Copper-Tin Alloys for the Electrocatalytic Reduction of CO2 in an Imidazolium-Based Non-Aqueous Electrolyte

Tailoring Borate Mediator Species Enables Industrial CO Production with Improved Overall Energy Efficiency by Sustainable Molten Salt CO2 Electrolysis

The electrochemical conversion of CO2 into CO represents a promising strategy for mitigating excessive global greenhouse gas emissions. Nevertheless, achieving industrial-scale electrochemical CO2-to-CO conversion with enhanced selectivity and reduced energy consumption presents significant challenges. In this study, a borate-enhanced molten salt process for CO2 capture and electrochemical transformation is employed, achieving over 98% selectivity for CO and over 55% energy efficiency without the necessity for complex and costly electrocatalysts. Cathodic CO2 electro-reduction (CO2ER) with the anodic oxygen evolution reaction (OER) at an overall current density of 500 mA cm-2 using non-nanostructured transition-metal plate electrodes at 650 °C is coupled. By regulating the electrolyte's oxo-basicity with earth-abundant borax (Na2B4O7), a borate-enhanced electrolyte is established that accelerates the overall electrochemical reaction efficiently. This system involved a series of well-designed target borate species (BO3 3-, BO2 -, and B4O7 2-) that acted as mediators shuttling between the cathode and anode, favoring CO as the primary cathodic product. Manipulating the atmosphere above the anode facilitated a spontaneous transformation of borates, further enhancing OER performance with long-term operational stability over a cumulative period of 50 h, while also reducing overall energy consumption. This work presents a cost-effective strategy for the industrial-scale production of CO derived from CO2, contributing to a lower carbon footprint by establishing a sustainable borate-mediated closed loop.

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.

Main group elements in electrochemical hydrogen evolution and carbon dioxide reduction

Main-group elements are renowned for their versatile reactivities in organometallic chemistry, including CO2 insertion and H2 activation. However, electrocatalysts comprising a main-group element active site have not yet been widely developed for activating CO2 or producing H2. Recently, research has focused on main-group element-based electrocatalysts that are active in redox systems related to fuel-forming reactions. These studies have determined that the catalytic performances of heavier main-group element-based electrocatalysts are often similar to those of transition-metal-based electrocatalysts. Our group has recently reported the scope of including the main-group elements in the design of molecular catalysts and explored their applications in redox catalysis, such as the generation of H2 upon coupling of two protons (H+) and two electrons (e-). This feature article summarizes our research efforts in developing molecular electrocatalysts comprising main-group elements at their active sites. Furthermore, we highlight their influence on the rate-determining step, thereby enhancing the reaction rate and product selectivity for multi-H+/multi-e- transfer catalysis. Particularly, we focus on the performance of our recently reported molecular Sn- or Sb-centered macrocycles for electrocatalytic H2 evolution reaction (HER) and on how their mechanisms resemble those of transition-metal-based electrocatalysts. Moreover, we discuss the CO2 reduction reaction (CO2RR), another promising fuel-forming reaction, and emphasize the recent progress in including the main-group elements in the CO2RR. Although the main-group elements are found at the active sites of the molecular catalysts and are embedded in the electrode materials for studying the HER, molecular catalysts bearing main-group elements are not commonly used for CO2RR. However, the main-group elements assist the CO2RR by acting as co-catalysts. For example, alkali and alkaline earth metal ions (e.g., Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, and Ba2+) are known for their Lewis acidities, which influence the thermodynamic landscape of the CO2RR and product selectivity. In contrast, the elements in groups 13, 14, and 15 are primarily used as dopants in the preparation of catalytic materials. Overall, this article identifies main-group element-based molecular electrocatalysts and materials for HER and CO2RR.

Ionic liquid-based electrolytes for CO2 electroreduction and CO2 electroorganic transformation

CO₂ is an abundant and renewable C1 feedstock. Electrochemical transformation of CO₂ can integrate CO₂ fixation with renewable electricity storage, providing an avenue to close the anthropogenic carbon cycle. As a new type of green and chemically tailorable solvent, ionic liquids (ILs) have been proposed as highly promising alternatives for conventional electrolytes in electrochemical CO₂ conversion. This review summarizes major advances in the electrochemical transformation of CO₂ into value-added carbonic fuels and chemicals in IL-based media in the past several years. Both the direct CO₂ electroreduction (CO₂ER) and CO₂-involved electroorganic transformation (CO₂EOT) are discussed, focusing on the effect of electrocatalysts, IL components, reactor configurations and operating conditions on catalytic activity, selectivity and reusability. The reasons for the enhanced CO₂ conversion performance by ILs are also discussed, providing guidance for the rational design of novel IL-based electrochemical processes for CO₂ conversion. Finally, the critical challenges remaining in this research area and promising directions for future research are proposed.

Reversible Proton-Coupled Reduction of an Iron Nitrosyl Porphyrin within [DBU-H]+-Based Protic Ionic Liquid Nanodomains

The reduction of [Fe(OEP)(NO)] has been studied in the presence of aprotic room-temperature ionic liquids (RTIL) and protic (PIL) ionic liquids dissolved within a molecular solvent (MS). The cyclic voltammetric results showed the formation of RTIL nanodomains at low concentrations of the RTIL/PIL solutions. The pK_a values of the two PILs studied (i.e., trialkylammonium and [DBU-H]⁺-based ionic liquids) differed by four units in THF. While voltammetry in solutions containing all three RTILs showed similar potential shifts of the first reduction of [Fe(OEP)(NO)] to [Fe(OEP)(NO)]^- at low concentrations, significant differences were observed at higher concentrations for the ammonium PIL. The trialkylammonium cation had previously been shown to protonate the {FeNO}⁸ species at room temperature. Visible and infrared spectroelectrochemistry revealed that the [DBU-H]⁺-based PIL formed hydrogen bonds with [Fe(OEP)(NO)]^- rather than formally protonating it. Despite these differences, both PILs were able to efficiently reduce the nitrosyl species to the hydroxylamine complex, which could be further reduced to ammonia. On the voltammetric time scale and when the switching potential was positive of the Fe(II)/Fe(I) potential, the hydroxylamine complex was re-oxidized back to the NO complex via direct oxidation of the coordinated hydroxylamine at low scan rates or initial oxidation of the ferrous porphyrin at high scan rates. The results of this work show that, while [DBU-H]⁺ does not protonate electrochemically generated [Fe(OEP)(NO)]^-, it still plays an important role in efficiently reducing the nitroxyl ligand via a series of proton-coupled electron transfer steps to generate hydroxylamine and eventually ammonia. The overall reaction rates were independent of the PIL concentration, consistent with the nanodomain formation being important to the reduction process.

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.