Electrochemical Reduction of CO2 at Metal Electrodes in a Distillable Ionic Liquid

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.

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.

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

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

Electrochemical conversion of CO2 requires selective catalysts and high solubility of CO2 in the electrolyte to reduce the energy requirement and increase the current efficiency. In this study, the CO2 reduction reaction (CO2RR) over Ag electrodes in acetonitrile-based electrolytes containing 0.1 M [EMIM][2-CNpyr] (1-ethyl-3-methylimidazolium 2-cyanopyrolide), a reactive ionic liquid (IL), is shown to selectively (>94%) convert CO2 to CO with a stable current density (6 mA·cm-2) for at least 12 h. The linear sweep voltammetry experiments show the onset potential of CO2 reduction in acetonitrile shifts positively by 240 mV when [EMIM][2-CNpyr] is added. This is attributed to the pre-activation of CO2 through the carboxylate formation via the carbene intermediate of the [EMIM]+ cation and the carbamate formation via binding to the nucleophilic [2-CNpyr]- anion. The analysis of the electrode-electrolyte interface by surface-enhanced Raman spectroscopy (SERS) confirms the catalytic role of the functionalized IL where the accumulation of the IL-CO2 adduct between -1.7 and -2.3 V vs Ag/Ag+ and the simultaneous CO formation are captured. This study reveals the electrode surface species and the role of the functionalized ions in lowering the energy requirement of CO2RR for the design of multifunctional electrolytes for the integrated capture and conversion.

Electrochemical organic reactions: A tutorial review

Although the core of electrochemistry involves simple oxidation and reduction reactions, it can be complicated in real electrochemical organic reactions. The principles used in electrochemical reactions have been derived using physical organic chemistry, which drives other organic/inorganic reactions. This review mainly comprises two themes: the first discusses the factors that help optimize an electrochemical reaction, including electrodes, supporting electrolytes, and electrochemical cell design, and the second outlines studies conducted in the field over a period of 10 years. Electrochemical reactions can be used as a versatile tool for synthetically important reactions by modifying the constant electrolysis current.

High-efficient carbon dioxide-to-formic acid conversion on bimetallic PbIn alloy catalysts with tuned composition and morphology

CO₂ electroreduction to value-added chemicals and fuels has gained increasing attention; however, there are only a few catalysts with high performance under mild conditions that can be used in this technique. In this study, single metal Pb, In and bimetallic PbIn catalysts for aqueous CO₂ electroreduction were prepared using a facile 3-step process including PbIn granulation by reducing Pb(NO₃)₂/In(NO₃)₃ aqueous solution with NaBH₄, calcination in air, and in situ electroreduction. The bimetallic PbIn catalysts had better catalytic performance on CO₂ electroreduction than single metal catalysts. The bimetallic Pb₇In₃ catalyst (atomic ratios of Pb and In is 7:3) presented the highest formic acid faradaic efficiency of 91.6% at -1.26 V vs reversible hydrogen electrode in a 0.5 M CO₂-saturated KHCO₃ aqueous solution, which was 13% and 9.7% higher than that of single Pb and In catalysts, respectively. Moreover, the catalyst remained active after 10 h of continuous CO₂ electrolysis with a stale current density of -17 mA cm^-2. The experimental results showed that the excellent catalytic performance of Pb₇In₃ catalyst may stem from its higher electrochemical active surface area, lower charge-transfer resistance and the synergistic effect of Pb and In in the catalyst. The presented bimetallic PbIn catalysts may have a wide of application prospect, and they may be synthesized from heavy metals in industrial wastewaters.

Emerging interstitial/substitutional modification of Pd-based nanomaterials with nonmetallic elements for electrocatalytic applications

Palladium (Pd)-based nanomaterials have been identified as potential candidates for various types of electrocatalytic reaction, but most of them typically exhibit unsatisfactory performances. Recently, extensive theoretical and experimental studies have demonstrated that the interstitial/substitutional modification of Pd-based nanomaterials with nonmetallic atoms (H, B, C, N, P, S) has a significant impact on their electronic structure and thus leads to the rapid development of one kind of promising catalyst for various electrochemical reactions. Considering the remarkable progress in this area, we highlight the most recent progress regarding the innovative synthesis and advanced characterization methods of nonmetallic atom-doped Pd-based nanomaterials and provide insights into their electrochemical applications. What's more, the unique structure- and component-dependent electrochemical performance and the underlying mechanisms are also discussed. Furthermore, a brief conclusion about the recent progress achieved in this field as well as future perspectives and challenges are provided.

Electroreduction of CO2 in Ionic Liquid-Based Electrolytes

Electroreduction of carbon dioxide (CO₂) to value-added chemicals and fuels is a promising approach for sustainable energy conversion and storage. Many electrocatalysts have been designed for this purpose and studied extensively. The role of the electrolyte is particularly interesting and is pivotal for designing electrochemical devices by taking advantage of the synergy between electrolyte and catalyst. Recently, ionic liquids as electrolytes have received much attention due to their high CO₂ adsorption capacity, high selectivity, and low energy consumption. In this review, we present a comprehensive overview of the recent progress in CO₂ electroreduction in ionic liquid-based electrolytes, especially in the performance of different catalysts, the electrolyte effect, as well as mechanism studies to understand the reaction pathway. Perspectives on this interesting area are also discussed for the construction of novel electrochemical systems.

Sustainable and efficient electrosynthesis of naproxen using carbon dioxide and ionic liquids

The use of CO₂ as a C1 carbon source for synthesis is raising increasing attention both as a strategy to bring value to carbon dioxide capture technologies and a sustainable approach towards chemicals and energy. The presented results focus on the application of electrochemical methods to incorporate CO₂ into organic compounds using ionic liquids as electrolytes, which provides a green alternative to the formation of C-C bonds. In this sense, the current manuscript shows that Naproxen (6-Methoxy-α-methyl-2-naphthaleneacetic acid) can be synthetizing in high yield (89%) and conversion rates (90%) through an electrocarboxylation process using CO₂ and ionic liquids. The role of the cathode and solvent, which can potentially enhance the synthesis, is also discussed. The "green" route described in the current work would open a new sustainable strategy for the electrochemical production of pharmaceutical compounds.

Insights into Carbon Dioxide Electroreduction in Ionic Liquids: Carbon Dioxide Activation and Selectivity Tailored by Ionic Microhabitat

Electroreduction of carbon dioxide (CO₂ ) into high value-added products is a potential solution to a reduction in CO₂ levels and its utilization. One major challenge is the lack of an efficient system that can highly selectively reduce CO₂ into desirable products with low energy consumption. Ionic liquids (ILs) have been used as electrolytes for the electroreduction of CO₂ , and it has been proven that the CO₂ -cation complex results in a low-energy pathway. In this work, an ionic microhabitat (IMH) has been built for CO₂ electroreduction, and a novel anion-functionalized IL, 1-butyl-3-methylimidazolium 1,2,4triazolide ([Bmim][124Triz]), has been designed as the reaction medium. The results showed that the IMH played a key role in enhancing the performance of CO₂ electroreduction, especially in dominating the product selectivity, which is recognized to be a great challenge in an electroreduction process. New insights into the role of the IMH in higher CO₂ solubility, bending linear CO₂ by forming the [124Triz]-CO₂^- adduct, and transferring activated CO₂ into the cathode surface easily were revealed. The Faradaic efficiency for formic acid is as high as 95.2 %, with a current density reaching 24.5 mA cm^-2 . This work provides a promising way for the design of robust and highly efficient ILs for CO₂ electroreduction.

Zinc-Coordinated Nitrogen-Codoped Graphene as an Efficient Catalyst for Selective Electrochemical Reduction of CO2 to CO

Electrochemical reduction of CO₂ to value-added chemicals by using renewable electricity offers a promising strategy to deal with rising CO₂ emission and the energy crisis. Single-site zinc-coordinated nitrogen-codoped graphene (Zn-N-G) catalyzes the electrochemical reduction of CO₂ to CO. The Zn-N-G catalyst exhibits excellent intrinsic activity toward CO₂ reduction, reaching a faradaic efficiency of 91 % for CO production at a low overpotential of 0.39 V. X-ray absorption fine structure and X-ray photoelectron spectroscopy both confirm the presence of isolated Zn-N_x moieties, which act as the key active sites for CO formation. DFT calculations reveal the origin of enhanced activity for CO₂ reduction on Zn-N-G catalysts. This work provide further understanding of the active centers on transition metal-nitrogen-carbon (M-N-C) catalysts for electrochemical reduction of CO₂ to CO.

Voltammetric Perspectives on the Acidity Scale and H+/H2 Process in Ionic Liquid Media

Nonhaloaluminate ionic liquids (ILs) have received considerable attention as alternatives to molecular solvents in diverse applications spanning the fields of physical, chemical, and biological science. One important and often overlooked aspect of the implementation of these designer solvents is how the properties of the IL formulation affect (electro)chemical reactivity. This aspect is emphasized herein, where recent (voltammetric) studies on the energetics of proton (H⁺) transfer and electrode reaction mechanisms of the H⁺/H₂ process in IL media are highlighted and discussed. The energetics of proton transfer, quantified using the p K_a (minus logarithm of acidity equilibrium constant, K_a) formalism, is strongly governed by the constituent IL anion, and to a lesser extent, the IL cation. The H⁺/H₂ process, a model inner-sphere reaction, also displays electrochemical characteristics that are strongly IL-dependent. Overall, these studies highlight the need to carry out systematic investigations to resolve IL structure and function relationships in order to realize the potential of these diverse and versatile solvents.

Recent advances in the nanoengineering of electrocatalysts for CO2 reduction

Emissions of CO2 from fossil fuel combustion and industrial processes have been regarded as the dominant cause of global warming. Electrochemical CO2 reduction (ECR), ideally in aqueous media, could potentially solve this problem by the storage of energy from renewable sources in the form of chemical energy in fuels or value-added chemicals in a sustainable manner. However, because of the sluggish reaction kinetics of the ECR, efficient, selective, and durable electrocatalysts are required to increase the rate this reaction. Despite considerable progress in using bulk metallic electrodes for catalyzing the ECR, greater efforts are still needed to tackle this grand challenge. In this Review, we highlight recent progress in using nanoengineering strategies to promote the electrocatalysts for the ECR. Through these approaches, considerable improvements in catalytic performance have been achieved. An outlook of future developments in applying and optimizing these strategies is also proposed.

Heterogeneous electrochemical CO2 reduction using nonmetallic carbon-based catalysts: current status and future challenges

Electrochemical CO₂ reduction (ECR) offers an important pathway for renewable energy storage and fuels production. It still remains a challenge in designing highly selective, energy-efficient, robust, and cost-effective electrocatalysts to facilitate this kinetically slow process. Metal-free carbon-based materials have features of low cost, good electrical conductivity, renewability, diverse structure, and tunability in surface chemistry. In particular, surface functionalization of carbon materials, for example by doping with heteroatoms, enables access to unique active site architectures for CO₂ adsorption and activation, leading to interesting catalytic performances in ECR. We aim to provide a comprehensive review of this category of metal-free catalysts for ECR, providing discussions and/or comparisons among different nonmetallic catalysts, and also possible origin of catalytic activity. Fundamentals and some future challenges are also described.

Electrochemical Reduction of Carbon Dioxide in a Monoethanolamine Capture Medium

The electrocatalytic reduction of CO₂ in a 30 % (w/w) monoethanolamine (MEA) aqueous solution was undertaken at In, Sn, Bi, Pb, Pd, Ag, Cu and Zn metal electrodes. Upon the dissolution of CO₂ , the non-conducting MEA solution is transformed into a conducting one, as is required for the electrochemical reduction of CO₂ . Both an increase in the electrode surface porosity and the addition of the surfactant cetyltrimethylammonium bromide (CTAB) suppress the competing hydrogen evolution reaction; the latter has a significantly stronger impact. The combination of a porous metal electrode and the addition of 0.1 % (w/w) CTAB results in the reduction of molecular CO₂ to CO and formate ions, and the product distribution is highly dependent on the identity of the metal electrode used. At a potential of -0.8 V versus the reversible hydrogen electrode (RHE) with an indium electrode with a coralline-like structure, the faradaic efficiencies for the generation of CO and [HCOO]^- ions are 22.8 and 54.5 %, respectively compared to efficiencies of 2.9 and 60.8 % with a porous lead electrode and 38.2 and 2.4 % with a porous silver electrode. Extensive data for the other five electrodes are also provided. The optimal conditions for CO₂ reduction are identified, and mechanistic details for the reaction pathways are proposed in this proof-of-concept electrochemical study in a CO₂ capture medium. The conditions and features needed to achieve industrially and commercially viable CO₂ reduction in an amine-based capture medium are considered.

Porous dendritic copper: an electrocatalyst for highly selective CO2 reduction to formate in water/ionic liquid electrolyte

Copper is currently extensively studied because it provides promising electrodes for carbon dioxide electroreduction. The original combination, reported here, of a nanostructured porous dendritic Cu-based material, characterized by electron microcopy (SEM, TEM) and X-ray diffraction methods, and a water/ionic liquid mixture as the solvent, contributing to CO2 solubilization and activation, results in a remarkably efficient (large current densities at low overpotentials), stable and selective (large faradic yields) electrocatalytic system for the conversion of CO2 into formic acid, a product with a variety of uses. These results provide new directions for the further improvement of Cu electrodes.