Research on the Characteristics of Electrolytes in Integrated Carbon Capture and Utilization Systems: The Key to Promoting the Development of Green and Low-Carbon Technologies

Abstract

The core challenge of integrated carbon capture and utilization (ICCU) technology lies in developing electrolytes that combine efficient carbon dioxide (CO₂) capture with electrocatalytic conversion capabilities. This review analyzes the structure–performance relationship between electrolyte properties and CO₂ electrochemical reduction (eCO₂RR), revealing the key regulatory mechanisms. Research shows that the performance of bicarbonate electrolytes heavily depends on the cation type, where Cs⁺ can achieve over 90% CO selectivity by suppressing the hydrogen evolution reaction (HER) and stabilizing reaction intermediates, though its strong corrosiveness limits practical applications. Although amine absorbents excel in carbon capture (efficiency > 90%), they tend to undergo competitive adsorption during electrocatalysis, making formic acid the primary product (FE = 15%); modifying electrodes with ionomers can enhance their activity by 1.15 times. Ionic liquids (ILs) demonstrate unique advantages due to their tunability: imidazolium-based ILs improve formate selectivity to 85% via carboxylate intermediate formation, while amino-functionalized task-specific ILs (TSILs) achieve a 1:1 stoichiometric CO₂ absorption ratio. Recent breakthroughs reveal that ternary IL hybrid electrolytes can achieve nearly 100% CO Faradaic efficiency (FE) through microenvironment modulation, while L-histidine additives boost CH₄ selectivity by 23% via interface modification. Notably, constructing a “bulk acidic–interfacial neutral” pH gradient system addresses carbonate deposition issues in traditional alkaline conditions, increasing C₂₊ product efficiency to 50%. Studies also highlight that cation–anion synergy (e.g., K⁺/I⁻) significantly enhances C-C coupling through electrostatic interactions, achieving 97% C₂₊ selectivity on Ag electrodes. These findings provide new insights for ICCU electrolyte design, with future research focusing on machine learning-assisted material optimization and reactor engineering to advance industrial applications.

1. Introduction

Global climate change has emerged as a critical challenge, marked by increasing extreme weather events [1]. Industrial carbon emissions are a primary driver of this crisis [2]. To address this, carbon capture and utilization (CCU) technologies have been developed. Based on process differences, carbon capture methods fall into three categories: pre-combustion, oxy-fuel combustion, and post-combustion [3]. Among these, post-combustion capture holds the greatest practical value due to its direct applicability to existing industrial infrastructure.

Among carbon conversion technologies, carbon dioxide (CO₂) electrochemical reduction (eCO₂RR) has attracted significant attention due to its mild reaction conditions and controllable product selectivity [4]. To improve the overall energy efficiency, researchers have proposed integrated carbon capture and utilization (ICCU) systems [5]. These systems impose unique requirements on electrolyte materials: they must simultaneously exhibit high CO₂ capture capacity and catalytic activity for electrochemical reactions.

The current electrolyte systems face several key challenges: bicarbonate electrolytes exhibit strong corrosivity [6]; amine-based absorbents are prone to side reactions during electrocatalysis [7]; and ionic liquids remain limited by high costs [8]. The fundamental challenge lies in balancing an electrolyte’s capture capacity, catalytic activity, and economic viability. Recent studies suggest that modulating the electrolyte microenvironment (e.g., through pH gradients) can significantly enhance reaction selectivity [9], though the underlying mechanisms require further investigation.

This review systematically examines recent advances in electrolyte development for ICCU systems, with particular focus on material design principles and performance optimization strategies, providing theoretical guidance for developing efficient and cost-effective carbon cycling technologies [10]. By analyzing the current technical challenges and the future research directions, this work aims to accelerate the practical implementation of ICCU systems [11].

2. Electrolyte for Carbon Capture and Electrocatalytic Applications

In contemporary research, the predominant electrolytes for eCO₂RR fall into three categories, i.e., (1) bicarbonate-based, (2) amine-based, and (3) ionic liquid electrolytes, along with some hybrid systems. These electrolytes play crucial roles in the following:

• Enhancing reaction efficiency through improved CO₂ adsorption [12,13] and facilitated electron transfer [7,14];
• Controlling reaction selectivity by suppressing side reactions [8,15,16,17,18] and directing reaction pathways [12,14,19,20];
• Expanding system compatibility with various electrode materials [7] and operational conditions.

The classification of carbon capture electrolytes and the investigation of their behavior during eCO₂RR constitute the fundamental research on electrocatalytic carbon conversion. This review will systematically examine both conventional and emerging carbon capture electrolyte systems.

2.1. Bicarbonate Electrolyte

Sodium hydroxide (NaOH) [21] and potassium hydroxide (KOH) [22] solutions are widely employed as strong alkaline CO₂ absorbents in laboratory settings, exhibiting rapid reaction kinetics and high absorption efficiency. The solid bicarbonate products can be thermally regenerated to release CO₂ while recovering the absorbent. As Group 1 elements, Na and K demonstrate increasing metallic character and hydroxide alkalinity with higher atomic numbers.

Studies reveal that the dissociation degree of bicarbonate ions (HCO₃⁻) in aqueous solutions (20 °C) of alkali metal bicarbonates (M_x(HCO₃)_y, where M = Li, Na, etc., significantly increases with the group number (Figure 1). Notably, cesium bicarbonate (CsHCO₃) shows approximately 60-fold greater solubility than lithium bicarbonate (LiHCO₃) [16]. This enhanced dissociation behavior makes higher-atomic-number alkali bicarbonates particularly suitable for eCO₂RR due to their superior ionic conductivity.

Endrodi and colleagues [16] developed a periodic Cs⁺ replenishment strategy (every 12 h) that maintained a CO Faradaic efficiency (FE) of >90% for 200 h at 400 ± 15 mA·cm⁻², demonstrating that dynamic cation supplementation effectively prevents cathode salt deposition and ensures eCO₂RR stability. The complementary in situ wide-angle X-ray scattering (WAXS) studies by Garg et al. [6] revealed distinct cation-dependent interfacial phenomena: Li⁺ rapidly forms passivating Li₂CO₃ layers that deactivate catalysts, while Na⁺ and K⁺ exhibit a dynamic deposition–dissolution equilibrium, and Cs⁺ shows no observable deposition (Figure 2).

Beyond deposition effects, alkali metal cations influence eCO₂RR selectivity through three primary mechanisms:

• Electrostatic stabilization of key reaction intermediates [23];
• Cation hydrolysis-mediated local pH modulation [24];
• Induced interfacial electric fields that alter reaction pathways [25].

Endrodi et al. [16] systematically investigated product distributions on Cu gas diffusion electrodes in 1M alkali bicarbonate electrolytes, revealing a clear periodic trend: as the atomic number increases from Li to Cs, the FE of carbonaceous products significantly improves while the competing hydrogen evolution reaction (HER) is progressively suppressed. Multiple research groups have confirmed that CsHCO₃ exhibits the optimal HER suppression, demonstrating the highest eCO₂RR selectivity among alkali bicarbonate systems (Figure 1). However, the practical application of strong alkaline absorbents (e.g., NaOH/KOH) faces substantial challenges. Their high corrosivity may damage reactor components, and the potential leakage poses serious ecological risks, significantly limiting their viability for industrial-scale implementation.

Ammonia solution, a widely used industrial chemical with mild alkalinity, has demonstrated exceptional potential for CO₂ capture due to its high absorption efficiency, strong CO₂ capture capacity, excellent oxidation resistance, and low regeneration energy requirements. In ammonia-based CO₂ capture systems, the resulting ammonium bicarbonate-rich solution can be regenerated through a combined crystallization–thermal decomposition process. Research has shown that the decomposition kinetics of ammonium bicarbonate crystals are significantly influenced by temperature and heating rate: while slow decomposition begins at room temperature, the rate increases dramatically with rising temperatures and faster heating rates, achieving an activation energy as low as 48.38 kJ/mol. Notably, an optimal regeneration rate is attained at 80 °C [26].

Studies by Li et al. [27,28] and Yu [26] have demonstrated that employing ammonium bicarbonate as an electrolyte in eCO₂RR systems can triple the CO₂ concentration at the electrode interface (Figure 3), attributable to the rapid equilibrium reactions of HCO₃⁻ [29,30]. This innovative strategy of directly utilizing ammonium bicarbonate-rich solutions from ammonia-based CO₂ capture for eCO₂RR not only reduces the energy consumption for absorbent regeneration but also simultaneously produces valuable carbon-containing energy storage products.

However, ammonia-based CO₂ capture technology still faces several challenges. Process simulations conducted by Xiaoxu et al. [31] using ChemDraw Professional (Version: 20.0) for a 350 MW coal-fired flue gas CO₂ removal system revealed two critical trade-offs: (1) increasing ammonia concentration under optimal flow conditions exacerbates ammonia slip, and (2) it simultaneously reduces CO₂ removal efficiency (Figure 4). To establish an efficient closed-loop technological system, coordinated optimization is required between upstream carbon capture processes (controlling ammonia slip) and downstream electrochemical conversion (regulating electrolyte concentration). This systems-level approach is essential to ensure both the operational safety and economic viability of the integrated process.

2.2. Amine-Based Electrolytes

Amine-based compounds, as the most mature chemical absorbents in post-combustion carbon capture, have attracted extensive attention due to their diverse molecular structures (e.g., primary, secondary, and sterically hindered amines) and high CO₂ affinity [32]. Monoethanolamine (MEA), a typical unhindered primary amine, forms stable carbamates with CO₂ through its exposed amino group (-NH₂), exhibiting strong CO₂ binding characteristics. In contrast, 2-amino-2-methyl-1-propanol (AMP), a sterically hindered secondary amine, shows significantly weaker CO₂ binding capacity due to the steric hindrance effect of the α-carbon methyl group. These structure–performance differences directly influence their behavior in subsequent electrochemical conversion processes.

The Boundary Dam coal-fired power plant in Saskatchewan, Canada, serves as a representative industrial application [33]. This facility employs MEA as the core absorbent, achieving 90% CO₂ capture efficiency in commercial operation and utilizing the captured CO₂ for enhanced oil recovery (EOR). However, the high energy consumption during amine regeneration (primarily due to the thermal decomposition energy barrier of carbamates) remains a critical bottleneck for large-scale implementation.

A recent work by Bruggeman et al. [7] using in situ characterization techniques has elucidated the transformation mechanism of amine-CO₂ complexes at electrochemical interfaces. Employing copper (Cu) as a model catalyst—whose unique d-electron configuration facilitates inner-sphere electron transfer through surface coordination bonds with reactants—the study combined cyclic voltammetry (CV) and Fourier-transform infrared spectroscopy (FTIR) to sequentially identify key intermediates via their characteristic vibrational signals:

• 2345 cm⁻¹: gaseous CO₂ consumption (O=C=O asymmetric stretching, v_as);
• 1675 cm⁻¹: carboxyl group formation at the electrode interface (C=O stretching, v);
• 1607 cm⁻¹: carboxylate ion generation (COO⁻ asymmetric stretching, v_as);
• 1604 cm⁻¹: free amine regeneration (N-H in-plane bending, δ).

Notably, despite their differing CO₂ capture capacities, both AMP and MEA participate in reactions through carbamate intermediates on Cu surfaces (Figure 5). The reaction pathway involves the following: (1) amine-CO₂ complexes accepting electrons via Cu 3d orbitals, (2) C-N bond cleavage to form formate, and (3) amine desorption from the electrode surface. These findings provide crucial theoretical foundations for developing integrated “capture–conversion” processes.

The Bruggeman research group [7] further investigated the influence of different electrode materials on the electrochemical reduction behavior of amine-CO₂ complexes. When replacing the Cu electrode with lead (Pb), experimental results demonstrated significantly enhanced eCO₂RR activity for both MEA-CO₂ and AMP-CO₂ complexes on Pb surfaces. This phenomenon can be explained by two key mechanisms: (1) on Cu electrodes, the strong interaction between MEA-H⁺ and Cu preferentially occupies active sites, competing with MEA-CO₂ adsorption and consequently inhibiting the eCO₂RR process (Figure 6), and (2) the outer-sphere electron transfer characteristics of Pb electrodes result in weaker surface interactions with amine molecules, creating a more flexible reaction interface that facilitates eCO₂RR (Figure 6).

To address challenges such as high reaction barriers and low product selectivity in MEA-CO₂ systems during eCO₂RR, Li [34] developed an innovative electrode material. The study employed Sustainion ionomer as a binder to fabricate hydrophilic nano-silver electrodes, which significantly improved the wettability of MEA-CO₂ complexes on the electrode surface and effectively increased the electrochemical active area. Experimental results showed that, at −1.3 V vs. Ag/AgCl, this electrode system achieved an FE_CO of 15.02%, representing a 1.15-fold improvement compared to conventional electrodes (Figure 7). This enhancement primarily stems from improved charge transfer efficiency and mass transport rates.

These findings collectively demonstrate that organic amines represented by MEA and AMP exhibit remarkable potential as carbon source materials for eCO₂RR. To achieve the direct application of these CO₂-capturing organic amines as carbon sources in electrolysis processes, it is essential to thoroughly investigate the microscopic interaction mechanisms between electrode catalysts and electrolytes. By systematically studying the structure–activity relationships between organic amine molecular characteristics and eCO₂RR performance, and establishing scientifically robust screening criteria, the most promising carbon source materials can be selected from numerous amine candidates. This multi-scale optimization strategy not only enables the synergistic optimization of efficient CO₂ capture and directional conversion but also holds promise for constructing a complete closed-loop “capture–conversion–utilization” carbon cycle system. The breakthroughs in this technological approach will provide innovative solutions for advancing carbon neutrality goals while establishing important theoretical foundations for developing next-generation carbon-neutral technologies.

2.3. Ionic Liquid Electrolyte

Ionic liquids (ILs), a class of molten salt systems composed entirely of ions at or near room temperature [35], have emerged as novel CO₂ capture materials to replace traditional alkanolamine absorbents [36] due to their unique physicochemical properties. These materials demonstrate remarkable advantages in carbon capture, including negligible volatility, exceptional thermal stability, and high CO₂ affinity. Studies show that 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) can achieve a CO₂ molar absorption ratio of 1:0.75 under 8.3 MPa high-pressure conditions [37]. Systematic gas solubility comparative studies by Anthony et al. [38] (Figure 8) confirmed that ILs exhibit significantly superior selective absorption capacity for CO₂ over other gas components like O₂.

Among various ILs, imidazolium-based ionic liquids have attracted particular attention [39,40,41] owing to their low viscosity, rapid mass transfer characteristics, and excellent CO₂ solubility. Research reveals that imidazole–carboxylate complexes formed between imidazolium groups and CO₂ play a pivotal role in eCO₂RR processes (Figure 9). This unique interaction not only reduces the reaction energy threshold but also effectively suppresses CO₂ conversion to oxalic acid (C₂H₂O₄), thereby promoting formic acid (HCOOH) production and significantly enhancing product selectivity. These distinctive characteristics endow imidazolium-based ILs with exceptional potential for developing integrated carbon capture and conversion systems.

Numerous studies have demonstrated that ILs as electrolytes exhibit remarkable advantages in eCO₂RR systems. Their exceptional CO₂ capture capacity provides abundant active sites for reactions, effectively reducing overpotential, suppressing the HER, and enhancing target product selectivity. Under mild conditions, ILs enable simultaneous efficient CO₂ capture and directional conversion, making them ideal electrolyte candidates for eCO₂RR.

The current research has identified three primary reduction products in IL-based eCO₂RR systems: HCOOH, CO, and C₂H₂O₄. While the formation mechanism of HCOOH via imidazolium-based ILs has been previously detailed, the CO generation pathway involves distinct intermediates. Chen et al. [42] revealed that imidazolium ILs can form [CO₂-bmim] complexes with CO₂ radical anions (CO₂⁻), which subsequently react with additional CO₂ to yield (CO₂)₂⁻ adducts before ultimately reducing to CO and carbonate ions (CO₃²⁻) at the cathode surface (Figure 10). Through the density functional theory (DFT) calculations, Hu et al. [43] systematically compared the effects of different anions (Cl⁻, [PF₆]⁻, and [Tf₂N]⁻), discovering that Cl⁻ significantly alters the linear configuration of CO₂, thereby improving reaction efficiency.

In the mechanistic study of C₂H₂O₄ formation, Yang et al. [8] systematically evaluated various ILs using nuclear magnetic resonance (NMR) and identified tetraethylammonium 4-(methoxycarbonyl)phenolate ([TEA][4-MF-PhO]) as the optimal catalyst. The reaction proceeds through the following steps: (1) the reduction of CO₂ to CO₂^*− radicals, (2) protonation to form [4-MF-PhO-COOH] intermediates, and (3) subsequent dimerization to yield C₂H₂O₄ (Figure 11). This system exhibits three distinctive features: (1) the proton-deficient environment effectively suppresses the HER, (2) aromatic ester anions provide dual active sites, and (3) significantly enhanced C₂H₂O₄ selectivity.

These findings provide crucial guidance for designing functionalized ILs. Future research should focus on elucidating how different functional groups influence catalytic performance to achieve precise control over eCO₂RR product selectivity. The strategic molecular engineering of ILs will advance integrated carbon capture and conversion technologies.

Inspired by the carbamate formation mechanism between amine solvents (-NH₂ groups) and CO₂, researchers have developed task-specific ILs (TSILs) to enhance CO₂ capture performance. These TSILs feature covalently tethered functional groups (e.g., -NH₂) on either cations or anions. Bates et al. [44] synthesized 1-butyl-2-bromopropylamine imidazolium ([NH₂p-bim][BF₄]), which demonstrated a CO₂ absorption ratio of 1:0.5 (mol/mol) with characteristic carbamate peaks confirmed by FT-IR and NMR. Zhang et al. [45] further developed dual-amine functionalized TSILs ([DNH₂EMIM]Br) achieving near 1:1 CO₂ absorption capacity, significantly outperforming mono-amine systems.

To address the mass transfer limitations caused by the high viscosity of conventional ILs, Wang et al. [46,47] systematically investigated the influence of component ratios in a [bmim][PF₆]/H₂O/MeCN ternary system on eCO₂RR performance. The following is evident from Figure 12: (a) when the mass fraction of [bmim][PF₆] increased from 5% to 60%, the Faradaic efficiency and current density exhibited a “volcano-type” trend, peaking at 30% [bmim][PF₆] content, and (b) with fixed [bmim][PF₆] content, similar behavior was observed as water content increased from 0% to 15%, with optimal performance at 5% water content. These results demonstrate that the appropriate regulation of IL/water ratios can effectively optimize interfacial mass transport. In catalyst modification, Sun et al. [13] achieved 91.9% FE_CO and 120 mA·cm⁻² current density using an IL-modified Ni/Fe dual-atom catalyst, attributed to enhanced CO₂ adsorption and reduced onset potential. Vichou et al. [18] found that [Emim][BF₄]-modified copper nanoparticles significantly improved C₂₊ product selectivity under acidic conditions (0.05 and 0.1 M H₂SO₄, Figure 13), demonstrating the effective HER suppression through IL modification.

These studies provide critical guidance for IL applications in eCO₂RR:

• Functional modification enables the precise control of CO₂ capture capacity;
• Solvent system optimization improves mass transport properties;
• Interface engineering enhances catalyst selectivity;

The current applications of ILs in CO₂ capture and eCO₂RR face several technical limitations. First, their high viscosity, particularly in amino-functionalized ILs due to hydrogen bonding networks, reduces mass transfer efficiency and conductivity while requiring energy-intensive agitation to maintain adequate current densities. Second, their relatively high production costs hinder large-scale implementation. Third, conventional ILs exhibit limited CO₂ adsorption capacities, often necessitating energy-consuming pressure or temperature swings. Fourth, the complex synthesis and elevated costs of functionalized ILs impede industrial adoption. Finally, most ILs show poor product selectivity in eCO₂RR, typically yielding mixed C₁, C₂, and C₂₊ products.

Ongoing research aims to address these challenges through strategic improvements in IL design and system optimization. Key priorities include elucidating ILs’ mechanistic roles in modifying reaction pathways and enhancing electrode stability, investigating synergistic effects between ILs and electrode materials, and developing cost-effective synthesis routes. Further advancements require optimized electrode–electrolyte combinations characterized by advanced techniques, along with scalable reactor designs to bridge laboratory and industrial applications. These efforts may enable more efficient CO₂ capture and conversion systems based on IL technology.

2.4. Other Electrolytes

Beyond the three mainstream electrolytes for eCO₂RR, researchers are exploring alternative carbon sources for capture and conversion, including ternary electrolytes and amino acids. Ternary electrolytes, composed of three organic or inorganic components, represent a promising direction. Yang et al. [48] developed an IL-based ternary system containing 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]), 1-dodecyl-3-methylimidazolium tetrafluoroborate ([C₁₂mim][BF₄]), and acetonitrile (MeCN). Using small-angle X-ray scattering (SAXS) and molecular dynamics simulations, they investigated the electrolyte’s microstructure and its interfacial interactions with electrodes. This optimized system enhanced current density, improved CO₂ adsorption, facilitated proton diffusion, and suppressed HER, achieving nearly 100% FE_CO within −2.2 to −2.5 V vs. Ag/Ag⁺ (

Table 1. The total current density and FEs of all carbon-containing products over Cu electrode at −2.5 V vs. Ag/Ag+ in different electrolytes (reproduced with permission from [48], J. Am. Chem. Soc., 2023).

Entry	Electrolytes	j (mA·cm−2)	FECO (%)	FEHCOOH (%)
1	0.3 M BminBF4/0.2 M C12minBF4/MeCN (1)	83.6 ± 1.3	99.5 ± 0.2	0
2	0.5 M C12minBF4/MeCN (2)	48.3 ± 2.6	99.2 ± 0.4	0
3	0.5 M BminBF4/MeCN (3)	63.2 ± 3.2	78.2 ± 1.3	3.6 ± 2.6
4	0.4 M BminBF4/0.1 M C12minBF4/MeCN	77.8 ± 1.5	98.3 ± 0.8	1.0 ± 0.5
5	0.2 M BminBF4/0.3 M C12minBF4/MeCN	72.4 ± 1.0	98.8 ± 0.2	0
6	0.1 M BminBF4/0.4 M C12minBF4/MeCN	66.5 ± 3.1	98.3 ± 0.5	0

).

In biochemical systems, amino acids—organic compounds containing both amino (-NH₂) and carboxyl (-COOH) groups—demonstrate remarkable CO₂ capture potential in their salt forms. These amino acid salts exhibit four key advantages: (1) negligible volatility, (2) excellent environmental compatibility, (3) strong oxidation resistance, and (4) low dissociation energy barriers, making them particularly suitable for oxygen-rich carbon capture applications while significantly reducing regeneration energy requirements.

Tsuda et al. [49] systematically investigated five amino acids (L-histidine, L-tryptophan, L-citrulline, L-phenylalanine, and L-arginine) as electrolyte additives for Cu-catalyzed eCO₂RR using energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and linear sweep voltammetry (LSV). Experiments conducted in CO₂-saturated 0.5 M KHCO₃ electrolyte at 1.27 V (vs. RHE) and 3 C·cm⁻² revealed that amino acids specifically adsorb on electrodeposited Cu surfaces, modifying reaction pathways by altering interfacial potentials and reducing average Cu grain sizes. Significantly, L-histidine containing an imidazole group showed optimal catalytic modification, enhancing FE_CH4 from 55% to 67.6% (Figure 14), highlighting amino acids’ unique role in tuning eCO₂RR selectivity and activity.

This section reviews the key characteristics, mechanisms, and applications of three major electrolyte systems (bicarbonates, amines, and ionic liquids) and emerging alternatives for ICCU (

Table 2. Performance comparison of electrolyte systems for CO₂ electroreduction.

Electrolyte Type	Advantages	Limitations
Bicarbonates	Fast reaction kinetics and high CO2 absorption efficiency [21,22] Easy CO2 desorption and absorbent regeneration Superior eCO2RR selectivity (e.g., CsHCO3 [6])	Highly corrosive (equipment challenges) Environmental risks if leaked Metal deposition issues (e.g., Li+) [6]
Amines	Mature technology (e.g., MEA) with high CO2 capture efficiency [33] Direct carbon source for eCO2RR Primary formate production [7]	High regeneration energy Competitive adsorption on electrodes (e.g., Cu [7]) High reaction barriers
ILs	Negligible volatility and thermal stability High CO2 affinity/selectivity Tunable product distribution (HCOOH [39]/CO [42]) HER suppression	High viscosity limits mass/charge transfer Prohibitive costs Moderate CO2 capacity Complex synthesis
Emerging Systems	Ternary electrolytes (e.g., IL mixtures) enhance current density/selectivity (FECO ≈ 100%) [48] Amino acid salts: eco-friendly, low-energy	Requires precise composition tuning [48] Complex amino acid–electrode interactions [49]

). Bicarbonate electrolytes (e.g., CsHCO₃) demonstrate efficient CO₂ conversion but face corrosion issues, while mature amine-based systems (e.g., MEA) require high regeneration energy. Although ionic liquids offer tunable selectivity and stability through functionalization, their high viscosity and cost remain challenges. Promising alternatives like ternary electrolytes and amino acid salts show synergistic potential. Future development should focus on electrolyte–electrode interactions, cost-effective synthesis, and scalable designs to enable closed-loop ICCU technologies.

3. Electrolyte Engineering for Product-Selective eCO₂RR

3.1. pH-Mediated Control of Electrolytes in CO₂ Electroreduction

Electrolyte pH critically determines eCO₂RR selectivity and reaction kinetics through distinct dual mechanisms. Under strongly acidic conditions (pH < 1), carbonate deposition is effectively suppressed, achieving 77% single-pass CO₂ conversion, though requiring high K⁺ concentrations (e.g., 3 M KCl) to maintain interfacial charge balance while enhancing C₂₊ production to 50% [9]. In 1 M H₃PO₄ electrolyte, high current density (1 A/cm²) establishes a pH gradient (bulk pH = 1 → local pH = 6.3 at 33 μm from electrode), which critically governs C-C coupling. Moderately acidic conditions (pH 2–4) alter proton donor species (H₃O⁺→H₂O), substantially modifying reaction barriers as demonstrated by the increased HER activation energy on Pt(111) from 0.04 to 0.42 eV [50]. While neutral/alkaline conditions (pH 6.8–13) promote C₂H₄ formation (40% selectivity on Cu(100) at pH 6.8), carbonate precipitation reduces CO₂ utilization below 25% [9,51].

A breakthrough strategy involves establishing “acidic bulk–neutral interface” pH gradients. Phosphate buffer systems (1 M H₃PO₄ + 3 M KCl) maintain a bulk pH of <1 while generating neutral interfacial conditions through OH⁻ accumulation at high current densities [9,52]. Microkinetic modeling indicates a mechanistic transition from CPET to SPET when pH exceeds 4.66 (HCOO^* pK_a), favoring HCOOH production [53], thereby providing quantitative criteria for pH-zone control.

3.2. Concentration Effects of Electrolytes on eCO₂RR

Electrolyte concentration profoundly influences eCO₂RR performance by modulating the electric double-layer structure and interfacial reaction kinetics [54,55]. The primary mechanism involves local electric field regulation—for instance, 2.5 M KAc enhances C₂H₄ selectivity to 44.2% on Cu catalysts [55] through cation enrichment (K⁺/Cs⁺) that stabilizes OCCO intermediates [54,56] and modifies CO coverage to promote C-C coupling [57].

Mass transport effects are equally significant. While 1.0 M KHCO₃ achieves 60% FE for HCOOH [58], 0.1 M CsHCO₃ yields a C₂H₄/CH₄ selectivity ratio of 4.8 [57]. However, excessive concentration (e.g., 3 M KHCO₃) reduces C₂₊ FE from 70% to 30% due to HCO₃⁻-mediated competing reactions [59,60]. Optimal ranges are 0.1–0.5 M KHCO₃ for C₂₊ products, 1.0 M KHCO₃ for HCOOH pathways [58,59], and 0.7–0.9 M for ILs to balance activity and selectivity [56,60].

3.3. Cation–Anion Synergistic Effects in eCO₂RR

Cations modulate reaction pathways through three primary mechanisms: (1) interfacial field effects—large-radius cations (Cs⁺, K⁺) induce a 3.7 cm⁻¹ redshift in C≡O vibration frequency [19]; (2) proton transfer mediation—acidic cations (Li⁺) optimize proton-coupled electron transfer, reducing methanol (CH₃OH) production Tafel slope to 47 mV/dec [14]; and (3) surface reconstruction—K⁺ enrichment forms high-activity Cu facets, achieving a C₂₊ current density of 225 mA/cm² [20]. Multivalent cations (e.g., Sr²⁺) further enhance CO₂ adsorption via oxygen vacancy generation [12].

Anions primarily modify interfacial microenvironments through specific adsorption: in the competitive sequence I⁻ > Br⁻ > Cl⁻, chloride electrolytes enable 97% C₂₊ FE on Ag electrodes [61]. ^*OH adlayers reduce C-C coupling barriers by 34% via K⁺ electrostatic attraction [62]. ILs (e.g., [EMIm]⁺) maintain >100 mA/cm² steady current by preventing OH⁻ interference [63], while organic additives (e.g., T-Pyr) boost C_≥2 products to 80% efficiency through local pH regulation [64].

Synergistic cation–anion interactions operate through the following: (1) electrostatic coupling—K⁺ enrichment and I⁻ adsorption jointly enhance C₂₊ selectivity (97% FE on Ag) [61]; (2) dynamic proton management—multivalent cations (Al³⁺) provide secondary proton sources at pH = 3, while buffer anions (HCO₃⁻) maintain optimal microenvironments [65]; and (3) cooperative surface restructuring—K⁺-induced facet reconstruction and Br⁻ site-selective passivation synergistically optimize Cu active sites, increasing C₂H₄ FE from 3.1% to 9.3% [9,52].

3.4. Design Strategies for Dopant Engineering

Dopants enable selective control by reconstructing interfacial microenvironments. In aprotic solvents (e.g., acetonitrile), proton concentration management suppresses the HER while achieving near 100% C₂₊ FE [15,17]. Hydrophobic PVDF–Nafion membranes similarly restrict proton transport [17]. Under alkaline conditions, OH⁻-induced ^*OH adlayers promote K⁺ enrichment, reducing C-C coupling barriers from 0.68 to 0.45 eV [62].

Surfactant design demonstrates precise modulation:

• Cationic CTAB lowers the proton concentration via hydrophobic chains, yielding 50% HCOOH FE [15];
• Double-charged quaternary ammonium salts tune product selectivity (28% C₂H₄ with C₈ chains vs. 41% CO with C₂ chains) [66];
• Hydrophobic [NTF₂]⁻ enriches CO₂ for 38.7% HCOOH FE [15];
• Immobilized imidazolium cations achieve 85% HCOOH selectivity in acidic media [67];
• Lewis acids (e.g., boric acid) form [B(OCH₂CH₂O)₂]⁻ chelates for 85% ethylene glycol FE [68].

3.5. Rational Screening Strategies

Three targeted electrolyte systems have been established based on the reaction mechanisms:

(1)

For C₁ products (HCOOH/CO), optimal performance requires pH > 4.66 buffered systems [53] with 1.0 M KHCO₃ [58], combined with hydrophobic anions ([NTF₂]⁻) [15] or immobilized imidazolium cations [67] to promote SPET pathways.

(2)

C₂₊ production necessitates constructing acidic bulk–neutral interface gradients (1 M H₃PO₄ + 3 M KCl) [9,52] with Cs⁺/K⁺ cations and specifically adsorbed I⁻/Br⁻ anions [19,61], supplemented by organic additives like T-Pyr [64].

(3)

Alcohol synthesis (CH₃OH/C₂H₅OH) benefits from Li⁺-containing electrolytes [14] at concentrations of 0.1–0.5 M paired with Lewis acid additives for directed synthesis [68].

Future advancements should integrate operando characterization with multiphysics modeling to establish comprehensive reaction phase diagrams across pH, ionic composition, and potential parameters, enabling precise electrolyte engineering.

4. Application of ICCU

Electrochemical cells serve as fundamental components in eCO₂RR systems, playing a pivotal role in both experimental research and industrial applications. The current eCO₂RR studies primarily utilize five cell configurations: H-type cells, flow cells, solid oxide electrolysis cells, membrane electrode assemblies, and microfluidic cells. Among these, liquid-phase flow cells with continuous material renewal demonstrate particular advantages for ICCU systems due to their superior mass transfer capabilities (Figure 15).

The electrolyte functions as a critical CO₂ carrier throughout the capture–conversion process, operating dually as follows: (1) an absorption medium in upstream capture towers, and (2) a liquid-phase CO₂ transporter to downstream flow cells for eCO₂RR. This creates an efficient industrial synergy between liquid electrolytes, absorption towers, and flow electrolyzers.

As illustrated in Figure 15a, conventional CCU systems employing H-cells require complete “capture–regeneration–separation” processes to supply pure CO₂ for eCO₂RR, resulting in significant energy penalties and operational complexity. To address this, researchers have developed directly coupled ICCU systems (Figure 15b) where CO₂ desorption and conversion occur in situ within a single reactor [69,70]. This innovative design eliminates energy-intensive CO₂ storage, transportation, and regeneration steps, establishing a streamlined technical pathway for carbon utilization.

The ICCU system faces three primary challenges: (1) flue gas impurity interference, (2) absorber–electrolyzer coordination, and (3) system-wide efficiency optimization. SO_x/NO_x impurities significantly degrade electrolyte performance: SO₂ dissolution (SO₂ + H₂O→H₂SO₃) reduces pH and competes for active sites, causing a U-shaped trend in CO₂ reduction selectivity (minimum 53.7% at 0.6% SO₂) [71,72]. Solutions include Ni single-atom catalysts (Ni-NC), which maintain 92% CO selectivity under 0.3% SO₂ [72], and functionalized ILs (e.g., [aP₄₄₄₃][2-Op]) with 1.57 mol CO₂/mol IL capacity, though the challenges of viscosity and impurity sensitivity remain [73,74]. Electrochemical desulfurization reduces H₂S from 1330 ppm to <0.5 ppm at 29% current efficiency [75].

Absorber–electrolyzer synergy and process optimization: The effective integration of absorbers and electrolyzers requires precise capacity matching. Conventional amine regeneration methods demand significant energy inputs (3.5–4.0 GJ/tCO₂ at 120 °C steam) [76]. In contrast, electrochemically mediated amine regeneration (EMAR) in ICCU systems reduces this energy requirement to 40–80 kJ/molCO₂ [11]. Advanced EMAR reactors employing multi-electrode configurations with gradient potential (ΔE = −0.04 V) demonstrate 20.05% enhanced desorption rates with only 4.04% additional energy consumption [11]. Flow dynamics optimization reveals that increasing flow velocity from 0.005 to 0.01 m/s improves desorption rates by 2.91‰ [11]. Similarly, in PEM water electrolysis (PEMWE) systems, membrane electrode assemblies (MEAs) show reduced temperature rise rates (from 69.6 to 50.4 °C/min) when flow rates increase from 20 to 100 mL/min [77]. Furthermore, multi-cycle topological optimization through density optimization and k-medoid clustering algorithms significantly improves system performance, increasing waste heat utilization from 8.5% to 51.3% while reducing operational costs by 17.9% [78].

Economic viability and carbon footprint optimization for ICCU systems: Techno-economic analysis (TEA) reveals that HCOOH and CO production achieve competitive levelized costs of USD 0.468/kg and USD 0.449/kg, respectively, while C₂ products (e.g., C₂H₄) require >90% FE at <2 V to become economically viable [79]. Significant energy reductions have been demonstrated through heat pump-assisted pressure-swing distillation (HPA-PSD), lowering HCOOH separation energy from 19.85 to 12.2 kWh/kg [79]. From an emission perspective, concentrated solar power (CSP)-driven ICCU systems achieve negative carbon footprints (GWP = −1.05 kg CO₂-eq) [79]. Operational efficiency gains are further achieved through intelligent control systems, with the Autoformer–DQN–Deep Forest model demonstrating 97.83% prediction accuracy for real-time carbon capture optimization using reinforcement learning on the CARMA dataset [80]. These advancements collectively establish a robust foundation for industrial-scale ICCU implementation.

5. Conclusions

While most eCO₂RR research focuses on catalyst development, systematic electrolyte studies remain limited. This work comprehensively analyzes three major electrolyte types for ICCU systems, providing critical insights for efficient carbon cycling.

Bicarbonate electrolytes are widely used due to their excellent pH buffering and rapid CO₂ equilibrium, with NH₄HCO₃ showing particular promise owing to its low regeneration energy (48.38 kJ/mol) and dual hydrolysis properties, though ammonia escape remains an engineering challenge. Amine-based electrolytes like MEA demonstrate commercial viability in carbon capture (>90% efficiency), but their eCO₂RR reaction mechanisms require further elucidation through advanced characterization techniques. ILs represent highly tunable candidates, with functionalized ILs achieving both efficient CO₂ capture and enhanced catalytic selectivity. For instance, imidazolium-based ILs enable 85% HCOOH selectivity, while amino-modified TSILs achieve 1:1 CO₂ absorption ratios. Although viscosity and cost limitations persist, strategies like aqueous blending and anion optimization can improve performance. Additionally, emerging amino acid salts show unique potential—additives like L-histidine significantly enhance product selectivity—though their standalone use as electrolytes warrants further investigation.

Recent studies demonstrate the significant potential of machine learning for electrolyte optimization. Salomone et al. [10] developed a two-step approach combining gradient boosting classifiers (GBCs) and regressors (GBRs) to predict CO adsorption energies on Cu-based bimetallic alloys with high accuracy (error < 0.05 eV²). By analyzing 13 chemical and 2 geometric descriptors, the model achieves precise CO adsorption energy predictions (F1-score > 96%), enabling the rapid screening of efficient CO₂RR electrocatalysts. This breakthrough highlights machine learning’s capability to accelerate the optimization of electrolyte–catalyst systems.

When applied to process simulation, Aspen Plus demonstrates distinct capability variations across different electrochemical systems. While Aspen Plus is well established for water electrolysis, its application to electrocatalytic CO₂ conversion faces limitations. The software performs system-level process simulations but cannot model micro-scale electrochemical processes. Dedicated modules for this application are also lacking. The introduction of AI Model Builder in Aspen Plus V12 addresses these challenges by combining machine learning with process simulation. This integration enables cross-scale optimization from molecular to system levels, offering new possibilities for eCO₂RR development.

Effective ICCU systems demand electrolytes that simultaneously satisfy multiple criteria: high CO₂ capture capacity, direct participation in electrocatalytic conversion, balanced conductivity and fluidity, suppressed side reactions, and environmental compatibility with recyclability. Emerging machine learning approaches, particularly graph neural networks (GNNs) for predicting functionalized ILs’ performance and reinforcement learning for dynamic reaction condition optimization, offer promising avenues to accelerate electrolyte development.

6. Future Prospects

Future research on electrolytes for ICCU systems will witness a deepening interdisciplinary convergence of electrochemistry, artificial intelligence, and materials engineering. The integration of in situ electrochemical characterization techniques, multi-scale computational modeling, and intelligent optimization algorithms is expected to enable comprehensive innovation spanning from molecular design to industrial-scale implementation.

At the fundamental level, advanced characterization methods such as time-resolved spectroscopy and synchrotron radiation should be prioritized to elucidate the solvation structures of ions and the dynamic evolution of interfacial electric double layers, particularly the competitive adsorption behavior between amine-based electrolytes and electrode surfaces. Concurrent efforts should focus on developing cost-effective synthesis routes for ionic liquids while employing machine learning-assisted high-throughput screening to optimize their viscosity, conductivity, and CO₂ affinity.

For practical applications, bridging atomic-scale DFT calculations with system-level Aspen Plus simulations is critical. When complemented by the COMSOL multiphysics coupling analysis, this approach establishes reliable cross-scale correlations from molecular structures to reactor design. With advancements in adaptive control algorithms and modular electrolyzer technologies, ICCU systems are poised to achieve intelligent integration with renewable power generation, ultimately achieving industrial-scale carbon cycling solutions with high selectivity and low energy consumption. This approach will not only reduce the operational energy demands of conventional carbon capture but also facilitate the transition of electrolyte materials from lab-scale gram-level production to industrial manufacturing, providing critical technological support for global carbon neutrality goals.