An annual increase of CO₂ concentrations in the atmosphere causes global environmental problems, addressed by systematic research to develop effective technologies for capturing and utilizing carbon dioxide. Electrochemical catalytic reduction is one of the effective directions of CO₂ conversion into valuable chemicals and fuels. The electrochemical conversion of CO₂ at catalytically active electrodes in aqueous solutions is the most studied. However, the problems of low selectivity for target products and hydrogen evolution are unresolved. Literature sources on CO₂ reduction at catalytically active cathodes in nonaqueous mediums, particularly in organic aprotic solvents, are analyzed in this article. Two directions of cathodic reduction of CO₂ are considered—nonaqueous organic aprotic solvents and organic aprotic solvents containing water. The current interpretation of the cathodic conversion mechanism of carbon (IV) oxide into CO and organic products and the main factors influencing the rate of CO₂ reduction, Faradaic efficiency of conversion products, and the ratio of direct cathodic reduction of CO₂ are given. The influence of the nature of organic aprotic solvent is analyzed, including the topography of the catalytically active cathode, values of cathode potential, and temperature. Emphasis is placed on the role of water impurities in reducing CO₂ electroreduction overpotentials and the formation of new CO₂ conversion products, including formate and H₂.

1. Introduction

Since 1958, when systematic direct measurements of CO₂ in the atmosphere began, there has been a steady annual increase in its concentration. In 1958, its content was ∼315 ppm and in 2021, ∼415 ppm [1]. The tendency to further increase CO₂ concentration in the atmosphere causes such negative planetary consequences as the greenhouse effect and increasing acidity of ocean and sea waters. To alleviate such global problems, research has been intensively conducted in the last decade to develop effective technologies for capturing and disposing of atmospheric CO₂. The most efficient direction is converting this gas into valuable chemicals and fuels. The best known and most studied are the following four methods (Figure 1): thermocatalytic [2, 3], photocatalytic [4–6], enzymatic [7, 8], and electrochemical catalytic reduction [9–14].

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Electrochemical reduction of CO₂ is marked by vast possibilities in terms of the range of conversion products, including CO, CH₄, C₂H₄, CH₃OH, CH₃COOH, HCOOH, and (COOH)₂. This fact and the increased use of renewable energy sources give grounds to consider the up-and-coming electrochemical methods, particularly in “green” technologies of valuable substances (Figure 2).

Electrochemical conversion of CO₂ at catalytically active electrodes in aqueous solutions has been studied the most [9–14]. Moreover, the transformation mechanism has been studied in detail at copper and copper-containing cathodes [11–14, 16, 17]. Despite the efficiency of such and other metal-containing electrodes, unresolved problems are low selectivity for target products and hydrogen evolution in aqueous solutions. The first is due to the simultaneous course of many electrochemical and chemical reactions due to the relative values of the standard potentials for the formation of essential compounds by the reduction of CO₂ in acidic (1)–(5) and neutral (6)–(10) solutions [18]. In the range E = 0..−1 V, there is also a parallel cathode reaction of hydrogen evolution (11) and (12).

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Electrochemical CO₂ conversion at catalytically active cathodes is investigated in aqueous and nonaqueous solutions. They significantly differ in the limiting electrode potentials and the values of the cathode current densities, the course of electrochemical processes and reduction products, and the selectivity of the latter (Figure 3). Nonaqueous include ionic liquids, methanol solutions, organic aprotic solvents, and salt melts.

Electrochemical reduction of CO₂ in nonaqueous media or solutions with low water content makes it possible to eliminate or reduce these negative factors of aqueous solutions. In addition, CO₂ has a high solubility in organic solutions. Among them, the most studied are ionic liquids [19–26], methanol solutions [27–29], and organic aprotic solvents [30–40]. In recent years, there has also been engagement in the CO₂ electrochemical reduction process in molten salts [41–46]. The following valuable nanomaterials can be obtained in such an environment: carbon nanofibers [43], carbon nanotubes (CNTs), carbon spheres (CSs), and honeycomb carbon [44]. In addition, the simplicity of the design of the diaphragm-free electrolyzer (Figure 4), high conversion speed (i_cathode = 100 mA × cm⁻²), and high Faradaic efficiency (80–90%) of products [42] make this method promising. However, high-cost ionic liquids and decomposition at a very negative potential of CO₂ [37] limit their practical application. Although CO₂ conversion in salt melts is unique in carbon nanomaterials synthesis, high temperature (≥500°С) is an energy-holding factor.

The relatively low cost of organic aprotic solvents, wide “electrochemical windows,” and high solubility make them a promising medium in the electrochemical conversion of CO₂. Such a medium is also effective for studying the catalytic activity of cathodes in a wide range of electrode potentials. The purpose of this review is to summarize the scientific literature on the mechanism of cathodic conversion of carbon (IV) oxide into CO and organic products, the impact of water impurities, cathode material and topography of its surface, and the main factors of electrolysis (electrode potential, electrolyte composition, and temperature), list of products, and their Faradaic efficiency. This paper also aims to focus on the problematic issues of electrochemical conversion of CO₂ in organic aprotic solvents and ways to solve them.

2. Features of Electrochemical Recovery of CO₂ in Organic Aprotic Solvents

Specifics of CO₂ conversion at catalytically active electrodes in organic aprotic solvents are primarily due to (1) deficiency or absence of protons in nonaqueous solution and (2) high solubility of carbon (IV) oxide. These factors cause new electrochemical and chemical processes, changes in the spectrum of CO₂ conversion products, and even the formation of new compounds and significant acceleration of cathode processes.

2.1. Cathodic Reduction of CO₂ in Nonaqueous Organic Aprotic Solvents

In aqueous solutions at E_cathode = −1.1…− 1.3 V, intensive hydrogen evolution begins in reaction (12), which limits the values of cathode potentials during electrochemical reduction of CO₂. In the absence of water and due to the high electrochemical stability of molecules of organic aprotic solvents, electrolysis can be carried out at E_cathode = −2.5…−3.0 V. However, there are reactions to form only two products of CO₂ conversion—oxalate anion and CO [27, 47]. Oxalate anion is formed by reaction (13) due to the formation of solvated radicals ⋅CO₂⁽⁻⁾ [48], followed by their connection with each other (14). Reaction (15) may occur in parallel with the formation of CO.

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The proportion of reactions (13)–(15), their rate, i.e., the value of i_cathode, Faradaic efficiency of conversion products, and, accordingly, the ratio of products of direct cathodic reduction of CO₂, i.e., oxalate and CO, depend on many factors. Among them, the main ones are as follows: the nature of the organic aprotic solvent; nature and topography of the catalytically active cathode; the value of the cathode potential; temperature.

2.1.1. Influence of the Nature of the Organic Aprotic Solvent

AN, DMF, DMSO, and PC (Table 1) media are most often used for cathodic CO₂ reduction, which, in addition to high electrochemical resistance, are characterized by high values of Donor Number and CO₂ solubility [27]. DN (kJ·mol^-1): AN—59.0; DMF—111.4; DMSO—124.8; PC—63.2. CO₂ Solubility (mmol·L^-1) is an order of magnitude greater than the value of this value in water: AN—314 ± 6; DMF—194 ± 14; DMSO—131 ± 7; PC—134 ± 9; Н₂О—34.5 ± 4.46.

1. Examples of nonaqueous aprotic solvents used in CO₂ electrochemical reduction by catalytic active cathodes.

Electrode	Electrolyte	E, V	i _cathode, mA⋅cm⁻²	CO₂ conversion products	Faradaic efficiency, %	References
Сu on basal Pt(hkl) single crystal faces	TBAPF₆ in AN or PC	−1.8	0.88 0.88 in AN 0.28 in PC	CO	−	[31]
Au foil	TBAP in PC	−2.8	16.0	CO	84.9	[32]
Cathode: lead gas diffusion electrode Anode: sacrificial zinc	TEAB in AN TBAP in AN	−2.5 −2.2…−4.0	80.0 20.0…80.0	Zinc oxalate	80.0 53.0	[33]
Cathode: Au foil Anode: graphite rod Electrolysis cell with proton exchange membrane	Catholyte: 0.1 M TBAP in PC Anolyte: 0.1 M H₂SO₄	−3.0	2.8	CO	91.8	[49]
Bulk gallium	TBACl, TBABr, TBAI and TBAP in DMSO, DMF, NMP, PC and AN	−2.2 −3.0	0.7 in NMP 0.7 in DMF 1.4 in DMSO 1.8 in PC 2.3 in AN	CO	26.0 in NMP 21.0 in DMF 14.0 in DMSO 32.0 in PC 83.0 in AN	[50]
Cathode: stainless steel	TBAP in AN	−2.5 −3.5	2.7…19.6	Zinc oxalate	62.4	[51]
Anode: sacrificial zinc	TBAP in AN	−2.5 −3.5	5.0…15.0	Zinc oxalate	73.9	[52]
Cathode: lead Anode: sacrificial zinc	TBAP in AN	−2.2 −2.8	4.0…12.0	Zinc oxalate	96.8	[53]
Cathode: MoO₂ microparticles on Pb supporting substrate	TBAPF₆ in AN	−2.4	25.0	CO oxalic acid	30.0 45.0	[54]

Molecules of the aprotic solvent, due to the indivisible electron pair, act as a Lewis base, causing the donor-acceptor interaction with the carbon atom of the low-polar molecule CO₂ L:⟶CO₂ and the cathode surface L: ⟶cathode surface.

(1) Influence of Donor Number. The donor-acceptor interaction L: ⟶CO₂ causes inhibition of electron transfer, which affects the ratio of oxalate and CO as products of cathodic reduction of CO₂; namely, with a decreasing electron-donor capability, increased oxalate formation is observed [27]. Therefore, AN, which has the lowest DN value, is often used as a medium for oxalate obtaining with a high value of Faradaic efficiency—∼80% [33], ∼62% [51], ∼74% [52], and ∼97% [53]. However, this dependence [oxalate]: [CO] on DN of aprotic solvent should be considered as a trend because the course of electrochemical reactions (13) and (15) and, accordingly, the value of FE are influenced by the values of cathode potential, cathode nature, and temperature. The electron-donor capability of the aprotic solvent also determines the adsorption of organic molecules on the cathode surface [55–57]. Reactions (13) and (15) take place on the latter, the course of which depends on the adsorption-desorption of CO₂ and CO molecules, CO₂ radicals, and CO₃²⁻ anions. Therefore, due to the predominant content of molecules of an aprotic solvent, the value of their DN should affect the ratio [oxalate]: [CO].

(2) Influence of CO₂ Solubility. Solubility of CO₂ is one of the factors of mass transport, concentration cathodic polarization, and, accordingly, the reaction rates (13) and (15). It is the highest CO₂ solubility in AN that mainly determines the highest values of cathode currents among organic aprotic solvents, for example, at Ga cathode, E = −3.0 V in AN i_cathode = 2.3 mA·cm ⁻² (CO₂ solubility ∼314 mmol·L⁻¹), in DMF—0.7 (∼111.4), and DMSO—1.4 (∼194) [50]. A similar pattern is observed at other catalytically active cathodes. Thus, E = −3.0 V on a boron-doped diamond working electrode in AN, DMF, and PC i_cathode = ∼5.5, ∼2.3, and ∼1.2 mA⋅cm ⁻², respectively [58]; on Au cathode—∼24, ∼15, and ∼7 [59].

2.1.2. Nature and Topography of Catalytically Active Cathode

Electrochemical reduction of CO₂ involves the adsorption of carbon (IV) oxide, solvent molecules, ions, intermediates, and end products on the cathode surface. The nature of the latter significantly affects the equilibrium processes of adsorption↔desorption due to inappropriate adsorption energies of key reaction intermediates [60]. The result is the dependence of the rate of certain electrochemical reactions and, accordingly, the product selectivities and Faradaic efficiencies of the CO₂ conversion process on the catalytic activity of the cathode surface. Therefore, systematic studies of cathode production ⟶ surface structure ⟶ catalytic activity ⟶ product selectivities and Faradaic efficiencies are a priority in the direction of electrochemical conversion of carbon (IV) oxide.

The influence of the nature of the cathode surface, namely, its electrocatalytic action on the rate of carbon dioxide reduction, selectivity of product selectivities, and Faradaic efficiencies, is most studied for aqueous solutions [9, 11, 14, 16, 27, 61], which allowed to form scientific principles of CO₂ conversion. The peculiarity of the environment of anhydrous organic aprotic solvents determines the course of fundamentally different electrode processes and, accordingly, reduction products. Thus, electron-donor molecules of organic solvent (L) are adsorbed on the electrode surface in parallel with CO₂ molecules (Figure 5). However, due to the high electrochemical stability of organic molecules, their destruction does not occur even at E = −3.0 V, while in aqueous solutions, there are reactions (11) and (12). This significantly affects product selectivities and Faradaic efficiencies. Adsorption of R₄N⁺ cations is also possible on the cathode surface, as conductive additives, such as R₄NClO₄, R₄NPF₆, and R₄NCl (Table 1), are present in aprotic solvents, which are also characterized by high electrochemical resistance [58]. Thus, the adsorption of organic solvent molecules and tetraalkylammonium cations causes only cathodic polarization. Accordingly, the nature of the cathode surface affects the adsorption energies L, R₄N⁺, and CO₂ (Figure 5), as well as intermediate (·CO₂) and final (CO) products.

Depending on the nature of the cathode surface in organic aprotic solvents, the reduction of CO₂ with the formation of oxalate or CO is possible. Thus, according to scheme (Figure 5(a)) on copper [31], gold [32, 49], and gallium electrodes [50], CO is formed, according to scheme (Figure 5(b)) on stainless steel [51, 52] and lead [33, 53] electrodes—oxalate with high values of Faradaic efficiency (Table 1). On the electrode based on MoO₂ [31], the parallel reduction is possible according to schemes (a) and (b) with the formation of two products. However, the amount of literature is still little to discuss the system dependencies of CO₂ conversion products on the nature of the cathode surface.

The use of a lead gas diffusion electrode [33] makes it possible to achieve values of cathode current densities much higher than on those using a lead plate [53] due to the large specific surface area. However, the product of CO₂ conversion is not affected by the topography of the cathode. Even higher efficiency is observed using Cu nanoparticles embedded in N-doped carbon (Cu@NC) arrays (Figure 6).

In nonaqueous media, the effect of nanostructured cathodes on the rate of CO₂ reduction and selectivity of the obtained products is little studied. However, positive results of the use of nanometals in aqueous solutions [63, 64] can also be expected in the environment of organic aprotic solvents. The highest efficiency is manifested in the size of metal nanoparticles of several nanometers and especially nanoclusters [65, 66], the synthesis of which is characterized by simplicity in the technological aspect [67].

2.1.3. Cathodic Potential Values and Temperature

The formation of the ⋅CO₂⁽⁻⁾ radical by the cathodic reaction (13) takes place in the environment of organic aprotic solvents at low values of electrode potentials. Thus, in DMF, E⁰CO₂/⋅CO₂ = −2.21 V according to [68] and −1.97 V according to [58]. Since, in such a nonaqueous medium, ⋅CO₂⁽⁻⁾ is the starting electrochemical stage of CO₂ reduction (Figure 5), the quantitative formation of conversion products begins at approximately −2.0 V [32, 33, 58–60, 69] on electrodes of different nature. A typical Е−i_cathode dependence is shown in Figure 6. The value of Е_cathode is limited by the electrochemical stability of organic aprotic solvents, and in AN, DMF, and PC solutions, it takes values not lower than −3.5…−4.0 V.

The environment of some organic aprotic solvents makes it possible to carry out the electrochemical reduction of CO₂ in a wide temperature range due to the low values of their melting point and high boiling point. Thus, for AN, DMF, and PC, these values are −46 and 82°C, −61 and 155°C, and −49 and 242°C, respectively. The authors [48] showed that on the inert (Mercury) cathode in the DMF medium, there is an increase in CO yield and, accordingly, a decrease in oxalate yield with decreasing temperature. Thus, at a CO₂ concentration of 152 mmol·dm ⁻³, the distribution of products is as follows: oxalate, 67%; CO, 25% and oxalate, 11%; CO, 89% at temperatures of 25 and −20°C, respectively.

2.2. Cathodic Reduction of CO₂ in Organic Aprotic Solvents Containing Water

CO₂ electroreduction occurs at high cathode overpotentials in anhydrous organic aprotic solvents in the absence of any proton source, as indicated in Section 2.1.3. High energy consumption does not contribute to technological attractiveness. The addition of water significantly reduces the overpotentials of CO₂ electroreduction but creates conditions for forming new products of CO₂ conversion, including formate and H₂ (Table 2). This is due to the possibility of reactions (2), (11), (12), (16)–(18).

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2. Examples of CO₂ electrochemical reduction by catalytic active cathodes in aprotic solvents with the addition of water.

Electrode	Electrolyte	E, V	i_cathode, mA⋅cm⁻²	CO₂ conversion products (Faradaic efficiency, %)	References
Cathode: (Cu, Pt, Au, and Pb discs) Anode: Pt coil	“Wet” AN solutions of: 0.1 M TEABF₄ (46 ppm H₂O) 0.1 M TEATfO (184 ppm H₂O) 0.1 M NaTfO (528 ppm H₂O)	−2.4	0.5 1.3 0.3	CO	[34]
Cathode: WO₃ nanoparticulate thin films on FTO glass Anode: Pt wire Electrolysis cell with proton exchange membrane	0.1 M TBAP in dry AN with <10 ppm H₂O in humid AN	−1.2 (CV)	0.3 (CV) 0.8 (CV)	CO (42%) CO + formate	[35]
Cathode: Nanoporous TiO₂ Anode: Pt wire Electrolysis cell with proton exchange membrane	0.1 M TBAP in dry AN AN + 0.5 M H₂O	−1.8	0.8 (CV) 1.0 (CV)	CO (48%), oxalate (12%) CH₃OH (60%), CO (17%)	[36]
Cathode: Ag-[PMo₁₂O₄₀]ⁿ⁻ on GC Anode: Pt wire	0.1М TBAPF₆ in DMF + 0.5 vol% H₂O	−2.5 (CV)	20.0 (CV)	CO (90%)	[37]
Cathode: Zn foil Anode: a graphite rod Electrolysis cell with proton exchange membrane	Catholyte: 0.1 M TBAP in PC + 6.8 vol% H₂O Anolyte: 0.1 M H₂SO₄	−2.3	6.72	CO (83%), H₂ (15%)	[38]
Cathode: MoO₂ microparticles on Pb supporting substrate	TBAPF₆ in AN + 0.1 1.4 M H2O	−2.32 2.10	25.0 (const)	CO formate	[54]
Cathode: Au foil Anode: a graphite rod Electrolysis cell with proton exchange membrane	Catholyte: 0.1 M TBAP in AN, DMF, DMSO, or PC + (0 6.8 vol% H₂O) Anolyte: 0.1 M aqueous H₂SO₄	−3.0	3.0 (without H₂O) 6.3 (6.8 vol% H₂O)	CO (84.3%), H₂ (15%)	[59]
Cu@NC (СuNPs, ∅ 4 nm)	0.1 M TBAPF₆ in DMF + 0.5% H₂O	−3.0 (CV) −2.5 (const)	18.0 (CV) 5.5 (const)	Formate (64%), CO (20%), H₂ (13%)	[62]
Cathode: Semiconducting Co₃O₄ nanofibers Anode: platinum Electrolysis cell with membrane	Catholyte: 0.1 M TBAPF₆ in AN + 1.0 vol% H₂O Anolyte: AN-H₂O	−1.56	0.5	CO (65%), formate (27%)	[70]
Ag₂S, deposited on Ag electrode (two-compartment electrolysis cell)	TBAP in PC + ≤ 6.8 wt% H₂O	−2.35	9.85	CO (92%)	[71]
Pt plate	0.1 M TEAP in AN + 4 mM H₂O	−3.2 (CV)	5 (const)	Oxalate (71%), formate (11%), CO (8%), H₂ (2%)	[72]
Au microelectrode	TBAPF6 in AN + 0.003 1.0 M H₂O	−1.8	0.5 10.0	CO	[73]

With the water increase in the solution composition, cathode currents increase significantly [34, 36, 38, 59, 60, 73]. Thus, for E = −2.5 V in anhydrous acetonitrile solution, i_cathode = 0.4 mA·cm ⁻², while for the water content of 0.01, 0.25, and 2 mol·L⁻¹, it is 0.75, 0.95, and 1.3 mol·L⁻¹, respectively [34]. This is also identical to the decrease in cathode potentials by i_cathode = const (Figure 7). Low water concentration in the solution composition practically does not reduce the high solubility of CO₂ provided by organic aprotic solvents. The low content of H₂O also does not significantly affect the course of parallel cathodic reactions (11) and (12) with the electrochemical release of H₂. This is due to these organic solvents high electron-donor properties, which promote the formation of stable associates (L:⋅⋅⋅Н−О−Н) or (L:⋅⋅⋅Н⁺), weakening the electrochemical activity of water molecules. The water in the solution composition will be limited to 6.8 vol% because, at higher concentrations, the electroconductive organic component of the nonaqueous solution begins to precipitate [59]. In addition, high water concentrations increase the proportion of reactions (11) and (12).

The water content in the aprotic solvent as a source of protons is an important factor influencing the course of reactions (11, 12, 16, 17) and, accordingly, the distribution of the products of cathodic reduction of CO₂ (Figure 8(a)). Moreover, it is natural to increase the share of H₂ with increasing concentrations of H₂O in the solution. Therefore, it is limited to 1-2%. An important factor influencing the distribution of CO₂ conversion products is the value of the cathode potential (Figure 8(b)).

For the controlled synthesis of H₂, it is necessary to ensure a relatively stable concentration of the proton-containing component during long-term electrolysis. One way to solve this problem is to use an electrolyzer separated into two compartments (Figure 9). Proton exchange membranes are preferably used as separators [38, 49, 59, 70, 71]. The cathode part of the cell is filled with the working electrolyte, the anode—with an aqueous solution of H₂SO₄. Due to the anodic reaction (19), H⁺ protons are generated, which penetrate the catholyte through the membrane and participate in the cathodic reactions of formation of CO₂ conversion products. Such electrolyzers make it possible to ensure catholyte circulation and implement a continuous process. The authors [49, 59] proposed a basic technological scheme, showing the practical possibility of industrial synthesis of valuable products by CO₂ conversion shortly.

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The mechanism of the effect of water in aprotic solvent − H₂O solutions on CO₂ reduction products is limited to cathodic reactions involving proton-donor components, in particular (16)–(18). However, as shown in [37], the water factor also depends on the nature of the catalytically active cathode. For E = −3.0 V in 0.1 M TBAP nonaqueous PC solution using Ag, Zn, and Au cathode i_cathode = ∼4, ∼8, and ∼9 mA × cm⁻², respectively. In solutions containing 6.8 wt % H₂O, i_cathode = ∼5, ∼14, and ∼15 mA × cm⁻². Therefore, the effect of water on the growth of i_cathode is much higher on Zn and Au cathode than Ag, which is obviously due to the different interaction of water molecules with the electrode surface. This issue is practically not covered in the literature. Still, its importance for understanding the mechanism of CO₂ reduction in aprotic solvent-H₂O solutions is confirmed by recent work, which considers, for example, microkinetic models of interaction of solution components with the cathode surface depending on its nature [74] and influences the nature of the electrode for the distribution of products in anhydrous and water-containing aprotic solvents (Figure 10).

In [75], a systematic analysis of the nature influence of the metal cathode on the selectivity and Faradaic efficiencies of CO₂ reduction reaction products in aqueous solutions and nonaqueous aprotic solvents is presented. However, due to the growing interest in aprotic solvent−H₂O solutions, studies aimed at establishing the dependence of the distribution of CO₂ conversion products on the nature of the cathode are relevant.

3. Conclusions

The capture and conversion of CO₂ are one of the areas aiming to solve a complex environmental problem. The most studied methods of converting this gas into valuable chemicals and fuels are thermocatalytic, enzymatic, photocatalytic, and electrochemical catalytic reduction. The latter best meets the criteria of “green” technologies and cost-effectiveness and is characterized by various conversion products: CO, CH₄, C₂H₄, CH₃OH, CH₃COOH, HCOOH, (COOH)₂, etc. However, several unresolved problems of electrochemical reduction of CO₂ in aqueous solutions, primarily low speed, low selectivity for target products, and Faradaic efficiency, constrain its widespread industrial application.

One of the ways to solve the problems caused by aqueous solutions is the electrochemical conversion of CO₂ in a nonaqueous medium, mainly organic aprotic solvents. High electrochemical stability of the latter and high solubility of carbon (IV) oxide allows conducting electrochemical processes at cathode potentials −2.5… −3.5 V, providing a high conversion rate and selectivity. Two directions of cathodic reduction of CO₂ are studied—nonaqueous organic aprotic solvents and organic aprotic solvents containing water.

The mechanism of CO₂ reduction in nonaqueous aprotic solvents involves the formation of only two products—CO and oxalate. The values of the cathode current density, Faradaic efficiency of conversion products, and, accordingly, the ratio of direct CO₂ reduction products depend on the following main factors: the nature of the organic aprotic solvent; nature and topography of the catalytically active cathode; values of cathode potential and temperature. The most effective among aprotic solvents is acetonitrile, which has the highest solubility of CO₂ and provides the highest rate of oxalate and CO with Faradaic efficiency up to 90%. The nature of the cathode significantly affects the mechanism of CO₂ reduction, which causes the formation of oxalate or CO: it produces mainly CO on copper, gold, and gallium electrodes; it produces oxalate with high Faradaic efficiency on stainless steel and lead electrodes. A highly developed and nanostructured cathode surface contributes to higher values of cathode current density but has virtually no effect on product selectivity. As the values of the cathode potentials increase, starting from −1.8…−2.0V, the cathode currents increase, which makes this parameter one of the main ones to ensure a high rate of CO₂ conversion.

In the environment of organic aprotic solvents with low water content, the cathode overpotentials of CO₂ electroreduction are significantly reduced, which makes it possible to carry out the process at high cathode currents. At the same time, the presence of water causes the formation of additional products—formate and H₂. The water content in the aprotic solvent is a factor influencing the course of cathodic reactions and, accordingly, the distribution of CO₂ reduction products. To date, the interdependence between the nature of the organic solvent, water content, the nature of the cathode, and the value of the cathode potential is little studied, which constrains the practical application of CO₂ electroreduction in aprotic solvent-H₂O.

Abbreviations

AN:: Acetonitrile
DMF:: N, N-Dimethylformamide
DMSO:: Dimethylsulfoxide
PC:: Propilencarbonate
NMP:: N-Methyl-2-pyrrodione
IL:: Ionic liquid
L:: Electron-donor molecules of organic aprotic solvent
FE:: Faradaic efficiency
Е⁰:: Standard electrode potential
E:: Electrode potential
Еcathode:: Cathode electrode potential
icathode:: Cathode current density
MNCs:: Metal nanoclusters
MNPs:: Metal nanoparticles
NWs:: Nanowires
DN:: Donor number
FTO:: Fluorine doped tin oxide
TBAP:: Tetrabutylammonium perchlorate
TEAB:: Tetraethylammonium tetrafluoroborate
TBACl:: Tetrabutylammonium chloride
TBABr:: Tetrabutylammonium bromide
TBAI:: Tetrabutylammonium iodide
TBAPF⁶:: Tetrabutylammonium hexafluorophosphate
TEABF⁴:: Tetraethylammonium tetrafluoroborate
TEATfO:: Tetraethylammonium trifluoromethanesulfonate.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this article.

Acknowledgments

This work was carried out with the partial financial support of the National Research Foundation of Ukraine; Project registration no.: 2020.02/0309 (“Design of polyfunctional nanostructured mono- and bimetals with electrocatalytic and antimicrobial properties”).

Open Research

Data Availability

All related data used to support the findings of the study are mentioned within the article with references.

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CO₂ Electroreduction in Organic Aprotic Solvents: A Mini Review

Abstract

1. Introduction

2. Features of Electrochemical Recovery of CO₂ in Organic Aprotic Solvents

2.1. Cathodic Reduction of CO₂ in Nonaqueous Organic Aprotic Solvents

2.1.1. Influence of the Nature of the Organic Aprotic Solvent

2.1.2. Nature and Topography of Catalytically Active Cathode

2.1.3. Cathodic Potential Values and Temperature

2.2. Cathodic Reduction of CO₂ in Organic Aprotic Solvents Containing Water

3. Conclusions

Abbreviations

Conflicts of Interest

Acknowledgments

Open Research

Data Availability

References

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CO2 Electroreduction in Organic Aprotic Solvents: A Mini Review

Abstract

1. Introduction

2. Features of Electrochemical Recovery of CO2 in Organic Aprotic Solvents

2.1. Cathodic Reduction of CO2 in Nonaqueous Organic Aprotic Solvents

2.1.1. Influence of the Nature of the Organic Aprotic Solvent

2.1.2. Nature and Topography of Catalytically Active Cathode

2.1.3. Cathodic Potential Values and Temperature

2.2. Cathodic Reduction of CO2 in Organic Aprotic Solvents Containing Water

3. Conclusions

Abbreviations

Conflicts of Interest

Acknowledgments

Open Research

Data Availability

References

Citing Literature

Figures

References

Related

Information

CO₂ Electroreduction in Organic Aprotic Solvents: A Mini Review

2. Features of Electrochemical Recovery of CO₂ in Organic Aprotic Solvents

2.1. Cathodic Reduction of CO₂ in Nonaqueous Organic Aprotic Solvents

2.2. Cathodic Reduction of CO₂ in Organic Aprotic Solvents Containing Water