are summarized in Fig. 1b.

Initially, Sun et al.⁸⁴ searched for efficient IL-based electrolytes among [BMIM][BF₄], [BMIM][PF₆], [BMIM][ClO₄], and [BMIM][NTf₂] in MeCN for electrocatalytic CO₂ conversion to CH₃OH over a Mo–Bi bimetallic chalcogenide on the carbon paper (CP) (Mo–Bi BMC/CP) electrocatalyst. As a result, the highest selectivity of CH₃OH was obtained with an FE of 71.2% and a current density of 12.1 mA cm⁻² at −0.7 V (vs. the standard hydrogen electrode (SHE)) when [BMIM][BF₄] was the supporting electrolyte. On the contrary, CH₃OH was not detected in the tetrabutylammonium/tetraethylammonium salts as the supporting electrolytes, suggesting the important role of ILs in the electrolyte. Additionally, different concentrations of [BMIM][BF₄] led to different FEs of CH₃OH and current density, further indicating the vital role of the IL in CO₂R. Similarly, Yang et al.⁸⁵ reported that [BMIM][BF₄] was the most efficient supporting electrolyte in the MeCN/H₂O solution for CO₂R to CH₃OH among the [BMIM]-based ILs with various anions (PF₆⁻, NTf₂⁻, OAC⁻, NO₃⁻, and ClO₄⁻). Over the Cu_1.63Se electrode, the highest FE of CH₃OH and current density were 77.6% and 41.5 mA cm⁻², respectively.

Hereafter, Han's group further explored the electrocatalytic CO₂ conversion to CH₃OH in the [BMIM][BF₄] aqueous solution by designing and synthesizing efficient electrocatalysts. Lu et al.⁸⁶ synthesized a kind of bimetallic catalyst of Pd₈₃Cu₁₇ on CP as the electrode, which successfully enhanced the FE of CH₃OH and current density up to 80.0% and 31.8 mA cm⁻², respectively. Guo et al.⁸⁷ designed the atomically dispersed Sn site anchored on the defective CuO catalysts (Sn₁/V₀–CuO) with high conductivity in the [BMIM][BF₄]/H₂O electrolyte. The high selectivity of CH₃OH (FE of 88.6%) and current density (67.0 mA cm⁻²) were obtained. In the work of Li et al.,⁸⁸ the current density surpassed 100 mA cm⁻² for the first time with a value of 122.7 mA cm⁻² by developing the Ag, S–Cu₂O/Cu electrocatalyst combined with [BMIM][BF₄]/H₂O as the electrolyte. During this process, the S anion could regulate the morphology and electron structure of the electrocatalyst, making it more efficient for CO₂R to CH₃OH, and, at the same time, the Ag cation could suppress the competing HER. The synergistic effect of the Ag and S heteroatoms and the Cu₂O/Cu host greatly contributed to the dramatic enhancement of the current density.

Sun et al.⁸⁴ studied CO₂R to CH₃OH using Mo–Bi BMC as the electrocatalyst in the [BMIM][BF₄]/H₂O electrolyte. They proposed that, in this system, firstly, the [BMIM–CO₂]⁺ complex was quickly formed, reducing the reaction barrier. Furthermore, the synergistic effect of Mo and Bi atoms in Mo–Bi BCM/CP electrocatalyst greatly promoted the CO₂ reduction to CH₃OH. The Bi sites favored the formation of CO, and the Mo sites were for the generation of H₂, conducive to the further hydrogenation of CO to form CH₃OH. By using Sn₁/V₀–CuO as the catalyst, Guo et al.⁸⁷ found that the formation of Lewis acid–base interaction between the Sn single atom and the oxygen vacancy of CuO reduced the energy barrier for the dissociation of *COOH and conversion to *CO, which was confirmed via in situ X-ray absorption (XAS) spectra, in situ Raman spectra, and DFT. Subsequently, the formed *CO free radical was combined with Cu to generate *CHO, and the moderate binding energy of *CHO with the electrocatalysts promoted the production of *OCH₂ and further conversion to CH₃OH.

2.3. CH₄ as the target product

CH₄ is more difficult to produce via CO₂R compared to the other C1 products due to the high C–H bond energy (434 kJ mol⁻¹) and the 8e⁻-transfer demand.^{33,40,89–93} Therefore, it is a challenge to achieve high FE and current density. To the best of our knowledge, only three articles have reported satisfactory FE and current density in the IL-based electrolytes, as summarized in Fig. 1c.

Kang et al.⁹⁴ deposited Zn–1,3,5-benzenetricarboxylic acid metal–organic frameworks on carbon paper (Zn–BTC-MOFs/CP) as the electrocatalyst, and relatively high product selectivity (FE of 80.1%) and current density (3.1 mA cm⁻²) in the pure [BMIM][BF₄] electrolyte at a low overpotential of 0.25 V were obtained. In this work, the combinations of different catalysts (Au, Ag, Pt, Fe, Zn) and electrolytes ([TBA][BF₄]/dimethylformamide (DMF), [TBA][PF₆]/MeCN, [BMIM][BF₄]/MeCN) were conducted, and the desired performance was attributed to the optimal compatibility between Zn–BTC-MOFs/CP and [BMIM][BF₄] as Zn–BTC-MOFs/CP was synthesized in the imidazolium-based ILs.

Sun et al.⁹⁵ reported an efficient catalyst, the metal-free electrode N-doped graphene-like materials (NGMs), for CO₂R to CH₄ in the [BMIM][BF₄]/H₂O electrolyte. It was found that FE was improved from 20.8 to 93.5% by increasing the content of the doped N from 1.8 to 4.8%, implying the vital importance of the active-doped N for the selective production of CH₄. A possible pathway (shown in Fig. 3) was proposed as follows: (1) CO₂˙⁻ was firstly generated via CO₂ adsorbed on the active N sites of the electrode. (2) CO₂˙⁻ was coupled with the CO₂ molecule dissolved in the electrolyte and then reduced to CO₂–CO₂˙⁻. (3) after obtaining the second electron, the adsorbed CO was formed, i.e., CO_ads. (4) The formed CO_ads was converted to CHO_ads by accepting the proton. (5) the formed CHO_ads was further transformed to CH₄ after obtaining six electrons and protons. During this pathway, the strong interaction between CO_ads and electrons favored the hydrogenation of CO_ads to generate CHO_ads, an important intermediate of CO₂R to CH₄. On the other hand, [BMIM][BF₄]/H₂O as the electrolyte improved the solubility of CO₂, driving the transformation of CO₂ to CO₂˙⁻.

Fig. 3 Mechanism schematic diagram of CO₂ reduction to CH₄ at the NGM/CP electrode.⁹⁵ Copyright 2016 Royal Society of Chemistry.

In the same electrolyte, [BMIM][BF₄]/H₂O, by using MoTe₂ nanoflakes as the catalyst, Liu et al.⁹⁶ greatly improved the current density to 25.6 mA cm⁻² along with the FE of 83%. Considering both the current density and FE of CH₄, the performance in this work was the best. Here, the MoTe₂ nanosheets provided more active sites and stronger adsorption capacity of CO₂ compared to the bulk materials, being beneficial for the CO₂ conversion, and the Tafel slope was very close to the theoretical value (68 mV dec⁻¹), indicating the ultrathin MoTe₂ to be an ideal catalyst. Additionally, the DFT calculations also suggested that the interaction between the MoTe₂ nanosheets and CO₂ molecules might be formed during CO₂R through the formation of intermediates.

2.4. Syngas as the target product

Syngas compositing of H₂ and CO, as an important feedstock in industry, is mainly derived from coal or natural gas, which are non-renewable resources.^97–99 CO₂R potentially provided a more energy-efficient method to produce syngas with different ratios by adjusting the reaction environment, including the electrolyte and catalyst.^100–106 Among them, H₂/CO with a ratio of 1 : 1, i.e., H₂/CO (1 : 1) is suitable for the Fischer–Tropsch hydrocarbon synthesis, while H₂/CO (2 : 1) can be used to synthesize formaldehyde via hydroformylation.^107–111 Herein, H₂/CO (1 : 1) and H₂/CO (2 : 1) were mainly discussed. The related available results are summarized in Fig. 1d.

The structures and properties of the catalysts greatly affect the ratio of syngas. Xu et al.¹¹² compared the efficiency of CO₂R to syngas by using the MoSeS alloy as well as the MoSe₂ and MoS₂ monolayers in the [EMIM][BF₄]/H₂O electrolyte. The MoSeS alloy monolayer exhibited the best catalytic effect, leading to the current density of 43 mA cm⁻² along with the FE for CO of 45.2% at −1.15 V vs. the reversible hydrogen electrode (RHE). For comparison, the FE of CO for the MoS₂ and MoSe₂ catalysts was 16.6% and 30.5%, respectively. The high catalytic activities of the MoSeS alloy monolayer were attributed to its strong absorption ability of CO₂, high intrinsic conductivity, and low charge transfer resistance. Moreover, the desorption of CO* from the electrode was also an important influencing factor, and the MoSeS alloy monolayer exhibited low CO onset desorption temperature.

As reported, the synergistic effect of Au and Ni could be in favor of syngas generation under suitable conditions.^113–115 Inspired by this, Yang et al.¹¹⁶ designed a kind of catalyst where the Au nanowires (Au NWs) were grown on the porous nickel foam. They found that the porous nickel foam was mainly responsible for the electrocatalytic HER, while the Au NWs with large active surface areas and abundant edge sites were beneficial to the CO₂ conversion to CO. The longer and thinner the Au NW was, the higher selectivity of CO was obtained. On the contrary, when Au NWs were grown tightly on the nickel foam, the active sites would be covered, reducing the selectivity of CO and the overall catalytic performance.

Apart from the catalyst, the ratio of syngas also strongly depends on the applied potential and electrolytes. Yang et al.¹¹⁷ synthesized a nanoflower-like catalyst γ-In₂Se₃ (F-γ-In₂Se₃/CP) for CO₂R in different IL-based electrolytes. Within the applied potential (−1.8 to 2.3 V), the ratio of H₂/CO was changed from 1 : 24 to 3 : 1. For the electrolyte with ([BMIM][PF₆], H₂O, and MeCN), upon increasing the [BMIM][PF₆] content from 5 to 70 wt%, the ratio of H₂/CO was changed from 1 : 24 to 16 : 9; upon increasing the H₂O content from 0 to 20 wt%, the ratio was varied from 1 : 24 to 3 : 2. Significantly, H₂/CO (1 : 1) was obtained with a current density of 90.1 mA cm⁻² at the applied potential of −2.3 V in the electrolyte composed of (30 wt% [BMIM][PF₆] + 65 wt% MeCN + 5 wt% H₂O).

Similarly, Yang and co-workers¹¹⁶ showed that the addition of H₂O into the electrolyte composed of [TBA]Br and DMF would greatly affect the ratio of syngas. This can be explained as follows. Without the addition of water, DMF could be oxidized to reduce protons in the anode chamber, thereby producing abundant H₂. After the addition of H₂O, the oxygenolysis of DMF could be suppressed, stabilizing the DMF. Besides, the addition of H₂O could also generate proton-coupled electron transfer (PCET) in favor of CO₂ activation, and enhance HER, which would have a great influence on the ratio of H₂/CO. The applied potential was also found to take an important role. H₂/CO (2 : 1) and H₂/CO (1 : 1) were obtained at −1.6 and −1.8 V, respectively, when the concentration of H₂O was 1 M. Qin et al.¹¹⁸ also tuned the ratio of H₂/CO from 0.15 : 1 to 4 : 1 by controlling the applied potential in the [BMIM][PF₆]/MeCN electrolyte, where H₂/CO (1 : 1) was obtained with the current density of 65.6 mA cm⁻² at −1.395 V vs. RHE.

2.5. Identification of the state-of-the-art research results

According to the reviews, when CO, CH₄, and syngas were chosen as the target products, their state-of-the-art research results can be identified according to their FE and current density. The corresponding results, together with the cell voltages, are listed in Table 1. While for CO₂R to CH₃OH, there were two potential cases, of which one is with an FE of 88.6% along with a current density of 67.0 mA cm⁻², the other showed a current density of 122.7 mA cm⁻² along with an FE of 67.4%. Thus, a preliminary study on economic benefit was conducted (the detailed comparison can be found in the ESI†), showing the case with the FE of 88.6% along with the current density of 67.0 mA cm⁻² as included in Table 1 was more promising and thus was chosen for further study.

Table 1 The best available performance of CO₂R to CO, CH₃OH, CH₄, and syngas

	FE (%)	Current density (mA cm^−2)	Cell voltage (V)
a The value was calculated on the basis of ref. 82 and experimental data; b The value was assumed based on ref. 96, 116 and 117.
CO	99.7	182.2	−3.29^a
CH3OH	88.6	67	−3.28
CH4	83	25.6	−1.37^b
Syngas (H2/CO (1 : 1))	47 (CO)	90.1 (CO)	−3.14^b
Syngas (H2/CO (2 : 1))	33.1 (CO)	11.4 (CO)	−2.19^b

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According to the study conducted, the recent research is more on CO, followed by methanol, syngas, and methane (Fig. 1). The performance of CO₂R to CO is much better compared to targeting other C1 products (Table 1), especially, the current density for CO₂R to CO is approaching 200 mA cm⁻². However, for CO₂R to CH₄, the FE value is lower than 85%, and the current density is only 25.6 mA cm⁻², calling for more research concerns. For the syngas of H₂/CO (2 : 1), the current density is the lowest among the studied C1 product, which is as low as 11.4 mA cm⁻².

IL-based electrolytes have been developed for CO₂ to C1. Currently, the ILs with imidazolium cation are the most commonly used electrolytes and are usually beneficial to the improvement of CO₂R performance through the formation of the CO₂-imidazolium intermediate compound, which can help to reduce the overpotential of CO₂R. The viscosity, conductivity, and interaction between cations and anions of ILs can all influence the efficiency, which can be optimized by adding water or organic solvents, changing the carbon chain length of the imidazole cation, and replacing the suitable anions. Regrettably, most studied IL-based electrolytes are concentrated on the imidazolium-based ones, and the ILs other than those of imidazolium-based as well as the effect of IL-anions have not been studied sufficiently, calling for more research. Also, the mechanism of ILs as the electrolyte in CO₂R should be deeply explored, and novel, clean, and highly efficient IL-based electrolytes with functional cations and anions need to be developed to improve the performance of CO₂R.

3. Evaluation of IL-based electrochemical CO₂ reduction to C1 products

Both, economic analysis and environmental impact depend on the performance of CO₂R and other related processes, such as the separation to obtain pure products. In this part, the identified parameters listed in Table 1 were used for evaluating CO₂R, and the cost estimation was based on the specific process for a different target product, and then the energy demand and environmental impact were further estimated for comparison and discussion.

3.1. Processes

As shown in Fig. 4, the whole process for all cases can be divided into two parts, namely, CO₂R and product separation, which can be liquid or gas separation, depending on the target products.

Fig. 4 The flow diagram of CO₂R to CH₃OH (a) and gaseous products (b).

For CO₂R, a CO₂ gas stream was used as the feedstock and injected into the cathode chamber for the reduction to obtain the products. The unit of CO₂R was assumed as a black-box model since it is still at the bench scale, in which the main and parasitic reaction equations are listed in Table 2. In general, by-products can be generated. In this work, when CH₃OH was targeted, H₂ was considered to be the only by-product. On the contrary, the production of H₂ was ignored due to the extremely high CO selectivity when CO was the targeted product. For the case of CH₄ production via CO₂R, H₂, and CO were considered as the by-products, while no other by-products were considered. As for syngas comprising CO and H₂, there were no other gaseous and liquid products generated. For all the cases, oxygen evolution took place in the anode chamber.

Table 2 Reactions during CO₂R

Products	Cathode	Anode
CO	CO2 + H^+ + 2e → CO + H2O	2H2O − 4e^− → O2 + 4H^+
	2H^+ + 2e → H2
CH3OH	CO2 + 6H^+ + 6e → CH3OH + H2O
	2H^+ + 2e → H2
CH4	CO2 + 8H^+ + 8e → CH4 + 2H2O
	CO2 + H^+ + 2e → CO + H2O
	2H^+ + 2e → H2
Syngas	CO2 + H^+ + 2e → CO + H2O
	2H^+ + 2e → H2

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After CO₂R, the separation unit was followed for different purposes. When the gaseous product (CO, CH₄, or syngas) was obtained as the products from CO₂R, the unreacted CO₂ was separated from the targeted product with pressure swing adsorption (PSA) and returned to the cathode compartment; while in terms of CH₃OH as the targeted product, an additional unit of distillation was used to separate the liquid product (CH₃OH) from the electrolyte. Notably, the further purification of CH₄, i.e., the removal of the small amounts of H₂ and CO was excluded as H₂ and CO could also be considered as energy gases or reducing agents. For all the cases, it was assumed that the (regenerated) electrolyte was recycled back to the electrolyzer without any waste and loss.

3.2. Scenarios of CO₂R

CO₂R is a relatively new technology, which has been investigated intensively with great and rapid progress, as summarized in section 2. It is expected that more progress will be achieved in the coming years. To evaluate the process fairly, in this work, both the current status and future potential cases were considered, i.e., one base case using the identified parameters listed in Table 1, which are also summarized in Table 3, and three future cases with further development of CO₂R, e.g., cell voltage, current density, and FE (shown in Table 3), which was based on the development in recent two years as well as the performance of CO₂R with other electrolytes.^119,120 The other used parameters and assumptions are listed in Table S1.†

Table 3 The parameters used in the base and future cases

	Base case	Case 1	Case 2	Case 3
CO as the target product
Current density (mA cm^−2)	182.2	200	400	600
FE (%)	99.7	99.7	99.7	99.7
Cell voltage (V)	3.29	3	2.5	2
CH3OH as the target product
Current density (mA cm^−2)	67	200	400	600
FE (%)	88.6	90	95	99
Cell voltage (V)	3.28	3	2.5	1.5
CH4 as the target product
Current density (mA cm^−2)	25.6	200	400	600
FE (%)	83	90	95	99
Cell voltage (V)	1.37	1.3	1.25	1.2
Syngas (H2/CO (1 : 1)) as the target product
Current density (mA cm^−2), CO	90.1	200	400	600
FE (%), CO	47	47	47	47
Cell voltage (V)	3.14	2.5	2	1.5
Syngas (H2/CO (2 : 1)) as target product
Current density (mA cm^−2), CO	11.4	200	400	600
FE (%), CO	33.1	33.1	33.1	33.1
Cell voltage (V)	2.19	2	1.5	1.34

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3.3. Economic analysis

The total production cost (TPC) was the summation of the annual capital cost (ACC) and the total operating cost (TOC),¹²¹ which was used as the criterion to evaluate the economic benefit of CO₂R by comparing it with the current market price for each case.

3.3.1 Annual capital cost. ACC was related to the total capital cost (TCC), interest rate (i), and project lifetime (L). In this work, ACC was estimated according to eqn (1).⁵⁸ The components for calculating TCC are listed in Table 4.

Table 4 The components of TCC and TOC

Total capital cost (TCC)		Total operating cost (TOC)
Direct cost (DC)^60	Percentage of DC (%)	Variable operating cost value (VOC)^58
Electrolyzer cost	65	CO2 feed cost	The amount of CO2 consumed × price
BoP cost	35	IL make-up	The amount of IL make-up × price
Indirect cost (IC)^122	Percentage of DC (%)	Organic solvent make-up	The amount of organic solvent make-up × price
Contingency	15	H2O cost	The amount of H2O consumed × price
Site preparation	2	Electricity cost	The power demand × electricity price
Engineering and design	8
Up-front permitting	15
	Percentage of FCC^a (%)	Fixed operating cost (FOC)	Percentage of DC (%)
Working capital	5	Operation and maintenance	3.2
PSA	Calculated based on Table S2†
Distillation	Calculated based on Table S3†	PSA^123	Calculated based on Table S2†
Initial solvent cost	Initial input quantity × price	Distillation^124	Calculated based on Table S3†
Initial catalyst cost	Initial input quantity × price
TCC	Sum of the above costs	TOC	Sum of the above costs

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3.3.2 Total capital cost. The total capital cost briefly includes two parts: the CO₂R electrolyzer and other auxiliary equipment.⁵⁸ For the CO₂R electrolyzer, no industrial scale has been implemented, making it difficult to determine its capital cost directly. Since CO₂R is similar to water electrolysis, the main components of the water electrolysis electrolyzer were used as the reference for those in the CO₂R electrolyzer. Currently, there are two types of electrolyzers for commercial water electrolysis, i.e., alkaline and proton exchange membrane (PEM) electrolysis, where the current densities are usually <1 and 1–2 A cm⁻², respectively.^125,126 Given that the highest current density is below 1 A cm⁻², similar to the previous work,⁵⁸ in this work, the capital cost of the CO₂R electrolyzer was calculated according to the H2A model for an alkaline water electrolyzer from the US Department of Energy (DOE).^60,127 The total required electrolyzer area (TEA), as an important parameter, can be calculated using the following eqn (2) and (3).⁵⁸where m_p and M_p are the production rate and molar mass of the product, respectively, FE_p and z_p are the faradic efficiency (FE) and demanded electrons number, respectively, for each product, F is the faradic's constant (96 485 C mol⁻¹), and I and j are the total current and current density through the electrolysis, respectively.

The equipment costs for PSA and distillation were calculated on the basis of ref. 123 and 124 according to eqn (4).¹²⁸

where C and S are the investment cost and the scaling size of the PSA/distillation unit, respectively, while C_ref and S_ref correspond to the data from references, and n is the scale factor. The chemical engineering plant cost index (CEPCI) was used to extrapolate the cost estimations to the price level of 2022.

The expenditure of the initial electrolyte and cathode catalyst was considered in the capital cost, which was independently calculated by multiplying the univalence of the item and its mass consumption. The main parameters and the counting process are described in the ESI.†

3.3.3 Total operating cost. The total operating cost (TOC) consists of variable and fixed operating costs. The components for calculating TOC are listed in Table 4. Here, we considered that the cost for operation and maintenance, as the only fixed operating cost, was 3.2% of the direct cost (DC). The variable operating cost was from materials, such as the CO₂ feed, electrolyte, H₂O, and electricity. Our preliminary study showed that the electricity price was a major contributor, which was calculated based on the basis of the demanded power (P) and electricity price in this work. The demanded power can be obtained by the following eqn (5).⁵⁸where V is the cell voltage.

Additionally, the operating costs for PAS and distillation were calculated independently, as shown in the ESI.†

3.3.4 Results and discussion on economic analysis. The values of TPC of CO, CH₃OH, CH₄, and syngas were calculated under the condition of the base case. The estimation results are shown in Fig. 5a, where the market price for each product is also illustrated as a “box”, i.e., in a range, according to the literature survey combined with the further explanation as follows.^{58,60,129–133} For the syngas, both its TPC and the market price were assessed via those of CO in the syngas, and the upper limit value represents the syngas from the basic oxygen furnace gas (BOFG) and the low limit value was for the syngas from corn stover.¹²⁹

Fig. 5 (a) TPC; (b) capital cost and (c) operating cost for CO₂R to CO, CH₃OH, CH₄, and syngas (H₂/CO (1 : 1) and H₂/CO (2 : 1)) in the base case; (d) TPC for each product in the future cases.

Among all the studied products, CO was found to be the only profitable product for CO₂R with ILs as electrolytes at the present status. While for other products, their TPCs were still too high to be profitable, and especially for CH₄ and H₂/CO (2 : 1), their TPCs were 4.09 and 2.99 € per kg, respectively, far away from the market prices (0.18–0.35 and 0.03–0.54 € per kg, respectively). This observation was, on the one hand, in line with the achievement of CO₂R, where the current density and FE for CO were already up to 182.2 mA cm⁻² and 99.7%, respectively, while for CH₄ and H₂/CO (2 : 1), their current densities were as low as 25.6 and 11.4 mA cm⁻², respectively, together with low FE (Table 1). On the other hand, the relatively low market price of CH₄ and H₂/CO (2 : 1) can be another reason, which was further discussed later in this work.

To further analyze, the detailed capital cost is shown in Fig. 5b. As we can see, the TCC of CO was quite low, followed by H₂/CO (1 : 1) and CH₃OH, while those of CH₄ and H₂/CO (1 : 2) were much higher. Compared to CO, the BoP and stack costs for CH₃OH, CH₄, H₂/CO (1 : 1) and H₂/CO (2 : 1) were much higher, owing to the lower current density and thus higher electrolyzer area. The high costs of BoP and stack lead to high indirect costs. The summations of (indirect, BoP, and stack costs) for H₂/CO (1 : 1), CH₃OH, CH₄, and H₂/CO (2 : 1) contribute to 67.6, 87.0, 91.7, and 91.8% in TCC, respectively, and this trend is opposite to that of their current densities in CO₂R, further indicating the vital role of the current density on TCC. As the current densities for products other than CO are still low, it is important to further develop CO₂R to improve the performance and then make it possible to achieve desirable results in the near future.

According to the detailed operating cost shown in Fig. 5c, the most intensive part was the electricity usage, which was closely linked to the cell voltage and FE. The yearly (operation and maintenance) cost was also the main expenditure for CH₄ and H₂/CO (1 : 1) due to the high investment cost reflected by the direct capital cost. As to CH₃OH, the capital cost for the distillation was also the obvious part due to the low concentration of CH₃OH (30 wt%) in the electrolyte that requires high energy demand for its separation.¹³⁴

As discussed above, the performance of CO₂R, including current density, cell voltage, and FE, had a great influence on TPC. This implied that the development status of CO₂R was of importance in cost estimation. According to section 2, the development of CO₂R was vital all these years, and performance improvement was rapid, making it essential to predict the cost based on the results that can be achieved in the near future. Hence, in this part, three future cases for each product, along with the improvement of current density and FE as well as the diminishing of cell voltage were assumed, according to the research results in recent years and those for the CO₂R using electrolytes other than ILs.^119,120 The results are shown in Fig. 5d.

Overall, TPCs evidently declined with the hypothetical augmented performance of CO₂R. For CO, TPC was decreased to 0.32 € per kg, approaching half of the low limit market price (0.5 € per kg) when the current density was enhanced to 600 mA cm⁻² and the cell voltage descended to 2 V, manifesting the perspective to set CO as the target product in CO₂R. As for CH₃OH, it can be profitable when the current density and FE reach 600 mA cm⁻² and 99%, respectively, and the cell voltage drops to 1.5 V. For the syngas of H₂/CO (1 : 1), under the situation of case 3, its TPC would be lower to the price for that from the corn stover (0.54 € per kg). However, for CH₄ and H₂/CO (2 : 1), their TPCs were still much higher than the market price even though with a dramatic improvement in their performance of CO₂R within the created scenarios.

To find out how the CH₄ and syngas (H₂/CO (2 : 1)) could be profitable, the current density of 1000 mA cm⁻² (the industrial performance of PEM for commercial water electrolysis),^125,135 FE of 100%, and the cell voltage of 1.06 V for CH₄ and 1.34 V for syngas (the standard cell voltage)¹³⁶ were assumed, and the corresponding TPCs were estimated. As a result, the TPC of H₂/CO (2 : 1) just reached the upper limit market price, but still, it remains unprofitable for CH₄. This observation suggested that CH₄ as the target product was an undesirable path of CO₂R. There may be two reasons for the unprofitable of CO₂R to CH₄. One is the low market price, and the other is the highest electron demand (8e⁻) for the formation of CH₄ among the different paths of CO₂R to C1 products.^{9,38,89,95,96}

3.3.5 Sensitivity analysis and discussion. To obtain a comprehensive insight into how the key parameters affect the TPCs of C1 products generated from CO₂R, sensitivity analyses were conducted under two assumed scenarios (better and worse cases) based on the base case. The parameters, including current density, FE, cell voltage, CO₂ price, electricity price, and stack price, were considered. Compared to the base case, all the parameters varied up and down by 20% in the better and worse cases, respectively. In the calculation, one parameter changed while the other parameters remained consistent with the base case. It should be pointed out that FE was set as 100% in the better case when CO and CH₃OH were the target products because there was only a small space for further improvement. For syngases of H₂/CO (1 : 1) and H₂/CO (2 : 1) as the target products, FE was not considered due to the fixed ratio.

In Fig. 6, the results of TPC for each product under the base, better and worse cases are exhibited via orange, green, and purple bars, respectively. Overall, FE was the most influential factor, while the effect of CO₂ price can be negligible for all the products. The effect degrees of current density and cell voltage on the TPC of each product depended on the performance of CO₂R itself. For example, when the current density improved by 20%, the TPCs of CO (the current density of 182.2 mA cm⁻² in the base case) and syngas of H₂/CO (2 : 1) (the current density of 11.4 mA cm⁻² in the base case) decreased by 1.6 and 12.3%, respectively. Similarly, when the cell voltage decreased by 20%, the TPCs of CO (the cell voltage of 3.29 V in the base case) and syngas of H₂/CO (2 : 1) (the cell voltage of 2.19 V in the base case) decreased by 13.3 and 4.2%, respectively. These results suggest that the slight change in the current density and cell voltage can lead to an obvious variation in TPCs when the performance of CO₂R itself is relatively poor. Additionally, the 20% fluctuation in electricity and stack prices can cause an obvious change in TPC, as shown in Fig. 6 for all the studied products.

Fig. 6 Sensitivity analysis for CO₂R to C1 products under better and worse cases with different parameters, (a) CO; (b) CH₃OH; (c) CH₄; (d) syngas of H₂/CO (1 : 1) and (e) syngas of H₂/CO (2 : 1).

3.4. Environment assessment

The life cycle assessment (LCA) was used to assess the environmental impacts of “CO₂R to product” based on the ISO 14040/14044 framework, and the global warming impact (GWI) was selected as the main indicator.¹³⁴ Herein, the total amount of CO₂ emitted from the CO₂R process was evaluated via the cradle-to-gate LCA method to account for all sources of CO₂ emissions in each unit. Finally, the amounts of CO₂ emissions were obtained for the assessment. It should be noted that the environmental burdens of by-products (such as H₂ and O₂) were also considered. The system boundary is shown in Fig. 7. GWI was calculated based on the electricity generation intensity and electrical energy required in each unit. The calculation process is described in the ESI.†

Fig. 7 System boundary for the CO₂R to CH₃OH (a) and gaseous products (b).

As described in the above paragraph, the calculation of GWI was based on electricity, i.e., LCA strongly depends on energy usage.^137–139 Here the energy demands, including CO₂R, BoP, and PSA units and the additional distillation unit for CH₃OH, for producing one kg of CO, CH₃OH, CH₄, and syngas were calculated. The results are depicted in Fig. 8a. Obviously, the electricity usage of the CO₂R unit was the major part for all the products, accounting for more than 80% when CO, CH₄, or syngas was the product and more than 50% for CH₃OH as the target product. Different from other products, the production of CH₃OH requires separating the liquid product CH₃OH from the electrolyte, and the corresponding energy demand was another main contributor. Comparatively, the energy usages for BoP and gas separation were insignificant for all the products.

Fig. 8 The demanded energy (a) and GWI breakdown (b) for each product under the base case; as well as net GWI (c) for each product under the base and future cases.

Overall, the energy demand for producing one kg-CH₃OH was higher than that of CO or syngas, which was ascribed to the additional energy usage from the distillation unit due to its low concentration (30 wt%).¹³⁴ For CH₄, the total energy usage was also higher than that for CO and syngas, and the corresponding energy demand of the CO₂R unit was higher than that for CH₃OH, which was attributed to its suboptimal performance, such as low current density, unsatisfactory FE, and high cell voltage.

The CO₂ emissions per kW h, containing the CO₂ footprint of all the co-product credits, energy usage, and raw materials, were calculated to appraise their GWI, thereby, their environmental impact, owing to its tight relationship with the global warming potential. Noteworthily, in this evaluation process, the GWI of CO₂ feedstock was calculated based on the carbon footprint of −0.5 kg-CO₂e per kg-captured-CO₂ taken from the literature.¹⁴⁰Fig. 8b shows the GWI breakdown for the five products in CO₂ feedstock, CO₂R, and separation under the base case, where the solid circle represents the net GWI. As indicated in Fig. 8b, the GWI for producing CH₃OH was the highest (0.88 kg-CO₂e per kW h) among the studied products, which was also higher than that of the coal-to-CH₃OH process (0.47 kg-CO₂e per kW h) due to the intensive energy usages for CO₂R and liquid-separation. The GWI of CH₄ was the lowest (0.27 kg-CO₂e per kW h) among the five products, which was also comparable to that of the thermochemical CO₂ conversion (0.19–0.32 kg-CO₂e per kW h).^134,141 This result suggested that CH₄ was the most suitable product from the environment-friendly aspect. For the syngas of H₂/CO (2 : 1), the GWI (0.29 kg-CO₂e per kW h) of the CO₂R route was higher than that of the traditional method (0.14 kg-CO₂e per kW h).¹⁴² As for CO and H₂/CO (1 : 1), to the best of our knowledge, no GWI value has been reported. Since the GWI of CO (0.30 kg-CO₂e per kW h) was similar to that of CH₄ and syngas of H₂/CO (2 : 1), CO was a more attractive product than CH₃OH and H₂/CO (1 : 1), indicating that the path of CO₂R to CO was also beneficial for the environment.

For the future cases, with the hypothetically improved CO₂R performance, the net GWI was distinctly decreased, as shown in Fig. 8c. For example, for CH₄, when the FE and current density were increased to 99% and 600 mA cm⁻², respectively, as well as the cell voltage was decreased to 1.2 V, the net GEI of CH₄ was sharply declined from 0.27 to 0.17 kg-CO₂e per kW h, which was lower than that of thermochemical CO₂ conversion (0.19 kg-CO₂e per kW h). When CH₃OH and H₂/CO (2 : 1) were set as the target products, under the most desirable case (case 3), the net GWI of CO₂R was almost comparable to the traditional method. The net GWIs of the CO₂R to CO and H₂/CO (1 : 1) were also greatly decreased from 0.3 and 0.44 kg-CO₂e per kW h to 0.08 and 0.14 kg-CO₂e per kW h, respectively. Therefore, CO₂R is a promising path to reaching environment-friendly requirements.

Combining the environmental assessment with the economic analysis, CO₂R to CO is a desirable pathway for CO₂R, even in the current situation. In the future, with the further improvement of CO₂R, CH₃OH and H₂/CO (1 : 1) will be desirable routes. However, for CH₄ and H₂/CO (2 : 1), the unfavorable economic results of CO₂R make it challenging to compete with other routes. Adding H₂ produced from other routes to H₂/CO (1 : 1) and thus forming H₂/CO (2 : 1) can be an alternative way, which will be studied in our future work.

This work summarized and evaluated the CO₂R using IL-based electrolytes. There has been much research work conducted on other systems using electrolytes other than IL-based, and the performance can be better than that with ILs listed in this work. For example, for CO, the current density was over 300 mA cm⁻².^119,120 Such results further indicate the prospectives of CO₂R and the reasonability of the assumed parameters for the future case. We expect that the development of CO₂R, together with ILs, will greatly improve the performance in the foreseen future.

It should also be pointed out that for future cases (cases 1 to 3), in this work, only the parameters linked to the CO₂R performance were adjusted, while in principle, other parameters, such as CO₂ price, electricity, and product price also need to be considered. However, based on the sensitivity analysis, the CO₂ price has a limited influence on the TPC for all studied products. Given the unstable variation tendency of electricity prices last year and other uncertainties linked to war and politics, it is difficult to make reasonable assumptions about electricity prices in the future. Similarly, reasonably assuming future product prices is another puzzle, which involves a deep knowledge of different industrial technologies. Therefore, it is reasonable to fix the parameters as the base case for the parameters of CO₂ price, electricity, and product price in future cases.

Additionally, industrialization is mandatory for CO₂R to achieve commercial value. The current density and product selectivity are required to be over 200 mA cm⁻² and 90%, respectively, for industrial applications. This is in line with the results obtained from the section on economic analysis. Therefore, except for CO, the performance of CO₂R to other products needs to be greatly improved. Besides, research on the stability of CO₂R systems at the industrial level should be performed in future studies.¹⁴³ Meanwhile, from the sensitivity analysis, the enhancement of CO₂R performance, including current density, FE, and cell voltage, are the keys to reducing TPC, especially for CH₃OH and syngas of H₂/CO (1 : 1). While it is difficult for CH₄ and syngas H₂/CO (2 : 1) to be profitable via only improving the performance of CO₂R due to the low energy efficiency. Developing a novel electrolyzer might be a choice for CH₄ to be profitable. As to syngas H₂/CO (2 : 1), integrating CO₂R with commercial hydrogen production may maximize its economic benefits.

4. Comparison of traditional and ionic liquids-based electrolytes

The electrolyte is a vital consideration apart from the extensively studied electrocatalysts to achieve the satisfactory performance of CO₂R via controlling reduction surrounding at the interface between electrocatalyst and electrolyte, dissolving the reactant CO₂, and stabilizing the reaction intermediates.¹⁰ Traditional electrolytes, including the aqueous of sodium or potassium salts and organic solvents (e.g., MeCN, DMF, DMSO, and acetone), have been widely used in CO₂R. Potassium or sodium solutions have good electrical conductivity while a fairly limited ability to dissolve CO₂ (<34 mM under the standard condition).¹⁴⁴ Besides, the undesirable HER side reaction is easier to occur in these aqueous solutions. Normally, the solubility of CO₂ in the organic solvent is higher than that in the aqueous electrolyte solutions. For example, the solubility of CO₂ in MeCN (270 mM) is approximately 9 times as high as that in aqueous electrolyte solutions.¹⁴⁵ However, the current density of CO₂R with organic solvents is low, although they can effectively inhibit HER, which limits their further large-scale application.¹⁴⁶ IL-based electrolytes with high CO₂ solubility, as discussed above, not only have unique properties in promoting the performance of CO₂R but also can be the activator of CO₂ and stabilizer of CO₂˙⁻, which have been increasingly studied and developed in these years, as summarized in this work and other review articles.^{13,26,42,46,67} Nevertheless, when IL-based electrolytes are used to replace traditional electrolytes, the economic (cost, stability) and environmental influence are usually questioned. Therefore, traditional and IL-based electrolytes were further analyzed and compared from economic and environmental aspects.

The market price of traditional electrolytes is indeed lower than that of ILs, which is about 3400, 2876, and 205 € per ton to MeCN,¹⁴⁷ DMF,¹⁴⁸ and NaCO₃,¹⁴⁹ respectively, while it is 5800 € per ton to ILs.⁵⁸ However, when CO is set as the target product of CO₂R, the proportions of IL cost are as low as 0.3 and 0.02% in TCC and TOC, respectively. This situation is similar to the other products, suggesting that the market price of ILs will not hamper the use of ILs as electrolytes in CO₂R. Additionally, the long-term stability of electrolytes is beneficial for cost saving. For industrialization, the stability of the CO₂R system should reach at least 80 000 h.¹⁴³ Regrettably, there is no study on the stability of electrolytes at the industrial level. While CO₂R with IL-based electrolytes shows comparable stability to that with traditional electrolytes in a large number of studies. Moreover, Yuan et al.¹⁵⁰ found that the CO selectivity in the IL-based electrolytes is 51.3% higher than that in 0.1 M KHCO₃ aqueous electrolytes after 10 h of continuous operation in an upscaling modified H-type cell, suggesting the higher stability of IL-based electrolytes. Therefore, IL-based electrolyte is not inferior to traditional electrolytes from the economic aspects.

As to the environment, ILs are considered “green solvents” because of their negligible volatility and flammability as well as high stability. However, the statement on “green solvents” for ILs has been questioned in recent years, because it lacks a comprehensive understanding of their toxicity and biodegradability. Given this, Costa and co-workers¹⁵¹ summarized and updated the relevant data on the toxicity and (bio)degradability of ILs. This work showed that the commonly used ILs with N-substitution cations or fluoride anions were resistant to biodegradation, which, however, can be improved through functionalizing the imidazole cations. Bystrzanowska et al.¹⁵² evaluated the greenness of more than 300 commercially available ILs and ranked them with organic solvents from the toxicity to organisms, biodegradability, hazard statements, and precautionary measures when handling them. As a result, the most commonly used ILs, including 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][Tf₂N]), [Bmim][BF₄], and [Bmim]Br, were ranked between polar and nonpolar molecular solvents in terms of greenness. Comprehensively, ILs appear to be more resistant to biodegradation and toxicity than traditional electrolytes. Nevertheless, traditional electrolytes, such as DMF and MeCN, can cause direct harm to human beings through touch and breath.¹⁵³ On the contrary, ILs have little impact on researchers, especially under normal pressure and temperature as those operated within CO₂R. Moreover, ILs as the electrolyte in CO₂R are usually used in small quantities and can be recycled. Therefore, devoting more efforts to designing and synthesizing novel effective ILs with low toxicity and high biodegradability as electrolytes are necessary as they will be meaningful for the future development of CO₂R considering the unique superiority of ILs in promoting the performance of CO₂R.

5. Conclusions

An instant literature survey on the CO₂R to C1 products (CO, CH₃OH, CH₄, and syngas) with ILs-based electrolytes was conducted summarizing the most recent results from the past two years, and the best performance for each target product was identified. The literature survey shows that CO₂R to CO was mostly studied in the recent two years, and desirable performance has been achieved (FE of 99.7% and current density of 182.2 mA cm⁻²); for CO₂R to other studied C1 products, much fewer studies were conducted, and the performance needs to be further improved.

The economic analysis and environmental assessment of CO₂R to C1-product were estimated, where both the parameters of the state-of-the-art (base case) and future cases with improved performance (cases 1 to 3) were used. Currently, CO is the only profitable product, in the future, CH₃OH and the syngas of H₂/CO (1 : 1) will achieve profitability, but CH₄ and the syngas of H₂/CO (2 : 1) will always be unprofitable. For the environmental impact, CH₄ is the most environmentally friendly product from CO₂R, followed by the syngas of H₂/CO (2 : 1) and CO, and then CH₃OH, and the desirable CO₂R performance will make CO₂R to C1-products an environmentally friendly pathway. Overall, CO₂R-to-CO is the most profitable path considering both economic and environmental aspects.

Summarily, for CO₂R with IL-based electrolytes, the pathway of CO₂R to CO has shown commercial potential based on the state-of-the-art achievement at the laboratory level from both economic and environmental aspects. For the other products, more efforts are needed to be implemented to improve the CO₂R performance or develop more advanced electrolyzers (e.g., GDE-type electrolyzer, membrane electrode assembly-type electrolyzer, microfluidic-type electrolyzer, and solid-state electrolyte-type electrolyzer). Furthermore, ILs should be further exploited in future CO₂R as follows: (1) the adjustable feature of ILs in the structure and properties provides unique advantages and feasibilities for designing more efficient and suitable electrolytes of CO₂R; (2) the capability of ILs to dissolve a variety of solvents and electrolytes can integrate other solvents and electrolytes, further improving the performance of CO₂R; (3) the cleaner ILs can be designed and synthesized applying into CO₂R to mitigate the environmental burden; (4) except as electrolytes, ILs can also be the co-catalyst or modifier for the catalyst exhibiting prominent performance.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was financially supported by the Swedish Energy Agency (51239-1 or P2020-90066). AL acknowledges the financial support from the Swedish Research Council, from Kempe Foundation, and from European Union's Horizon Europe research and innovation programme under grant agreement No. 101086667, project BioMat4CAST (BioMat4CAST – “Petru Poni” Institute of Macromolecular Chemistry Multi-Scale in silico Laboratory for Complex and Smart Biomaterials).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2im00055e

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