A Bifunctional Ionic Liquid for Capture and Electrochemical Conversion of CO2 to CO over Silver

Electrochemical valorization of captured CO2: recent advances and future perspectives

The excessive emission of CO2 has led to severe climate change, prompting global concern. Capturing CO2 and converting it through electrochemistry into value-added products represent promising approaches to mitigating CO2 emissions and closing the carbon cycle. Traditionally, these two processes have been performed independently, involving multiple steps, high energy consumption, and low efficiency. Recently, the electrochemical conversion of captured CO2, which integrates the capture and conversion processes (also referred to as electrochemically reactive CO2 capture), has garnered increasing attention. This integrated approach bypasses the energy-intensive steps involved in the traditional independent process, including CO2 release, purification, compression, transportation, and storage. In this review, we discuss recent advances in the electrochemical conversion of captured CO2, focusing on four key aspects. First, we introduce various capture media, emphasizing the thermodynamic aspects of carbon capture and their implications for integration with electrochemical conversion. Second, we discuss product control mediated by the selection of different catalysts, highlighting the connections between the conversion of captured CO2 and gas-fed CO2. Third, we examine the effect of reactor systems and operational conditions on the electrochemical conversion of captured CO2, shedding light on performance optimization. Finally, we explore real integration systems for CO2 capture and electrochemical conversion, revealing the potential of this new technology for practical applications. Overall, we provide insights into the existing challenges, potential solutions, and thoughts on opportunities and future directions in the emerging field of electrochemical conversion of captured CO2.

Polymer Sorbent Design for the Direct Air Capture of CO2

Anthropogenic activities have resulted in enormous increases in atmospheric CO2 concentrations particularly since the onset of the Industrial Revolution, which have potential links with increased global temperatures, rising sea levels, increased prevalence, and severity of natural disasters, among other consequences. To enable a carbon-neutral and sustainable society, various technologies have been developed for CO2 capture from industrial process streams as well as directly from air. Here, direct air capture (DAC) represents an essential need for reducing CO2 concentration in the atmosphere to mitigate the negative consequences of greenhouse effects, involving systems that can reversibly adsorb and release CO2, in which polymers have played an integral role. This work provides insights into the development of polymer sorbents for DAC of CO2, specifically from the perspective of material design principles. We discuss how physical properties and chemical identities of amine-containing polymers can impact their ability to uptake CO2, as well as be efficiently regenerated. Additionally, polymers which use ionic interactions to react with CO2 molecules, such as poly(ionic liquids), are also common DAC sorbent materials. Finally, a perspective is provided on the future research and technology opportunities of developing polymer-derived sorbents for DAC.

Carboxylative cyclization of atmospheric CO2 with alkynol catalyzed by a 1-methylhydantoin anion-functionalized ionic liquid via chelative interactions

Metal- and solvent-free carboxylative cyclization of atmospheric CO2 with alkynol can be achieved using a 1-methylhydantoin anion-functionalized ionic liquid. 1H NMR, in situ FT-IR and DFT calculations indicate that the 1-methylhydantoin anion acts as a "pincer ligand" to form chelative interactions with the hydroxyl group, thereby effectively activating the alkynol.

Ag Atom Induces Microstrain Environment around Cd Sites to Construct Diatomic Sites for Almost 100% CO2-to-CO Electroreduction

Deeply understanding how local microstrain environment around diatomic sites influences their electronic state and adsorption is crucial for improving electrochemical CO2 reduction (eCO2R) reaction; however, precise engineering of the atomic microstrain environment is challenging. Herein, we fabricate Ag-CdTMT electrocatalysts with AgN2S2-CdN2S2 diatomic sites by anchoring Ag to the nodes of CdTMT (TMT = 2,4,6-trimercaptotriazine anion) coordination polymers. The Ag-CdTMT catalysts achieve approximately 100% Faradaic efficiency for CO reduction with an industrial level current density (∼200 mA cm-2 in H-cell). The embedded Ag atoms induce the formation of Ag-Cd diatomic sites with local microstrain, stretching Cd-N/S bonds, and reinforcing electron localization at Cd sites. The microstrain engineering and adjacent Ag atoms synergistically reduced Cd 4d-C 2p antibonding orbital occupancy for intensifying *COOH adsorption as the rate-determining step. This study provides novel insights into customizing the electronic structure of diatomic sites through strain engineering.

Plasmon-Driven Highly Selective CO2 Photoreduction to C2H4 on Ionic Liquid-Mediated Copper Nanowires

Selective CO2 photoreduction to value-added multi-carbon (C2+) feedstocks, such as C2H4, holds great promise in direct solar-to-chemical conversion for a carbon-neutral future. Nevertheless, the performance is largely inhibited by the high energy barrier of C-C coupling process, thereby leading to C2+ products with low selectivity. Here we report that through facile surface immobilization of a 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) ionic liquid, plasmonic Cu nanowires could enable highly selective CO2 photoreduction to C2H4 product. At an optimal condition, the resultant plasmonic photocatalyst exhibits C2H4 production with selectivity up to 96.7 % under 450 nm monochromatic light irradiation, greatly surpassing its pristine Cu counterpart. Combined in situ spectroscopies and computational calculations unravel that the addition of EMIM-BF4 ionic liquid modulates the local electronic structure of Cu, resulting in its enhanced adsorption strength of *CO intermediate and significantly reduced energy barrier of C-C coupling process. This work paves new path for Cu surface plasmons in selective artificial photosynthesis to targeted products.

Customizing Ionic Liquids Functionalized MOFs Composites with Hydrophobic Interface for Electrochemical CO2 Reduction

The electrochemical carbon dioxide reduction reaction (CO2RR) to generate feedstocks for chemical products (e.g., carbon monoxide, CO) offers a highly attractive method for achieving the closure of the carbon cycle. Ionic liquids (ILs)-functionalized Cu-based catalyst Cu2O-HKUST-1/IL1/PTFE was developed, configuring metal-organic frameworks(MOFs) based materials with high adsorption and multiple active sites. The modified electrocatalysts exhibited high specific surface area, strong CO2 adsorption capacity, abundant active sites, and fast charge transfer rate. The nucleophilic active site of deprotonation at the C2 site in imidazole ILs further improved the selectivity of proton migration and CO product generation, which was verified through DFT calculations for the low Gibbs free energy of the generated intermediate interactions. In addition, the hydrophobic interface constructed by PTFE facilitated the inhibition of the hydrogen evolution reaction (HER) and significantly improved the efficiency of CO2 electroreduction. The Cu2O-HKUST-1/IL1/PTFE catalyst manifested a high C1 Faraday efficiency (FE) up to 96.5% and in particular 92.7% for FECO at -1.7 V vs RHE. The present work provides an efficient strategy for configuring ILs-functionalized MOFs-based materials with good hydrophobic interfaces to enhance the efficiency of CO2 electroreduction to C1 products.

Strong effect-correlated electrochemical CO2 reduction

Electrochemical CO2 reduction (ECR) holds great potential to alleviate the greenhouse effect and our dependence on fossil fuels by integrating renewable energy for the electrosynthesis of high-value fuels from CO2. However, the high thermodynamic energy barrier, sluggish reaction kinetics, inadequate CO2 conversion rate, poor selectivity for the target product, and rapid electrocatalyst degradation severely limit its further industrial-scale application. Although numerous strategies have been proposed to enhance ECR performances from various perspectives, scattered studies fail to comprehensively elucidate the underlying effect-performance relationships toward ECR. Thus, this review presents a comparative summary and a deep discussion with respect to the effects strongly-correlated with ECR, including intrinsic effects of materials caused by various sizes, shapes, compositions, defects, interfaces, and ligands; structure-induced effects derived from diverse confinements, strains, and fields; electrolyte effects introduced by different solutes, solvents, cations, and anions; and environment effects induced by distinct ionomers, pressures, temperatures, gas impurities, and flow rates, with an emphasis on elaborating how these effects shape ECR electrocatalytic activities and selectivity and the underlying mechanisms. In addition, the challenges and prospects behind different effects resulting from various factors are suggested to inspire more attention towards high-throughput theoretical calculations and in situ/operando techniques to unlock the essence of enhanced ECR performance and realize its ultimate application.

Synergistic Effects of the Electric Field Induced by Imidazolium Rotation and Hydrogen Bonding in Electrocatalysis of CO2

The roles of the ionic liquid (IL), 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), and water in controlling the mechanism, energetics, and electrocatalytic activity of CO2 reduction to CO on silver in nonaqueous electrolytes were investigated. The first electron transfer occurs to CO2 at reduced overpotentials when it is trapped between the planes of the [EMIM]+ ring and the electrode surface due to cation reorientation as determined from voltammetry, in situ surface-enhanced Raman spectroscopy, and density functional theory calculations. Within this interface, water up to 0.5 M does not induce significant Faradaic activity, opposing the notion of it being a free proton source. Instead, water acts as a hydrogen bond donor, and the proton is sourced from [EMIM]+. Furthermore, this study demonstrates that alcohols with varying acidities tune the hydrogen bonding network in the interfacial microenvironment to lower the energetics required for CO2 reduction. The hydrogen bonding suppresses the formation of inactive carboxylate species, thus preserving the catalytic activity of [EMIM]+. The ability to tune the hydrogen bonding network opens new avenues for advancing IL-mediated electrocatalytic reactions in nonaqueous electrolytes.

Reactive capture and electrochemical conversion of CO2 with ionic liquids and deep eutectic solvents

Ionic liquids (ILs) and deep eutectic solvents (DESs) have tremendous potential for reactive capture and conversion (RCC) of CO2 due to their wide electrochemical stability window, low volatility, and high CO2 solubility. There is environmental and economic interest in the direct utilization of the captured CO2 using electrified and modular processes that forgo the thermal- or pressure-swing regeneration steps to concentrate CO2, eliminating the need to compress, transport, or store the gas. The conventional electrochemical conversion of CO2 with aqueous electrolytes presents limited CO2 solubility and high energy requirement to achieve industrially relevant products. Additionally, aqueous systems have competitive hydrogen evolution. In the past decade, there has been significant progress toward the design of ILs and DESs, and their composites to separate CO2 from dilute streams. In parallel, but not necessarily in synergy, there have been studies focused on a few select ILs and DESs for electrochemical reduction of CO2, often diluting them with aqueous or non-aqueous solvents. The resulting electrode-electrolyte interfaces present a complex speciation for RCC. In this review, we describe how the ILs and DESs are tuned for RCC and specifically address the CO2 chemisorption and electroreduction mechanisms. Critical bulk and interfacial properties of ILs and DESs are discussed in the context of RCC, and the potential of these electrolytes are presented through a techno-economic evaluation.

Key Experimental Considerations When Evaluating Functional Ionic Liquids for Combined Capture and Electrochemical Conversion of CO2

Ionic liquids (ILs) are considered functional electrolytes for the electrocatalytic reduction of CO2 (ECO2R) due to their role in the double-layer structure formation and increased CO2 availability at the electrode surface, which reduces the voltage requirement. However, not all ILs are the same, considering the purity and degree of the functionality of the IL. Further, there are critical experimental factors that impact the evaluation of ILs for ECO2R including the reference electrode, working electrode construction, cosolvent selection, cell geometry, and whether the electrochemical cell is a single compartment or a divided cell. Here, we describe improved synthesis methods of imidazolium cyanopyrrolide IL for electrochemical studies in consideration of precursor composition and reaction time. We explored how IL with cosolvents (i.e. acetonitrile, dimethylformamide, dimethyl sulfoxide, propylene carbonate, and n-methyl-2-pyrrolidone) affects conductivity, CO2 mass transport, and ECO2R activation overpotential together with the effects of electrode materials (Sn, Ag, Au, and glassy carbon). Acetonitrile was found to be the best solvent for lowering the onset potential and increasing the catalytic current density for the production of CO owing to the enhanced ion mobility in combination with the silver electrode. Further, the ECO2R activity of molecular catalysts Ni(cyclam)Cl2 and iron tetraphenylsulfonato porphyrin (FeTPPS) on the carbon cloth electrode maintained high Faradaic efficiencies for CO in the presence of the IL. This study presents best practices for examining nontraditional multifunctional electrolytes amenable to integrated CO2 capture and conversion technologies for homogeneous and heterogeneous ECO2R.

Understanding the Electrode-Electrolyte Interfaces of Ionic Liquids and Deep Eutectic Solvents

Developing unconventional electrolytes such as ionic liquids (ILs) and deep eutectic solvents (DESs) has led to remarkable advances in electrochemical energy storage and conversion devices. However, the understanding of the electrode-electrolyte interfaces of these electrolytes, specifically the liquid structure and the charge/electron transfer mechanism and rates, is lacking due to the complexity of molecular interactions, the difficulty in studying the buried interfaces with nanometer-scale resolution, and the distribution of the time scales for the various interfacial events. This Feature Article outlines the standing questions in the field, summarizes some of the exciting approaches and results, and discusses our contributions to probing the electrified interfaces by electrochemical impedance spectroscopy (EIS), surface-enhanced Raman spectroscopy (SERS), and neutron reflectivity (NR). The related findings are analyzed within electrical double-layer models to provide a framework for studying ILs, DESs, and, more broadly, the concentrated hydrogen-bonded electrolytes.

Tailoring Electrochemical CO2 Reduction on Copper by Reactive Ionic Liquid and Native Hydrogen Bond Donors

Electrochemical CO2 reduction (CO2 RR) on copper (Cu) shows promise for higher-value products beyond CO. However, challenges such as the limited CO2 solubility, high overpotentials, and the competing hydrogen evolution reaction (HER) in aqueous electrolytes hinder the practical realization. We propose a functionalized ionic liquid (IL) which generates ion-CO2 adducts and a hydrogen bond donor (HBD) upon CO2 absorption to modulate CO2 RR on Cu in a non-aqueous electrolyte. As revealed by transient voltammetry, electrochemical impedance spectroscopy (EIS), and in situ surface-enhanced Raman spectroscopy (SERS) complemented with image charge augmented quantum-mechanical/molecular mechanics (IC-QM/MM) computations, a unique microenvironment is constructed. In this microenvironment, the catalytic activity is primarily governed by the IL and HBD concentrations; former controlling the double layer thickness and the latter modulating the local proton availability. This translates to ample CO2 availability, reduced overpotential, and suppressed HER where C4 products are obtained. This study deepens the understanding of electrolyte effects in CO2 RR and the role of IL ions towards electrocatalytic microenvironment design.

Cd/Cd(OH)2 Nanosheets Enhancing the Electrocatalytic Activity of CO2 Reduction to CO

Electric-driven conversion of carbon dioxide (CO2 ) to carbon monoxide (CO) under mild reaction conditions offers a promising approach to mitigate the greenhouse effect and the energy crisis. Surface engineering is believed to be one of the prospective methods for enhancing the electrocatalytic activity of CO2 reduction. Herein, hydroxyl (OH) groups were successfully introduced to cadmium nanosheets to form cadmium and cadmium hydroxide nanocomposites (i. e. Cd/Cd(OH)2 nanosheets) via a facile two-step method. The as-prepared Cd/Cd(OH)2 /CP (CP indicates carbon paper) electrode displays excellent electrocatalytic activity for CO2 reduction to produce CO. The Faradaic efficiency of CO reaches 98.3 % and the current density achieves 23.8 mA cm-2 at -2.0 V vs. Ag/Ag+ in a CO2 -saturated 30 wt% 1-butyl-3-methylimidazole hexafluorophosphate ([Bmim]PF6 )-65 wt% acetonitrile (CH3 CN)-5 wt% water (H2 O) electrolyte. And the CO partial current density can reach up to 71.6 mA cm-2 with the CO Faradaic efficiency of more than 85 % at -2.3 V vs. Ag/Ag+ , which stands out against Cd/CP, Cd(OH)2 /CP, and Cd/CdO/CP electrodes. The excellent electrocatalytic performance of the Cd/Cd(OH)2 /CP electrode can be attributed to its unique structural properties, suitable OH groups, perfect interaction with electrolyte, abundant active sites and fast electron transfer rate.